Photosynthesis Flashcards
Chloroplast
- Found mainly in mesophyll cells
- Composed of a double lipid bilayer: outer membrane very permeable, inner membrane has selective permeability (surrounds fluid stroma).
- Stroma: photosynthetic enzymes, thykaloid membrane sacs (contain chlorophyl
Photosynthesis
Transformation of solar light energy into chemical energy
6 CO2 + 12 H2O + light -> C6H12O6 + 6O2 + 6 H2O
Photosynthesis is a redox reaction
- Electrons and H+ are transferred to CO2, reducing it to sugar.
- Electrons come from water H2O
- The electrons increase in potential energy (simple->complex/water->sugar), light supplies the energy
Photosynthesis is comprised of 2 processes
Light reaction and Calvin cycle
Summary of light reaction
Converts solar energy into chemical energy, occurs in the thykaloids.
1. Production if NADPH
NADP+ + 2e + 2H+ -> NADPH + H+
NADP+: nicotamide adenine dinucleotide phosphate.
NADPH: reducing power for the Calvin cycle
2. Production of ATP
phosphorylation
ADP+Pi->ATP
Summary of Calvin cycle
In stroma, converts CO2 into sugar.
- Carbon fixation: CO2 into organic material
- Filled carbon reduced to sugar: NADPH electron source
- Regeneration of the CO2 acceptor
Energy source for light reaction
- Photons (sunlight, amount of energy inversely proportional to wavelength, visible light)
- Pigments: light receptors that absorb photon energy, embedded in thykaloid membranes (chlorophyl a (most important) & b (carotenoids))
Photosystem
Light harvesting complexes, found embedded in thykaloid membrane, composed of light-harvesting molecules & reaction centres.
Light-harvesting complexes
Pigment molecules, absorb photon energy, passed from pigment to pigment.
Reaction centre
Contains a pair of chlorophyl a molecules and a primary electron acceptor (removes excited electron)
Types of reaction centres
- Photosystem 1 (PS1):
- Photosystem 2 (PS2):
Noncyclic electron flow
Produces ATP & NADPH
1) Photosynthesis II & I absorbs light: electrons go to a higher energy state, trapped by the primary electron acceptor
2) Electrons from water replace the ones lost in PSII: photolysis
3) Electrons pass from PSII TO PSI: via electron transport chain
4) Electrons from PSI are transferred to NAPP+: via electron transport chain, enzyme NADP+ reductase
Cyclic electron flow
Produces on ATP -> Calvin cycle uses more ATP than NADPH. Process involves only PSI.
-excited electron cycle: PSI primary acceptor, cytochrome complex (ETC) produces ATP, P700.
Chemiosmosis
The energy coupling mechanism
1. electron flow down an electron transport chain: creation of an electrochemical proton gradient.
2. ATP production: photophosphorylation
ADP + Pi -> ATP
Proton (H+) gradient
[H+] in thylakoid space > [H+] in stoma
- water is split: protons released in the thylakoid space.
- protons in the stoma are removed: pumped into the thylakoid space.
- protons in the stroma are removed (NADP+ -> NADPH)
Production of ATP
- ATP synthase: ADP + Pi -> ATP
- Proteins embedded in the thylakoid membrane.
- Source of energy is proton gradients
- As [H+] moves down ****
Calvin cycle
- Production of a three carbon sugar: glyceraldehyde 3 phosphate.
- ATP is the energy source, NADPH is the reducing agent.
- 3 phases
2 Reduction
-3-phosphoglycerate receives a phosphate group from ATP.
-1,3-bis-phosphoglycerate.
NADPH (2e) reduces 1,3-bisphosphateglycerate to glyceraldehyde-3-phosphate (G3P)
-3CO2 + 3 ribulose biphosphate -> 6 G3P
1 Carbon fixation (C3 plants)
- 3 CO2 enter cycle (one at a time)
- CO2 is attached to a five carbon sugar (ribulose bisphosphate)
- Enzyme: ribulose bisphosphate carboxylase -> rubisco
- 6 carbon intermediate splits in two
- Per CO2, get 2x3-phosphoglycerate
3 Regeneration of ribulose bisphosphate
One G3P will exit the cycle, five G3P are used to regenerate ribulose bisphosphate, requires more ATP.
Alternative mechanisms of carbon fixation
- Hot & dry climates: to conserve water, stomata close. Problem: decreased gas exchange (CO2 levels decrease, O2 increases).
- Result: photorespiration. rubisco adds O2 to RuBP -> phosphoglycolate (2C). Decreases photosynthesis.
- C4 plants & CAM plants.
C4 plants
Spatial separation of steps
- Cytoplasm of mesophyll cells: CO2 is fixed into a 4 carbon compound. *drawing.
- Malate transported into bundle-sheath cells: lack PSII, therefore no O2 present. Malate -> pyruvate (3C) and CO2 -> Calvin cycle.
CAM plants
- Stomata open during the night.
- CO2 fixation into malate
- Stored in vacuoles
- Stomata closed during the day
- Malate releases CO2
