Biological molecules

Question 1

What is a monomer?

Answer 1

(a monomer is a smaller / repeating) unit / molecule from which larger molecules / polymers are made;

Question 2

Lactulose is a disaccharide formed from one molecule of galactose and one molecule of fructose.
Other than both being disaccharides, give one similarity and one difference between the structures of lactulose and lactose.

Answer 2

Similarity
1. Both contain galactose / a glycosidic bond;
Ignore references to hydrolysis and / or condensation
Difference
2. Lactulose contains fructose, whereas lactose contains glucose;

Question 3

Following digestion and absorption of food, the undigested remains are processed to form faeces in the parts of the intestine below the ileum.
The faeces of people with constipation are dry and hard. Constipation can be treated by drinking lactulose. Lactulose is soluble, but is not digested or absorbed in the human intestine.
Use your knowledge of water potential to suggest why lactulose can be used to help people suffering from constipation.

Answer 3

1. (Lactulose) lowers the water potential of faeces / intestine / contents of the intestine;
   Accept Ψ for water potential
2. Water retained / enters (due to osmosis) and softens the faeces;

Question 4

Give two features of DNA and explain how each one is important in the semi-conservative replication of DNA.

Answer 4

1. Weak / easily broken hydrogen bonds between bases allow two strands to separate / unzip;
   may appear in the same feature
2. Two strands, so both can act as templates;
   may appear in the same feature
3. Complementary base pairing allows accurate replication;

Question 5

Describe how you would test a liquid sample for the presence of lipid and how you would recognise a positive result.

Answer 5

1. (Mix / shake sample) with ethanol, then water;
   Sequence is important
2. White / milky (emulsion);

Question 6

State and explain the property of water that can help to buffer changes in temperature.

Answer 6

1. (water has a relatively) high (specific) heat capacity;
2. Can gain / lose a lot of heat / energy without changing temperature;
   OR
   Takes a lot of heat / energy to change temperature;

Question 7

Water is used to hydrolyse ATP.

Name the two products of ATP hydrolysis.

Answer 7

Adenosine diphosphate and (inorganic) phosphate;

Question 8

Describe how a peptide bond is formed between two amino acids to form a dipeptide.

Answer 8

1. Condensation (reaction) / loss of water;
   Accept each marking point if shown clearly in diagram.
2. Between amine / NH2 and carboxyl / COOH;

Question 9

The secondary structure of a polypeptide is produced by bonds between amino acids.
Describe how.

Answer 9

1. Hydrogen bonds;
   Accept as a diagram
   Reject N - - - C / ionic / disulfide bridge / peptide bond
2. Between NH (group of one amino acid) and C=O (group);
   OR
   Forming β pleated sheets / α helix;

Question 10

Two proteins have the same number and type of amino acids but different tertiary structures.
Explain why.

Answer 10

1. Different sequence of amino acids
   OR
   Different primary structure;
   If candidate assumes proteins are the same, accept effect of different pH/ temperature
2. Forms ionic / hydrogen / disulfide bonds in different places;

Question 11

What is digestion?

Answer 11

1. Hydrolysis (of);

2. (Large / insoluble substances) to small(er) / soluble substances;

Question 12

One species of fungus digests cellulose using two types of enzyme, endocellulases and exocellulases.
Endocellulases act in the middle of the cellulose molecule and exocellulases act at the ends of the cellulose molecule.
(b) Endocellulases and exocellulases act at different places on cellulose molecules.
Suggest why.

Answer 12

1. Active sites are different shapes;
2. So different enzyme-substrate complexes (are formed);
   OR
   So complementary to different parts of cellulose / substrate;

Question 13

Formation of an enzyme-substrate complex increases the rate of reaction.
Explain how.

Answer 13

1.      Reduces activation energy;
Accept ‘reduces Ea’.
2.      Due to bending bonds
OR
Without enzyme, very few substrates have sufficient energy for reaction;

Question 14

The genetic code is described as degenerate.

What is meant by this? Use an example from Table 1 to illustrate your answer.

Answer 14

1. More than one codon codes for a single amino acid;

2. Suitable example selected from Table 1;

Question 15

Describe the role of two named enzymes in the process of semi-conservative replication of DNA.

Answer 15

1. (DNA) helicase causes breaking of hydrogen/H bonds (between DNA strands);
   Reject ‘helicase hydrolyses hydrogen bonds’.
2. DNA polymerase joins the (DNA) nucleotides;
   Reject if suggestion that DNA polymerase joins the complementary nucleotides or forms H bonds.
   Reject if joining RNA nucleotides or forming RNA.
3. Forming phosphodiester bonds;

Question 16

Describe the gross structure of the human gas exchange system and how we breathe in and out.

Answer 16

1. Named structures – trachea, bronchi, bronchioles, alveoli;
   Reject mp1 if structures from other physiological systems are named but award mp2 if the correct structures are in the correct order.
2. Above structures named in correct order
   OR
   Above structures labelled in correct positions on a diagram;
   Reject mp1 if structures from other physiological systems are named but award mp2 if the correct structures are in the correct order.
3. Breathing in – diaphragm contracts and external intercostal muscles contract;
4. (Causes) volume increase and pressure decrease in thoracic cavity (to below atmospheric, resulting in air moving in);
   For thoracic cavity accept ‘lungs’ or ‘thorax’.
   Reference to ‘thoracic cavity’ only required once.
5. Breathing out - Diaphragm relaxes and internal intercostal muscles contract;
   Accept diaphragm relaxes and (external) intercostal muscles relax and lung tissue elastic (so recoils).
6. (Causes) volume decrease and pressure increase in thoracic cavity (to above atmospheric, resulting in air moving out);

Question 17

Mucus also contains glycoproteins. One of these glycoproteins is a polypeptide with the sugar, lactose, attached.
Describe how lactose is formed and where in the cell it would be attached to a polypeptide to form a glycoprotein.

Answer 17

1. Glucose and galactose;
   Ignore α or β for glucose
2. Joined by condensation (reaction);
3. Joined by glycosidic bond;
4. Added to polypeptide in Golgi (apparatus);;

Question 18

Mucus produced by epithelial cells in the human gas exchange system contains triglycerides and phospholipids.
Compare and contrast the structure and properties of triglycerides and phospholipids.

Answer 18

1. Both contain ester bonds (between glycerol and fatty acid);
   All statements must be clearly comparative or linked by the candidate, not inferred from separate statements.
   Accept mark points shown on adjacent annotated diagrams.
2. Both contain glycerol;
3. Fatty acids on both may be saturated or unsaturated;
4. Both are insoluble in water;
5. Both contain C, H and O but phospholipids also contain P;
   Must relate to element.
6. Triglyceride has three fatty acids and phospholipid has two fatty acids plus phosphate group;
7. Triglycerides are hydrophobic/non-polar and phospholipids have hydrophilic and hydrophobic region;
   Accept ‘non-polar’ for hydrophobic and ‘polar’ for hydrophilic.
8. Phospholipids form monolayer (on surface)/micelle/bilayer (in water) but triglycerides don’t;

Question 19

If most of the mitochondria in a cell are faulty, this prevents many important enzyme-catalysed reactions taking place or slows them down.
Suggest and explain one reason why.

Answer 19

1. Not enough / little ATP produced;
2. ATP provides energy for (enzyme) reactions
   OR
   ATP phosphorylates substrates / enzymes, so making them (more) reactive;

Question 20

The enzymes DNA helicase and DNA polymerase are involved in DNA replication.
Describe the function of each of these enzymes.

Answer 20

1. DNA helicase – (unwinding DNA and) breaking hydrogen bonds / bonds between chains / bases / strands;
2. DNA polymerase – joins (adjacent) nucleotides OR forms phosphodiester bond / sugar-phosphate backbone

Question 21

Adenosine triphosphate (ATP) is a nucleotide derivative.
Contrast the structures of ATP and a nucleotide found in DNA to give two differences.

Answer 21

1. ATP has ribose and DNA nucleotide has deoxyribose;
2. ATP has 3 phosphate (groups) and DNA nucleotide has 1 phosphate (group);
3. ATP – base always adenine and in DNA nucleotide base can be different / varies;

Question 22

Glycogen and cellulose are both carbohydrates.

Describe two differences between the structure of a cellulose molecule and a glycogen molecule.

Answer 22

1. Cellulose is made up of β-glucose (monomers) and glycogen is made up of α-glucose (monomers);
2. Cellulose molecule has straight chain and glycogen is branched;
3. Cellulose molecule has straight chain and glycogen is coiled;
4. glycogen has 1,4- and 1,6- glycosidic bonds and cellulose has only 1,4- glycosidic bonds;

Question 23

Starch is a carbohydrate often stored in plant cells.

Describe and explain two features of starch that make it a good storage molecule.

Answer 23

1. Insoluble (in water), so doesn’t affect water potential;
2. Branched / coiled / (α-)helix, so makes molecule compact;
   OR
   Branched / coiled / (α-)helix so can fit many (molecules) in small area;
3. Polymer of (α-)glucose so provides glucose for respiration;
4. Branched / more ends for fast breakdown / enzyme action;
5. Large (molecule), so can’t cross the cell membrane

Question 24

Give the two types of molecule from which a ribosome is made.

Answer 24

One of RNA / ribonucleic acid(s) / nucleotide(s)/nucleic acid(s) / rRNA / ribosomal RNA / ribosomal ribonucleic acid
and
one of protein(s) / polypeptide(s) / amino acid(s) / peptide(s) / ribosomal protein;

Question 25

Describe the role of a ribosome in the production of a polypeptide. Do not include transcription in your answer.

Answer 25

1. mRNA binds to ribosome;
2. Idea of two codons / binding sites;
3. (Allows) tRNA with anticodons to bind / associate;
4. (Catalyses) formation of peptide bond between amino acids (held by tRNA molecules);
5. Moves along (mRNA to the next codon) / translocation described;

Question 26

In a eukaryotic cell, the base sequence of the mRNA might be different from the sequence of the pre-mRNA.
Explain why.

Answer 26

1. Introns (in pre-mRNA);

2. Removal of sections of (pre-mRNA) / splicing;

Question 27

In mammals, in the early stages of pregnancy, a developing embryo exchanges substances with its mother via cells in the lining of the uterus. At this stage, there is a high concentration of glycogen in cells lining the uterus.
(a) Describe the structure of glycogen.

Answer 27

1.      Polysaccharide of α-glucose;
OR
polymer of α-glucose;
2.      (Joined by) glycosidic bonds
OR
Branched structure;

Question 28

During early pregnancy, the glycogen in the cells lining the uterus is an important energy source for the embryo.
Suggest how glycogen acts as a source of energy.
Do not include transport across membranes in your answer.

Answer 28

1. Hydrolysed (to glucose);

2. Glucose used in respiration;

Question 29

Suggest and explain two ways the cell-surface membranes of the cells lining the uterus may be adapted to allow rapid transport of nutrients.

Answer 29

1. Membrane folded so increased / large surface area;
   OR
   Membrane has increased / large surface area for (fast) diffusion / facilitated diffusion / active transport / co-transport;
2. Large number of protein channels / carriers (in membrane) for facilitated diffusion;
3. Large number of protein carriers (in membrane) for active transport;
4. Large number of protein (channels / carriers in membrane) for co-transport;

Question 30

Bacteria are often used in industry as a source of enzymes. One reason is because bacteria divide rapidly, producing a large number of them in a short time.
Describe how bacteria divide.

Answer 30

1. Binary fission;
2. Replication of (circular) DNA;
3. Division of cytoplasm to produce 2 daughter cells;
4. Each with single copy of (circular) DNA;

Question 31

Some proteases are secreted as extracellular enzymes by bacteria.
Suggest one advantage to a bacterium of secreting an extracellular protease in its natural environment.
Explain your answer.

Answer 31

1. To digest protein;
2. (So) they can absorb amino acids for growth / reproduction / protein synthesis / synthesis of named cell component;
   OR
   (So) they can destroy a toxic substance / protein;

Question 32

Mammals have some cells that produce extracellular proteases. They also have cells with membrane-bound dipeptidases.
Describe the action of these membrane-bound dipeptidases and explain their importance.

Answer 32

1. Hydrolyse (peptide bonds) to release amino acids;
2. Amino acids can cross (cell) membrane;
   OR
   Dipeptides cannot cross (cell) membrane;
   OR
   Maintain concentration gradient of amino acids for absorption;
   OR
   Ensure (nearly) maximum yield from protein breakdown;

Question 33

Farmers use artificial fertilisers to maintain or increase yield from grain-producing crop plants such as wheat.
(a) Artificial fertiliser is used to replace mineral ions removed from the land when crops are harvested. One of the mineral ions is nitrate.
Give two examples of biological molecules containing nitrogen that would be removed when a crop is harvested.

Answer 33

Two suitable examples;
Examples
1.      amino acid / protein / polypeptide / peptide;
2.      nucleic acid / nucleotide / base;
3.      DNA;
4.      RNA;
5.      ATP / ADP;
6.      NAD / NADP (reduced or not);
7.      Cyclic AMP / cAMP;
8.      Chlorophyll;

Question 34

The genetic code is degenerate and non-overlapping.

Explain the meaning of:

Answer 34

1. Degenerate: more than one (base) triplet for each amino acid;
2. Non-overlapping: each base is part of only one triplet.