8.2 Photosynthesis summary of mark schemes

8.2 Photosynthesis ? summary of mark schemes

8.2.3 Explain the light-dependent reactions.

Mark Scheme

A. chlorophyll / photosystem absorbs light; B. electron raised to higher energy level / photoactivated; C. splitting of water / photolysis replaces electron; D. passing of excited electrons between chlorophyll molecules in photosystems; E. electron passed from photosystem II to carriers (in thylakoid membrane); F. production of ATP in this way is called photophosphorylation; G. electron causes pumping of protons into the thylakoid; H. proton gradient used by ATPase to drive ATP production; I. lectron passes to photosystem I at end of carrier chain; J. electron re-excited and emitted by photosystem I; K. electron passed to / used to reduce NADP+; L. NADPH + H+ / reduced NADP produced; M. cyclic photophosphorylation using photosystem I, electron carriers and ATPase only;

8.2.4 Explain photophosphorylation in terms of chemiosmosis.

Mark Scheme

A. chemiosmosis is synthesis of ATP coupled to electron transport and proton movement; B. photophosphorylation is the production of ATP with energy from light; C. light energy causes photolysis / splitting of water; D. electrons energized (from chlorophyll) / photoactivation; E. photolysis provides (replacement) electrons for those lost from excited chlorophyll; F. photolysis provides protons / H+ (for thylakoid gradient); G. electron transport (carriers on membrane of thylakoid;) H. causes pumping of protons / H+ across thylakoid membrane / into thylakoid space; I. protons / H+ accumulate in thylakoid space / proton gradient set up; J. protons / H+ move down concentration gradient; K. into stroma; L. flow through ATPase / synthetase; M. leading to ATP formation;

8.2.5 Explain the light-independent reactions.

Mark Scheme

A. light-independent reaction fixes CO2; B. to make glycerate 3-phosphate; C. glycerate 3-phosphate / GP / phosphoglyceric acid becomes reduced; D. to triose phosphate / phosphoglyceraldehyde / glyceraldehyde 3-phosphate; E. using NADPH; F. using ATP; G. ATP needed to regenerate RuBP; H. ATP is made in light-dependent reactions; I. light causes photoactivation / excitation of electrons; J. flow of electrons causes pumping of protons into thylakoid; K. ATP formation when protons pass back across thylakoid membrane; L. electrons are passed to NADP / NADP+; M. NADPH produced in the light dependent reactions;

8.2.6 Explain the relationship between the structure of the chloroplast and its function.

Mark Scheme

A. light dependent reaction occurs in the thylakoid membrane; B. thylakoids provide a large surface area; C. pigments / chlorophyll is located in the membrane; D. in groups of (hundreds of) molecules called photosystems; E. folds in thylakoid allow photosystems to be close to each other; F. electron carriers embedded in membrane;

G. NADP+ accepts two high energy electrons and an H+ from stroma to form NADPH; H. electron flow causes H+ to be pumped into thylakoid space; I. proton gradient formed in space between thylakoids; J. H+ flow back through ATP synthase / synthetase channels to produce ATP; K. coupling of electron transport produces ATP / chemiosmosis; L. ATP synthase / synthetase embedded in thylakoid; M. photolysis of water occurs in thylakoid space;

8.2.7 Explain the relationship between the action spectrum and the absorption spectrum of photosynthetic pigments in green plants.

Mark Scheme

N. x-axis labelled light wavelength / frequency and y-axis labelled rate of photosynthesis; O. curve increases, decreases and then increases again to decrease again; P. peak approximately at 450 nm / blue region; Q. peak approximately at 670 nm / red region; R. first peak higher than second peak;

Rate of photosynthesis

400 450 blue

500 550 600 green

Light wavelength / nm

650 700 red

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