A Level Biology
CIE A Level Biology Solved Past Paper Oct/Nov 2020 P22

CIE A Level Biology Solved Past Paper Oct/Nov 2020 P22

Q1a(i) Plasmodesmata

(ii) Water potential gradient

(iii) Symplast

b) Cell walls of spongy mesophyll cells provide a large surface area for the evaporation of water vapour into air space. Water vapour diffuses down the water potential gradient through stomata from leaf to air outside.

c) 36000/25 = X 1440

Q2a) Long-term treatment involving a combination of drugs ensures that all bacteria are killed and none remain to form a reservoir of infection or develop resistance to antibiotics which could be transmitted to other bacteria as well. Even if some bacteria have developed resistance to one of the four antibiotics, they would still be susceptible to the other antibiotics, and therefore be killed by combination.

b(i) Transcription

(ii) It could have altered the shape of the enzyme’s active site. This prevents RNA nucleotides from binding to the enzyme, hence phosphodiester bonds do not form.

c(i) Phe, tyr

(ii) B, D, F, G

(iii) Change in codon means the formation of hydrogen bonds with the new anticodon of a new tRNA carrying a different amino acid. The new amino acid sequence in polypeptide leads to the formation of new bonds between the R group of the polypeptide that determines its shape as well as that of the enzyme. The shape of the binding site of rifampicin on enzyme changes, so it cannot bind to enzyme leading to resistance development. Multiple mutations involved have varying impacts on the shape of the binding site for rifampicin, which causes different levels of resistance.

Q3a) Plates of rings, layers of tissue, large lumen

b) Vein, irregular shape

c) Goblet cells secret mucus that traps dust and pathogens. Cilia sweep away this mucus from the gas exchange surface to prevent infection

Q4a(i) Nuclear pores, outer membrane continuous with the rough endoplasmic reticulum, nuclear envelope with two membranes.

 (ii) Greatest resolution using a light microscope is 200nm. All the structures mentioned in the question are smaller than that, so resolution is not enough to distinguish between them as two different points.

b) A,F,C,B,E,D

c) Due to clonal expansion by mitosis, large number of plasma cells are produced capable of synthesizing and secreting a particular type of antibody. Memory cells provide long term immunity as they remain in body and can be activated upon re-entry of same antigen to carry secondary immune response. Due to memory cells, in secondary response antibodies are produced faster and in a much greater quantity.

d) Body carries out immune response against self antigens. Helper T-lymphocytes stimulate secretion of antibodies that bind to receptors on muscle fibers. This prevents impulse transmission from motor neuron to muscle fibers which would earlier have caused muscle contraction. 

Q5a) Fructose

b) Enzyme lower activation energy, put strain on bonds making it easier to break them.

c) Immobilised enzyme has greater percentage activity during most of the range in pH shown. It also has a higher optimum temperature due to the greater thermostability of its structure. This makes it a better option.

Q6a(ii) Respiring tissues have the greatest carbon dioxide partial pressure. Carbon dioxide forms carbonic acid and then hydrogen ions dissociate from acid to displace oxygen from oxyhemoglobin. This supplies respiring tissues with oxygen and increases pH of blood as hydrogen ions bind with hemoglobin.

b) CO2 – simple diffusion

Ions – facilitated diffusion through transport protein that provides the aqueous channel for movement of polar ions as they are repelled by hydrophobic fatty acid tails.

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