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AQA A-Level Biology > [3.1] Biological Molecules > Flashcards

[3.1] Biological Molecules Flashcards

Monomers & Polymers, Carbohydrates, Lipids, Proteins & Enzymes, Nucleic Acids, ATP, Water & Inorganic Ions

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1
Q

What are monomers & polymers?

A
  • Monomers are the smaller units from which larger molecules are made.
  • Polymers are molecules made from a large number of monomers joined together.
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2
Q

What happens in condensation & hydrolysis reactions?

A

CONDENSATION REACTION

  • 2 molecules join together
  • Forming a chemical bond
  • Releasing a water molecule

HYDROLYSIS REACTION

  • 2 molecules separated
  • Breaking a chemical bond
  • Using a water molecule
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3
Q

Give examples of polymers and the monomers from which they’re made.

A
  • Polynucleotide (DNA or RNA) - made up of nucleotides.
  • Polysaccharide (starch, cellulose, glycogen) - made up of monosaccharides.
  • Polypeptide - made up of amino acids.
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4
Q

Are lipids polymers?

A
  • Lipids are not polymers as they are not made up of repeating monomers.
  • Instead lipids are known as macromolecules.
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5
Q

What are monosaccharides? Give 3 common examples.

A
  • Monosaccharides are the monomers from which larger carbohydrates are made.
  • Glucose, galactose and fructose are common monosaccharides.
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6
Q

What are disaccharides and how are they formed?

A
  • Two monosaccharides joined together with a glycosidic bond
  • Formed by a condensation reaction, releasing a water molecule.
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7
Q

List 3 disaccharides and the monosaccharides from which they’re made.

A
  • Maltose - glucose + glucose
  • Sucrose - glucose + fructose
  • Lactose - glucose + galactose
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8
Q

Draw the structure of an alpha-glucose molecule.

A
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9
Q

Draw the structure of a beta-glucose molecule.

A
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10
Q

Draw a diagram to show how two molecules of alpha-glucose join together.

A
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11
Q

What are polysaccharides and how are they formed?

A
  • Many monosaccharides joined together with glycosidic bonds
  • Formed by many condensation reactions, releasing water molecules.
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12
Q

Describe the basic function and structure of starch.

A

FUNCTION OF STARCH

  • Energy store in plant cells.

STRUCTURE OF STARCH

  • Polysaccharide formed by the condensation of alpha-glucose molecules.
  • Amylose - 1,4-glycosidic bonds, unbranched.
  • Amylopectin - 1,4- & 1,6-glycosidic bonds, branched.
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13
Q

Describe the basic function and structure of glycogen

A

FUNCTION OF GLYCOGEN

  • Energy store in animal cells.

STRUCTURE OF GLYCOGEN

  • Polysaccharide formed by the condensation of alpha-glucose
    molecules    .
  • 1,4- & 1,6-glycosidic bonds, branched.
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14
Q

Explain how the structures of starch and glycogen relate to their functions.

A

STARCH (AMYLOSE)

  • Helical - compact for storage in cell.
  • Large, insoluble polysaccharide molecule - can’t leave cell/cross cell membrane.
  • Insoluble in water - water potential of cell not affected.

STARCH (AMYLOPECTIN) & GLYCOGEN

  • Branched - compact so fits more molecules in small area.
  • Branched - more ends for faster hydrolysis to release glucose for respiration to make ATP for the release of energy.
  • Large, insoluble polysaccharide molecule - can’t leave cell/cross cell membrane.
  • Insoluble in water - water potential of cell not affected.
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15
Q

Describe the basic function and structure of cellulose.

A

FUNCTION OF CELLULOSE

  • Provides strength and structural support to plant/agal cell walls.

STRUCTURE OF CELLULOSE

  • Polysaccharide formed by the condensation of beta-glucose molecules.
  • 1,4-glycosidic bond - straight, unbranched chains.
  • Chains linked in parallel by hydrogen bonds forming microfibrils.
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16
Q

Explain how the structure of cellulose relates to its function.

A
  • Every other beta-glucose molecule is inverted in a long, straight,
    unbranched chain.
  • Many hydrogen bonds link parallel strands, known as crosslinks, to
    form microfibrils.
  • Hydrogen bonds are strong in high numbers.
  • So provides strength to plant cell walls.
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17
Q

Name some reducing sugars and describe how to test for them.

A

Reducing sugars = monosaccharides, maltose, lactose.

  1. Add Benedict’s solution to sample.
  2. Heat in a boiling water bath.
  3. Positive result = green/yellow/orange/red precipitate.

(Darker precipitate indicates a higher quantity of sugar)

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18
Q

Name a non-reducing sugar and describe how to test for it.

A

Non-reducing sugar = sucrose

  1. Do Benedict’s test and stays blue indicates a negative result.
  2. Heat in a boiling water bath with acid (to hydrolyse into reducing sugars).
  3. Neutralise with an alkali such as sodium hydrogencarbonate.
  4. Heat in a boiling water bath with Benedict’s solution.
  5. Positive result = green/yellow/orange/red precipitate.
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19
Q

Describe the biochemical test for starch.

A
  1. Add iodine dissolved in potassium iodide to sample and shake/stir.
  2. Brown/orange to blue/black indicates positive result.
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20
Q

Suggest two methods to measure the quantity of sugar in a solution.

A

METHOD 1

  1. Carry out a Benedict’s test.
  2. Filter and dry the precipitate.
  3. Find the mass/weight.

METHOD 2

  1. Make sugar solutions of known concentrations - dilution series.
  2. Heat a set volume of each sample with a set volume of Benedict’s solution for the same time.
  3. Use colourimeter to measure absorbance of each known concentration.
  4. Plot calibration curve - concentration x-axis and absorbance on y-axis and draw line of best fit.
  5. Repeat Benedict’s test with unknown sample and measure absorbance.
  6. Read off calibration curve to find concentration associated with unknown sample’s absorbance.
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21
Q

Name two groups of lipids.

A

Triglycerides and phospholipids.

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22
Q

Describe how triglycerides form.

A
  • 1 glycerol molecule and 3 fatty acids
  • Condensation reaction
  • Removing 3 water molecules
  • Forming 3 ester bonds
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23
Q

Describe the structure of a fatty acid.

A
  • Variable R-group - hydrocarbon chain which can be saturated or unsaturated.
  • -COOH - carboxyl group
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24
Q

Describe the difference between saturated and unsaturated fatty acids.

A
  • Saturated: no C=C double bonds in hydrocarbon chain; all fully saturated with hydrogen.
  • Unsaturated: one or more C=C double bond in hydrocarbon chain which creates a bend/kink.
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25
Q

Describe the difference between the structure of triglycerides & phospholipids.

A

One of the fatty acids of a triglyceride is substituted by a phosphate-containing group (PO₄³⁻).

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26
Q

What is the function of triglycerides?

Explain how the properties of triglycerides are related to their structure.

A

FUNCTION

  • Energy storage

HOW THE PROPERTIES OF TRIGLYCERIDES RELATE TO THEIR STRUCTURE

  • High ratio of C-H bonds to carbon atoms in hydrocarbon chain
    • So used in respiration to release more energy than same mass of carbohydrates.
  • Hydrophobic/non-polar fatty acids so insoluble in water
    • So no effect on water potential of cell.
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27
Q

What is the function of phospholipids?

Explain how the properties of phospholipids are related to their structure.

A

FUNCTION OF PHOSPHOLIPIDS

  • Form a bilayer in cell membrane, allowing diffusion of lipid-soluble (non-polar) or very small substances and restricting movement of water-soluble (polar) or larger substances.
  • Phosphate heads are hydrophilic
    • Attracted to water so point to water either side of membrane.
  • Fatty acid tails are hydrophobic
    • Repelled by water so point away from water and point towards the interior of the membrane instead.
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28
Q

Describe the test for lipids.

A
  1. Add ethanol, shake and then add water.
  2. A milky white emulsion indicates a positive test.
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29
Q

Draw and describe the general structure of an amino acid.

A
  • COOH - represents a carboxyl group.
  • H₂N - represents an amine group
  • R = variable side chain/group
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30
Q

How many amino acids are common in all organisms? How do they vary?

A

There are 20 amino acids that are common in all organisms and they only vary in their side group (R).

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31
Q

Describe how amino acids join together.

A
  • Condensation reaction.
  • Removing a water molecule.
  • Between carboxyl/COOH group on one amino acid and amine/NH₂ group of another.
  • Forming a peptide bond.
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32
Q

What are dipeptides and polypeptides?

A
  • Dipeptide - formed by the condensation of two amino acids.
  • Polypeptides - formed by the condensation of many amino acids.
33
Q

Describe the primary structure of a protein.

A

Sequence of amino acids in a polypeptide chain, joined by peptide bonds.

34
Q

Describe the secondary structure of a protein.

A
  • Folding of polypeptide chain into an alpha helix or beta pleated sheet
  • Due to hydrogen bonding between amino acids
  • Between NH and C=O.
35
Q

Describe the tertiary structure of a protein.

A
  • 3D folding of polypeptide chain
  • Due to interactions between acid R groups
  • Forming hydrogen bonds, ionic bonds and disulfide bridges
36
Q

Describe the quaternary structure of a protein.

A
  • More than one polypeptide chain
  • Formed by interactions between polypeptides (hydrogen bonds, ionic bonds, disulfide bridges).
37
Q

Describe the test for proteins.

A
  1. Add biuret reagent (sodium hydroxide + copper (II) sulphate)
  2. Purple/lilac colour is a positive result as it indicates the presence of
    peptide bonds (negative stays blue)
38
Q

How do enzymes act as biological catalysts?

A

Each enzyme lowers activation energy of reaction it catalyses to speed up the rate of reaction.

39
Q

Describe the induced-fit model of enzyme action.

A
  1. Substrate binds to active site of enzyme.
  2. Causing active site to change shape so it is complementary to substrate.
  3. So enzyme-substrate complex forms.
  4. Causing bonds in substrate to bend/distort, lowering activation energy.
40
Q

Explain the specificity of enzymes.

A
  • Specific tertiary structure determines the shape of active site
    • Dependent on sequence of amino acids (primary structure).
  • Active site is complimentary to specific substrate.
  • Only this substrate can bind to active site, inducing fit and forming an enzyme-substrate complex.
41
Q

Describe how models of enzyme action have changed over time.

A
  • Initially lock and key model
    • Active site is a fixed shape, complementary to one substrate.
  • Now induced-fit model.
42
Q

Describe and explain the effect of enzyme concentration on the rate of enzyme-controlled reactions.

A
  • As enzyme concentration increases, rate of reaction increases
    • Enzyme concentration = limiting factor
    • More enzymes so more available active sites
    • So more enzyme-substrate complexes form
  • At a certain point, the rate of reaction stops increasing and levels off
    • Substrate concentration has become a limiting factor as all substrates are in use
43
Q

Describe and explain the effect of substrate concentration on the rate of enzyme-controlled reactions.

A
  • As substrate concentration increases, rate of reaction increases
    • Substrate concentration = limiting factor
    • More enzyme-substrate complexes form
  • At a certain point, the rate of reaction stops increasing and levels off
    • Enzyme concentration becomes a limiting factor
    • As all active sites are saturated/occupied
44
Q

Describe and explain the effect of temperature on the rate of enzyme-controlled reactions.

A
  • As temperature increases up to optimum, rate of reaction increases
    • More kinetic energy
    • So more enzyme-substrate complexes form
  • As temperature increases above optimum, rate of reaction decreases
    • Enzymes denature - tertiary structure and active site change shape
    • As hydrogen bonds/ionic bonds break
    • So active site no longer complementary
    • So fewer enzyme-substrate complexes form
44
Q

Describe and explain the effect of pH on the rate of enzyme-controlled reactions.

A
  • As pH increases/decreases above/below an optimum, rate of
    reaction decreases
    • Enzymes denature - tertiary structure and active site change shape
    • As hydrogen/ionic bonds break
    • So active site no longer complementary
    • So fewer enzyme-substrate complexes form
44
Q

Describe and explain the effect of concentration of competitive inhibitors on the rate of enzyme-controlled reactions.

A
  • As concentration of competitive inhibitor increases, rate of reaction
    decreases
    • Similar shape to substrate
    • Competes for/binds to/blocks active site
    • So substrate can’t bind and fewer enzyme-substrate complexes form
  • Increasing substrate concentration reduces effect of inhibitors.
45
Q

Describe and explain the effect of concentration of non-competitive inhibitors on the rate of enzyme-controlled reactions.

A
  • As concentration of non-competitive inhibitor increases, rate of reaction decreases
    • Binds to site other than the active site
    • Changes enzyme tertiary structure thus changing the shape of the active site
    • So active site no longer complementary to substrate
    • So substrates can’t bind and fewer enzyme-substrate complexes form
  • Increasing substrate concentration has no effect on the rate of
    reaction as change to active site is permanent
45
Q

Describe the basic functions of DNA and RNA in all living cells.

A
  • DNA - holds genetic information which codes for polypeptides (proteins).
  • RNA - transfers genetic information from DNA to ribosomes.
46
Q

Name the two types of molecules from which a ribosome is made.

A

RNA and proteins.

47
Q

Draw and label a DNA nucleotide and an RNA nucleotide.

A
48
Q

Describe the difference between a DNA nucleotide and an RNA nucleotide.

A

DNA NUCLEOTIDE

  • Pentose sugar is deoxyribose.
  • Base can be thymine.

RNA NUCLEOTIDE

  • Pentose sugar is ribose.
  • Base can be uracil.
49
Q

Describe how nucleotides join together to form polynucleotides.

A
  • Condensation reactions, removing water molecules
  • Between phosphate group of one nucleotide and deoxyribose/ribose of another
  • Forming phosphodiester bonds
50
Q

Why did many scientists initially doubt that DNA carried the genetic code?

A

The relative simplicity of DNA - chemically simple molecule with few components.

51
Q

Describe the structure of DNA.

A
  • Polymer of nucleotides (polynucleotide).
  • Each nucleotide formed from deoxyribose, a phosphate group and a nitrogen-containing organic base.
  • Phosphodiester bonds join adjacent nucleotides.
  • 2 polynucleotide chains held together by hydrogen bonds between specific complimentary base pairs - adenine/thymine and cytosine/guanine.
  • Double helix.
52
Q

Describe the structure of (messenger) RNA

A
  • Polymer of nucleotides (polynucleotides).
  • Each nucleotide formed from ribose, a phosphate group and a nitrogen-containing organic base.
  • Bases - uracil, adenine, cytosine and guanine.
  • Phosphodiester bonds join adjacent nucleotides.
  • Single helix.
53
Q

Compare and contrast the structure of DNA and (messenger) RNA

A

DNA NUCLEOTIDE

  • Pentose sugar is deoxyribose.
  • Has the base thymine.
  • Double-stranded/double helix.
  • Long (many nucleotides).
  • Has hydrogen bonds/base pairing.

RNA NUCLEOTIDE

  • Pentose sugar is ribose.
  • Has the base uracil.
  • Single-stranded/single helix.
  • Shorter (fewer nucleotides).
  • Does not have hydrogen bonds/base pairing.
54
Q

Suggest how the structure of DNA relates to its functions.

A
  • Two strands so both can act as template strands for semi-
    conservative replication    .
  • Hydrogen bonds between bases are weak so strands can be
    separated for replication.
  • Complementary base pairing so accurate replication.
  • Many hydrogen bonds between bases so stable/strong molecule.
  • Double helix with sugar-phosphate backbone protects bases/hydrogen bonds.
  • Long molecule so stores lots of genetic information that codes for polypeptides.
  • Double helix so compact.
55
Q

Suggest how you can use incomplete information about the frequency of bases on DNA strands to find the frequency of other bases.

A
  1. % of adenine in strand 1 = % of thymine in strand 2 (vice versa).
  2. % of guanine in strand 1 = % of cytosine in strand 2 (vice versa).
    • Because of specific complementary base pairing between 2 strands.
56
Q

Why is semi-conservative replication important?

A

Ensures genetic continuity between generations of cells.

57
Q

Describe the process of semi-conservative DNA replication.

A
  1. DNA helicase breaks hydrogen bonds between complementary bases, unwinding the double helix.
  2. Both strands act as templates.
  3. Free DNA nucleotides attracted to exposed bases and join by specific complementary base pairing.
  4. Hydrogen bonds form between adenine-thymine and guanine-cytosine.
  5. DNA polymerase joins adjacent nucleotides on new strand by condensation reactions.
  6. Forming phosphodiester bonds.
58
Q

What does semi-conservative mean in terms of DNA?

A

Each new DNA molecule consists of one original/template strand and one new strand.

59
Q

Name the two scientists who proposed models of the chemical structure of DNA and of DNA replication.

A

Watson and Crick.

60
Q

Describe the work of Meselson and Stahl in validating the Watson-Crick model of semi-conservative DNA replication.

A
  1. Bacteria grown in medium containing heavy nitrogen (¹⁵N) and nitrogen is incorporated into DNA bases.
    • DNA extracted & centrifuged where it settles near bottom as all DNA molecules contain two ‘heavy’ strands.
  2. Bacteria transferred to medium containing light nitrogen (¹⁴N) and allowed to divide once
    • DNA extracted and centrifuged where it settles in middle as all DNA molecules contain 1 original ‘heavy’ strand and 1 new ‘light’ strand.
  3. Bacteria in light nitrogen (¹⁴N) allowed to divide again.
    • DNA extracted and centrifuged where half settles in middle as it contains 1 original ‘heavy’ strand and 1 new ‘light’ strand.
    • The other half settles near top as it contains 2 ‘light strands’.

(Other models not supported - bands would be in different places)

61
Q

What is ATP?

A

Adenosine triphosphate.

62
Q

Describe the structure of ATP.

A
  • Ribose bound to a molecule of adenine and 3 phosphate groups (PO₄³⁻).
  • Nucleotide derivative.
63
Q

Describe how ATP is broken down.

A
  1. ATP + (water) -> ADP (adenosine diphosphate) + Pi (inorganic phosphate)
  2. Hydrolysis reaction, using a water molecule.
  3. Catalysed by ATP hydrolase.
64
Q

Give two ways in which the hydrolysis of ATP is used in cells.

A
  1. Coupled to energy-requiring reactions within cells (releases/provides energy)
    • E.g. active transport, protein synthesis.
  2. Inorganic phosphate released can be used to phosphorylate (add phosphate to) other compounds, making them more reactive.
65
Q

Describe how ATP is resynthesised in cells.

A
  1. ADP + Pi -> ATP (+ water)
  2. Condensation reaction, removing a water molecule.
  3. Catalysed by ATP synthase.
  4. During respiration and photosynthesis.
66
Q

Suggest how the properties of ATP make it a suitable immediate source of energy for cells.

A
  1. Releases energy in small amounts/little energy lost as heat.
  2. Single reaction/one bond hydrolysed to release energy so immediate release.
  3. Cannot pass out of cell.
67
Q

Explain how hydrogen bonds occur between water molecules.

A
  1. Water is a polar molecule.
  2. Slightly negative charged oxygen atoms attract slightly positive
    charged hydrogen atoms of other water molecules.
68
Q

Explain 5 properties of water that are important in biology.

A
  1. Metabolite.
    • Used in condensation / hydrolysis /photosynthesis / respiration.
  2. Solvent (dissolves solutes).
    • Allows metabolic reactions to occur (faster in solution).
    • Allows transport of substances e.g. nitrates in xylem, urea in blood.
  3. High specific heat capacity
    • Buffers changes in temperature as water can gain/lose a lot of heat/energy without changing temperature.
      • This makes it a good habitat for aquatic organisms as temperature is more stable than land.
      • Helps organisms maintain a constant internal body temperature.
  4. High latent heat of vaporisation.
    • Provides a cooling effect with little loss of water through
      evaporation (e.g. sweat)
    • So helps organisms maintain a constant internal body temperature.
  5. Strong cohesion between water molecules.
    • Supports columns of water (e.g. transpiration stream) through xylem in plants.
    • Produces surface tension where water meets air, supporting small organisms (to walk on water).
69
Q

Where are inorganic ions found in the body?

A

In solution in cytoplasm and body fluid, some in high concentrations and others in very low concentrations.

70
Q

Describe the role of hydrogen, iron, sodium and phosphate ions.

A

HYDROGEN IONS (H⁺)

  • Maintain pH levels in the body.
    • High concentration = acidic, low pH.
    • Low concentration = alkali, high pH.
  • Affects enzyme rate of reaction as can cause enzymes to denature.

IRON IONS (Fe²⁺)

  • Component of haem group of haemoglobin.
    • Allowing oxygen to bind/associate for transport as oxyhaemoglobin.

SODIUM IONS (Na⁺)

  • Involved in co-transport of glucose/amino acids into cells.
  • Involved in action potentials in neurons.
  • Affects water potential of cells/osmosis.

PHOSPHATE IONS (PO₄³⁻)

  • Component of nucleotides allowing phosphodiester bonds to form in DNA & RNA.
  • Component of ATP, allowing energy release.
  • Phosphorylates other compounds making them more reactive.
  • Hydrophilic part of phospholipids, allowing a bilayer to form.
71
Q

Describe the difference between the structure of a-glucose and b-glucose.

A
  • OH group is below carbon 1 in alpha-glucose but above carbon 1 in beta-glucose.

(Alpha & beta glucose are isomers - same molecule formula, differently arranged atoms)

72
Q

Use your knowledge of enzyme action to suggest why DNA polymerase moves in opposite directions along DNA strands.

A
  1. DNA has antiparallel strands.
  2. So shapes/arrangements of nucleotide on two ends are different.
  3. DNA polymerase is an enzyme with a specific shaped active site.
  4. So can only bind to substrate with complementary shape.
73
Q

What equation would you use to calculate pH if you were given the hydrogen ion concentration of a solution?

A

pH = -log₁₀[H⁺]

74
Q

Draw a diagram to show the formation of a dipeptide.

A
75
Q

Draw a diagram to show the structure of an ATP molecule.

A

AQA A-Level Biology (5 decks)

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