Photosynthesis

Question 1

Where do the light-dependent and light-independent reactions occur in plants?

Answer 1

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

Question 2

Explain the role of light in photoionisation.

Answer 2

Chlorophyll molecules absorb energy from photons of light.
This ‘excited 2 electrons (raises them to a higher energy level), causing them to be released from the chlorophyll.

Question 3

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

Answer 3

1. Electron transfer chain
2. Chemiosmosis

Question 4

What happens in the electron transfer chain (ETC)?

Answer 4

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

Question 5

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

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.

Question 7

Explain the role of light in photolysis.

Answer 7

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

Question 8

What happens to the 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

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

Answer 9

• NADP + 2H+ + 2e- —> NADP.H
• catalysed by dehydrogenase enzymes
• stroma of chloroplasts

Question 10

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

Name the 3 main stages in the carbon cycle.

Answer 11

1. Carbon fixation
2. Reduction
3. Regeneration

Question 12

What happens during carbon fixation?

Answer 12

• reaction between CO2 and ribulose biphosphate (RuBP) catalysed by rubisco.
• forms unstable 6C intermediate that breaks down into 2x glycerate-3-phosphate

Question 13

What happens during reduction (in the calvin cycle)?

Answer 13

• 2x GP are reduced to 2x TP
• requires 2x reduced NADP & 2x ATP
• forms 2x NADP & 2x ADP

Question 14

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 TP is converted into useful organic molecules)

Question 15

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

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

Answer 16

• ATP: reduction of GP to TP & provides phosphate group to convert RuP into RuBP
• (reduced) NADP: coenzyme transports electrons needed for reduction of GP to TP

Question 17

State the no. Carbon atoms in RuBP, GP & TP.

Answer 17

RuBP: 5
GP: 3
TP: 3

Question 18

How does the structure of the chloroplast maximise the rate of the light-dependent reaction?

Answer 18

• ATP synthase channels within granal membrane
• large surface area of thylakoid membrane for ETC
• photosystems position chlorophyll to enable maximum absorption of light

Question 19

How does the structure of the chloroplast maximise the rate of the light-independent reaction?

Answer 19

• own DNA & ribosomes of synthesis of enzymes e.g. rubisco
• concentration of enzymes & substrates in stroma is high

Question 20

Define ‘limiting factor’.

Answer 20

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

Question 21

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

Answer 21

• light intensity (ld)
• CO2 levels (ld)
• temperature (enzyme controlled steps)
• mineral/ magnesium levels (maintain normal functioning of chlorophyll)

Question 22

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

Answer 22

• artificial light, especially at night
• artificial heating
• addition of CO2 to greenhouse atmosphere

Question 23

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

Answer 23

• to increase yield
• additional cost must be balanced with yield to ensure maximum profit

Question 24

Suggest how a student could investigate the effect of a named variable on the rate of photosynthesis.

Answer 24

1. Use a potometer
2. Place balls of calcium alginate containing green algae in hydrogencarbonate indicator ( colour change orange —> magenta as CO2 is consumed & pH increases)

Question 25

State the purpose and principle of paper chromatography.

Answer 25

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

Question 26

Outline a method for extracting photosynthetic pigments.

Answer 26

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

Question 27

Outline how paper chromatography can be used to separate photosynthetic pigments.

Answer 27

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

Question 28

What are Rf values? How can they be calculated?

Answer 28

• ratios 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.