Photosynthesis

Question 0

Chloroplast

Answer 0

• 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

Question 1

Photosynthesis

Answer 1

Transformation of solar light energy into chemical energy

6 CO2 + 12 H2O + light -> C6H12O6 + 6O2 + 6 H2O

Question 2

Photosynthesis is a redox reaction

Answer 2

• 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

Question 3

Photosynthesis is comprised of 2 processes

Answer 3

Light reaction and Calvin cycle

Question 4

Summary of light reaction

Answer 4

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

Question 5

Summary of Calvin cycle

Answer 5

In stroma, converts CO2 into sugar.

1. Carbon fixation: CO2 into organic material
2. Filled carbon reduced to sugar: NADPH electron source
3. Regeneration of the CO2 acceptor

Question 6

Energy source for light reaction

Answer 6

• 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))

Question 7

Photosystem

Answer 7

Light harvesting complexes, found embedded in thykaloid membrane, composed of light-harvesting molecules & reaction centres.

Question 8

Light-harvesting complexes

Answer 8

Pigment molecules, absorb photon energy, passed from pigment to pigment.

Question 9

Reaction centre

Answer 9

Contains a pair of chlorophyl a molecules and a primary electron acceptor (removes excited electron)

Question 10

Types of reaction centres

Answer 10

• Photosystem 1 (PS1):
• Photosystem 2 (PS2):

Question 11

Noncyclic electron flow

Answer 11

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

Question 12

Cyclic electron flow

Answer 12

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.

Question 13

Chemiosmosis

Answer 13

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

Question 14

Proton (H+) gradient

Answer 14

[H+] in thylakoid space > [H+] in stoma

1. water is split: protons released in the thylakoid space.
2. protons in the stoma are removed: pumped into the thylakoid space.
3. protons in the stroma are removed (NADP+ -> NADPH)

Question 15

Production of ATP

Answer 15

• ATP synthase: ADP + Pi -> ATP
• Proteins embedded in the thylakoid membrane.
• Source of energy is proton gradients
• As [H+] moves down ****

Question 16

Calvin cycle

Answer 16

• Production of a three carbon sugar: glyceraldehyde 3 phosphate.
• ATP is the energy source, NADPH is the reducing agent.
• 3 phases

Question 17

2 Reduction

Answer 17

-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

Question 18

1 Carbon fixation (C3 plants)

Answer 18

• 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

Question 19

3 Regeneration of ribulose bisphosphate

Answer 19

One G3P will exit the cycle, five G3P are used to regenerate ribulose bisphosphate, requires more ATP.

Question 20

Alternative mechanisms of carbon fixation

Answer 20

• 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.

Question 21

C4 plants

Answer 21

Spatial separation of steps

1. Cytoplasm of mesophyll cells: CO2 is fixed into a 4 carbon compound. *drawing.
2. Malate transported into bundle-sheath cells: lack PSII, therefore no O2 present. Malate -> pyruvate (3C) and CO2 -> Calvin cycle.

Question 22

CAM plants

Answer 22

• Stomata open during the night.
• CO2 fixation into malate
• Stored in vacuoles
• Stomata closed during the day
• Malate releases CO2