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Chloroplasts: catching light and turning it into ATP questions
Chloroplast structure with each part linked to its function, photosynthetic pigments and the absorption and action spectra, separating pigments by chromatography and calculating Rf, and the light-dependent reactions: photoionisation, the electron transport chain, chemiosmosis through ATP synthase, photolysis of water, and the difference between cyclic and non-cyclic photophosphorylation.
6 original questions · 20 marks · the chloroplasts: catching light and turning it into atp notes · Photosynthesis and primary productivity
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Describe the light-dependent reactions of photosynthesis, in the order in which they happen, from the absorption of a photon to the formation of reduced NADP.
Mark scheme
- B1 a photon is absorbed by the pigments of photosystem II and the energy is funnelled to the chlorophyll a at the reaction centre, from which an electron is raised to a higher energy level and lost altogether, leaving the molecule positively charged
- B1 the lost electron is replaced by the photolysis of water on the inner face of the thylakoid membrane, which gives protons, electrons and oxygen
- B1 the excited electron is passed from carrier to carrier along the thylakoid membrane, losing energy at each transfer
- B1 that energy drives protons from the stroma into the thylakoid space, so a steep proton gradient builds across the membrane
- B1 protons flow back into the stroma through ATP synthase, driving photophosphorylation, while the electron reaches photosystem I, is re-energised by light and is passed with a proton to NADP, giving reduced NADP
Compare non-cyclic photophosphorylation with cyclic photophosphorylation.
Mark scheme
- B1 non-cyclic uses photosystem II and photosystem I, whereas cyclic uses photosystem I alone
- B1 in non-cyclic the electron ends up on NADP and never returns, whereas in cyclic it is passed to a carrier and returned to the same photosystem
- B1 non-cyclic needs photolysis of water to replace the electrons photosystem II loses, so it releases oxygen, whereas cyclic splits no water and releases none
- B1 non-cyclic produces both ATP and reduced NADP, whereas cyclic produces ATP only, which suits a job such as pumping potassium ions into a guard cell
Explain why a leaf appears green, referring to the absorption spectrum of chlorophyll.
Mark scheme
- B1 chlorophyll absorbs strongly in the blue region and in the red region of the spectrum
- B1 its absorption falls to a small fraction of that peak in the green band around 550 nm, so green light is barely absorbed
- B1 the green light is reflected and transmitted instead, so it is the light that reaches the eye, and a leaf lit only by green light photosynthesises slowly
On a chromatogram of leaf pigments the solvent front has run 84 mm from the pencil origin line, and the centre of the carotene spot lies 79 mm from that line. Calculate the Rf value of carotene.
A plant is watered with water in which the oxygen is the heavy isotope oxygen-18, while the carbon dioxide it is given contains only ordinary oxygen. Suggest where the oxygen-18 will first be detected, and justify your answer.
State what an absorption spectrum measures, and state what an action spectrum measures.
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