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AS Biology Module 1 - Molecules and Cells > AS1 Chapter 1 - Molecules > Flashcards

AS1 Chapter 1 - Molecules Flashcards

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1
Q

Why is water a very good solvent?

A

Water is polar and is therefore very effective at dissolving other polar substances.

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2
Q

What does the term ‘polar’ describe?

A

Polar - A term used to describe molecules with an uneven distribution of charge.

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3
Q

The polar nature of water results in the formation of ________ _____ between adjacent water molecules.

A

Hydrogen bonds

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4
Q

The value of water as a solvent in living organisms includes:

A
  1. The fact that most of a cell’s reactions take place in an aqueous solution.
  2. It’s ability to act as a transport medium in living organisms.
  3. The different effects it has on hydrophilic (‘water loving’) and hydrophobic (‘water hating’) molecules. For example, lipids are hydrophobic and do not dissolve in water.
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5
Q

What factor affects protein shape and consequently function?

A

Changes in pH

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6
Q

What is a buffer?

A

A buffer is a chemical or substance that resists changes to pH (i.e. Minimises pH changes) and ensures that a particular environment maintains a particular pH.

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7
Q

Give two examples of buffers

A
  1. Hydrogencarbonate ions

2. Blood proteins such as albumin

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8
Q

What are ions?

A

Ions are atoms or a group of atoms that have an electrical charge due to the number of protons and electrons not balancing.

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9
Q

Name some biologically important ions.

A
  1. Calcium
  2. Iron
  3. Magnesium
  4. Potassium
  5. Nitrate
  6. Phosphate
  7. Hydrogen carbonate
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10
Q

Why is calcium an important biological ion?

A

Calcium (Ca2+) forms calcium pectate in plants, which is an essential component middle lamella of plant cell walls (e.g. It acts like a biological cement which sticks cells together).
Calcium is an essential component of bones and teeth in animals, and is essential in blood clotting and muscle contraction.

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11
Q

Why is iron an important biological ion?

A

Iron (Fe2+) is part of the haem group in haemoglobin and an important constituent of electron carriers in respiration.

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12
Q

Why is magnesium an important biological ion?

A

Magnesium (Mg2+) is essential in giving chlorophyll its light-absorbing properties.

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13
Q

Why is potassium an important biological ion?

A

Potassium (K+) is important in maintaining electrical gradients across neurones.

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14
Q

Why is nitrate an important biological ion?

A

Nitrate (NO3 -) is a component of amino acids, nucleic acids (which form RNA and DNA) and chlorophyll.

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15
Q

Why is phosphate an important biological ion?

A

Phosphate (PO4 3-) is a component of phospholipids which is an important component of cell membranes. It is also a major component of other important biological molecules including adenosine triphosphate (ATP) and nucleic acids.

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16
Q

Why is hydrogen carbonate an important biological ion?

A

Hydrogen carbonate (HCO3 -) is important as a natural buffer.

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17
Q

What is the name of the process by which monomers join together to form larger polymers?

A

Polymerisation

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18
Q

Polymerisation definition

A

The process by which monomers join together to form larger polymers.

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19
Q

What elements make up carbohydrates?

A

Carbon
Hydrogen
Oxygen

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20
Q

What is the ratio of hydrogen atoms to oxygen atoms in carbohydrates?

A

Hydrogen : Oxygen

2:1

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21
Q

What is a glycosidic bond?

A

A bond at is formed between two hexose sugars in the formation of a disaccharide.

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22
Q

What type of bond links adjacent glucose molecules in maltose and amylose?

A

Alpha 1-4 glycosidic bonds

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23
Q

What is the general formula for glucose?

A

C6H12O6

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24
Q

What are the three main types of carbohydrates?

A

Monosaccharides
Disaccharides
Polysaccharides

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25
Q

Name three monosaccharides

A

Glucose
Fructose
Galactose

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26
Q

Name three disaccharides

A

Maltose
Sucrose
Lactose

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27
Q

Name three polysaccharides

A

Cellulose
Starch
Glycogen

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28
Q

Substances such as the monosaccharides just discussed, which have the same molecular formula, but different structural formulae, are described as being _______ of each other.

A

Isomers

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29
Q

What are isomers?

A

Isomers are substances which share the same molecular formula, but have different structural formulae.

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30
Q

What is the difference between alpha glucose and beta glucose?

A

In B-glucose the hydrogen and hydroxyl group (-OH) groups at carbon 1 are reversed.

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31
Q

What forms maltose?

A

a-glucose + a-glucose

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32
Q

Name two storage polymers of alpha glucose.

A

Starch and glycogen

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33
Q

What general formula do all disaccharides share?

A

C12H22O11

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34
Q

What forms sucrose?

A

a-glucose + fructose

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35
Q

What forms lactose?

A

a-glucose + galactose

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36
Q

In what form are carbohydrates transported in the phloem of plants?

A

Sucrose

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37
Q

What is the general formula for polysaccharides?

A

(C6H10O5)n

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38
Q

What are the two polymers which make up starch, and give their relative abundance.

A

Amylose (20%)

Amylopectin (80%)

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39
Q

How often do branches occur in amylopectin?

A

The branches may occur as often as one branch every ten a-glucose monomers.

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40
Q

What type of bond links unbranded amylose chains together to form branched amylopectin?

A

Alpha 1-6 glycosidic bonds

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41
Q

Name a structural polymer of beta glucose.

A

Cellulose

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42
Q

Why is starch such a good storage molecule

A
  1. The molecules of both amylose and amylopectin are very compact (aided by the coiled configuration due to hydrogen bonds), therefore they contain a rich store of glucose in a small space.
  2. As it is insoluble it will not affect the water relations of the cell - if significant quantities of carbohydrate were stored as glucose this would cause a lot of water to enter by osmosis.
  3. Being a large molecule it can be retained in the cell and will not easily pass through the cell membrane.
  4. The branching nature of amylopectin creates many terminal ends that are easily hydrolysed. This aids in the rapid enzymatic breakdown of starch into its constituent glucose molecules at times of high respiratory demand.
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43
Q

Where is starch stored in plants?

A

Starch is stored as solid grains in plant chloroplasts.

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44
Q

Where is glycogen found?

A

Glycogen is found as small granules in animals (liver and muscle cells) and fungal cells.

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45
Q

What is the difference between glycogen and amylopectin?

A

Structurally, glycogen is very similar to amylopectin, being formed of chains of a-glucose monomers. Glycogen also has a-1,4 glycosidic and a-1,6 glycosidic bonds but the chains are more branched and shorter than the amylopectin chains.

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46
Q

Why is glycogen hydrolysed faster than amylopectin?

A
  1. All of the chains in glycogen are branched.
  2. The branches are shorter and more common.

Therefore it has proportionally more terminal ends for enzymatic action and faster hydrolysis.

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47
Q

What elements do proteins contain?

A 
Carbon
Hydrogen
Oxygen
Nitrogen
Sometimes sulfur
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48
Q

How many different types of amino acids are there?

A

20

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49
Q

What is the generalised structure of amino acids?

A

Amino group attached to the alpha carbon
Carboxyl group attached to the alpha carbon
Hydrogen atom attached to the alpha carbon
Varying R-group attached to the alpha carbon

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50
Q

What bonds link amino acids together?

A

Peptide bonds

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51
Q

What type of reaction forms peptide bonds between amino acids in a polypeptide chain? Name the products formed.

A

Condensation reactions

Water

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52
Q

What two groups on the amino acids structure form the peptide bond?

A

The amino group of one amino acid and the carboxylic group of another amino acid together form a peptide bond with the release of water.

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53
Q

Two amino acids joined together form a (blank).

A

Dipeptide

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54
Q

Many amino acids can be joined together in a chain through peptide bonding to form a (blank).

A

Polypeptide

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55
Q

What is a proteins primary structure?

A

The primary structure of a polypeptide (protein) is the sequence of the amino acids in the polypeptide chain.

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56
Q

What is a proteins secondary structure?

A

The amino acids in a polypeptide contain -NH and -C=O groups on either side of each peptide bond. The O of the -C=O group has a negative charge, whereas the H of the -NH group has a positive charge. This enables the formation of hydrogen bonds, which in turn cause generalised secondary structures to be formed.

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57
Q

What are the two most common types of generalised secondary structure?

A
  • a-helix - In the a-helix the hydrogen bonds are formed between amino acids occurring at regular intervals in the sequence. The bonds twist the chain of amino acids into a spiral or helical shape. Small sections (or the entire polypeptide) can be held in an a-helix shape by the hydrogen bonds involved.
  • B-pleated sheets - These are more rigid and less flexible configurations than the a-helix. They are formed by sections of the polypeptide chain, oriented in opposite (anti-parallel) directions, lying adjacent to each other. Hydrogen bonds form between the C=O and NH groups.
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58
Q

What is a proteins tertiary structure?

A

Tertiary structure - This involves the further folding of the secondary structure. This additional folding gives each protein it’s unique 3-D shape and is a consequence of the range of bonds formed between the R-groups of amino acids in the chain.

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59
Q

Name some of the bonds involved in the formation of the tertiary structure of a protein.

A
  • Hydrogen bonds form between R-groups in the chain.
  • Ionic bonds are formed between amino and carboxyl groups in some of the amino acid R-groups.
  • Disulfide bonds (bridges) are covalent bonds formed between R-groups of sulfur-containing amino acids.
  • Hydrophobic interactions involving amino acids with hydrophobic R-groups, which tend to take up positions within the molecule surrounded by other parts of the polypeptide.
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60
Q

What is a proteins quaternary structure?

A

Some proteins consist of two or more polypeptides bonded together (largely by disulfide bonds). Some quaternary proteins contain non-protein components (prosthetic groups) that are integral in their function.

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61
Q

Give two examples of conjugated proteins and state their function.

A

Glycoprotein - Important in membrane structure

Haemoglobin - Consists of four polypeptide chains (two each of two different polypeptides). Each chain is attached to an iron-rich haem group, which is an essential part of the molecule in the transport of oxygen.

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62
Q

What are the two groups of proteins?

A

Fibrous and globular

Fibrous proteins - consist of polypeptides arranged in chains that form fibres or sheets. The parallel chains are linked by cross-bridges to form very strong and stable molecules. Fibrous proteins are invariably structural in function.

Globular proteins - have a metabolic role and include enzymes and antibodies. The ability of protein to form very specific 3-D shapes is crucial to their role as enzymes and antibodies.

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63
Q

Give an example of a fibrous protein and explain its function.

A

Collagen - Each collagen molecule consists of three identical polypeptides wound round each other and held together by hydrogen bonds. Collagen is found in tendons that link muscle to bone. Obviously, it is very important that it is a very strong molecule and one that does not stretch when tension is applied.

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64
Q

Give an example of a globular protein and explain its function.

A

Haemoglobin

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65
Q

What is a prion?

A

Prion are a particular type of protein found in mammals and some other animal groups. They are found in the nervous system and it is thought they are involved in synaptic transmission.

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66
Q

How can disease-causing prions arise?

A
  1. The normal prion protein present (PrP) can ‘spontaneously’ adopt the PrPsc form.
  2. Mutations in the DNA that codes for the prion protein. Therefore the disease-causing form can be passed from parent to offspring.
  3. Through eating contaminated food that contains the disease-causing form.
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67
Q

Give the normal form of a prion protein.

A

PrP or PrPc

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68
Q

Give the disease-causing form of a prion protein.

A

PrPsc

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69
Q

What is the structural difference between normal prion proteins and disease-causing prion proteins?

A

Disease-causing prion proteins have a higher proportion of B-pleated sheets (compared to a-helices).

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70
Q

Name some prion diseases.

A
  • Scrapie that affects sheep.
  • Bovine spongiform encephalopathy (BSE) or ‘mad cow disease’, normally cattle are affected through eating contaminated food products.
  • Variant Creutzfeldt-Jakob Disease (vCJD), a human version of BSE that is normally acquired through eating contaminated beef products containing the PrPsc form.
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71
Q

How do disease-causing forms of prion proteins ‘replicate’?

A

If the disease-causing form is present in an individual, it acts as a template causing other prion proteins to convert to the disease-causing form. Once the disease-causing form is present in an organism, it leads to a chain reaction causing the number of other disease-causing prions to increase, ie the prion protein progressively becomes misfolded and changes from the PrP to the PrPsc form.

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72
Q

What is the sub-unit of nucleic acid?

A

The nucleotide

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73
Q

What components make up a nucleotide?

A
  1. A pentose (5 carbon) sugar
  2. An inorganic phosphate group
  3. An organic nitrogenous base
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74
Q

What type of reaction combines the components of a nucleotide?

A

Condensation reactions

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75
Q

What bond(s) links the pentose sugar to the base and phosphate in a nucleotide?

A

Phosphoester (in one nucleotide)

Phosphodiester (in a nucleic acid strand, polynucleotide)

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76
Q

A chain of nucleotides is known as …

A

A polynucleotide (nucleic acid)

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77
Q

Adjacent nucleotides can be combined by (blank) reactions to create the nucleic acid.

A

Condensation

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78
Q

What is a 5’ end and a 3’ end?

A

5’ end = Free phosphate

3’ end = Free sugar

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79
Q

What are the two types of nucleic acid?

A

Deoxyribonucleic acid

Ribonucleic acid

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80
Q

What is DNA?

A

DNA consists of two anti-parallel strands with the two strands being held together by hydrogen bonds between adjacent bases.

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81
Q

Name the four bases for DNA.

A

Adenine
Thymine
Guanine
Cytosine

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82
Q

What base pairs with adenine?

A

Thymine

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83
Q

What base pairs with thymine?

A

Adenine

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84
Q

What base pairs with guanine?

A

Cytosine

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85
Q

What base pairs with cytosine?

A

Guanine

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86
Q

How many hydrogen bonds connect adjacent adenine and thymine bases?

A

2

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87
Q

How many hydrogen bonds connect adjacent cytosine and guanine bases?

A

3

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88
Q

What is the shape of DNA?

A

A double helix

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89
Q

What are the three types of RNA?

A
  1. Messenger RNA (mRNA)
  2. Transfer RNA (tRNA)
  3. Ribosomal RNA (rRNA)
90
Q

Name five differences between DNA and RNA.

A
  1. Deoxyribose/ ribose
  2. Double stranded/ mainly single stranded
  3. Longer molecule/ relatively shorter
  4. Thymine / Uracil
  5. Chemically very stable/ chemically less stable
  6. Permanent/ exists for short periods of time
  7. Ratio of adenine to thymine and guanine to cytosine is constant/ ratio of adenine to uracil and guanine and cytosine varies.
91
Q

What is the role of RNA?

A

RNA is involved in protein synthesis.

92
Q

What is the role of messenger RNA in protein synthesis?

A

Carries the code from the DNA in the nucleus (the DNA remains protected in the nucleus) to a ribosome in the cytoplasm where protein synthesis takes place.

93
Q

What is the role of transfer RNA in protein synthesis?

A

Carries the amino acids to the mRNA/ribosome where protein synthesis takes place. It is a single chain folded into a ‘clover leaf’ shape. There are as many different types of tRNA as there are amino acids. Their structure is similar except for the part that links with the appropriate amino acid and the part that links with the mRNA.

94
Q

What is the role of ribosomal RNA in protein synthesis?

A

Ribosomal RNA is made in the nucleolus and forms over half the mass of each ribosome.

95
Q

What reaction forms glycosidic, ester, peptide and phosphodiester bonds?

A

Condensation reactions

96
Q

Draw an alpha glucose molecule.

A

Textbook

97
Q

Draw a beta glucose molecule.

A

Textbook

98
Q

Draw a fructose molecule.

A

Textbook

99
Q

Pentose sugars play important roles as …

A
  • Components of nucleic acids (ribose and deoxyribose)

* ATP (ribose)

100
Q

What reaction breaks down glycosidic, ester, peptide and phosphodiester bonds?

A

Hydrolysis reactions

101
Q

What are the main types of lipids?

A

Triglycerides (fats and oils)
Phospholipids
Waxes
Steroids

102
Q

What are triglycerides?

A

Triglycerides are condensation products of glycerol and fatty acids.

103
Q

What is released when a triglyceride is hydrolysed?

A

Three molecules of fatty acids

One molecule of glycerol

104
Q

What type of reaction forms ester bonds?

A

Condensation reactions

105
Q

What is the name of the bonds which link fatty acids to glycerol in triglycerides?

A

Ester bonds

106
Q

Draw the structure of a fatty acid.

A

Textbook

107
Q

Draw the structure of glycerol.

A

Textbook

108
Q

Draw the structure of a triglyceride.

A

Textbook

109
Q

What are the two types of fatty acid?

A

Saturated and unsaturated

110
Q

What is a saturated fatty acid?

A

Saturated fatty acids contain the maximum number of hydrogen atoms. Also, the carbon atoms are linked by C-C single bonds.

111
Q

What is an unsaturated fatty acid?

A

Unsaturated fatty acids have at least one C=C double bond in the chain.

112
Q

If there is one double bond in a fatty acid it is (blank). If there is more than one double bond in a fatty acid it is (blank).

A

Monounsaturated

Polyunsaturated

113
Q

Fats and oils are both …

A

Triglycerides

114
Q

Name physical and chemical properties of fats and oils

A
  • Fats are solid at room temperature
  • Fats are formed from saturated fatty acids or longer chains
  • Oils are liquid at room temperature
  • Oils are formed unsaturated fatty acids or shorter chains
115
Q

What is the structure of a phospholipid?

A

Phospholipids are similar to triglycerides, except that one of the fatty acids molecules is replaced by a phosphate group.

116
Q

What are the properties of phospholipids?

A

The fatty acid molecules repel water and are insoluble in water, forming hydrophobic ‘tails’, whereas the phosphate gives the glycerol part (‘head’) of the molecule hydrophobic properties and it is soluble in water. Phospholipids are polar molecules.

117
Q

Give a comparison between DNA and RNA.

A 
1. Subunits:
DNA - Deoxyribonucleotides (contain deoxyribose and thymine)
RNA - Ribonucleotides (contain ribose and uracil)
2. Length:
DNA - Very long
RNA - Relatively short
3. Strands:
DNA - Double stranded
RNA - Single stranded
4. Base pairing:
DNA - A with T and G with C
RNA - No base pairing
118
Q

Give three functions of triglycerides in living organisms

A
  1. Triglycerides are an excellent energy store as they release more energy per unit of mass than carbohydrate. Oils act as energy stores in plant seeds.
  2. Fats are important for insulation and are stored in a layer below the body surface in many animals.
  3. Many body organs are protected by a layer of fat.
  4. Buoyancy
  5. Waterproofing
119
Q

Past Paper Question June 2017 Q5 b) (Revised Spec. + Legacy Material)
Plant cells are composed of cellulose. Explain how the structure of cellulose provides plant cell walls with high tensile strength. [2]

A

b) It consists of beta glucose monomers which join together to make straight chains. Beta 1-4 glycosidic bonds form. Hydrogen bonds form cross-links between adjacent chains.

120
Q

DNA replication is described as a …

A

Semi-conservative mechanism

121
Q

Describe the processes involved in DNA replication

A

The sequence of events in DNA replication is as follows:

  1. The enzyme DNA helicase breaks the hydrogen bonds holding the base pairs together and ‘unzips’ part of the DNA double helix, revealing two strands.
  2. The enzyme DNA polymerase moves along each strand, which acts as a template for the synthesis of a new strand.
  3. DNA polymerase catalyses the joining of free deoxyribonucleotides to each of the exposed original strands, according to base pairing rules, so that new complementary strands form.
  4. The process of unzipping and joining new nucleotides continues along the whole length of the DNA molecule.

Each DNA molecule formed is identical to the other and to the original DNA (and contains one strand of the original).

122
Q

Draw a nucleotide

A

Textbook

123
Q

What experiment provides evidence for the semi-conservative model?

A

The Meselson and Stahl Experiment

124
Q

Outline the steps taken in the Meselson and Stahl Experiment

A

Meselson and Stahl cultured bacteria in heavy isotopes of nitrogen (15N). The bacteria were then transferred to a medium containing the lighter (normal) 14N. Key stages were:

  • Bacteria growing in 14N (before transfer to 15N).
  • Bacteria growing in 15N (many generations after transfer from 14N).
  • One generation after transfer back to 14N.
  • Two generations after transfer back to 14N.

Density-gradient centrifugation was used to separate the bacterial DNA following sampling at the stages listed above. DNA containing the lighter 14N accumulated in a zone near the top of the centrifuge tube, whereas DNA consisting of ‘heavy’ 15N formed a zone near the bottom of the centrifuge tube.

Explanation of Meselson and Stahl’s results:
• After one generation - The intermediate position of the DNA can be explained by all the DNA consisting of one strand that has bases containing 15N and one strand having bases containing 14N.
• After two generations - About half the DNA consisted of ‘mixed’ DNA of both 14N and 15N but the other half was DNA that only contained 14N.

125
Q

What are the condensation products of a pentose sugar, an organic nitrogenous base and an inorganic phosphate group?

A

Water

Nucleotide

126
Q

What are the condensation products of nucleotides?

A

Water

Nucleic acid

127
Q

Practical Work

What test can be carried out to detect the presence of a reducing sugar? Describe this test.

A

Benedict’s test
When a reducing sugar is mixed with Benedict’s reagent and heated in a hot water bath, the blue reagent will change the through the sequence blue-green-yellow-orange to form a brick red precipitate.

128
Q

What is a reducing sugar?

A

A reducing sugar is one which can donate electrons (or reduce) Benedict’s reagent.

129
Q

Name some reducing sugars

A

All monosaccharides

Maltose

130
Q

Name some non-reducing sugars

A

Sucrose

131
Q

Practical Work

What test can be carried out to detect the presence of a non-reducing sugar? Describe this test.

A

A non-reducing sugar can be identified through giving a negative Benedict’s test result initially, but if hydrolysed into its constituent monosaccharides with dilute hydrochloric acid (and subsequently neutralised with, for example, sodium hydrogen carbonate), it gives a positive result.

132
Q

Practical Work

Name three carbohydrate tests and the substance which they identify.

A
  1. Benedict’s Test = Reducing sugar (and non-reducing sugar if test is modified).
  2. Glucose-Specific Tests = Clinistix test identifies the presence of glucose.
  3. Iodine Test = Detects the presence of starch.
133
Q

Practical Work

What test can be carried out to detect the presence of glucose? Describe this test.

A

Clinistix/diastix test

The Clinistix strip is dipped in the test solution and if glucose is present, it will change colour.

134
Q

Practical Work

What test can be carried out to detect the presence of starch? Describe this test.

A

Iodine Test - Starch changes the colour of iodine from yellow- brown to blue- black.

135
Q

Practical Work

What test can be carried out to detect the presence of protein in a food sample? Describe this test.

A

Biuret Test - Add potassium/sodium hydroxide to the test sample then add a few drops of copper sulfate solution. Positive test = Blue —> Mauve/lilac

136
Q

Practical Work - Sample Question

Describe how you would compare the amount of sugar in chocolate and onion.

A

Procedure:

  1. Weigh 5g of white chocolate and 5g of onion. Grind each into a paste (add equal volumes of water if necessary).
  2. Add the chocolate and onion separately to equal volumes of Benedict’s reagent (eg 5cm^3) in two boiling tubes.
  3. Heat the boiling tubes in the same beaker which acts as a water bath.
  4. Compare the colours of the Benedict’s reagent in the two boiling tubes after the same length of time or when there is no further colour change.
  5. Weigh another 5g of white chocolate and 5g of onion, and grind each into a paste (add equal volumes of water if necessary).
  6. Heat with hydrochloric acid in a water bath.
  7. Neutralise with sodium hydrogencarbonate.
  8. Redo the Benedict’s test.

Comment:

  1. Measuring mass is more accurate than volume as it can be difficult to cut to an exact size. Also a top-pan balance can measure mass to one or two decimal places making it very precise. White chocolate will mask the colour of Benedict’s reagent much less than brown chocolate.
  2. Equal volumes of Benedict’s reagent as a controlled variable.
  3. Ensures that each boiling tube is heated at the same temperature for the same time.
  4. This indicates the relative amounts of reducing sugar in white chocolate and onion.
  5. This hydrolyses any non-reducing sugar present.
  6. Comparison with tubes from the first Benedict’s test will indicate if non-reducing sugar is present.

Note 1: this procedure allows comparison of both reducing and non-reducing sugar between the two foods.

Note 2: if both the chocolate and the onion contain enough sugar to make the Benedict’s solution turn brick red, you can measure the time taken to turn brick red.

Note 3: you could use a colorimeter to more accurately compare the colour changes. If doing so you would need to filter the solutions to remove the precipitate.

137
Q

Past Paper - January 2014 AS1 (Legacy Material)
Q2 The technique of chromatography can be used to separate, and subsequently identify, substances in solution. When setting up a chromatogram, certain procedures are followed.
a) Describe two procedures, relevant to setting up a chromatogram, that would ensure valid results. [2]

A

Any two from:
• Solvent should be placed in sealed chromatography jar/container beforehand (to allow vapour to saturate jar).
• Avoid touching chromatography paper with bare hands.
• All marks on chromatography paper should be in pencil.
• Solutions under investigation should not be placed too close together/ too close to the edge of chromatography paper/ samples are same distance from bottom (starting from same point)
• Solutions should be concentrated by repeated application in the same position.
• Chromatography paper should not touch the sides of the jar/ container.

138
Q

Describe how one would prepare a chromatogram

A
  1. The chromatography paper is cut to fit the tank/vessel used to hold it in a vertical position.
  2. The paper (chromatogram) should be long enough to allow attachment to the lid of the apparatus and to drop to just above the base of the tank.
  3. A horizontal line should be drawn in pencil a few centimetres above the base of the chromatogram. It is important for the line to be drawn in a position that will lie above the solvent when the chromatogram is placed in the solvent.
  4. The solution(s) containing amino acids to be tested need to be ‘spotted’ on the pencil line. Each solution is added to a pre-determined position by a micro-pipette. After adding a drop of the solution it is then dried before the process is repeated. This allows the solution to be concentrated.
  5. It is important to ensure that the concentrated spot forms as small an area as possible. This can be aided by ensuring that the spot is dry before adding additional drops of solution. In addition, it is important to avoid contamination of the chromatogram. Only hold the chromatogram at the edges and avoid setting it on laboratory benches that could be contaminated with a range of chemicals. Before placing the prepared chromatogram in the tank, the solvent should be added and the lid placed on top to allow the atmosphere to become saturated.
139
Q

Describe how one would run a chromatogram

A

The chromatogram is carefully suspended into the solvent and attached to the lid of the tank. It is important to ensure that:

  • The line (and concentrated spot(s)) do not make contact with the solvent.
  • The chromatogram is securely attached.
  • The chromatogram is not suspended at an angle - if at an angle the solvent cannot ‘run’ the length of the chromatogram.

As the solvent ‘runs’ up the chromatogram, it carries the amino acids, although they will not be visible at this stage. The ‘run’ of the solvent should be stopped when it is well up the chromatogram but before it reaches the top.

140
Q

Describe how one would develop the chromatogram

A

The chromatogram should then be dried. Before the solvent is dry, it is important to mark the ‘solvent front’ on the chromatogram with a pencil. As amino acids are colourless on the chromatogram, it should be sprayed with ninhydrin in a fume cupboard (this is necessary as ninhydrin can be harmful if breathed in).
Following staining, the chromatogram should be re-dried (with a hairdryer or in an oven) and the amino acids will appear as purple spots (proline is an exception as it appears yellow). The spots should be encircled with a pencil as they subsequently fade.

141
Q

What is the Rf value?

A

The Rf value is the distance moved by a solute divided by the distance moved by the solvent front.

142
Q

Past Paper Question June 2017 Q7 a)ii) (Revised Spec.)
When a tadpole changes to an adult frog, its tail is broken down and reabsorbed into the body. A major component of the tail is the protein collagen, which provides support.
Collagen in the tail is broken down by the enzyme collagenase, which has a zinc cofactor.
a)ii) Describe how the structure of the protein collagen is related to its function of support. [3]

A

a)(ii) Any three from:
• individual collagen chains show α-helix polypeptide chains/secondary structure helix present in chains/functional collagen
comprised of three polypeptide chains;
• held by hydrogen bonds;
• three chains twisted around each other;
• very strong/does not stretch with tension/insoluble; [3]

143
Q

Past Paper Question - June 2017 Q2 a) AS1 (Revised Spec. + Legacy Material)
Q2 Fatty acids can be saturated or unsaturated. Palmitic acid is a saturated fatty acid (C15H31COOH) and alpha-linolenic acid is an unsaturated fatty acid (C17H29COOH).
a) Define the term ‘unsaturated’. [1]

A

Q2 a) Has one or more carbon-carbon double bonds/ contains double bonds between carbon atoms. [1]

144
Q

Past Paper Question - June 2017 Q2 b)ii) AS1 (Revised Spec. + Legacy Material)
Q2 Fatty acids can be saturated or unsaturated. Palmitic acid is a saturated fatty acid (C15H31COOH) and alpha-linolenic acid is an unsaturated fatty acid (C17H29COOH).
b) The diagram below represents a triglyceride containing three palmitic acid chains.
ii) Describe how the structure of a phospholipid differs from a triglyceride. [1]

A

b)ii) One fatty acid chain replaced with a phosphate group. [1]

145
Q

Past Paper Question - June 2017 Q2 c)i) AS1 (Revised Spec. + Legacy Material)
Q2 Triglycerides perform several functions in living organisms. They are synthesised in the smooth endoplasmic reticulum of cells and can be broken down in a number of metabolic processes.
c)i) Name the types of reactions that occur during the synthesis and breakdown of triglycerides.
Synthesis __________________
Breakdown _________________ [2]

A

Q2 c)i) Synthesis; Condensation

Breakdown; Hydrolysis [2]

146
Q

Past Paper Question - June 2017 Q2 c)ii) AS1 (Revised Spec. + Legacy Material)
Q2 Triglycerides perform several functions in living organisms. They are synthesised in the smooth endoplasmic reticulum of cells and can be broken down in a number of metabolic processes.
c)ii) State two functions of triglycerides in living organisms. [2]

A 
Q2 c)ii) Any two from:
• Energy store/ respiratory substance
• Insulation
• Buoyancy
• Organ protection
• Waterproofing [2]
147
Q

Past Paper Question - June 2017 Q5 a) AS1 (Revised Spec. + Legacy Material)
Q5 a) Plants require a number of inorganic ions in order to synthesise biologically important compounds. State the name of one biologically important compound in plants which contains calcium and one which contains magnesium.
Calcium ______________
Magnesium ____________ [2]

A

Calcium; Calcium Pectate

Magnesium; Chlorophyll [2]

148
Q

Past Paper Question - June 2017 Q7a) AS1 (Legacy Material)
Q7 Chromatography is a technique which can be used to identify amino acids in a solution. The solution under investigation is analysed and compared with known amino acids.
a) Describe the process used to prepare a chromatogram before it is placed in the solvent. [4]

A

a) Any four from:
• Make sure the chromatography paper is only handled at the edges/ use gloves when handling the chromatography paper/ cut paper to fit the chromatography jar.
• Draw a line a set distance from the bottom of the chromatography paper, above the solvent level.
• In pencil.
• Ensure adequate spacing between each spot.
• Make a concentrated spot for each amino acid/ spot each amino acid three times.

149
Q

Past Paper Question - June 2017 Q7b)ii) AS1 (Legacy Material)
Q7 b)ii) In another investigation it was found that the amino acid serine had an Rf value of 0.31. Suggest an explanation for the different values of serine. [1]
Note: table shows the Rf value for serine to be 0.27.

A

b)ii) A different solvent was used to run this chromatogram/ other appropriate response. [1]

150
Q

Past Paper Question - January 2014 Q2a) AS1
Q2 The technique of chromatography can be used to separate, and subsequently identify, substances in solution. When setting up a chromatogram, certain procedures are followed.
a) Describe two procedures, relevant to setting up a chromatogram, that would ensure valid results. [2]

A

Q2 a) Any two from:
• Solvent should be placed in sealed chromatography jar/container beforehand (to allow vapour to saturate jar).
• Avoid touching chromatography paper with bare hands.
• All marks on chromatography paper should be in pencil.
• Solutions under investigation should not be placed too close together/ too close to the edge of chromatography paper/ samples are same distance from bottom (starting from same point).
• Solutions should be concentrated by repeated application in the same position.
• Chromatography paper should not touch the sides of the jar/ container. [2]

151
Q

Past Paper Question - June 2017 Q7c)i) AS1 (Legacy Material)
Q7 Chromatography is a technique which can be used to identify amino acids in a solution. The solution under investigation is analysed and compared with known amino acids. The mass of an amino acid can be measured in Daltons (Da). The table below shows the mass of the amino acids in the chromatogram and their Rf values.

Amino Acid Mass/Da Rf Value
Taurine 125 0.33
Glycine 75 0.16
Phenylalanine 165 0.68
Methionine 149 0.55
Serine 105 0.27

c)i) Using the information for mass and Rf values in the table plot a scatter diagram on the graph paper provided. (Include a caption). [4]

A

Q7c)i) Caption = Scatter diagram showing the relationship between mass and Rf of amino acids;
Appropriate scaling;
Axes labelled including units;
Corrects plotting of points (not joined by lines), i.e. Do not connect points with straight lines. Do not draw a line of best fit either, it is only a scatter diagram. [4]

152
Q

Past Paper Question - June 2017 Q7c)ii) AS1 (Legacy Material)
Q7 Chromatography is a technique which can be used to identify amino acids in a solution. The solution under investigation is analysed and compared with known amino acids. The mass of an amino acid can be measured in Daltons (Da). The table below shows the mass of the amino acids in the chromatogram and their Rf values.

Amino Acid Mass/Da Rf Value
Taurine 125 0.33
Glycine 75 0.16
Phenylalanine 165 0.68
Methionine 149 0.55
Serine 105 0.27

c)i) Using the information for mass and Rf values in the table plot a scatter diagram on the graph paper provided. (Include a caption). [4]
c)ii) Describe and suggest an explanation for the trend shown in the scatter diagram. [2]
Note: Do not answer c)i)

A

Q7 c)ii) Positive correlation between mass and Rf/ the amino acids with the larger mass travel further up the chromatogram;
Larger amino acids are more soluble in the solvent used; [2]

153
Q

Past Paper Question - June 2016 Q2 b) AS1 (Legacy Material)
Q2 The diagram below illustrates a reversible reaction involving two amino acids.
b) Suggest a possible role of the R-groups shown on the amino acids. [1]

A

Q2 b) Bonding properties/ solubility/ polarity/ identity/ pH/ formation of secondary, tertiary or quaternary structure. [1]

154
Q

Past Paper Question - June 2016 Q4 a)i) AS1 (Legacy Material)
Q4 In order to complete its life cycle, a virus must enter a host cell via the cell surface membrane. Several types of molecules are found in the cell membranes of animal cells, including cholesterol and glycoproteins.
a)i) Identify the group of proteins to which glycoproteins belong. Explain your answer. [2]

A

Q4 a)i) Conjugated proteins;

Structure consists of a polypeptide combined with a non-protein group/ sugar/carbohydrate residue; [2]

155
Q

Past Paper Question - June 2016 Q6 b) AS1 (Legacy Material)
Q6 b) Both lipids and carbohydrates are used as energy storage molecules. Lipids store more energy per gram than carbohydrates but are not as easily broken down. Using this information, suggest why animals use both glycogen and lipids as energy stores. [2]

A

Q6 b) Glycogen is hydrolysed rapidly/provides rapid energy source;
Animals move so therefore cannot be too massive/heavy/movement is not restricted by mass/ same amount of energy is stored in a smaller mass; [2]

156
Q

Past Paper Question - June 2016 Q6 c) AS1 (Legacy Material)
Q6 c) A student tested a sample of a carbohydrate solution for the presence of reducing sugar and obtained a negative result. It was suggested that the solution contained sucrose. Describe the procedure the student would use in order to show that sucrose was present. Explain a suitable safety precaution in your procedure. [5]

A

Q6 c) Appropriate safety precaution explained;
Any four from:
• Add dilute (1M) hydrochloric acid to sample
• Heat in a water bath
• Neutralise with dilute sodium hydrogencarbonate/sodium hydroxide
• Add Benedict’s reagent and heat again in a water bath
• (Green/yellow)/ brick red precipitate indicates presence of reducing sugar [5]

157
Q

Past Paper Question - June 2015 Q4 a) AS1 (Legacy Material)

Q4 a) Describe the process of DNA replication in cells. [4]

A

Q4 a) Any four from:
• DNA is unzipped by breaking of hydrogen bonds
• this is catalysed by DNA helicase
• each strand acts as a template/semi-conservative mechanism
• free nucleotides are attracted to their complementary bases/base
pairing occurs, A with T and C with G
• (phosphodiester) bonds form between the deoxyribose and the
phosphate of adjacent nucleotides
• catalysed by DNA polymerase [4]

158
Q

Past Paper Question - June 2015 Q4 c)i) AS1 (Legacy Material)
Q4 c) Cancer occurs when cells divide more frequently than normal, resulting in a mass of cells called a tumour. One type of treatment for cancer is chemotherapy, which involves drugs which interfere with the cell cycle in cancer cells.
Two chemotherapy drugs, A and B, are effective because they prevent DNA replication within the cancer cells. This reduces the rate of cell division within a tumour.
Drug A contains platinum, a metallic element, that forms strong cross-links between the two strands in a DNA molecule.
Drug B is similar in structure to a nucleotide but contains three phosphates instead of one.

(i) Suggest how drug A and drug B prevent DNA replication within the cancer cells. [2]

Drug A _______________________________________

Drug B _______________________________________

A

Q4 c)i) Drug A: prevents the DNA being unzipped (due to cross linkage);
Drug B: prevents addition of further nucleotides to the spine (due to three phosphates instead of one); [2]

159
Q

Past Paper Question - June 2015 Q5 a)i) AS1 (Legacy Material)
Q5 a) Plant cells contain a variety of carbohydrate molecules, including starch and cellulose.
(i) Name the type of bond which is found between the monomers (subunits) in both starch and cellulose. [1]

A

Q5 a)i) Glycosidic; [1]

160
Q

Past Paper Question - June 2015 Q5 a)ii) AS1 (Legacy Material)
Q5 a) Plant cells contain a variety of carbohydrate molecules, including starch and cellulose.
ii) Complete the table below to show the function of each of these molecules, and their location within a cell.

Carbohydrate Function Location in cell

Starch

Cellulose

[4]

A

Q5 a)ii)

Carbohydrate Function Location in cell

Starch energy store/ storage of glucose chloroplasts/ cytoplasm

Cellulose provide support/ strength/rigidity cell walls

[4]

161
Q

Past Paper Question - June 2015 Q5 b)i)+ii)+iii) AS1 (Legacy Material)
Q5 b) Sucrose is another carbohydrate found in plants and is abundant in nectar. Honeybees collect nectar and take it back to their hive, where they convert it into honey. This process involves the breakdown of some of the sucrose into its monomers.

(i) State precisely the carbohydrate group to which sucrose belongs. ______________________________ [1]

(ii) Name the type of reaction involved in the breakdown of sucrose into its monomers.
______________________________ [1]

(iii) Identify the products of this reaction.
_______________________ and _______________________ [1]

A

Q5 (b) (i) Disaccharides; [1]
(ii) Hydrolysis; [1]
(iii) Glucose and fructose; [1]

162
Q

Past Paper Question - June 2015 Q5 c) AS1 (Legacy Material)
Q5 Sucrose is another carbohydrate found in plants and is abundant in nectar. Honeybees collect nectar and take it back to their hive, where they convert it into honey. This process involves the breakdown of some of the sucrose into its monomers.

c) In addition to sucrose, honey also contains other carbohydrates made by honeybees. These include maltose and erlose, a molecule first discovered in honey.

The structure of erlose is shown in the diagram below.
Diagram: Sucrose molecule attached to a glucose monomer.

It is thought that the honeybees use sucrose to make erlose. Describe precisely how the structure of erlose differs from that of sucrose. [2]

A

Q5 c)
• Erlose has one more monomer than sucrose/consists of three monosaccharides (rings)/is a trisaccharide;
• Erlose consists of two glucose plus a fructose/a glucose molecule
has been added to the sucrose; [2]

163
Q

Past Paper Question - January 2014 Q1 AS1 (Legacy Material)
Q1 Read the following passage about the structure of DNA and chromosomes, and write the most appropriate word in each blank space to complete the account.

A DNA molecule consists of many repeating units called _________ which are joined by _________ reactions to form the sugar-phosphate backbone. The double helix consists of two single _________ strands held together by __________ bonds between ___________
nitrogenous bases. Chromosomes are formed when the DNA coils round proteins called _________. [5]

A 
Q1 Nucleotides 
condensation
antiparallel 
hydrogen 
complementary 
histones [5]
164
Q

Past Paper Question - January 2014 Q2 b)+c) AS1 (Legacy Material)
Q2 A chromatogram was prepared to identify four carbohydrates. The following carbohydrate solutions were applied to the origin:

● Glucose
● Fructose
● Maltose treated with a-glucosidase (hydrolyses glycosidic bonds)
● Sucrose treated with a-glucosidase (hydrolyses glycosidic bonds)

One solution was added to each of the four lanes on the origin. The resulting chromatogram is shown below.

b) Calculate the Rf value for the carbohydrate in Lane 1. (Show your working.) [2]

c) Lanes 1 and 2 contain the monosaccharides. Lanes 3 and 4 contain the hydrolysed disaccharides.

Identify the carbohydrate added to each of the lanes 1–4.

1 _______________________________
2 _______________________________
3 _______________________________
4 _______________________________ [3]

(Go do this past paper question)

A

Q2 b) Distance to solvent front = 74 mm, distance to middle of spot = 37 mm;
Rf = 37/74 = 0.5 [consequential to values above]; [2]

c) Fructose
Glucose
Maltose
Sucrose [3]

165
Q

Past Paper Question - June 2015 Q2 d) AS1 (Legacy Material)
Q2 d) A starch solution was treated with a-glucosidase (hydrolyses glycosidic bonds), so that the starch was fully broken down. Give the full name of the monosaccharide which would be present in the resulting solution. [1]

A

Q2 d) Alpha-glucose; [1]

166
Q

Past Paper Question - January 2014 Q6 a) AS1 (Legacy Material)
Q6 Proteins can be described as organic macromolecules.
a) Explain the term ‘organic macromolecule’. [2]

A

Q6 a) A large molecule/molecule consisting of subunits;
including carbon (and hydrogen)/originating in living organisms; [2]

167
Q

Past Paper Question - January 2014 Q6 b)i+ii)
Q6 Proteins can be described as organic macromolecules.
b) Gluten is a type of protein found in grains such as wheat, barley and rye. It is a ‘composite’ protein, made up of two simpler proteins called glutenin and gliadin. The following diagram represents the primary structure of part of a gliadin molecule.

i) Identify the building blocks of proteins, as represented by circles in the diagram. [1]
ii) Identify the type of bond labelled X. [1]

(Go do this past paper question)

A

Q6 b)i) Amino acid; [1]

ii) Peptide; [1]

168
Q

Past Paper Question - June 2014 Q5 a)i) AS1 (Legacy Material)
Q5 a) A student tested five solutions (A–E) with Benedict’s reagent, Biuret reagent, iodine solution and Clinistix. The student recorded the results in the following manner:

• When tested with Benedict’s reagent, solutions A and C both produced a brick-red precipitate.

• When tested with Biuret reagent, only solution B produced a purple colour.

• When tested with iodine solution, only solution D turned blue-black.

• Solution E produced no colour change with any of the reagents.

• When tested with Clinistix, only solution A gave a positive result.

i) In the space below, construct a table of the results obtained by this student. Your table should include the following:

• appropriate column headings
• positive test results recorded with a (tick) and negative results
with an X. All boxes should be filled.

No caption is required. [3]

A

Q5 a)i) Test solutions in left-hand column/top row;
reagents in next four columns/rows (in any order);
results as (tick) or X; [3]

|Test solution|Benedict’s reagent|Biuret reagent|Iodine|Clinistix|
A Tick X X Tick
B X Tick X X
C Tick X X X
D X X Tick X
E X X X X [3]

169
Q

Past Paper Question - June 2014 Q5 a)ii) AS1 (Legacy Material)
Q5 a) A student tested five solutions (A–E) with Benedict’s reagent, Biuret reagent, iodine solution and Clinistix. The student recorded the results in the following manner:

• When tested with Benedict’s reagent, solutions A and C both produced a brick-red precipitate.

• When tested with Biuret reagent, only solution B produced a purple colour.

• When tested with iodine solution, only solution D turned blue-black.

• Solution E produced no colour change with any of the reagents.

• When tested with Clinistix, only solution A gave a positive result.

ii) Suggest a possible identification for each of the substances present in solutions A to D. [4]

A

Q5 a)ii) A: glucose; B: protein;
C: fructose/galactose/maltose/lactose/other mono- or disaccharide (except sucrose);
D: starch; [4]

170
Q

Past Paper Question - June 2014 Q5 a)iii) AS1 (Legacy Material)
Q5 a) A student tested five solutions (A–E) with Benedict’s reagent, Biuret reagent, iodine solution and Clinistix. The student recorded the results in the following manner:

• When tested with Benedict’s reagent, solutions A and C both produced a brick-red precipitate.

• When tested with Biuret reagent, only solution B produced a purple colour.

• When tested with iodine solution, only solution D turned blue-black.

• Solution E produced no colour change with any of the reagents.

• When tested with Clinistix, only solution A gave a positive result.

iii) Describe how the test with Benedict’s reagent would have been carried out. [1]

A

Q5 a)iii) Mix a small amount of the test solution with an equal volume of Benedict’s reagent and heat; [1]

171
Q

Past Paper Question - June 2014 Q5 b) AS1 (Legacy Material)
Q5 a) A student tested five solutions (A–E) with Benedict’s reagent, Biuret reagent, iodine solution and Clinistix. The student recorded the results in the following manner:

• When tested with Benedict’s reagent, solutions A and C both produced a brick-red precipitate.

• When tested with Biuret reagent, only solution B produced a purple colour.

• When tested with iodine solution, only solution D turned blue-black.

• Solution E produced no colour change with any of the reagents.

• When tested with Clinistix, only solution A gave a positive result.

b) After carrying out the tests outlined in (a), the student wished to identify substance E. After hydrolysis of E, it was found that the resulting solution now tested positive with both Benedict’s reagent and Clinistix.
Suggest which substance was originally present in solution E and give a reason for your answer.

Substance E ____________________
Reason ________________________ [2]

A

Q5 b) Sucrose;
a non-reducing disaccharide/sugar which hydrolyses into glucose and fructose (reducing sugars); [2]

172
Q

Past Paper Question - June 2014 Q6 a)i) AS1 (Legacy Material)
Q6 Proteins comprise a large group of organic molecules with a wide variety of functions. The specific function of a protein depends on its shape which is determined by its sequence of amino acids.

(a) (i) Identify the elements which are present in all proteins. [1]

A

Q6 a)i) C, H, O, N (not S); [1]

173
Q

Past Paper Question - June 2014 Q6 a)ii) AS1 (Legacy Material)
Q6 Proteins comprise a large group of organic molecules with a wide variety of functions. The specific function of a protein depends on its shape which is determined by its sequence of amino acids.

ii) Explain what is meant when a protein is said to have a quaternary structure. [1]

A

Q6 a)ii) The protein consists of two or more polypeptide chains; [1]

174
Q

Past Paper Question - June 2014 Q6 b)i) AS1 (Legacy Material)
Q6 Proteins comprise a large group of organic molecules with a wide variety of functions. The specific function of a protein depends on its shape which is determined by its sequence of amino acids.

Some features of four human proteins are described below.

Keratin is the major component of hair and nails. Its structure consists of a repeating pattern of a sequence of seven amino acids.

Trypsin is an enzyme found in the small intestine, where it is involved in the digestion of proteins in food.

Collagen is found in skin, where it maintains elasticity, and in tendons, where it provides strength.

Mucin is found in saliva, where it makes food slippery and thus assists its passage from the mouth to the stomach. Its structure includes many carbohydrate chains attached to the protein.

b)i) From the list above, select a protein which could be categorised as follows.
(Each protein may be used once, more than once, or not at all.)

  • a conjugated protein ____________________________
  • a fibrous protein _________________________________
  • a globular protein ________________________________
  • a protein which catalyses hydrolysis ____________________ [4]
A

Q6 b)i) Mucin;

keratin/collagen;
mucin/trypsin;
trypsin; [4]

175
Q

Past Paper Question - June 2014 Q6 b)ii) AS1 (Legacy Material)
Q6 Proteins comprise a large group of organic molecules with a wide variety of functions. The specific function of a protein depends on its shape which is determined by its sequence of amino acids.

Some features of four human proteins are described below.

Keratin is the major component of hair and nails. Its structure consists of a repeating pattern of a sequence of seven amino acids.

Trypsin is an enzyme found in the small intestine, where it is involved in the digestion of proteins in food.

Collagen is found in skin, where it maintains elasticity, and in tendons, where it provides strength.

Mucin is found in saliva, where it makes food slippery and thus assists its passage from the mouth to the stomach. Its structure includes many carbohydrate chains attached to the protein.

b)ii) Shampoo manufacturers sometimes state that their product contains amino acids.
Suggest why amino acids in shampoo are unlikely to be of use in the production of keratin in hair. [1]

A

Q6 b)ii) The keratin is produced in cells, not within the hair/ amino acids are obtained from the diet, rather than applied to the body/other appropriate response; [1]

176
Q

Past Paper Question - June 2014 Q6 e) AS1 (Legacy Material)
Q6 Proteins comprise a large group of organic molecules with a wide variety of functions. The specific function of a protein depends on its shape which is determined by its sequence of amino acids.

e) Research in protein biochemistry has been greatly enhanced by the use of molecular modelling software which allows users to view the three-dimensional structure of a protein. Often, scientists researching a particular protein will make the molecular modelling file for that protein available to download via the Internet.
Suggest an advantage of this file-sharing for scientific research. [1]

A

Q6 e) Research groups working in different parts of the world can study the findings of other teams easily/may be of use to teams working on different areas of biology (or by example)/other appropriate response; [1]

177
Q

Past Paper Question - January 2013 Q2 a) AS1 (Legacy Material)
Q2 a) Identify two similarities and two differences between polysaccharides and triglycerides.

Similarities:

  1. __________________________
  2. __________________________

Differences:

  1. __________________________
  2. __________________________ [4]
A

Q2 a) Similarities
Any two from:
• both contain only carbon, hydrogen and oxygen
• both are produced by condensation reactions/are broken down by
hydrolysis reactions
• both may be used as energy storage/release energy during respiration [both contain/produce energy is not acceptable]
• both are insoluble/have no osmotic effect

Differences
Any two from:
• polysaccharides are carbohydrates (starch, glycogen and cellulose) while triglycerides are lipids (fats and oils)
• polysaccharides are made from monosaccharides (glucose) while triglycerides contain glycerol and fatty acids
• polysaccharides are polymers/many monomers, triglycerides are not
• polysaccharides contain glycosidic bonds, while triglycerides contain ester bonds
• fats contain much less oxygen than polysaccharides
• fats contain more energy per gram than polysaccharides
• triglycerides are hydrophobic, polysaccharides are not

178
Q

Past Paper Question - January 2013 Q2 b) AS1 (Legacy Material)
Q2 b) Name the biochemical tests used specifically to identify starch (a polysaccharide) and glucose. [1]

Starch ____________________
Glucose __________________

A

Q2 b) Iodine (for starch) and Clinistix (for glucose); [1]

179
Q

Past Paper Question - January 2013 Q5 a)i)+ii)+b) AS1 (Legacy Material)
Q5 Proteins may be classified as globular or fibrous. The structural properties of globular and fibrous proteins are related to their functions.

a) Collagen is a fibrous protein and comprises approximately 35% of the total protein in the human body. It is a major constituent of connective tissues, including tendons.

The diagram below represents the structure of collagen.

i) Using the information in the diagram, identify evidence for the presence of a secondary and a quaternary structure in collagen. [2]

Secondary structure __________________________________________
________________________________________________________________________

Quaternary structure __________________________________________
________________________________________________________________________

ii) Suggest how the structure of collagen, as shown in the diagram, relates to its function. [1]

b) In both fibrous and globular proteins, hydrophobic and hydrophilic interactions between the amino acids and water play a part in maintaining structure.

In which part of the collagen molecule, X (exterior) or Y (interior), would you expect amino acids with hydrophobic R-groups to be found? Explain your answer. [2]

(Go do this past paper question)

A

Q5 a)i) Secondary structure: helices evident in polypeptide chains;
Quaternary structure: three (more than one) polypeptide chains
present; [2]

ii) The arrangement of polypeptide chains/triple helix provides (tensile) strength [must relate structure to strength]; [1]
b) Hydrophobic amino acids found in interior/at Y;
orientated away from cytoplasm/extracellular fluid/because water present to exterior; [2]

180
Q

Past Paper Question - January 2013 Q7 a) AS1 (Legacy Material)
Q7 The diagram below represents a short section of DNA.

a) Identify the type of bonds labelled A and B. [2]

(Go do this past paper question)

A

Q7 a) A: hydrogen bond;
B: phosphodiester bond/covalent; [2]

181
Q

Past Paper Question - January 2013 Q7 b) AS1 (Legacy Material)
Q7 b) Replication of DNA is described as ‘semi-conservative’. Explain this term. [2]

A

Q7 b) In a new DNA molecule, one strand is conserved from the parent molecule; while the other strand is newly synthesised (from free nucleotides); [2]

182
Q

Past Paper Question - January 2013 Q7 c)i) AS1 (Legacy Material)
Q7 c) Evidence in support of semi-conservative replication was provided by experiments carried out by Meselsöhn and Stahl in 1958. They grew bacteria on a nutrient medium containing ‘heavy’ nitrogen (15N) until it was assumed that all bacterial DNA was of the ‘heavy’ type. They then transferred some bacteria to a nutrient medium containing ‘light’ nitrogen (14N) and sampled the bacterial DNA at intervals corresponding to generation times for the bacteria. The DNA was subsequently separated using a centrifuge (which separates substances on the basis of density).
(i) Which part of the DNA molecule would be expected to incorporate the nitrogen?

A

Q7 c)i) (Nitrogenous/organic)bases; [1]

183
Q

Past Paper Question - January 2013 Q7 c)ii)+iii) AS1 (Legacy Material)
Q7 c) Evidence in support of semi-conservative replication was provided by experiments carried out by Meselsöhn and Stahl in 1958. They grew bacteria on a nutrient medium containing ‘heavy’ nitrogen (15N) until it was assumed that all bacterial DNA was of the ‘heavy’ type. They then transferred some bacteria to a nutrient medium containing ‘light’ nitrogen (14N) and sampled the bacterial DNA at intervals corresponding to generation times for the bacteria. The DNA was subsequently separated using a centrifuge (which separates substances on the basis of density).
As shown in the diagram below, the parental DNA was ‘heavy’ (with 15N only). The position of the ‘light’ DNA (with 14N only) is shown for comparison.
ii) Complete the diagram to show the position(s) which the bacterial DNA would occupy after successive generations in nutrient medium containing ‘light’ nitrogen (14N). [2]

iii) Explain the result obtained following one generation in 14N. [1]

(Go do this past paper question)

A

Q7 c)ii) Following one generation: one intermediate band drawn; Following two generations: two bands drawn – one intermediate and one light; [2]

iii) In each molecule of DNA, one strand contains heavy nitrogen (15 N)/is the parental strand, while the other contains light nitrogen (14 N)/is the new strand; [1]

184
Q

Past Paper Question - January 2012 Q1 AS1 (Legacy Material)
Q1 Identify the word or phrase that is described by each of the following statements.

• Carbohydrates which, when heated with Benedict’s reagent, cause the formation of a brick-red precipitate.
________________________________________________________

  • A cell which has two homologous copies of each chromosome.
    ________________________________________________________
  • A fatty acid chain which contains a number of double-bonded carbon atoms along its length.
    ________________________________________________________

• The enzyme used by HIV to synthesise viral DNA from RNA.
_______________________________________________________ [Out of 4]

A

Q1 Reducing sugars/minimum of two appropriate examples; diploid;
(poly)unsaturated;
reverse transcriptase; [Out of 4]

185
Q

Past Paper Question - January 2012 Q5 b) AS1 (Legacy Material)
Q5 b) In cells of the liver, excess glucose is converted into glycogen. Glycogen deposits in the liver are a means of storing glucose for future use in the body.
Identify one structural property of the glycogen molecule, and state how this relates to its role as an effective storage compound of glucose.

Property ____________________________________
Role ______________________________________________
___________________________________________________ [2]

A

Q5 b) Branched/has 1–6 bonds;
produces many terminal ends/aids hydrolysis/makes molecule more compact;
or
Large molecule;
(insoluble) so exerts no osmotic effect/does not pass through the cell membrane; [2]

186
Q

Past Paper Question - January 2012 Q8 a)i) AS1 (Legacy Material)
Q8 Haemoglobin is a protein which has a role in transporting oxygen around the body. It consists of four connected polypeptide chains, each of which contains a non-protein haem group, the site at which oxygen is known to bind. Some sections of the polypeptide chains are coiled into alpha-helices, and there is further folding of these chains to form a compact globular molecule.
Myoglobin is a different protein which also contains a haem group, but it consists of only one polypeptide chain containing 153 amino acid residues. A myoglobin molecule is very similar in structure to a polypeptide chain in haemoglobin, with alpha-helices and an overall globular shape.

a)i) Complete the grid below by placing a tick or cross in the appropriate cell. [3]

Do not leave any boxes blank.

                                                    | Haemoglobin | Myoglobin |  | Has a secondary structure |                | Has a quaternary structure|                 |   Is a conjugated protein     |
A

Q8 a)i) [3]
| Haemoglobin | Myoglobin |
| Has a secondary structure | Tick Tick
| Has a quaternary structure| Tick X
| Is a conjugated protein | Tick Tick

187
Q

Past Paper Question - January 2012 Q8 a)ii) AS1 (Legacy Material)
Q8 Haemoglobin is a protein which has a role in transporting oxygen around the body. It consists of four connected polypeptide chains, each of which contains a non-protein haem group, the site at which oxygen is known to bind. Some sections of the polypeptide chains are coiled into alpha-helices, and there is further folding of these chains to form a compact globular molecule.
Myoglobin is a different protein which also contains a haem group, but it consists of only one polypeptide chain containing 153 amino acid residues. A myoglobin molecule is very similar in structure to a polypeptide chain in haemoglobin, with alpha-helices and an overall globular shape.

a)ii) Name two types of bonds which may contribute to the tertiary structure of a protein. [2]

A

Q8 a)ii) Any two from
• ionic bonds
• hydrogen bonds
• disulphide bonds/bridges
• hydrophobic interactions [2]

188
Q

Past Paper Question - January 2012 Q8 b)i) AS1 (Legacy Material)
Q8 Haemoglobin is a protein which has a role in transporting oxygen around the body. It consists of four connected polypeptide chains, each of which contains a non-protein haem group, the site at which oxygen is known to bind. Some sections of the polypeptide chains are coiled into alpha-helices, and there is further folding of these chains to form a compact globular molecule.
Myoglobin is a different protein which also contains a haem group, but it consists of only one polypeptide chain containing 153 amino acid residues. A myoglobin molecule is very similar in structure to a polypeptide chain in haemoglobin, with alpha-helices and an overall globular shape.

b) Myoglobin was hydrolysed into its constituent amino acids, using acid hydrolysis. Paper chromatography was carried out on the resulting mixture and the Rf value was calculated for each of the spots obtained on the chromatogram.
i) Given a concentrated solution of amino acids produced by the hydrolysis of myoglobin, outline the procedure used for preparing and running the chromatogram. (Do not describe how to develop or analyse the chromatogram). [4]

A

Q8 b)i) Any four from
• add solvent to the chromatography vessel and allow it to saturate the atmosphere
• draw a base line in pencil towards the bottom of the chromatography paper
• add a spot of the amino acid solution to the base line, allow it to dry and re-apply the solution to make a concentrated spot
• lower the paper into the vessel, ensuring the base line is above the level of the solvent
• allow sufficient time for the solvent to rise up the paper but not reach the top/ensure chromatography paper does not touch side of vessel
• remove the paper and mark the solvent front
• handle the paper using gloves/tongs/avoid touching the paper/
touching only sides or top [4]

189
Q

Past Paper Question - January 2012 Q8 b)ii) AS1 (Legacy Material)
Q8 Haemoglobin is a protein which has a role in transporting oxygen around the body. It consists of four connected polypeptide chains, each of which contains a non-protein haem group, the site at which oxygen is known to bind. Some sections of the polypeptide chains are coiled into alpha-helices, and there is further folding of these chains to form a compact globular molecule.
Myoglobin is a different protein which also contains a haem group, but it consists of only one polypeptide chain containing 153 amino acid residues. A myoglobin molecule is very similar in structure to a polypeptide chain in haemoglobin, with alpha-helices and an overall globular shape.

b) Myoglobin was hydrolysed into its constituent amino acids, using acid hydrolysis. Paper chromatography was carried out on the resulting mixture and the Rf value was calculated for each of the spots obtained on the chromatogram.

ii) The substance used to develop the spots on the chromatogram is ninhydrin.
State one safety precaution specific to the use of this substance, apart from the use of goggles. [1]

A

Q8 b)ii) Avoid breathing vapour/wear gloves/carry out procedure in a fume cupboard; [1]

190
Q

Past Paper Question - January 2012 Q8 c) AS1 (Legacy Material)
Q8 c) A diagrammatic representation of a developed chromatogram is shown below. For the sake of clarity only 3 spots are shown. The dotted line through each spot indicates its mid-point, which is used to measure the distance it has travelled.

Amino acid Rf value
alanine 0.38
arginine 0.20
asparagine 0.50
glutamic acid 0.30
leucine 0.73
lysine 0.14
valine 0.61

Using the table of Rf values shown above, identify amino acid X on the chromatogram. (Show your working). [3]

(Go do this past paper question)

A

Q8 c) Distance moved by spot 3cm, distance moved by solvent 6cm;
Rf value 3 ÷ 6 = 0.5 [consequential to answer above];
X corresponds to asparagine [consequential to R f value calculated]; [3]

191
Q

Past Paper Question - June 2012 Q2 a)i+ii) AS1 (Legacy Material)
Q2 Iodine, Benedict’s reagent, Biuret reagent and Clinistix were used in an investigation to identify certain biochemical substances.

a) i) Describe the colour change which indicates a positive result when using Biuret reagent. Your answer should state the initial and the final colour. [1]
ii) Which of the reagents used in the investigation (and listed above) needs to be heated when carrying out the test? [1]

A

Q2 a)i) Blue to purple/lilac; [1]

ii) Benedict’s (reagent); [1]

192
Q

Past Paper Question - June 2012 Q2 b) AS1 (Legacy Material)
Q2 Iodine, Benedict’s reagent, Biuret reagent and Clinistix were used in an investigation to identify certain biochemical substances.

b) The following statements indicate which tests gave positive results for four different substances.

  • Substance A gave a positive result with Biuret reagent.
  • Substance B gave a positive result with both Benedict’s reagent and with Clinistix.
  • Substance C initially gave a negative result with both Benedict’s reagent and iodine. However after hydrolysis with hydrochloric acid it gave a positive result with both Benedict’s reagent and Clinistix.
  • Substance D gave a positive result with Benedict’s reagent but a negative result with Clinistix.

Identify each of the substances:

Substance A: ___________________
Substance B: ___________________
Substance C: ___________________
Substance D: ___________________ [4]

A

Q2 b) A: protein;
B: glucose;
C: sucrose/non-reducing sugar;
D: fructose/maltose/lactose/galactose/reducing sugar
other than glucose; [4]

193
Q

Past Paper Question - June 2012 Q3 a)i) AS1 (Legacy Material)
Q3 DNA is the molecule of the gene and has a very specific structure.
a) The diagram below shows three nucleotides on one strand of a DNA molecule.
i) Draw the complementary strand of this section of the DNA molecule. [2]

(Go do this past paper question)

A

Q3 a)i) Correct pairing of bases;
new strand antiparallel to existing strand; [2]

194
Q

Past Paper Question - June 2012 Q3 a)i) AS1 (Legacy Material)
Q3 DNA is the molecule of the gene and has a very specific structure.
a)ii) In an analysis of the DNA in a cell nucleus, 21% of the bases was found to be guanine. Calculate the percentage of each of the other bases in the DNA. (Show your working). [2]

A

Q3 a)ii) Cytosine = guanine = 21%;
Adenine and thymine both = (100 – 42) ÷ 2 = 29%; [2]

195
Q

Past Paper Question - June 2012 Q6 c) AS1 (Legacy Material)
Q6 c) Calcium and magnesium are inorganic ions that are important for the synthesis of substances in plant cells.

State the substance which each of these ions is used to produce.

  • Calcium _______________________
  • Magnesium ____________________ [2]
A

Q6 c) Calcium: to make calcium pectate (for cell walls)/middle lamella;
Magnesium: to make chlorophyll; [2]

196
Q

Past Paper Question - June 2012 Q7 a)i)+ii) AS1 (Legacy Material)
Q7 Lipids are a group of organic molecules which include fats and oils.

The diagram below shows two component molecules, A and B, which make up lipids.

a) (i) Identify the type of reaction which is involved in bonding B to A. [1]

(ii) State the ratio of A to B in a fat. [1]

(Go do this past paper question)

A

Q7 a)i) Condensation (reaction); [1]

ii) 1:3; [1]

197
Q

Past Paper Question - June 2012 Q8 Section B AS1 (Legacy Material)
Q8 Starch, glycogen and cellulose are three polysaccharides found in living organisms. Give an account of the similarities and differences in their structure and describe their role and distribution. [13]
Quality of written communication [2]

A

Q8 Thirteen points, with a maximum of seven from each section

Similarities and differences in structure:
• all three have (1,4) glycosidic links/formed by condensation reactions
• starch and glycogen contain alpha-glucose
• there are two components of starch, amylose and amylopectin
• in amylose the long chain of monomers is wound into a helix
• amylopectin is branched due to additional 1,6 (glycosidic) links
• glycogen is similar to amylopectin but more branched
• cellulose is composed of beta-glucose
• and forms straight chains (because every second glucose is ‘upside-down’)
• parallel chains are held together by hydrogen bonds/cross linkage

Role and distribution:
• starch and glycogen are both glucose/energy stores
• starch is found in chloroplasts/storage tissue (e.g. seeds/roots/tubers)
• glycogen is found in muscle/liver cells/fungi
• the helix/branching makes the molecules more compact (for storage)
• glycogen/amylopectin are more readily hydrolysed due to more terminal ends (for enzymes to work on)
• starch/glycogen are insoluble so exert no osmotic effect/cannot leach out of the cell
• cellulose offers tensile strength to plant cell walls
• layers of cellulose are arranged at varying angles to each other, which adds to the tensile strength

198
Q

Past Paper Question - June 2011 Q3 AS1 (Legacy Material)
Q3 Using a series of tests, a key was produced to identify the following carbohydrates:
Cellulose, glucose, maltose, starch and sucrose.

Using the key identify each of the carbohydrates A to E. [5]

(Go do this past paper question)

A

Q3 A: Cellulose;

B: starch;
C: sucrose;
D: maltose;
E: glucose; [5]

199
Q

Past Paper Question - June 2011 Q5 a)+b)i)+ii) AS1 (Legacy Material)
Q5 a) The diagram below represents the structure of DNA.

Identify the part of the DNA structure shown in the box X.
___________________________________________________________ [1]

b) In a classic experiment, Matthew Meselsöhn and Frank Stahl grew bacteria in a medium where the nitrogen source contained the ‘light’ nitrogen isotope, 14N. The bacteria were then placed in a medium with a nitrogen source containing the ‘heavy’ nitrogen isotope, 15N, and allowed to reproduce. After many generations, all DNA in the bacteria contained this ‘heavy’ 15N isotope. This DNA was termed ‘heavy’ DNA.
DNA extracted from bacterial cells was centrifuged and observed under ultra-violet light. The DNA appeared as a black band in the centrifuge tube. The band produced by ‘heavy’ DNA was much lower in the centrifuge tube than that produced by ‘light’ DNA. They are shown in the diagrams below.

The bacteria containing ‘heavy’ DNA were then transferred to a medium with a nitrogen source containing the ‘light’ nitrogen isotope, 14N and allowed to reproduce. After one generation, samples of the bacteria were removed and their DNA was extracted and centrifuged. This process was repeated after a further generation.

(i) Complete the diagrams below to show the position of the extracted DNA by drawing appropriate bands. [2]

(ii) Explain the result produced after one generation.
__________________________________________________________
__________________________________________________________
__________________________________________________________
_______________________________________________________ [2]

(Go do this past paper question)

A

Q5 a) Nucleotide;
[Accept deoxyribose nucleotide, but NOT ribonucleotide] [1]

b)i) One intermediate band after one generation;
two bands, one light and one intermediate after two generations;
[2]
(ii) Semi-conservative replication of the DNA/each (heavy) strand acts as a template;
produces DNA with one heavy chain and one light chain; [2] [allow by means of a diagram] [Note: Diagrams may not be accepted on the current specification]

200
Q

Past Paper Question - June 2011 Q7 a) AS1 (Legacy Material)
Q7 An experiment was carried out on the rate of absorption of two monosaccharide sugars by living intestinal cells and intestinal cells that have been poisoned with cyanide (a chemical which inhibits production of ATP in cells by inhibiting cell respiration). The results are shown in the table below.

Monosaccharide Rate of absorption/arbitrary units
Living intestinal cells Cyanide-treated
intestinal cells
Glucose 1.00 0.33
Arabinose 0.29 0.29

(a) Describe the trends evident in the data above.
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
_____________________________________________________________
___________________________________________________________ [3]

A

Q7 a) Any three from
• more glucose is absorbed by living intestinal cells than arabinose
• similar amounts of glucose and arabinose are absorbed by
cyanide-treated cells
• treatment with cyanide causes a (significant) reduction in the rate
of absorption of glucose
• similar treatment with cyanide does not affect the rate of
arabinose absorption [3]

201
Q

Past Paper Question - June 2011 Q7 b) AS1 (Legacy Material)
Q7 An experiment was carried out on the rate of absorption of two monosaccharide sugars by living intestinal cells and intestinal cells that have been poisoned with cyanide (a chemical which inhibits production of ATP in cells by inhibiting cell respiration). The results are shown in the table below.

Monosaccharide Rate of absorption/arbitrary units
Living intestinal cells Cyanide-treated
intestinal cells
Glucose 1.00 0.33
Arabinose 0.29 0.29

b) Identify which of the two monosaccharides is entirely absorbed by diffusion. Explain your answer. [3]

A

Q7 b) Arabinose;
as the rate remains the same in the intestinal cells treated with cyanide;
suggesting that ATP/respiration is not needed for arabinose absorption/it is a passive process [NOT energy for ATP]; [3]

202
Q

Past Paper Question - June 2011 Q7 c) AS1 (Legacy Material)
Q7 An experiment was carried out on the rate of absorption of two monosaccharide sugars by living intestinal cells and intestinal cells that have been poisoned with cyanide (a chemical which inhibits production of ATP in cells by inhibiting cell respiration). The results are shown in the table below.

Monosaccharide Rate of absorption/arbitrary units
Living intestinal cells Cyanide-treated
intestinal cells
Glucose 1.00 0.33
Arabinose 0.29 0.29

c) Once absorbed into the intestinal cells, glucose may be converted into a polysaccharide for storage. Name the polysaccharide stored in animal cells and describe how it is synthesised from glucose. [4]

A

Q7 c) Glycogen;
Any three from
• contains alpha-glucose molecules
• joined by condensation reactions/glycosidic bonds
• both 1–4 and 1–6 bonds are present
• 1–4 bonds create the straight chains/1–6 bonds create
branching [4]

203
Q

Past Paper Question - January 2011 Q3 a) AS1 (Legacy Material)
Q3 a) Read the following passage which describes the primary structure of a protein and write the most appropriate word(s) in the blank spaces to complete the account.

Proteins are polymers, consisting of long chains of ____________ joined together by ______________ reactions to form numerous ____________ bonds. The sequence of monomers in a protein is known as its primary structure and is encoded in the ______________ of an organism. [4]

A

Q3 a) Amino acids;
condensation;
peptide;
genes/DNA/mRNA; [4]

204
Q

Past Paper Question - January 2011 Q3 b) AS1 (Legacy Material)
Q3 b) Proteins are complex molecules with a level of organisation beyond the primary structure. Describe how a final overall shape is produced in a protein. [4]

A

Q3 b) Any four from
• hydrogen bonds (between the amino acids) leads to twisting/folding of the chain/secondary structure
• and can take the form of an alpha-helix or a beta-pleated sheet
• a protein will then fold further onto itself to form a tertiary structure/
tertiary structure represented by a globular shape
• this involves H bonds, ionic bonds, disulfide bonds and
hydrophobic interactions (between the R groups of the individual
amino acids) [any two]
• in some proteins, a quaternary structure forms between two or more polypeptide chains
• this involves mainly disulfide bonds between the individual chains [4]

205
Q

Past Paper Question - January 2011 Q3 c) AS1 (Legacy Material)
Q3 c) Describe the colour change which indicates the presence of protein in a food sample when it is tested with Biuret reagent. [1]

A

Q3 c) Blue to mauve/lilac; [1]

206
Q

Past Paper Question - January 2011 Q7 a) AS1 (Legacy Material)
Q7 This question is about the digestion of jelly by protein-digesting enzymes.
a) Jelly contains the protein gelatine, which is broken down to amino acids by protein-digesting enzymes. State the type of reaction which takes place during digestion. [1]

A

Q7 a) Hydrolysis; [1]

207
Q

Past Paper Question - January 2011 Q8 a) Section B AS1 (Legacy Material)
Q8 a) Give an account of the structure of the nucleic acids, DNA and RNA. [8]
Quality of written communication is awarded a maximum of 2 marks in this section. [2]

A

Q8 a) Any eight points

Nucleic acids (common features):

• nucleic acids are chains of nucleotides
• (a nucleotide) consists of a pentose sugar, a phosphate and a
nitrogenous (organic) base/purine and pyrimidine [or by diagram]
• the sugars and phosphates are joined to form the spine [or by
diagram]
• by phosphodiester/condensation/covalent bonds

DNA and RNA (comparisons):

• in DNA the pentose sugar is deoxyribose, while in RNA it is ribose
• DNA is a double chain/double helix, while RNA is single stranded
• hydrogen bonds between the bases form the double chain (in DNA)
• in DNA the bases are adenine, thymine, cytosine and guanine
• in DNA adenine (A) always pairs with thymine (T), while cytosine (C)
pairs with guanine (G)/a purine binds to a pyrimidine
• in DNA the two strands run anti-parallel to each other
• a DNA molecule is much longer than an RNA molecule (allow converse)
• in RNA thymine is replaced by uracil
• there are three forms of RNA – ribosomal, messenger and transfer [8]

208
Q

Past Paper Question - January 2011 Q8 b) Section B AS1 (Legacy Material)
Q8 b) Describe the process of DNA replication. [5]
Quality of written communication is awarded a maximum of 2 marks in this section. [2]

A

Q8 b) Any five points
• DNA replication is said to be semi-conservative, because each new molecule contains one old strand and one new strand
• the two sides of the DNA molecule are “unzipped” from one end
• by DNA helicase
• each strand acts as a template for the formation of new strands
• free nucleotides enter opposite their complementary bases (A
opposite T and C opposite G)
• DNA polymerase catalyses the joining up of the nucleotides
• by condensation reactions [5]

209
Q

Past Paper Question - June 2013 Q3 AS1 (Legacy Material)
Q3 All organic macromolecules contain the elements carbon, hydrogen and oxygen. Some also contain a variety of other elements.

The following statements give information about the chemical composition and nature of five different macromolecules.

Molecules A and B contain only the elements carbon, hydrogen and oxygen.

  • When hydrolysed, molecule A produces monomers that reduce Benedict’s reagent.
  • When hydrolysed, molecule B produces some sub-units that lower the pH of the solution.

Molecules C, D and E all contain other elements in addition to carbon, hydrogen and oxygen.

When hydrolysed, molecule C produces sub-units, all of which contain nitrogen and some also contain sulfur.

Molecules D and E both contain phosphate.

  • When hydrolysed, molecule D creates two other types of sub-unit in addition to the phosphate. One type of sub-unit lowers the pH of the solution.
  • When hydrolysed, molecule E creates four types of sub-unit. Each sub-unit has a nitrogen containing molecule and a pentose, in addition to the phosphate.

Identify the macromolecules from the information that is given above. [5]

A

Q3 A: polysaccharides/starch/glycogen/cellulose/amylose/amylopectin;
B: triglycerides/lipids/fats/oils;
C: proteins/polypeptides;
D: phospholipid;
E: nucleic acids/DNA/RNA; [5]

211
Q

Past Paper Question - June 2018 Q2 a)i)+ii) AS1 (Revised Spec.)
Q2 Creutzfeldt-Jakob Disease (CJD) is a neurodegenerative disorder which results in the breakdown of nerve tissue in the brain of humans.

a)i) State the specific type of molecule that causes CJD and e larger biological group to which this molecule belongs.

  • Specific type of molecule ________________________
  • Larger biological group __________________________ [2]

ii) Name one similar disorder, caused by this type of molecule, which occurs in other mammals. [1]

A

Q2 a)i) Prion;
Protein; [2]

ii) BSE/Scrapie; [1]

212
Q

Past Paper Question - June 2018 Q2 b)i) AS1 (Revised Spec.)
Q2 Creutzfeldt-Jakob Disease (CJD) is a neurodegenerative disorder which results in the breakdown of nerve tissue in the brain of humans.

The incubation period of a disease is the time from infection to the appearance of symptoms. For CJD, this can be from 5-20 years.

b)i) Describe one way in which someone could become infected with CJD. [1]

A

Q2 b)i) Eating prion-rich food/spontaneous transformation from normal to disease-causing prion/inherited genetic mutation; [1]

213
Q

Past Paper Question - June 2018 Q2 b)ii) AS1 (Revised Spec.)
Q2 Creutzfeldt-Jakob Disease (CJD) is a neurodegenerative disorder which results in the breakdown of nerve tissue in the brain of humans.

The incubation period of a disease is the time from infection to the appearance of symptoms. For CJD, this can be from 5-20 years.

b)ii) State what happens during the incubation period. [1]

A

Q2 b)ii) Changes to secondary structure, forming more B-pleated sheets, leading to the replication of disease-causing prions/conversion of enough normal to disease-causing prions to cause symptoms; [1]

214
Q

Past Paper Question - June 2018 Q2 c) AS1 (Revised Spec.)
Q2 Creutzfeldt-Jakob Disease (CJD) is a neurodegenerative disorder which results in the breakdown of nerve tissue in the brain of humans.

The incubation period of a disease is the time from infection to the appearance of symptoms. For CJD, this can be from 5-20 years.

c) Viruses can also cause infectious diseases. Describe how viruses differ biochemically from the molecule that cause CJD. [1]

A

Q2 c) Contain DNA/RNA/nucleic acids; [1]

215
Q

Past Paper Question - June 2018 Q4 a) AS1 (Revised Spec.)
Q4 Each of the polysaccharides shown below is constructed from a single type of monomer.
[Polysaccharides shown are diagrammatic representations of starch, consisting of both amylose and amylopectin, glycogen, and cellulose. The diagram is present on page 4 of the Christmas test if necessary, avoid looking at the answers which are displayed below].

a) Name the type of reaction that occurs to join two monomers and the bond that is created between the monomers in these polysaccharides. [1]

Reaction _______________
Bond ___________________

A

Q4 a) Condensation reaction;

Glycosidic bond; [1]

216
Q

Past Paper Question - June 2018 Q4 b) AS1 (Revised Spec.)
Q4 Glycogen and starch are found in animal and plant cells respectively. They have similar biochemical structures and functions.

b) Apart from bonding, compare and contrast the structure and function of glycogen and starch. [4]

Structure
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________

Function
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________
_______________________________________________________________

A

Q4 b) Structure:
Made from a-glucose monomers;
Starch is comprised of two polymers while glycogen is comprised of only one polymer/glycogen is more highly branched than starch;

Function:
Storage polymers/store glucose for energy;
Glucose is more readily released from glycogen than starch; [4]

217
Q

Past Paper Question - June 2018 Q4 c) AS1 (Revised Spec.)
Q4 The cell wall of plant cells is composed of cellulose whereas the cell wall of fungal cells contains chitin. These polymers have similar functions. Chitin can also be found in some animals, such as the shore crab (Carcinus maenas).

[Diagram present on page 6 of the S2 Christmas Test (Assessment 2), avoid looking at answer below diagram]

c) Suggest a function for chitin in the shore crab and explain how its structure relates to this function. [2]

A

Q4 c) Provides support/structural polymer;

Long, parallel unbranched chains/fibrils joined by hydrogen bonds; [2]

218
Q

Past Paper Question - June 2018 Q6 c)i)+ii) AS1 (Revised Spec.)
Q6 c) The table below shows the approximate percentages of the four bases found in the DNA of five different species.

Organism Percentage of bases in organism’s DNA/%
Adenine Guanine Cytosine Thymine

Yeast 32 18 18 32
Rat 30 20 20 30
Human 30 20 20 30
Grasshopper 30 20 20 30
E. coli 26

i) Complete the table above to show the expected percentage of bases that would be found in DNA from E.coli. [1]
ii) The values shown for human and grasshopper DNA are the same, even though the two organisms are very different. Explain how their DNA differs. [1]

A

Q6 c)i) [1]

Organism Percentage of bases in organism’s DNA/%
Adenine Guanine Cytosine Thymine

E. coli 24 26 24

ii) Sequence of bases are different; [1]

219
Q

Past Paper Question - June 2018 Q6 d)i) AS1 (Revised Spec.)

Q6 d)i) DNA is replicated through semi-conservative replication. Explain the term ‘semi-conservative replication’. [1]

A

Q6 d)i) Each new DNA molecule contains one template strand and one new DNA strand. [1]

220
Q

Past Paper Question - June 2018 Q6 d)ii) AS1 (Revised Spec.)
Q6 d)ii) Name two of the enzymes involved in semi-conservative replication of DNA and describe the role of each. [4]

  1. Enzyme ________________
    Role ________________________________________________________
    ______________________________________________________________
    ______________________________________________________________
  2. Enzyme ________________
    Role ________________________________________________________
    ______________________________________________________________
    ______________________________________________________________
A

Q6 d)ii) DNA helicase;
Breaks the hydrogen bonds between strands;
DNA polymerase;
Joins the nucleotides of new strand together; [4]

221
Q

Past Paper Question - June 2018 Q6 e)i)+ii) AS1 (Revised Spec.)
Q6 e) In 1958 Meselson and Stahl carried out an experiment which demonstrated that replication of DNA was semi-conservative. They cultured bacteria on agar containing 15N, which is a heavy isotope of nitrogen. The bacteria were then moved to agar containing normal 14N (a light isotope) and allowed to reproduce. DNA was extracted from each generation of bacteria and centrifuged. The results for bacteria cultured on 15N are shown in the first diagram below.

i) Complete the second diagram to show the distribution of DNA extracted from the second generation grown on 14N. [1]

[First diagram shows DNA extracted from bacteria in 15N, the DNA has accumulated at the bottom of the test tube to form a single band]

ii) Meselson and Stahl grew bacteria on 14N for several more generations. Suggest and explain how the results after three generations would differ from that for two generations. [2]

A

Q6 e)i) The second diagram should show an intermediate (hybrid) band of DNA in the middle of the test tube and a light band of DNA situated at the top of the test tube. [1]

ii) The light band (top one) will be thicker (while the hybrid band (intermediate) will stay the same);
All new DNA produced will consist of two light (14N) strands (while the amount of hybrid (14N/15N) DNA will remain the same); [2]

AS Biology Module 1 - Molecules and Cells (6 decks)

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