Wrongly answered exam questions

Question 1

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

Answer 1

The order of the amino acids (primary structure) in the two proteins may be different. This can result in ionic, hydrogen and disulphide bonds to form in different locations in each protein.

Question 2

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

Answer 2

Hydrogen bonds form beta pleated sheets or alpha helixes.

Question 3

Draw an amino acid

Answer 3

R
H2N-C-COOH
H

Question 4

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

Answer 4

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

Question 5

Explain five properties that make water important for organisms

Answer 5

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 6

What is a gene?

Answer 6

A sequence of DNA bases that codes for a polypeptide

Question 7

Describe how HIV is replicated

Answer 7

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);

Question 8

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

Answer 8

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 cells

Question 9

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

Answer 9

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;
7. Do not accept unqualified references to thin membranes.
   Max 2 if any reference to blood Ignore references to spiracles 5. Accept ‘water’ for fluid. Accept ‘cells’ and ‘tissues’ as interchangeable words.

Question 10

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

Answer 10

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; Reject if absorbed by facilitated diffusion Ignore if micelles themselves are being absorbed. Ignore references to monoglycerides.

Question 11

Phloem pressure is reduced during the hottest part of the day explain why

Answer 11

1. High (rate of) transpiration/evaporation;
2. Water lost through stomata OR (High) tension in xylem;
3. (Causes) less water movement from xylem to phloem OR Insufficient water potential in phloem to draw water from xylem;

Question 12

Explain how the active site of an enzyme causes a high rate of reaction

Answer 12

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

Question 13

Why is ATP a good energy source?

Answer 13

(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

Question 14

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.

Answer 14

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

Question 15

Explain the role of ribosomes in Translation

Answer 15

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 16

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

Answer 16

1. Micelles contain bile salts and fatty acids/monoglycerides;
   Ignore other correct components of micelles
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 17

Define the term exon

Answer 17

Base/nucleotide/triplet sequence coding for polypeptide/sequence of amino
acids/primary structure;

Question 18

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

Answer 18

b) 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;
Accept the idea of not transcribed for ‘does not
code for’.
Do not credit ‘DNA that does not code for an amino
acid’.
Ignore reference to introns.
2. (Positioned) between genes;
Reject (positioned) ‘in introns’ or ‘between exons’.
Accept ‘(Positioned) at the end of chromosomes’ or
‘(Positioned) in the telomeres’.

Question 19

Describe how a polypeptide is formed by translation of mRNA.

Answer 19

(b) 1. (mRNA attaches) to ribosomes
OR
(mRNA attaches) to rough endoplasmic reticulum;
2. (tRNA) anticodons (bind to) complementary (mRNA) codons;
3. tRNA brings a specific amino acid;
4. Amino acids join by peptide bonds;
5. (Amino acids join together) with the use of ATP;
6. tRNA released (after amino acid joined to polypeptide);
7. The ribosome moves along the mRNA to form the polypeptide;

Question 20

Why can fertile offspring not reproduce?

Answer 20

(e) 1. Too many/extra set/three copies of chromosomes;
Accept 1 paternal, 2 maternal
2. (Homologous) chromosomes do not pair
OR
(Homologous) chromosomes do not separate (evenly);
Accept divide for separate
3. (So) no meiosis;

Question 21

Describe how he made a 1 in 10 dilution and then used this to make a 1 in
1000 dilution of the original liquid culture of bacteria.

Answer 21

1. Add 1 part (bacteria) culture to 9 parts (sterile) liquid (to make 10–1
   dilution);
   Accept water / nutrient / broth for liquid
2. Mix (well);
   Accept stir
3. Repeat using 9 parts fresh (sterile) liquid and 1 part of 10–1 and 10–2
   dilutions to make 10–3 dilution;
   OR
   Add 1 part 10–1 (suspension) to 99 parts (sterile) liquid (to make 10–3
   dilution);
   Accept water / nutrient / broth for liquid
   Reject 1 part (undiluted) culture added to 999 parts
   liquid

Question 22

Define homologus chromosomes

Answer 22

(Two chromosomes that) carry the same
genes;

Question 23

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

Answer 23

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

Question 24

Compare and contrast the structure and properties of triglycerides and phospholipids.

Answer 24

1. Both contain ester bonds (between glycerol and
   fatty acid);
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;
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;
8. Phospholipids form monolayer (on
   surface)/micelle/bilayer (in water) but
   triglycerides don’t;

Question 25

Describe the role of micelles in the absorption of fats into the cells lining the ileum (3 marks)

Answer 25

1. Micelles include bile salts and fatty acids;
2. Make the fatty acids (more) soluble in water;
3. Bring/release/carry fatty acids to cell/lining (of the
   ileum);
4. Maintain high(er) concentration of fatty acids to
   cell/lining (of the ileum);
5. Fatty acids (absorbed) by diffusion;

Question 26

Describe how a triglyceride is formed

Answer 26

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