▶️ Answer/Explanation
Detailed Solution
a.i.
- Label the polar phosphate head of the phospholipid as H.
- This is the part of the phospholipid that is attracted to water (hydrophilic).
a.ii.
- Label a synaptic vesicle inside the presynaptic neuron that contains neurotransmitter (acetylcholine) as E.
- This vesicle will move to the presynaptic membrane and release its contents by exocytosis.
a.iii.
- Label the acetylcholine receptor protein on the postsynaptic membrane as P.
- Neonicotinoid pesticides act by binding to these receptor sites, interfering with normal neurotransmission.
b.
- Voltage-gated sodium channels open in response to a stimulus.
- Sodium ions (Na⁺) rapidly enter the axon, causing the membrane potential to change from negative to positive (depolarization).
c.
- Acetylcholine binds to receptor proteins on the postsynaptic membrane.
- This binding causes ion channels to open, allowing sodium ions to enter the postsynaptic neuron, initiating an action potential.
d.i.
- Acetylcholinesterase breaks down acetylcholine into acetate and choline.
d.ii.
- A change in pH can alter the enzyme’s tertiary structure by affecting ionic and hydrogen bonds.
- This changes the shape of the active site, reducing the enzyme’s ability to bind substrates.
- As a result, the enzyme becomes less efficient or denatured, decreasing the rate of reaction.
Markscheme
a.i. Hydrophilic End of Phospholipid: [1]
hydrophilic end of phospholipid shown ✔
For 2a, i, ii and iii, accept letters written on the structure even if a line is not drawn.
Award the mark if the full word is used to label the structure. Labels must be unambiguous.
a.ii. Exocytic Vesicle: [1]
an exocytic vesicle shown ✔
For 2 a, i, ii and iii, accept letters written on the structure even if a line is not drawn.
Award the mark if the full word is used to label the structure. Labels must be unambiguous.
Accept any of the three intact circles or the exocytosis shown within the box.
a.iii. Neonicotinoid Pesticide Binding Location: [1]
any receptor protein on post synaptic membrane marked ✔
For 2 a, i, ii and iii, accept letters written on the structure even if a line is not drawn.
Award the mark if the full word is used to label the structure.
Must unambiguously be pointed to a receptor on the synapse side. Accept labels of acetylcholinesterase.
b. Depolarization of Axon Membrane: [2]
a. (local depolarization) causes ion / sodium / voltage gated channels to open ✔
b. altering membrane permeability to sodium ions/Na+/positive ions / Na+ diffuses into the cytoplasm ✔
c. membrane potential (of the axon) changes from negative to positive ✔
d. a threshold potential is reached and an action potential is generated ✔
Do not accept sodium pumps.
Mp b requires evidence that the student recognizes sodium is an ion.
Numerical values indicating change from −70 to +30/+40/+55 can be accepted.
c. Acetylcholine Initiating Action Potential: [2]
a. acetylcholine binds to the receptor protein ✔
b. (causing a) change in tertiary structure / conformation of protein ✔
c. diffusion/entry of ions / Na+ through the receptor/channel protein allowed ✔
d.i. Action of Acetylcholinesterase: [1]
breaks down acetylcholine (to acetyl and choline) ✔
d.ii. Effect of pH Change on Enzyme: [3]
a. change in pH can cause a change in the tertiary structure of the enzyme
OR
enzyme denatured ✔
OR
enzyme denatured ✔
b. causing a change in the active site / substrate not able to bind ✔
c. the enzyme will not work as efficiently / decreased rate of reaction ✔
d. (may no longer be) the optimum/optimal pH for enzyme activity ✔