Photosynthesis

Question 1

Describe the light-independent reactions of photosynthesis and explain how they allow the continued synthesis of hexose sugars.

Answer 1

• RuBP (5C) combines with CO2
• to form 2x GP (3C)
• 2 x TP (3C) to glucose
• using ATP
• and reduced NADP
• All RuBP is regenerated so glucose can be resynthesised

Question 2

Describe the role of electron transport chains in the light-dependent reactions of photosynthesis.

Answer 2

1. electron transport chain accepts excited electrons;
2. from chlorophyll / photosystem;
3. electrons lose energy along chain;
4. ATP produced;
5. from ADP and Pi;
6. reduced NADP formed;
7. when electrons (from transport chain) and H+
   combine with NADP;
8. H+ from photolysis;

Question 3

Explain why the increase in the dry mass of a plant over twelve months is less than the mass of hexose produced over the same period.

Answer 3

some hexose / biomass / eq. used in respiration;
CO2 produced (is lost to air);
some parts of the plant are eaten / some parts lost to decomposers / in leaf fall;

Question 4

the role of chlorophyll in photolysis;

Answer 4

Chlorophyll absorbs light energy resulting in it losing electrons and becoming oxidised. It then accepts electrons from water which causes more water to dissociate.

Question 5

how the energy of light is converted into chemical energy in the light-dependent reactions.

Answer 5

The electrons are raised to high energy levels, these then use electron transfer chains to transfer and diffuse down the ATP synthase channel to form ATP from ADP + Pi.

Question 6

Describe the part played by chlorophyll in photosynthesis.

Answer 6

light (energy) absorbed by chlorophyll;

raises energy level of electrons / electrons are excited / emitted; ATP formed;

Question 7

During the light-independent reaction of photosynthesis, carbon dioxide is converted into organic substances. Describe how.

Answer 7

1. Carbon dioxide combines with ribulose bisphosphate / RuBP;
   6.
2. Produces two glycerate (3-)phosphate / GP;
   Accept: any answer which indicates that 2 x as much GP produced from one RuBP.
3. GP reduced to triose phosphate / TP;
   Must have idea of reduction. This may be conveyed by stating m.p.4
4. Using reduced NADP;
   Reject: Any reference to reduced NAD for m.p.4 but allow reference to reduction for m.p. 3.
5. Using energy from ATP;
   Must be in context of GP to TP.
6. Triose phosphate converted to glucose / hexose / RuBP / ribulose bisphosphate / named organic substance;

Question 8

The photolysis of water is an important part of the process of photosynthesis. Describe what happens in the photolysis of water.

Answer 8

(Water) forms H+ / hydrogen ions and electrons / e– ;

O2 / oxygen formed; [NOT ‘O’, NOT ‘O –’]

(Light) excites electrons / raises energy level of electrons / electrons to
chlorophyll / to photosystem;

Question 9

ATP and reduced NADP are two products of the light-dependent reactions. Describe one function of each of these substances in the light-independent reactions.

Answer 9

(ATP) Provides energy for GP → TP

(Reduced NADP)Provides H / electrons for GP → TP / reduces GP to TP;

Question 10

In the light-dependent reaction of photosynthesis, light energy generates ATP.

Answer 10

1. Light (energy) excites / raises energy level of electrons in chlorophyll;

Electrons pass down electron transfer chain;
Q Accept any reasonable alternative for electron transfer chain.

(Electrons) reduce carriers / passage involves redox reactions;

Electron transfer chain / role of chain associated with chloroplast membranes / in thylakoids / grana;

Example such as chemiosmosis;

Energy released / carriers at decreasing energy levels;

ATP generated from ADP and phosphate / Pi / phosphorylation of ATP;

Question 11

Lowering the temperature has very little effect on the light-dependent reaction, but it slows down the light-independent reaction. Explain why the light-independent reaction slows down at low temperatures.

Answer 11

enzymes involved / not a photochemical reaction;

slow rate of enzyme / chemical reaction at low temperature / less kinetic energy / fewer collisions;

Question 12

protein binds to ETC

explain how this reduced the rate of photosynthesis

Answer 12

Reduced transfer of protons across thylakoid membrane
OR
Reduced chemiosomotic gradient / proton gradient across thylakoid membrane;
(So) less ATP produced;
(So) less reduced NADP produced;
Accept NADPH / NADPH2 / NADPH+ Reject reduced NAD
(So) light-independent reaction slows / stops;
OR
Less reduction of GP to triose phosphate.

Question 13

Crops use light energy to produce photosynthetic products.

Describe how crop plants use light energy during the light-dependent reaction.

Answer 13

Excites electrons / electrons removed (from chlorophyll);
Accept: higher energy level as ‘excites’.

Electrons move along carriers/electron transfer chain releasing energy;

Energy used to join ADP and Pi to form ATP;

Photolysis of water produces protons, electrons and oxygen; NADP reduced by electrons / electrons and protons / hydrogen;
Accept: NADP to NADPH (or equivalent) by addition of electrons/hydrogen.