Photosynthesis Flashcards
Classes of Photosynthetic Pigments
- chlorophylls
- carotenoids
Roles of Chlorophylls
- absorb mainly red and blue-violet light; reflect green light
Structure of Chlorophylls
- 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
Roles of Carotenoids
- 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
2 stages of photosynthesis
- light-dependent
- light-independent
Product(s) of light-dependent stage
ATP, NADPH
Product(s) of light-independent stage
Glucose
Location of Photophosphorylation
Thylakoid membrane
Location of Calvin Cycle
Stroma
Explain the light-dependent reactions of photosynthesis.
NON-CYCLIC PHOTOPHOSPHORYLATION
- 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
Explain the light-dependent reactions of photosynthesis.
CYCLIC PHOTOPHOSPHORYLATION
- 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
When does cyclic photophosphorylation occur?
- 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
Purpose of cyclic photophosphorylation
- produce more ATP for light-independent Calvin cycle
3 phases of the Calvin cycle
1) Carbon fixation
2) Reduction
3) Regeneration of RuBP
Outline CARBON FIXATION in the Calvin cycle
- 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)
Outline REDUCTION in the Calvin cycle
- 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)
Outline REGENERATION OF RuBP in the Calvin cycle
- 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
Principle of Limiting Factors
- 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
Limiting factors of photosynthesis
- CO2 concentration
- temperature
- wavelength of light / light quality
- light intensity
Effect of light intensity on rate of photosynthesis
- 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
Effect of temperature on rate of photosynthesis
- 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
