3.5.1 Photosynthesis

Question 1

(Photosynthesis) Where do the light-dependent & light-independent reactions occur in plants?

Answer 1

Light-dependent: in the thylakoids of chloroplasts.
Light-independent: stroma of the chloroplasts.

Question 2

(Photosynthesis) Explain the role of light in photoionisation.

Answer 2

Chlorophyll molecules absorb energy form photons of light.
This ‘excites’ 2 electron a (raise a them to a higher energy level), causing them to be released from the chlorophyll.

Question 3

(Photosynthesis) Name the 2 main stages involved in ATP production in the light-dependent reaction.

Answer 3

1) Electron transfer chain
2) Chemiosmosis

Question 4

(Photosynthesis) What happens in the electron transfer chain (ETC)?

Answer 4

Electrons released from chlorophyll move down a series of carrier proteins embedded in the thylakoid membrane & undergo a series of redox reactions, which releases energy.

Question 5

(Photosynthesis) How is a proton concentration gradient established during chemiosmosis?

Answer 5

Some energy released from the ETC is coupled to the active transport of H+ ions (protons) from the stroma into the thylakoid space.

Question 6

(Photosynthesis) How does chemiosmosis produce ATP in the light-dependent stage?

Answer 6

H+ ions (protons) move down their concentration gradient from the thylakoid space into the stroma via the channel protein, ATP synthase.
ATP synthases catalyses ADP + Pi ➡️ ATP.

Question 7

(Photosynthesis) Explain the role of light photolysis.

Answer 7

Light energy splits molecules of water.
2H2O ➡️ 4H+ + 4e- + O2

Question 8

(Photosynthesis) What happens to the 3 products of the photolysis of water?

Answer 8

H+ ions: move out of thylakoid space via ATP synthase & are used to reduce the coenzyme NADP.
e-: replace electrons lost from chlorophyll.
O2: used for respiration or diffuses out of leaf as waste gas.

Question 9

(Photosynthesis) How and where is reduced NADP produced in the light-dependent reaction?

Answer 9

• NADP + 2H+ + 2e- ➡️ reduced NADP
• Catalyses by dehydrogenase enzymes
• Stroma of chloroplasts

Question 10

(Photosynthesis) Where do the H+ ions and electrons used to reduce NADP come from?

Answer 10

H+ ions: photolysis of water
Electrons: NADP acts as the final electron acceptor of the electron transfer chain

Question 11

(Photosynthesis) name the 3 main stages in the Calvin cycle.

Answer 11

1) Carbon fixation
2) Reduction
3) Regeneration

Question 12

(Photosynthesis) What happens during carbon fixation?

Answer 12

Reaction between CO2 and RuBP catalysed by rubisco.
Forms unstable 6C intermediate that breaks down into 2x glycerate 3-phosphate (GP).

Question 13

(Photosynthesis) What happens during reduction (in the Calvin cycle)?

Answer 13

2x GP are reduced to 2x Triose phosphate
Requires 2x reduced NADP & 2x ATP
Forms 2x NADP & 2x ADP

Question 14

(Photosynthesis) How does the light-independent reaction result in the production of useful organic substances?

Answer 14

1C leaves the cycle (I.e. some of the Triose phosphate is converted into useful organic molecules).

Question 15

(Photosynthesis) What happens during regeneration (in the Calvin cycle)?

Answer 15

After 1C leaves the cycle, the 5C compound RuP forms.
RuBP is regenerated from RuP using 1x ATP.
Forms 1x ADP.

Question 16

(Photosynthesis) State the roles of ATP & (reduced) NADP in the light-independent reaction.

Answer 16

ATP: reduction of GP to Triose phosphate & provides phosphate group to convert RuP into RuBP.
(Reduced) NADP: coenzyme transports electrons needed for reduction of GP to Triose phosphate.

Question 17

(Photosynthesis) State the number of carbon atoms in RuBP, GP & Triose phosphate.

Answer 17

RuBP: 5
GP: 3
Triose phosphate: 3

Question 18

(Photosynthesis) Describe the structure of a chloroplast. (5)

Answer 18

1) Usually disc-shaped.
2) Double membrane (envelope).
3) Thylakoids: flattened discs stack to form grana.
4) Intergranal lamellae: tubular extensions attach thylakoids in adjacent grana.
5) Fluid-filled matrix.

Question 19

(Photosynthesis) How does the structure of the chloroplast maximise the rate of the light-DEPENDENT reaction?

Answer 19

ATP synthase channels within granal membrane.
Large surface area of thylakoid membrane for ETC.
Photosystems position chlorophyll to enable maximum absorption of light.

Question 20

(Photosynthesis) How does the structure of the chloroplast maximise the rate of the light-INDEPENDENT reaction?

Answer 20

Own DNA & ribosomes for synthesis of enzymes e.g. rubisco.
Concentration of enzymes & substrates in stroma is high.

Question 21

(Photosynthesis) Define ‘limiting factor’.

Answer 21

Factor that determines maximum rate of a reaction, even if other factors change to become more favourable.

Question 22

(Photosynthesis) Name 4 environmental factors that can limit the rate of photosynthesis.

Answer 22

1) Light intensity (light-dependent stage)
2) CO2 levels (light-independent stage)
3) Temperature (enzyme-controlled steps)
4) Mineral/magnesium levels (maintain normal functioning of chlorophyll)

Question 23

(Photosynthesis) Outline some common agricultural practices used to overcome the effect of limiting factors in photosynthesis.

Answer 23

• Artificial light, especially at night
• Artificial heating
• Addition of CO2 to greenhouse atmosphere

Question 24

(Photosynthesis) Why do farmers try to overcome the effect of limiting factors?

Answer 24

• To increase yield.
• Additional cost must be balanced with yield to ensure maximum profit.

Question 25

(Photosynthesis) Suggest how a student could investigate the effect of a name variable on the rate of photosynthesis.

Answer 25

DV: Rate of O2 production/CO2 consumption
1) Use a potometer.
2) Place balls of calcium alginate containing green algae in hydrogencarbonate indicator (colour change from orange to magenta as CO2 is consumed & ph increases)

Question 26

(Photosynthesis) State the purpose and principle of paper chromatography.

Answer 26

Molecules in a mixture are separated based on their relative attraction to the mobile phase (running solvent) vs the stationary phase (chromatography paper).

Question 27

(Photosynthesis) Outline a method for extracting photosynthetic pigments.

Answer 27

Use a pestle and mortar to grind a leaf with an extraction solvent, e.g. propanone.

Question 28

(Photosynthesis) Outline how paper chromatography can be used to separate photosynthetic pigments. (3)

Answer 28

1) Use a capillary tube to spot pigment extract onto pencil ‘start line’ (origin 1) 1cm above bottom of paper.
2) Place chromatography paper in solvent (origin should be about solvent level).
3) Allow solvent to run until it almost touches the other end of the paper. Pigments move different distances.

Question 29

(Photosynthesis) What are Rf values? How can they be calculated?

Answer 29

Ratio that allow comparison of how far molecules have moved in chromatograms.

Rf value = distance between origin and centre of pigment spot / distance between origin and solvent front