exam quesitons

Question 1

Describe how a phosphodiester bond is formed between two nucleotides within a DNA molecule (2)

Answer 1

1. Condensation (reaction)/loss of water;
2. (Between) phosphate and deoxyribose;
3. (Catalysed by) DNA polymerase;
   Reject if DNA polymerase joins AT/GC OR
   complementary nucleotides/bases OR forms
   hydrogen bonds

Question 2

differences between DNA in the nucleus of a plant cell and DNA in a prokaryotic cell. (3)

Answer 2

1. (Associated with) histones/proteins v no histones/proteins;
2. Linear v circular;
3. No plasmids v plasmids;
4. Introns v no introns;
5. Long(er) v short(er);

Question 3

Define ‘non-coding base sequences’ and describe where the non-coding multiple repeats are positioned in the genome. (2)

Answer 3

1. DNA that does not code for protein/polypeptides
   OR
   DNA that does not code for (sequences of) amino acids
   OR
   DNA that does not code for tRNA/rRNA;
2. (Positioned) between genes;

Question 4

Give three ways in which the DNA in a chloroplast is different from DNA in the nucleus. (3)

Answer 4

In chloroplasts
1. DNA shorter;
2. Fewer genes;
3. DNA circular not linear;
4. Not associated with protein/histones, unlike nuclear DNA;
5. Introns absent but present in nuclear DNA;

Question 5

Not all mutations in the nucleotide sequence of a gene cause a change in the structure of a polypeptide. (2)

Answer 5

1. Triplets code for same amino acid
2. Occurs in introns /non-coding sequence;

Question 6

Compare and contrast the DNA in eukaryotic cells with the DNA in
prokaryotic cells. (5)

Answer 6

Comparisons
1. Nucleotide structure is identical;
2. Nucleotides joined by phosphodiester bond;
OR
Deoxyribose joined to phosphate (in sugar, phosphate backbone);
3. DNA in mitochondria / chloroplasts same / similar (structure) to DNA in prokaryotes;

Contrasts
4. Eukaryotic DNA is longer;
5. Eukaryotic DNA contain introns, prokaryotic DNA does not;
6. Eukaryotic DNA is linear, prokaryotic DNA is circular;
7. Eukaryotic DNA is associated with / bound to protein / histones,
prokaryotic DNA is not;

Question 7

The dark stain used on the chromosomes binds more to some areas of the chromosomes than others, giving the chromosomes a striped appearance, why?

Answer 7

Differences in base sequences
OR
Differences in histones/interaction with histones
OR
Differences in condensation/(super)coiling;

Question 8

Messenger RNA (mRNA) is used during translation to form polypeptides.
Describe how mRNA is produced in the nucleus of a cell. (6)

Answer 8

1. Helicase;
2. Breaks hydrogen bonds;
3. Only one DNA strand acts as template;
4. RNA nucleotides attracted to exposed bases;
5. (Attraction) according to base pairing rule;
6. RNA polymerase joins (RNA) nucleotides together;
7. Pre-mRNA spliced to remove introns.

Question 9

Describe the structure of proteins. (5)

Answer 9

1. Polymer of amino acids;
2. Joined by peptide bonds;
3. Formed by condensation;
4. Primary structure is order of amino acids;
5. Secondary structure is folding of polypeptide chain due to hydrogen
   Accept alpha helix / pleated sheet
6. Tertiary structure is 3-D folding due to hydrogen bonding and ionic / disulfide bonds;
7. Quaternary structure is two or more polypeptide chains

Question 10

Describe how proteins are digested in the human gut. (4)

Answer 10

1. Hydrolysis of peptide bonds;
2. Endopeptidases break polypeptides into smaller peptide chains;
3. Exopeptidases remove terminal amino acids;
4. Dipeptidases hydrolyse / break down dipeptides into amino acids

Question 11

State three comparisons of genetic diversity

Answer 11

1. The (base) sequence of DNA;
2. The (base) sequence of mRNA;
3. The amino acid sequence (of proteins);

Question 12

Describe how breeding experiments could determine whether two
populations are from the same species.

Answer 12

1. Breed the two mice together;
2. (if Same species) produce fertile offspring.

Question 13

Give two differences between mitosis and meiosis

Answer 13

1. One division, two divisions in meiosis;
2. (Daughter) cells genetically identical, daughter cells genetically different in
   meiosis;
3. Two cells produced, (usually) four cells produced in meiosis;
4. Diploid to diploid/haploid to haploid, diploid to haploid in
   meiosis,
5. Separation of homologous chromosomes only in meiosis;
6. Crossing over only in meiosis;
7. Independent segregation only in meiosis;

Question 14

Explain the advantage for larger animals of having a specialised system
that facilitates oxygen uptake. (2)

Answer 14

1. Large(r) organisms have a small(er) surface area:volume (ratio);
   OR
   Small(er) organisms have a large(r) surface area:volume (ratio);
2. Overcomes long diffusion pathway
   OR
   Faster diffusion;

Question 15

Suggest and explain how a reduced tidal volume affects the exchange of
carbon dioxide between the blood and the alveoli.

Answer 15

1. Less carbon dioxide exhaled/moves out (of lung)
   OR
   More carbon dioxide remains (in lung);
2. (So) reduced diffusion/concentration gradient (between blood and
   alveoli);
3. Less/slower movement of carbon dioxide out of blood
   OR
   More carbon dioxide stays in blood;

Question 16

(a) Describe the gross structure of the human gas exchange system and how
we breathe in and out.(6)

Answer 16

1. Named structures – trachea, bronchi, bronchioles, alveoli;
2. Above structures named in correct order
   OR
   Above structures labelled in correct positions on a diagram;
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);
   For thoracic cavity accept ‘lungs’ or ‘thorax’.

Question 17

Explain three ways in which an insect’s tracheal system is adapted for
efficient gas exchange.

Answer 17

1. Tracheoles have thin walls so short diffusion distance to cells;
2. Highly branched / large number of tracheoles so short diffusion
   distance to cells;
3. Highly branched / large number of tracheoles so large surface
   area (for gas exchange);
4. Tracheae provide tubes full of air so fast diffusion (into insect
   tissues);
5. Fluid in the end of the tracheoles that moves out (into tissues)
   during exercise so faster diffusion through the air to the gas
   exchange surface;
   OR
   Fluid in the end of the tracheoles that moves out (into tissues)
   during exercise so larger surface area (for gas exchange);
6. Body can be moved (by muscles) to move air so maintains
   diffusion / concentration gradient for oxygen / carbon dioxide;

Question 18

Explain two ways in which the structure of fish gills is adapted for efficient
gas exchange.

Answer 18

1. Many lamellae / filaments so large surface area;
2. Thin (surface) so short diffusion pathway;

Question 19

Describe the processes involved in the absorption and transport of
digested lipid molecules from the ileum into lymph vessels. (5)

Answer 19

1. Micelles contain bile salts and fatty acids/monoglycerides;
2. Make fatty acids/monoglycerides (more) soluble (in water)
   OR
   Bring/release/carry fatty acids/monoglycerides to cell/lining (of the
   iluem)
   OR
   Maintain high(er) concentration of fatty acids/monoglycerides to
   cell/lining (of the ileum);
   Accept lipid/fat for fatty acid/ monoglyceride
3. Fatty acids/monoglycerides absorbed by diffusion;
   Reject if absorbed by facilitated diffusion
   Ignore if micelles themselves are being absorbed
4. Triglycerides (re)formed (in cells);
   Accept chylomicrons form
5. Vesicles move to cell membrane;
   Accept exocytosis for ‘vesicles move’

Question 20

Describe the role of micelles in the absorption of fats into the cells lining
the ileum.

Answer 20

1. Micelles include bile salts and fatty acids;
   Ignore other correct components of micelles.
2. Make the fatty acids (more) soluble in water;
   For ‘fatty acids’ accept fats / lipids.
3. Bring/release/carry fatty acids to cell/lining (of the ileum);
   For ‘fatty acids’ accept fats/lipids.
4. Maintain high(er) concentration of fatty acids to cell/lining (of the
   ileum);
5. Fatty acids (absorbed) by diffusion;

Question 21

Explain the advantages of lipid droplet and micelle formation

Answer 21

1. Droplets increase surface areas (for lipase /
   enzyme action);
2. (So) faster hydrolysis / digestion (of
   triglycerides / lipids);
3. Micelles carry fatty acids and glycerol /
   monoglycerides to / through membrane / to
   (intestinal epithelial) cell;

Question 22

suggest how golgi apparatus is involved in
the absorption of lipids.

Answer 22

2. Modifies / processes triglycerides;
3. Combines triglycerides with proteins;
4. Packaged for release / exocytosis
   OR
   Forms vesicles;

Question 23

Suggest two ways the student could improve the quality of his scientific
drawing of the blood vessels in this dissection.

Answer 23

1. Only use single lines/do not use sketching (lines)/ensure lines are
   continuous/connected;
2. Add labels/annotations/title;
3. Add magnification/scale (bar);
4. Draw all parts to same scale/relative size;
5. Do not use shading/hatching;

Question 24

Describe two precautions the student should take when clearing away after
the dissection.

Answer 24

1. Carry/wash sharp instruments by holding handle
   OR
   Carry/wash sharp instruments by pointing away (from body)/down;
   Accept for ‘instruments’, a suitable named example,
   eg. scalpel
2. Disinfect instruments/surfaces;
   Accept for ‘instruments’, a suitable named example,
   eg. scalpel
   Accept for ‘disinfect’, sanitise OR use antiseptic
3. Disinfect hands
   OR
   Wash hands with soap (and water);
   Accept for ‘disinfect’, sanitise OR use antiseptic
4. Put organ/gloves/paper towels in a (separate) bag/bin/tray to dispose;

Question 25

Explain how an arteriole can reduce the blood flow into capillaries

Answer 25

1. Muscle contracts; 2. Constricts/narrows arteriole/lumen;

Question 26

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

Answer 26

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;

Question 27

Describe and explain two features you would expect to find in a cell specialised for absorption.

Answer 27

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);

Question 28

The movement of substances across cell membranes is affected by membrane structure. Describe how.

Answer 28

1. Phospholipid (bilayer) allows movement/diffusion of nonpolar/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;

Question 29

Describe binary fission in bacteria

Answer 29

1. Replication of (circular) DNA; Accept nucleoid Reject chromosome Reject mitosis 2. Replication of plasmids; 3. Division of cytoplasm (to produce daughter cells);

Question 30

Suggest and explain how two environmental variables could be changed to increase the growth rate of these cells.

Answer 30

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;

Question 31

(a) Describe the roles of iron ions, sodium ions, and phosphate ions in cells.

Answer 31

Iron ions 1. Haemoglobin binds/associates with oxygen OR Haemoglobin transports/loads oxygen;Sodium ions 2. Co-transport of glucose/amino acids (into cells); 3. (Because) sodium moved out by active transport/Na – K pump; 4. Creates a sodium concentration/diffusion gradient; 5. Affects osmosis/water potential; Phosphate ions 6. Affects osmosis/water potential; Accept 5. OR 6. – not both 7. Joins nucleotides/in phosphodiester bond/in backbone of DNA/RNA/in nucleotides; 8. Used in/to produce ATP; Reject ‘energy produced’ 9. Phosphorylates other compounds (usually) making them more reactive; 10. Hydrophilic/water soluble part of phospholipid bilayer/membrane;

Question 32

Explain five properties that make water important for organisms

Answer 32

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;

Question 33

Describe the induced-fit model of enzyme action and how an enzyme acts as a catalyst.

Answer 33

1. Substrate binds to the active site/enzyme OR Enzyme-substrate complex forms; 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;