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AQA A level Biology > mark scheme standard > Flashcards

mark scheme standard Flashcards

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1
Q

Suggest one way the structure of the chromosome could differ along its length to result in the stain binding more in some areas (1 mark)

A

Different base sequence

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2
Q

What is a homologous pair of chromosomes

A

2 chromosomes that carry the same genes

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3
Q

Give 2 ways in which the arrangement of prokaryotic DNA is different from the arrangement of the human DNA (2 marks)

A

Prokaryotic DNA is :
- Circular ( as opposed to linear)
- Not associated with proteins
- Only one piece of DNA

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4
Q

Describe how 1 amino acid is added to a polypeptide that is being formed at a ribosome during translation (3 marks)

A
  • tRNA brings specific amino acid to ribosome
  • Anticodon on tRNA binds to codon on mRNA
  • Amino acids join by condensation reaction
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5
Q

Explain how the formation of an enzyme-substrate complex increase the rate of reaction

A
  • Reduces activation energy
  • Due to bending bonds / without enzyme, very few substrates have sufficient energy for reaction
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6
Q

Suggest why iron-deficient plants have a reduced growth rate

A
  • Less thylakoid membrane
  • Smaller SA / less chlorophyll
  • So reduced light absorption
  • So slower rate of photosynthesis
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7
Q

What is a monomers

A

Molecule from which larger molecules are made

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8
Q

Describe a biochemical test to show a solution contains a non-reducing sugar (3 marks)

A
  • Heat with acid and neutralise
  • Heat with Benedict’s solution
  • Red precipitate
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9
Q

A student carried out the Benedict’s test. Suggest a method, other than using a colorimeter, that this student could use to measure the quantity of reducing sugar in a solution. (2 marks)

A

Filter and dry (the precipitate);
Accept: correct reference to evaporation after filtration
2. Find mass/weight;

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10
Q

Describe two differences between the structure of a cellulose molecule and a glycogen molecule (2 marks)

A
  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;
    Ignore ref. to H bonds / microfibrils
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11
Q

Describe and explain two features of starch that make it a good storage molecule. (2 marks)

A
  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
    Require feature and explanation for 1 mark
    - Accept Ψ or WP
    - Accept Insoluble so doesn’t affect osmosis
    - Do not allow ref to ‘doesn’t affect water leaving cells
    - Ignore ‘surface area’
    - Accept ‘branched so glucose readily released’
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12
Q

Describe the structure of glycogen

A
  1. Polysaccharide of α-glucose;
    OR
    polymer of α-glucose;
  2. (Joined by) glycosidic bonds
    OR
    Branched structure
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13
Q

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.

A
  1. Hydrolysed (to glucose);
  2. Glucose used in respiration;
    1. Ignore ‘Broken down’
    2. ‘Energy produced’ disqualifies mp2
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14
Q

Name the monomers from which a maltose molecule is made

A

Glucose (and glucose);

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15
Q

Name the type of chemical bond that joins the two monomers to form maltose.

A

(α1,4) Glycosidic;

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16
Q

Explain the difference in the structure of the starch molecule and the
cellulose molecule (2 marks)

A
  1. Starch formed from α-glucose but cellulose formed from β-glucose;
  2. Position of hydrogen and hydroxyl groups on carbon atom 1
    inverted.
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17
Q

Explain one way in which starch molecules are adapted for their function in plant cells. (2 marks)

A
  1. Insoluble;
  2. Don’t affect water potential;
    OR
  3. Helical;
    Accept form spirals
  4. Compact;
    OR
  5. Large molecule;
  6. Cannot leave cell.
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18
Q

Explain how cellulose molecules are adapted for their function in plant cells. (3 marks)

A
  1. Long and straight chains;
  2. Become linked together by many hydrogen bonds to form
    fibrils;
  3. Provide strength (to cell wall).
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19
Q

Name the group represented by COOH

A

Carboxyl;

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20
Q

Describe how you would test for the presence of a lipid in a liquid sample of food.

A
  1. Add ethanol/alcohol then add water and shake/mix
    OR
    Add ethanol/alcohol and shake/mix then pour into/add water;
    Reject heating the emulsion test
    Accept ‘Add Sudan III and mix’
    Ignore a second shake
  2. White/milky (emulsion)
    OR
    (emulsion) test turns white/milky;
    Ignore cloudy
    Reject precipitate
    Accept (for Sudan III) top (layer) red
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21
Q

Describe how a triglyceride molecule is formed

A
  1. One glycerol and three fatty acids;
  2. Condensation (reactions) and removal of three molecules of water;
  3. Ester bond(s) (formed);
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22
Q

Describe how an ester bond is formed in a phospholipid molecule.

A
  1. Condensation (reaction)
    OR
    Loss of water;
  2. Between of glycerol and fatty acid;
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23
Q

Describe the induced-fit model of enzyme action and how an enzyme acts as a catalyst (3 marks)

A
  1. Substrate binds to the active site/enzyme
    OR
    Enzyme-substrate complex forms;
    Accept for ‘binds’, fits
  2. Active site changes shape (slightly) so it is complementary to
    substrate
    OR
    Active site changes shape (slightly) so
    distorting/breaking/forming bonds in the substrate;
  3. Reduces activation energy;
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24
Q

A competitive inhibitor decreases the rate of an enzyme-controlled
reaction.
Explain how.
(3 marks)

A
  1. Inhibitor similar shape to substrate;
    Reject same shape
    Accept ‘complementary to active site’
  2. Fits/binds to active site;
  3. Prevents/reduces enzyme-substrate complex forming
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25
Q

Describe how the structure of a protein depends on the amino acids it contains. (5 marks)

A
  1. Structure is determined by (relative) position of amino acid/R
    group/interactions;
    Accept for ‘interactions’, hydrogen bonds / disulfide
    bridges / ionic bonds / hydrophobic hydrophilic
    interactions
  2. Primary structure is sequence/order of amino acids;
  3. Secondary structure formed by hydrogen bonding (between amino
    acids);
    Accept alpha helix/β-pleated sheet for ‘secondary
    structure’
  4. Tertiary structure formed by interactions (between R groups);
    Accept for ‘interactions’, hydrogen bonds / disulfide
    bridges / ionic bonds / hydrophobic hydrophilic
    interactions
  5. Creates active site in enzymes
    OR
    Creates complementary/specific shapes in antibodies/carrier
    proteins/receptor (molecules);
  6. Quaternary structure contains >1 polypeptide chain
    OR
    Quaternary structure formed by interactions/bonds between
    polypeptides;
    Accept for ‘intereactions’, hydrogen bonds/ disulfide
    bridges/ionic bonds/hydrophobichydrophilic
    interactions
    Accept prosthetic (group)
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26
Q

Explain how the active site of an enzyme causes a high rate of reaction. (3 marks)

A
  1. Lowers activation energy;
  2. Induced fit causes active site (of enzyme) to change shape;
  3. (So) enzyme-substrate complex causes bonds to form/break;
    Accept: description, of induced fit
    Accept: enzyme-substrate complex causes stress/strain
    on bonds.
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27
Q

Describe a biochemical test to confirm the presence of protein in a
solution. (2 marks)

A
  1. Add biuret (reagent);
    Accept sodium hydroxide (solution) and copper sulphate
    (solution)
    Reject addition of other incorrect chemicals
  2. (Positive result) purple/lilac/violet /mauve;
    Reject other colours
    Ignore references to heating
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28
Q

A dipeptide consists of two amino acids joined by a peptide bond.
Dipeptides may differ in the type of amino acids they contain.

Describe two other ways in which all dipeptides are similar and one way in which they might differ. (3 marks)1

A

Similarities
1. Amine/NH2 (group at end);
Accept amino/NH3+
2. Carboxyl/COOH (group at end);
Accept carboxylic / COO−
3. Two R groups;
4. All contain C and H and N and O;

Differences
Variable/different R group(s);

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29
Q

Describe how a non-competitive inhibitor can reduce the rate of an
enzyme-controlled reaction. (3 marks)

A
  1. Attaches to the enzyme at a site other than the active site;
    Accept ‘attaches to allosteric/inhibitor site’
  2. Changes (shape of) the active site
    OR
    Changes tertiary structure (of enzyme);
  3. (So active site and substrate) no longer complementary so
    less/no substrate can fit/bind;
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30
Q

Describe how a peptide bond is formed between two amino acids to form a dipeptide. (2 marks)

A
  1. Condensation (reaction) / loss of water;
    Accept each marking point if shown clearly in diagram.
  2. Between amine / NH2 and carboxyl / COOH;
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31
Q

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

A
  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;
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32
Q

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

A
  1. Different sequence of amino acids
    OR
    Different primary structure;
  2. Forms ionic / hydrogen / disulfide bonds in different places;
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33
Q

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

A
  1. Reduces activation energy;
    Accept ‘reduces Ea’.
  2. Due to bending bonds
    OR
    Without enzyme, very few substrates have sufficient energy for
    reaction;
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34
Q

Describe the structure of DNA (5 marks)

A
  1. Polymer of nucleotides;
    Accept ‘Polynucleotide’
    Accept for ‘phosphate’. phosphoric acid
  2. Each nucleotide formed from deoxyribose, a phosphate (group) and an organic/nitrogenous base;
  3. Phosphodiester bonds (between nucleotides);
  4. Double helix/2 strands held by hydrogen bonds;
  5. (Hydrogen bonds/pairing) between adenine, thymine and
    cytosine, guanine;
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35
Q

Describe how a phosphodiester bond is formed between two nucleotides within a DNA molecule.

A
  1. Condensation (reaction)/loss of water;
  2. (Between) phosphate and deoxyribose;
  3. (Catalysed by) DNA polymerase;
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36
Q

In the process of semi-conservative DNA replication, the two strands within a DNA molecule are separated. Each then acts as a template for the formation of a new complementary strand.
Describe how the separation of strands occurs. (2 marks)

A
  1. DNA helicase;
  2. Breaks hydrogen bonds between base pairs/ AT and GC/complementary bases
    OR
    Breaks hydrogen bonds between polynucleotide strands;
    Reject hydrolysis of hydrogen bonds
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37
Q

Describe the role of DNA polymerase in the semi-conservative replication of DNA. (2 marks)

A
  1. Joins (adjacent DNA) nucleotides;
    Reject suggestions that it forms hydrogen bonds or
    joins complementary bases.
    Reject ‘nucleotide bases’.
  2. (Catalyses) condensation (reactions);
  3. (Catalyses formation of) phosphodiester bonds (between adjacent
    nucleotides)
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38
Q

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

A

Watson and Crick

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39
Q

Use your knowledge of semi-conservative replication of DNA to suggest:
1. the role of the single-stranded DNA fragments
2. the role of the DNA nucleotides
(3 marks)

A

Role of single-stranded DNA fragments
1. Template;
2. Determines order of nucleotides/bases;

Role of DNA nucleotides
3. Forms complementary pairs / A – T, G - C
OR
Forms complementary (DNA) strand;
Ignore forms complementary bases
Accept sequence/ chain for strand

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40
Q

Give two features of DNA and explain how each one is important in the semi-conservative replication of DNA (2 marks)

A
  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;
    Allow description of complementary base pairing
    and accurate replication.
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41
Q

Describe the role of two named enzymes in the process of semiconservative replication of DNA. (3 marks)

A
  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;
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42
Q

The enzymes DNA helicase and DNA polymerase are involved in DNA
replication.
Describe the function of each of these enzymes.
1. DNA helicase
2. DNA polymerase
(2 marks)

A
  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
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43
Q

Adenosine triphosphate (ATP) is a nucleotide derivative.

Contrast the structures of ATP and a nucleotide found in DNA to give two differences.

A
  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;
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44
Q

Describe how an ATP molecule is formed from its component molecules (4 marks)

A
  1. and 2. Accept for 2 marks correct names of three components adenine, ribose/pentose, three phosphates;;
    Accept for 1 mark, correct name of two components
    Accept for 1 mark, ADP and phosphate/Pi
    Ignore adenosine
    Accept suitably labelled diagram
  2. Condensation (reaction);
    Ignore phosphodiester
  3. ATP synthase;
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45
Q

Water is used to hydrolyse ATP.
Name the two products of ATP hydrolysis.

A

Adenosine diphosphate and (inorganic) phosphate;

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46
Q

ATP is an energy source used in many cell processes. Give two ways in which ATP is a suitable energy source for cells to use. (2 marks)

A
  1. Releases relatively small amount of energy / little energy lost as
    heat;
    Key concept is that little danger of thermal death of cells
  2. Releases energy instantaneously;
    Key concept is that energy is readily available
  3. Phosphorylates other compounds, making them more reactive;
  4. Can be rapidly re-synthesised;
  5. Is not lost from / does not leave cells
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47
Q

Cells constantly hydrolyse ATP to provide energy.
Describe how ATP is resynthesised in cells (2 marks)

A
  1. From ADP and phosphate;
    Reject P/Phosphorus
    Reject use of water in the reaction
  2. By ATP synthase;
  3. During respiration/photosynthesis;
48
Q

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

A
  1. To provide energy for other reactions/named process;
    Reject ‘produce’ energy
  2. To add phosphate to other substances and make them more
    reactive/change their shape;
49
Q

Explain five properties that make water important for organisms. (5 marks)

A
  1. A metabolite in condensation/hydrolysis/ photosynthesis/respiration;
  2. A solvent so (metabolic) reactions can occur
    OR
    A solvent so allowing transport of substances;
  3. High (specific) heat capacity so buffers changes in temperature;
    For ‘buffer’ accept ‘resist’.
  4. Large latent heat of vaporisation so provides a cooling effect (through evaporation);
    Reject latent heat of evaporation
  5. Cohesion (between water molecules) so supports columns of water
    (in plants);
    For ‘columns of water’ accept ‘transpiration stream’.
    Do not credit ‘transpiration’ alone but accept description of
    ‘stream’.
    For ‘columns of water’ accept ‘cohesion-tension (theory)’.
  6. Cohesion (between water molecules) so produces surface tension
    supporting (small) organisms;
    For cohesion accept hydrogen bonding
    Ignore reference to pH. Allow other suitable properties but
    must have a valid explanation.
    For example
    * ice floating so maintaining aquatic habitat beneath
    * water transparent so allowing light penetration for
    photosynthesis
50
Q

State and explain the property of water that helps to prevent temperature increase in a cell. (2 marks)

A
  1. High (specific) heat capacity;
  2. Buffers changes in temperature
51
Q

Give two properties of water that are important in the cytoplasm of cells.
For each property of water, explain its importance in the cytoplasm.
(4 marks)

A
  1. Polar molecule;
  2. Acts as a (universal) solvent;
    OR
  3. (Universal) solvent;
  4. (Metabolic) reactions occur faster in solution;
    OR
  5. Reactive;
  6. Takes place in hydrolysis / condensation / named reaction
52
Q

Explain why it is not possible to determine the identity of the structures within a cell using an optical microscope. (2 marks)

A
  1. Resolution (too) low;
  2. Because wavelength of light is (too) long;
53
Q

U. marinum cells ingest bacteria and digest them in the cytoplasm.
Describe the role of one named organelle in digesting these bacteria (3 marks)

A
  1. Lysosomes;
  2. Fuse with vesicle;
    Accept phagosome for vesicle
  3. (Releases) hydrolytic enzymes;
54
Q

Give two structures found in all prokaryotic cells and in all eukaryotic cells (2 marks)

A
  1. Cell(-surface) membrane;
  2. Ribosomes;
    Ignore 70S
  3. Cytoplasm;
  4. DNA;
55
Q

Describe how a sample of chloroplasts could be isolated from leaves. (4 marks)

A
  1. Break open cells/tissue and filter
    OR
    Grind/blend cells/tissue/leaves and filter;
    Accept homogenise and filter
  2. In cold, same water potential/concentration, pH controlled solution;
    Accept for ‘same water potential/ concentration’,
    isotonic
    Accept for ‘pH controlled’, buffered
  3. Centrifuge/spin and remove nuclei/cell debris;
  4. (Centrifuge/spin) at high(er) speed, chloroplasts settle out;
56
Q

Eukaryotic cells produce and release proteins.
Outline the role of organelles in the production, transport and release of proteins from eukaryotic cells.
Do not include details of transcription and translation in your answer
(4 marks)

A
  1. DNA in nucleus is code (for protein);
  2. Ribosomes/rough endoplasmic reticulum produce (protein);
    Accept rER for ‘rough endoplasmic reticulum’
  3. Mitochondria produce ATP (for protein synthesis);
  4. Golgi apparatus package/modify;
    OR
    Carbohydrate added/glycoprotein produced by Golgi apparatus;
    Accept body for ‘apparatus’
  5. Vesicles transport
    OR
    Rough endoplasmic reticulum transports;
  6. (Vesicles) fuse with cell(-surface) membrane;
    Accept exocytosis at cell membrane
57
Q

Give one advantage of viewing a biological specimen using a transmission electron microscope compared with using a scanning electron microscope (1 mark)

A

Higher resolution
OR
View internal structures;

58
Q

A biologist separated cell components to investigate organelle activity. She prepared a suspension of the organelles in a solution that prevented damage to the organelles.
Describe three properties of this solution and explain how each property prevented damage to the organelles. (3 marks)

A
  1. (Ice) cold to prevent/reduce enzyme activity;
    For 1, 2 and 3 reject context of cell
  2. Buffered to prevent denaturing of enzyme/protein;
    Accept description of buffer.
    Accept: prevent change of tertiary structure.
  3. Same water potential/ Ψ to prevent lysis/bursting (of organelle);
    Accept: isotonic for same water potential.
    Reject: references to turgor or plasmolysis or crenati
59
Q

Contrast how an optical microscope and a transmission electron
microscope work and contrast the limitations of their use when studying cells. (6 marks)

A
  1. TEM use electrons and optical use light;
  2. TEM allows a greater resolution;
  3. (So with TEM) smaller organelles / named cell structure can be
    observed
    OR
    greater detail in organelles / named cell structure can be
    observed;
  4. TEM view only dead / dehydrated specimens and optical (can)
    view live specimens;
  5. TEM does not show colour and optical (can);
  6. TEM requires thinner specimens;
  7. TEM requires a more complex/time consuming preparation;
  8. TEM focuses using magnets and optical uses (glass) lenses;
60
Q

Name two structures present in plant cells that are not present in animal cells (1 mark)

A
  1. Chloroplasts / plastids
  2. Cell wall
  3. Cell vacuole
  4. Starch grains / amyloplasts;
61
Q

Describe how you could make a temporary mount of a piece of plant tissue to observe the position of starch grains in the cells when using an optical (light) microscope (4 marks)

A
  1. Add drop of water to (glass) slide;
  2. Obtain thin section (of plant tissue) and place on slide / float on
    drop of water;
  3. Stain with / add iodine in potassium iodide.
    Allow any appropriate method that avoids trapping air bubbles
  4. Lower cover slip using mounted needle.
62
Q

Describe binary fission in bacteria (3 marks)

A
  1. Replication of (circular) DNA;
    Accept nucleoid
    Reject chromosome
    Reject mitosis
  2. Replication of plasmids;
  3. Division of cytoplasm (to produce daughter cells);
    Ignore genetically identical
63
Q

Suggest and explain how two environmental variables could be changed
to increase the growth rate of bacterial cells (4 marks)

A
  1. Increased (concentration of) glucose;
  2. Increased respiration;
  3. Increased (concentration of) oxygen;
  4. Increased respiration;
  5. Increased temperature;
  6. Increased enzyme activity;
  7. Increased (concentration of) phosphate;
  8. Increased ATP/DNA/RNA;
  9. Increased (concentration of) nucleotides;
  10. Increased DNA synthesis
64
Q

Describe the role of the spindle fibres and the behaviour of the
chromosomes during each of these phases, C-Prophase, D-Metaphase, E-Anaphase (4 marks)

A

(In) prophase, chromosomes condense;
Accept chromatin for ‘chromosomes’ and for ‘condense’, shorten and
thicken
3. (In) prophase OR metaphase, centromeres attach to spindle fibres;

  1. (In) metaphase, chromosomes/pairs of chromatids at equator/centre of spindle/cell;
  2. (In) anaphase, centromeres divide;
  3. (In) anaphase, chromatids (from each pair) pulled to (opposite) poles/ends (of cell);
    Accept for ‘chromatids’, chromosomes but reject homologous
    chromosomes
  4. (In) prophase/metaphase/anaphase, spindle fibres shorten
65
Q

A student prepared a stained squash of cells from the tip of an onion root
and observed it using an optical microscope.
During the preparation of the slide, he:
* cut the first 5 mm from the tip of an onion root and placed it on a
glass slide
* covered this tip with a drop of stain solution and a cover slip
* warmed the glass slide
* pressed down firmly on the cover slip.
He identified and counted nuclei in different stages of the cell cycle.

Explain why the student:
1. used only the first 5 mm from the tip of an onion root.
2. pressed down firmly on the cover slip.
(2 marks)

A
  1. Where dividing cells are found / mitosis occurs;
    OR
    No dividing cells / mitosis in tissue further away / more than 5 mm from tip;
    OR
    To get (soft) tissue that will squash;
    OR
    Length that will fit under cover slip;
    Accept most dividing cells
  2. Single / thin layer of cells / spread out cells so light passes through
    (making cells / nuclei visible);
    Accept thin layer of tissue
66
Q

What is the name given to the division of cytoplasm during the cell cycle

A

Cytokinesis

67
Q

Describe and explain what the student should have done when counting
cells to make sure that the mitotic index he obtained for this root tip was
accurate (2 marks)

A
  1. Examine large number of fields of view / many cells;
    Mark as pairs only
    Accept large number / 20 or more for many
  2. To ensure representative sample;
    Accept typical / reliable
    OR
  3. Repeat count;
  4. To ensure figures are correct;
    OR
  5. Method to deal with part cells shown at edge /count only whole cells;
  6. To standardise counting
68
Q

When preparing the cells for observation the scientist placed them in a
solution that had a slightly higher (less negative) water potential than the
cytoplasm. This did not cause the cells to burst but moved the
chromosomes further apart in order to reduce the overlapping of the
chromosomes when observed with an optical microscope.

Suggest how this procedure moved the chromosomes apart.
(2 marks)

A
  1. Water moves into the cells/cytoplasm by osmosis;
    Reject water moving into chromosomes/nucleus.
  2. Cell/cytoplasm gets bigger
69
Q

Name the process by which bacterial cells divide

A

Binary Fission

70
Q

A microbiologist investigated the ability of different plant oils to kill the bacterium
Listeria monocytogenes. She cultured the bacteria on agar plates. She obtained
the bacteria from a broth culture.

Describe two aseptic techniques she would have used when transferring a
sample of broth culture on to an agar plate.

Explain why each was important.
(4 marks)

A
  1. Keep lid on Petri dish
    OR
    Open lid of Petri dish as little as possible.
  2. To prevent unwanted bacteria contaminating the dish.
    OR
    L. monocytogenes may be dangerous / may get out.

OR

  1. Wear gloves
    OR
    Wear mask
    OR
    Wash hands;
  2. To prevent contamination from bacteria on hands / mouth
    OR
    Prevent spread of bacteria outside the lab;
    OR
  3. Use sterile pipette
    OR
    Flame the loop
    OR
    Flame the neck of the container of the culture;
  4. To maintain a pure culture of bacteria
71
Q

Describe the appearance and behaviour of chromosomes during mitosis
(5 marks)

A

(During prophase)
1. Chromosomes coil / condense / shorten / thicken / become visible;

  1. (Chromosomes) appear as (two sister) chromatids joined at the centromere;

(During metaphase)
3. Chromosomes line up on the equator / centre of the cell;

  1. (Chromosomes) attached to spindle fibres;
  2. By their centromere;

(During anaphase)
6. The centromere splits / divides;

  1. (Sister) chromatids / chromosomes are pulled to opposite poles / ends of the cell / separate;

(During telophase)
8. Chromatids / chromosomes
uncoil / unwind / become longer / thinner

72
Q

The student prepared a temporary mount of the onion tissue on a glass
slide. She covered the tissue with a cover slip. She was then given the
following instruction.

“Push down hard on the cover slip, but do not push the cover slip
sideways.”

Explain why she was given this instruction. (2 marks)

A
  1. Push hard – spread / squash tissue;
  2. Not push sideways – avoid rolling cells together / breaking
    chromosomes.
73
Q

Name and describe five ways substances can move across the
cell-surface membrane into a cell (5 marks)

A
  1. (Simple) diffusion of small/non-polar molecules down a concentration gradient;
    If no reference to ‘small/ non-polar’ for 1.
    accept this idea from ‘large/charged’ given in description of 2.
  2. Facilitated diffusion down a concentration gradient via protein carrier/channel;
    Reject if active rather than passive
  3. Osmosis of water down a water potential gradient;
  4. Active transport against a concentration gradient via protein carrier using ATP;
  5. Co-transport of 2 different substances using a carrier protein;
    For any answer accept a correct example
    For ‘carrier protein’ accept symport OR cotransport protein
74
Q

The movement of Na+ out of the cell allows the absorption of glucose into
the cell lining the ileum.

Explain how.
(2 marks)

A
  1. (Maintains/generates) a concentration/diffusion gradient for Na+ (from ileum into cell);
    Accept ‘(Maintains/generates) a lower concentration of Na+ inside the cell
    compared with outside the cell’.
  2. Na+ moving (in) by facilitated diffusion, brings glucose with it
    OR
    Na+ moving (in) by co-transport, brings glucose with it
75
Q

Describe and explain two features you would expect to find in a cell
specialised for absorption (2 marks)

A
  1. Folded membrane/microvilli so large surface area (for absorption);
    Reject references to ‘villi’.
    Accept ‘brush border’ for ‘microvilli’.
  2. Large number of co-transport/carrier/channel proteins so
    fast rate (of absorption)
    OR
    Large number of co-transport/carrier proteins for active transport
    OR
    Large number of co-transport/carrier/channel proteins for facilitated
    diffusion;
  3. Large number of mitochondria so make (more) ATP (by respiration)
    OR
    Large number of mitochondria for aerobic respiration
    OR
    Large number of mitochondria to release energy for active transport;
  4. Membrane-bound (digestive) enzymes so maintains concentration gradient (for fast absorption)
76
Q

The movement of substances across cell membranes is affected by
membrane structure. Describe how (5 marks)

A
  1. Phospholipid (bilayer) allows movement/diffusion of non-polar/lipid-soluble substances;
    1. and 2. Accept correct named examples
    1. and 2. Ignore water
    Accept phospholipid (bilayer) allows movement/diffusion of O2/CO2
    Accept water-insoluble
  2. Phospholipid (bilayer) prevents movement/diffusion of polar/
    charged/lipid-insoluble substances
    OR
    (Membrane) proteins allow polar/charged substances to cross the membrane/bilayer;
    Accept water-soluble
  3. Carrier proteins allow active transport;
  4. Channel/carrier proteins allow facilitated diffusion/co-transport;
    Accept aquaporins allow osmosis
  5. Shape/charge of channel / carrier determines which
    substances move;
  6. Number of channels/carriers determines how much movement;
  7. Membrane surface area determines how much diffusion/movement;
    6. and 7. Accept correct reference to faster/slower/rate for ‘how much
    movement’
    Accept microvilli / Golgi (apparatus) / ER / rER
    Accept surface area to volume for ‘surface area’
  8. Cholesterol affects fluidity/rigidity/permeability;
    Accept cholesterol affects vesicle formation/ endocytosis /exocytosis
    /phagocytosis
77
Q

Give two similarities in the movement of substances by diffusion and by
osmosis (2 marks)

A
  1. (Movement) down a gradient / from high concentration to low
    concentration;
    Ignore along / across gradient
    Reject movement from gradient to gradient
  2. Passive / not active processes;
    OR
    Do not use energy from respiration / from ATP / from metabolism;
    OR
    Use energy from the solution;
78
Q

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.

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

A
  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;
79
Q

High absorption of salt from the diet can result in a higher than normal
concentration of salt in the blood plasma entering capillaries. This can lead
to a build-up of tissue fluid.
Explain how.
(2 marks)

A
  1. (Higher salt) results in lower water potential of tissue fluid;
  2. (So) less water returns to capillary by osmosis (at venule end);
    OR
  3. (Higher salt) results in higher blood pressure / volume;
  4. (So) more fluid pushed / forced out (at arteriole end) of
    capillary;
80
Q

Compare and contrast the processes by which water and inorganic ions
enter cells.
(3 marks)

A
  1. Comparison: both move down concentration gradient;
  2. Comparison: both move through (protein) channels in membrane;
    Accept aquaporins (for water) and ion channels
  3. Contrast: ions can move against a concentration gradient by active
    transport
81
Q

Contrast the processes of facilitated diffusion and active transport.
(3 marks)

A
  1. Facilitated diffusion involves channel or carrier proteins whereas active transport only involves carrier proteins;
  2. Facilitated diffusion does not use ATP / is passive whereas active transport uses ATP;
  3. Facilitated diffusion takes place down a concentration gradient
    whereas active transport can occur against a concentration gradient.
82
Q

Describe how HIV is replicated
(4 marks)

A
  1. Attachment proteins attach to receptors on helper T cell/lymphocyte;
  2. Nucleic acid/RNA enters cell;
  3. Reverse transcriptase converts RNA to DNA;
  4. Viral protein/capsid/enzymes produced;
  5. Virus (particles) assembled and released (from cell)
83
Q

Describe how the human immunodeficiency virus (HIV) is replicated once
inside helper T cells (TH cells)
(4 marks)

A
  1. RNA converted into DNA using reverse transcriptase;
    Reject ‘messenger’ or ‘m’ before RNA
  2. DNA incorporated/inserted into (helper T cell)DNA /chromosome /genome/ nucleus;
  3. DNA transcribed into (HIV m)RNA;
    Accept descriptions of transcription
  4. (HIV mRNA) translated into (new) HIV/viral proteins (for assembly into viral particles);
    Accept descriptions of translation
    Accept named viral protein, eg capsid
    Reject viral cell
84
Q

Describe how a phagocyte destroys a pathogen present in the blood.
(3 marks)

A
  1. Engulfs;
    Accept endocytosis
    OR
    Description
    Ignore ‘taken in’
  2. Forming vesicle/phagosome and fuses with lysosome;
  3. Enzymes digest/hydrolyse;
85
Q

Give two types of cell, other than pathogens, that can stimulate an immune
response.
(2 marks)

A
  1. (Cells from) other organisms/transplants;
  2. Abnormal/cancer/tumour (cells);
  3. (Cells) infected by virus;
86
Q

What is the role of the disulfide bridge in forming the quaternary structure
of an antibody?
(1 mark)

A

Joins two (different) polypeptides;

87
Q

Explain how HIV affects the production of antibodies when AIDS develops
in a person.
(3 marks)

A
  1. Less/no antibody produced;
  2. (Because HIV) destroys helper T cells;
    Accept ‘reduces number’ for ‘destroys’
  3. (So) few/no B cells activated / stimulated
    OR
    (So) few/no B cells undergo mitosis/differentiate/form plasma cells;
88
Q

In Europe, viruses have infected a large number of frogs of different species. The
viruses are closely related and all belong to the Ranavirus group.
Previously, the viruses infected only one species of frog.

Suggest and explain how the viruses became able to infect other species
of frog
(2 marks)

A
  1. Mutation in the viral DNA/RNA/genome/genetic material;
    Accept named examples mutations
  2. Altered (tertiary structure of the) viral attachment protein;
    Accept ‘antigen’ for ‘attachment protein’
    Accept causes antigenic variability
  3. Allows it/attachment protein/virus to bind (to receptors of other species);
    Accept descriptions of binding eg is complementary
89
Q

Determining the genome of the viruses could allow scientists to develop a
vaccine.
Explain how.
(2 marks)

A
  1. (The scientists) could identify proteins (that derive from the genetic
    code)
    OR
    (The scientists) could identify the proteome;
  2. (They) could (then) identify potential antigens (to use in the vaccine);
    Reject if answer suggests vaccine contains antibodies
90
Q

Describe how the B lymphocytes of a frog would respond to vaccination
against Ranavirus.

You can assume that the B lymphocytes of a frog respond in the same way
as B lymphocytes of a human.

Do not include details of the cellular response in your answer
(3 marks)

A
  1. B cell (antibody) binds to (viral) specific/complementary receptor/antigen;
    Accept B cell forms antigen-antibody complex
  2. B cell clones
    OR
    B cell divides by mitosis;
  3. Plasma cells release/produce (monoclonal) antibodies (against the virus);
  4. (B/plasma cells produce/develop) memory cells;
91
Q

What is a monoclonal antibody?
(1 mark)

A

(Antibodies with the) same tertiary structure
OR
(Antibody produced from) identical/cloned plasma cells/B cells/B lymphocytes;

92
Q

After a disease is diagnosed, monoclonal antibodies are used in some
medical treatments.
Give one example of using monoclonal antibodies in a medical treatment
(1 mark)

A

Targets/binds/carries drug/medicine to specific cells/antigens/receptors
OR
Block antigens/receptors on cells

93
Q

Describe the role of antibodies in producing a positive result in an ELISA
test.
(4 marks)

A
  1. (First) antibody binds/attaches /complementary (in shape) to antigen;
  2. (Second) antibody with enzyme attached is added;
  3. (Second) antibody attaches to antigen;
    Accept (second) antibody attaches to (first) antibody (indirect ELISA test).
  4. (Substrate/solution added) and colour changes;
94
Q

Describe and explain the role of antibodies in stimulating phagocytosis.
Do not include details about the process of phagocytosis.
(2 mark)

A
  1. Bind to antigen
    OR
    Are markers;
    Accept opsonin for ‘marker’
    Accept form (antibody-antigen) complexes/are complementary to antigen
  2. (Antibodies) cause clumping/agglutination
    OR
    Attract phagocytes;
95
Q

Describe how phagocytosis of a virus leads to presentation of its antigens
(3 marks)

A
  1. Phagosome / vesicle fuses with lysosome;
  2. (Virus) destroyed by lysozymes / hydrolytic enzymes;
  3. Peptides / antigen (from virus) are displayed on the cell
    membrane;
96
Q

Describe how presentation of a virus antigen leads to the secretion of an
antibody against this virus antigen
(3 marks)

A
  1. Helper T cell / TH cell binds to the antigen (on the antigenpresenting cell / phagocyte);
  2. This helper T / TH cell stimulates a specific B cell;
  3. B cell clones
    OR
    B cell divides by mitosis;
  4. (Forms) plasma cells that release antibodies;
97
Q

What is an antigen
(2 marks)

A
  1. Foreign protein;
    Accept glycoprotein / glycolipid / polysaccharide
  2. (that) stimulates an immune response / production of antibody
98
Q

What is an antibody?
(2 marks)

A
  1. A protein / immunoglobulin specific to an antigen;
  2. Produced by B cells
    OR
    Secreted by plasma cells;
99
Q

Bacterial meningitis is a potentially fatal disease affecting the membranes around
the brain. Neisseria meningitidis (Nm) is a leading cause of bacterial meningitis.

In the UK, children are vaccinated against this disease. Describe how
vaccination can lead to protection against bacterial meningitis.
(6 marks)

A
  1. Antigen / epitope on surface of N. meninigitidis / bacterium binds to
    surface protein / surface receptor on a (specific / single) B cell.
    If answered in context of T cell, allow Antigen binds to (specific / single) T cell
  2. (Activated) B cell divides by mitosis / produces clone;
    If answered in context of T cell, allow (Activated) T cell releases cytokine.
  3. (Division) stimulated by cytokines / by T cells;
    If answered in context of T cell, allow (Cytokine) stimulates production of
    plasma cells;
  4. B cells / plasma cells release antibodies;
  5. (Some) B cells become memory cells;
  6. Memory cells produce plasma / antibodies faster
100
Q

When a vaccine is given to a person, it leads to the production of
antibodies against a disease-causing organism. Describe how.
(5 marks)

A
  1. Vaccine contains antigen from pathogen;
  2. Macrophage presents antigen on its surface;
  3. T cell with complementary receptor protein binds to antigen;
  4. T cell stimulates B cell;
  5. (With) complementary antibody on its surface;
  6. B cell secretes large amounts of antibody;
  7. B cell divides to form clone all secreting / producing same
    antibody.
101
Q

Describe the difference between active and passive immunity
(5 marks)

A
  1. Active involves memory cells, passive does not;
  2. Active involves production of antibody by plasma cells /
    memory cells;
  3. Passive involves antibody introduced into body from outside /
    named source;
  4. Active long term, because antibody produced in response to
    antigen;
  5. Passive short term, because antibody (given) is broken down;
  6. Active (can) take time to develop / work, passive fast acting.
102
Q

Describe the primary structure of all proteins ( 2 marks)

A
  • Sequence of amino acids
  • Joined by peptide bonds
103
Q

About the genetic code:
Define universal, non-overlapping and degenerate (3 marks)

A
  • Universal: The same codon always codes for the same amino acid
  • Non-overlapping: Each base is only part of one codon
  • Dengenerate: More than one codon codes for each amino acid
104
Q

Describe the mechanism of breathing that causes air to enter the lungs (3 marks)

A
  • Diaphragm contracts AND diaphragm flattens
  • External intercostal muscles contract AND ribcage pulled out / up
  • Causes volume increase AND pressure decrease in the thoracic cavity
105
Q

Explain why phospholipids can form a bilayer but triglycerides cannot (3 marks)

A
  • Phospholipid both hydrophobic and hydrophilic
  • Triglycerides only hydrophobic
  • Hydrophilic phosphate group attracts water
106
Q

State one role of helper T cells

A
  • Stimulating phagocytes
107
Q

Describe the role of tRNA in translation (2 marks)

A
  • Anticodon binds to codon
  • tRNA brings specific amino acid to ribosome
108
Q

Describe how gas exchange occurs in single-celled organisms AND explain why this method cannot be used by large, multicellular organisms ( 3 marks )

A
  • Diffusion
  • Large organisms have a smaller surface area to volume ratio
  • Diffusion pathway would be too long
109
Q

Name 2 enzymes that are involved in the replicating the DNA of HPV and describe their roles in the replication process ( 3 marks)

A
  • DNA helicase and DNA polymerase
  • Helicase breaks hydrogen bonds
  • Polymerase condensation reaction to join adjacent nucleotides
110
Q

Give 2 reasons why glucose uptake by muscle cells increases significantly during exercise ( 4 marks)

A
  • Increases respiration
  • To provide more ATP for muscle contraction
  • Concentration gradient for glucose
  • Glucose enters by facilitated diffusion
111
Q

Potato plants with fewer sucrose- transport proteins produce fewer potatoes and accumulate sugars in their leaves, explain why ( 2 marks )

A
  • Carbohydrates made by photosynthesis
  • Less sucrose transported into phloem at leaf
112
Q

Describe the structure of glycogen (2 marks)

A
  • Polysaccharide of alpha glucose
  • Joined by glycosidic bonds / branched structure
113
Q

Suggest how glycogen acts as a source of energy (2 marks)

A
  • hydrolysed to glucose
  • Glucose is used in respiration
114
Q

Describe how bacteria divide (2 marks)

A
  • Binary fission
  • Replication of circular DNA
  • Division of cytoplasm to produce 2 daughter cells
  • Each with single copy of circular DNA
115
Q
A

AQA A level Biology (33 decks)

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