8.2 Photosynthesis – summary of mark schemes
8.2.3
Explain the light-dependent reactions.
Mark Scheme
A.
B.
C.
D.
E.
F.
G.
H.
I.
J.
K.
L.
M.
8.2.4
chlorophyll / photosystem absorbs light;
electron raised to higher energy level / photoactivated;
splitting of water / photolysis replaces electron;
passing of excited electrons between chlorophyll molecules in photosystems;
electron passed from photosystem II to carriers (in thylakoid membrane);
production of ATP in this way is called photophosphorylation;
electron causes pumping of protons into the thylakoid;
proton gradient used by ATPase to drive ATP production;
lectron passes to photosystem I at end of carrier chain;
electron re-excited and emitted by photosystem I;
electron passed to / used to reduce NADP+;
NADPH + H+ / reduced NADP produced;
cyclic photophosphorylation using photosystem I, electron carriers and ATPase only;
Explain photophosphorylation in terms of chemiosmosis.
Mark Scheme
A.
B.
C.
D.
E.
F.
G.
H.
I.
J.
K.
L.
M.
8.2.5
chemiosmosis is synthesis of ATP coupled to electron transport and proton movement;
photophosphorylation is the production of ATP with energy from light;
light energy causes photolysis / splitting of water;
electrons energized (from chlorophyll) / photoactivation;
photolysis provides (replacement) electrons for those lost from excited chlorophyll;
photolysis provides protons / H+ (for thylakoid gradient);
electron transport (carriers on membrane of thylakoid;)
causes pumping of protons / H+ across thylakoid membrane / into thylakoid space;
protons / H+ accumulate in thylakoid space / proton gradient set up;
protons / H+ move down concentration gradient;
into stroma;
flow through ATPase / synthetase;
leading to ATP formation;
Explain the light-independent reactions.
Mark Scheme
A.
B.
C.
D.
E.
F.
G.
H.
I.
J.
K.
L.
M.
8.2.6
light-independent reaction fixes CO2;
to make glycerate 3-phosphate;
glycerate 3-phosphate / GP / phosphoglyceric acid becomes reduced;
to triose phosphate / phosphoglyceraldehyde / glyceraldehyde 3-phosphate;
using NADPH;
using ATP;
ATP needed to regenerate RuBP;
ATP is made in light-dependent reactions;
light causes photoactivation / excitation of electrons;
flow of electrons causes pumping of protons into thylakoid;
ATP formation when protons pass back across thylakoid membrane;
electrons are passed to NADP / NADP+;
NADPH produced in the light dependent reactions;
Explain the relationship between the structure of the chloroplast and its function.
Mark Scheme
A.
B.
C.
D.
E.
F.
light dependent reaction occurs in the thylakoid membrane;
thylakoids provide a large surface area;
pigments / chlorophyll is located in the membrane;
in groups of (hundreds of) molecules called photosystems;
folds in thylakoid allow photosystems to be close to each other;
electron carriers embedded in membrane;
G.
H.
I.
J.
K.
L.
M.
Explain the relationship between the action spectrum and the absorption spectrum of photosynthetic pigments in green plants.
Mark Scheme
N.
O.
P.
Q.
R.
x-axis labelled light wavelength / frequency and y-axis labelled rate of photosynthesis;
curve increases, decreases and then increases again to decrease again;
peak approximately at 450 nm / blue region;
peak approximately at 670 nm / red region;
first peak higher than second peak;
Rate of photosynthesis
8.2.7
NADP+ accepts two high energy electrons and an H+ from stroma to form NADPH;
electron flow causes H+ to be pumped into thylakoid space;
proton gradient formed in space between thylakoids;
H+ flow back through ATP synthase / synthetase channels to produce ATP;
coupling of electron transport produces ATP / chemiosmosis;
ATP synthase / synthetase embedded in thylakoid;
photolysis of water occurs in thylakoid space;
400
450
blue
500
550
600
green
Light wavelength / nm
650
700
red