Topic 5.7 Photosynthesis

Question 1

Describe the structure of a chloroplast

Answer 1

• Usually disc-shaped
• Double membrane (envelope)
• Thylakoids: flattened discs stack to form grana
• Intergranal lamellae: tubular extensions attach thylakoids in adjacent grana
• Stroma: fluid-filled matrix with high enzyme and subtrate concentraion

Question 2

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

Answer 2

Light-dependent: in the thylakoids of chloroplasts

Light-independent: stroma of chloroplasts

Question 3

Name the processes in the light-dependent reaction

Answer 3

• Photo ionisation
• Electron transport chain
• Chemiosmosis

Non-cyclic only:
* Reduction of NADP
* Photolysis of water

Question 4

Explain the role of light in photoionisation

Answer 4

Chlorophyll molecules absorb light from photons of light.

This ‘excites’ 2 electrons (raises them to a higher energy level), causing them to be released from the chlorophyll.

Question 5

What happens in the ETC?

Photosynthesis

Answer 5

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

Question 6

How is a proton concentration gradient established during chemiosmosis in photosynthesis?

Answer 6

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

Question 7

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

Answer 7

H⁺ ions move down their concentration gradient from the thylakoid space into the stroma via the transmembrane channel protein ATP synthase

ATP synthase catalyses ADP + Pi -> ATP

Question 8

Describe non-cyclic photophosphorylation

Answer 8

Uses photosystems I and II. Excited electrons enter ETC to produce ATP. NADP acts as final electron acceptor and is reduced. Water is photolysed to release electrons to replace those lost from PS II.

Question 9

Describe cyclic photophophorylation

Answer 9

Uses only Photosystem I. Excited electrons enter ETC to produce ATP then return directly to photosystem (so no reduction of NADP and no water needed to replace lost electrons).

Question 10

State the purpose of cyclic and non-cyclic photophosphorylation

Answer 10

Cyclic: produces additional ATP to meet surplus energy demands of cell.

Non-cyclic: produces ATP and reduced NADP for Calvin cycle to produce biological compounds.

Question 11

What happens in photolysis of water?

Answer 11

Light energy splits molecules of water

2H₂O -> 4H⁺ + 4e^- + O₂

Question 12

What happens to the products of the photolysis of water?

Answer 12

H⁺ ions: move out of thylakoid space via ATP synthase and 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 13

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

Answer 13

NADP + 2H⁺ + 2e^- -> reduced NADP.

Catalysed by dehydrogenase enzymes.

Stroma of chloroplasts.

Question 14

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

Answer 14

H⁺: photolysis of water.

Electrons: NADP acts as the final electron acceptor of the electron transport chain.

Question 15

Name the three stages in the Calvin cycle

Answer 15

1. Carbon fixation
2. Reduction
3. Regeneration

Question 16

What happens during carbon fixation?

Answer 16

Reacton between Co₂ and rubulose bisphophate (RuBP) catalysed by rubulose bisphosphate carboxylase (RUBISCO).

Forms unstable 6C intermediate that breaks down into 2x glycerate 3-phosphate (GP).

Question 17

What happens during reduction (in the Calvin cycle)?

Answer 17

2x GP are reduced to 2x glyceraldehyde phosphate (GALP).

Requires 2x reduced NADP and 2x ATP.

Forms 2x NADP and 2x ADP.

Question 18

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

Answer 18

GALP acts as raw material when 1C leaves the cycle to produce monosaccharides, amino acids and other biological molecules.

Question 19

What happens during regeneration (in the Calvin cycle)?

Answer 19

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

Question 20

Outline the sequence of events in the light-independent reaction (Calvin cycle).

Answer 20

Look in text book, or online

Question 21

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

Answer 21

ATP: reduction of GP to TP and provides phosphate group to convert RuP into RuBP.

(reduced) NADP: coenzyme transports electrons needed for reduction of GP to TP.

Question 22

State the number of carbon atoms in RuBP, GP and GALP

Answer 22

RuBP: 5

GP: 3

GALP: 3

Question 23

Define ‘limiting factor’

Answer 23

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

Question 24

Name 4 enviromental factors that can limit the rate of photosynthesis

Answer 24

• Light intensity (light-dependent stage)
• CO₂ levels (light-independent stage)
• Temperature (enzyme-controlled steps)
• Mineral/ magnesuim levels (maintain normal functioning of chlorophyll)