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Flashcards · 9700 · A Level Topic 13

Photosynthesis (A Level) flashcards

15 cards on A Level Topic 13, subtopics 13.1 to 13.2. Pick a subtopic or study them all.

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13.1 Photosynthesis as an energy transfer process

Sites of the two stages of photosynthesis in the chloroplast
Light-dependent stage: thylakoid membranes (stacked into grana), with a large surface area for pigments, electron carriers and ATP synthase. Light-independent stage (Calvin cycle): the stroma, which contains rubisco.
Photosynthetic pigments and their roles
Chlorophyll a: main pigment; emits electrons when photoactivated. Chlorophyll b, carotene and xanthophyll: accessory pigments that pass absorbed energy to chlorophyll a.
Absorption spectrum vs action spectrum
Absorption spectrum: light absorbed by a pigment against wavelength. Action spectrum: rate of photosynthesis against wavelength.
Rf value in chromatography of pigments
Rf = distance moved by the pigment ÷ distance moved by the solvent front, measured from the origin to the centre of the spot. It has no units.
Cyclic vs non-cyclic photophosphorylation
Cyclic: PSI only; electrons return to PSI; makes ATP only. Non-cyclic: PSII and PSI; photolysis of water; makes ATP and reduced NADP and releases O₂.
Photolysis of water
2H₂O → 4H⁺ + 4e⁻ + O₂, catalysed by the oxygen-evolving complex of PSII. The electrons replace those lost from PSII.
Chemiosmosis in the chloroplast
Energy from electrons passing along the electron transport chain moves protons from the stroma into the thylakoid space. They return to the stroma by facilitated diffusion through ATP synthase, making ATP.
The three stages of the Calvin cycle
Fixation: CO₂ + RuBP (5C) → 2 GP (3C), catalysed by rubisco. Reduction: GP → triose phosphate (TP), using reduced NADP and ATP. Regeneration: TP → RuBP, using ATP.
What happens to GP and RuBP if light intensity is reduced?
GP rises and RuBP falls. Less ATP and reduced NADP, so GP is not reduced to TP and RuBP is not regenerated, while fixation continues for a while.

13.2 Investigation of limiting factors

Limiting factor: how to read it from a graph
On the rising part of the curve, the factor on the x-axis is limiting. On the plateau another factor is limiting; a higher plateau when CO₂ or temperature is raised shows that factor was limiting.
Which stage does each limiting factor mainly affect?
Light intensity: light-dependent stage (less ATP and reduced NADP). CO₂ concentration: light-independent stage (less fixation by rubisco). Temperature: light-independent stage (enzymes such as rubisco).
Using DCPIP with a chloroplast suspension
DCPIP (or methylene blue) turns from blue to colourless when reduced by electrons from the light-dependent stage. Rate = 1 ÷ time to decolourise. Controls: a dark tube and a tube without chloroplasts.
Investigating the effect of light intensity on an aquatic plant
Pondweed in sodium hydrogencarbonate solution in a water bath, with a heat shield. Change lamp distance, acclimatise, collect gas over a fixed time (capillary tube or gas syringe), repeat and find mean rates.
Why is oxygen collected from a whole plant a net rate?
Some of the oxygen produced is used in the plant's own respiration, so the true rate of photosynthesis is higher than the rate measured.
Why must light intensity be checked when using coloured filters?
Filters change light intensity as well as wavelength, so intensity must be measured (light meter) and made equal before comparing wavelengths.

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