4,5,6 markers

Question 1

Describe the structure of proteins

Answer 1

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 bonding;
   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

Question 2

Explain how the structure of DNA is related to its functions

Answer 2

Sugar -phosphate (backbone) / double stranded / helix so provides strength / stability / protects bases / protects hydrogen bonds;
Must be a direct link / obvious to get the mark
Neutral: reference to histones
2. Long / large molecule so can store lots of information;
3. Helix / coiled so compact;
Accept: can store in a small amount of space for ‘compact’
Base sequence allows information to be stored / base sequence codes for amino acids / protein;
Accept: base sequence allows transcription
5. Double stranded so replication can occur semi-conservatively / strands
can act as templates / complementary base pairing / A-T and G-C so accurate replication / identical copies can be made;
6. (Weak) hydrogen bonds for replication / unzipping / strand separation / many hydrogen bonds so stable / strong;
Accept: ‘H-bonds’ for ‘hydrogen

Question 3

Describe and explain the processes that occur during meiosis that increase genetic variation

Answer 3

(b) 1. Homologous chromosomes pair up;

2. Independent segregation;

Maternal and paternal chromosomes are re-shuffled in any combination;

4. Crossing over leads to exchange of parts of (non-sister) chromatids / alleles
   between homologous chromosomes;
5. (Both) create new combinations of

Question 4

Describe and explain how the structure of the mammalian breathing system enables
efficient uptake of oxygen into the blood.

Answer 4

1. alveoli provide a large surface area;
2. walls of alveoli thin to provide a short diffusion pathway;
3. walls of capillary thin / close to alveoli provides
   a short diffusion pathway;
4. walls (of capillaries / alveoli) have flattened cells;
5. cell membrane permeable to gases;
6. many blood capillaries provide a large surface area;
7. intercostal / chest muscles / diaphragm muscles / to ventilate lungs /
   maintain a diffusion / concentration gradient;
8. wide trachea / branching of bronchi / bronchioles for efficient
   flow of air;
9. cartilage rings keep airways open;
   (reject moist and thin membranes)
   (a) exchange / diffusion across body surface / skin;