Photosynthesis

6CO2 + 12H20 C6H12O6 + 6O2 + 6 H2O

Overview

• Light-dependent reactions of photosynthesis

• Light-independent reactions of photosynthesis

• C3 versus C4 metabolism

• Write an essay on the light-dependent (light) reactions of photosynthesis

• Write an essay on the light-independent (Dark) reactions of photosynthesis

• Compare and contrast C3 and C4 metabolism

Photosynthesis

Light reactions Calvin/BensonCycle

(“Dark Reactions”)

ADP

ATP

NADP+

NADPH

CO2

Sucrose

H2O O2

Photosynthesis occurs only in the chloroplasts

Vacuole

Palisade cell

Stroma

ChloroplastThylakoids

Thylakoidspace

Grana

PSII PSI

The light reactions of photosynthesis

Plants absorb Light

Wavelength of light (nm)

400 500 600 700Green RedBlue

Abs

Chlorophyll a

Chlorophyll b

Carotenoids

Absorption of light leads to excited electrons

• When light energy is absorbed by chlorophyll, an electron is promoted to a higher (more energetic) orbital. This energy can be lost by:– from one orbital to the next with loss of heat– back to the ground state with loss of light

(fluorescence) and heat– transfer to another chlorophyll.

Chlorophylls are organised into photosystems to trap light energy

Light

Chlorophylls are organised into photosystems

Transfer of lightenergy from onechlorophyll to another

Special chlorophyll molecule- primary electron acceptor

Cyclic photosphorylation generates ATP

PQ

PQ

PSII PSI

Stromal side

Thylakoid space side

Fd

P700

Non-Cyclic photosphorylation generates ATP and NADPH

PQ

PQ

PSII PSI

Stromal side

Thylakoid space side

Fd

NADP+ NADPH

Q

P700P680

H20 2H+ & 2e- & 1/2 O2

Generation of ATP

• H+ gradient generated by:– PQ shuttle (H+)– Splitting of water (2H+)– NADPH production (2H+

removed from stroma)

• ATP generated by an ATP synthase as H+ ions flow through it

ADP+Pi

ATP

H+

H+ H+

H+

H+

H+

H+

Stroma

Light-independent reactions

The Calvin/Benson cycleRibulose biphosphate

CO2CarbonPhosphorus

3-phosphoglycerate

ATP

ADP

1,3 diphospho-glycerate

NADPH

NADP+Glyeraldehyde-3-phosphate

G-3-PSugars

ATP

ADP

C4 metabolism

C3 and C4 metabolism

• Inefficient as RUBISCO can act as both a carboxylase and as an oxygenase.

• Oxygenase activity leads to loss of carbon that has already been fixed.

• Some plants have an alternative pathway where CO2 is first fixed into C4 organic acids (C4 pathway) and then liberated later to undergo the Calvin/Benson cycle (C3 metabolism).

C4 metabolism

• 2 types

• C3 and C4 metabolism are separated in space (different cells) e.g. sugar cane Characteristic aggregation of cells around the vascular bundles - Krantz morphology

• C3 and C4 metabolism are separated in time (same cell) e.g. pineapple.

Comparison of C3 & C4 leaves• C3 - note the lack of

chloroplasts in the bundle sheath

• C4 - note the extensive chloroplasts in the bundle sheath

Separation in space

Vascular tissue

Bundle sheath cell

Mesophyll cellsurrounding bundle sheath

CO2

PEP (3C)

C3CO2

OAA (4C)

Pyruvate (C3)

ATPADP

sugar