(a) Explain how a cholinergic synapse functions. (b) Describe how you would carry out an investigation into the effect of wavelength of light on the rate of photosynthesis of a plant, using a redox indicator such as DCPIP.

Mark scheme

Marking points from the official mark scheme (May/June 2019). The mark or mark type is shown in brackets.

1. [7] any seven from: 1 impulse / action potential / depolarisation, reaches, synaptic knob ; 2 (voltage-gated) Ca2+ channels open (in pre-synaptic membrane) ; 3 Ca2+ enters (synaptic knob / pre-synaptic neurone) ; 4 vesicles with acetylcholine, move towards / fuse with, pre-synaptic membrane ; 5 (ACh) released / secreted / exocytosis ; 6 ACh diffuses across synaptic cleft; 7 binds to receptors on post-synaptic membrane ; 8 (ligand-gated) Na+ channels open and Na+ enters post-synaptic neurone ; 9 depolarisation / action potential / EPSP ; 10 acetylcholinesterase, breaks down ACh / recycles Ach ;
2. [8] any eight from: 1 Hill reaction; 2 oxidised DCPIP is blue; 3 (DCPIP) goes colourless when reduced ; 4 method for making chloroplast, extract / suspension ; 5 ref. to buffer solution / control osmosis / control pH; 6 expose chloroplasts + DCPIP to wavelength of light; 7 (method 1) measure time for, blue to disappear/blue-green → green / colour to match control with no DCPIP or (method 2) leave for fixed time and measure with colorimeter ; 8 calculate rate as, (method 1) 1/t or (method 2) change in colour value divided by time; 9 test five (or more), wavelengths / colours ; 10 do three repeats / calculate means; 11 plot wavelength on x-axis against, dependent variable / calculated rate, on y-axis ; 12 AVP;