Photosynthesis

Question 1

Classes of Photosynthetic Pigments

Answer 1

• chlorophylls

- carotenoids

Question 2

Roles of Chlorophylls

Answer 2

• absorb mainly red and blue-violet light; reflect green light

Question 3

Structure of Chlorophylls

Answer 3

• flat, light-absorbing head end containing Mg atom
• long hydrocarbon tail, hydrophobic in nature; projects into thylakoid membrane to anchor chlorophyll
• different side chains attached to head; widen range of wavelengths of light absorbed

Question 4

Roles of Carotenoids

Answer 4

• yellow, orange, red, brown pigments; absorb mainly blue-violet light
• accessory pigments; pass light energy absorbed to chlorophyll
• protect chlorophylls from excess light and oxygen produced in photophosphorylation

Question 5

2 stages of photosynthesis

Answer 5

• light-dependent

- light-independent

Question 6

Product(s) of light-dependent stage

Answer 6

ATP, NADPH

Question 7

Product(s) of light-independent stage

Answer 7

Glucose

Question 8

Location of Photophosphorylation

Answer 8

Thylakoid membrane

Question 9

Location of Calvin Cycle

Answer 9

Stroma

Question 10

Explain the light-dependent reactions of photosynthesis.

NON-CYCLIC PHOTOPHOSPHORYLATION

Answer 10

• photophosphorylation; involves flow of electrons from P680 in PSII and P700 in PSI
• e boosted to higher energy level through transfer of light energy from light harvesting complex to light reaction centre and become excited
• excited e accepted by primary e acceptors; photosystems oxidised, primary e acceptors reduced
• e from photolysis of H2O replace those lose in PSII
• e passed from one e carrier to another through series of progressively lower e carriers in the ETC; simultaneously pump H+ into thylakoid space, resulting in proton gradient and diffusion of H+ into stroma
• these e replace those lost in PSI
• H+ gradient drives ATP production by ATP sythases in stalked particles through chemiosmosis
• NADP+ reduced to NAPH

Question 11

Explain the light-dependent reactions of photosynthesis.

CYCLIC PHOTOPHOSPHORYLATION

Answer 11

• e boosted to higher energy level through transfer of light energy from light harvesting complex to light reaction centre and become excited
• excited e accepted by primary e acceptors; PSI oxidised, primary e acceptors reduced
• excited e recycled back to PSI in ETC involving ferredoxin
• ATP production

Question 12

When does cyclic photophosphorylation occur?

Answer 12

• lack of CO2 in light-independent Calvin cycle to reoxidise NADPH to NADP+
• lack of H20 to supply high energy e to replace those lost in PSII

Question 13

Purpose of cyclic photophosphorylation

Answer 13

• produce more ATP for light-independent Calvin cycle

Question 14

3 phases of the Calvin cycle

Answer 14

1) Carbon fixation
2) Reduction
3) Regeneration of RuBP

Question 15

Outline CARBON FIXATION in the Calvin cycle

Answer 15

• 5C ribulose bisphosphate accepts CO2 through carboxylation catalysed by RuBP carboxylase, forming unstable, intermediate 6C product
• intermediate broken down into 2 molecules of glycerate 3-phosphate (GP) / 3-phosphoglycerate (PGA)

Question 16

Outline REDUCTION in the Calvin cycle

Answer 16

• glycerate 3-phosphate phosphorylated by ATP to form 1,3-bisphosphoglycerate
• 1,3-bisphsphoglycerate reduced by NADPH to glyceraldehyde 3-phosphate (G3P or GALP) / triose phosphate (TP)

Question 17

Outline REGENERATION OF RuBP in the Calvin cycle

Answer 17

• 5 glyceraldehyde 3-phosphate used to synthesise 3 RuBP, using 3 ATPs in the process; 1 G3P enters biosynthesis of glucose
• to form a molecule of glucose, 2 G3Ps are needed and therefore 2 cycles are required

Question 18

Principle of Limiting Factors

Answer 18

• the rate of a biochemical process involving a series of reactions is limited by the rate of the slowest reaction
  OR
• when a biochemical process is affected by several factors, the rate is limited by the factor which is nearest its minimum value

Question 19

Limiting factors of photosynthesis

Answer 19

• CO2 concentration
• temperature
• wavelength of light / light quality
• light intensity

Question 20

Effect of light intensity on rate of photosynthesis

Answer 20

• at low light intensities, rate of photosynthesis increases as light intensity increases
• at high light intensities, other factors become limiting and rate of photosynthesis plateaus even with an increase in light intensity as light saturation point is reached

Question 21

Effect of temperature on rate of photosynthesis

Answer 21

• photosynthetic reactions are controlled by a series of enzymes and are thus sensitive to temperature
• as temperature is increased, rate of photosynthesis increases
• however, as temperature is increased beyond optimum temperature, rate of photosynthesis falls as enzymes become denatured