3.3 Photosynthsis

Photosynthesis

a process through which solar energy is converted to chemical energy

Endosymbiosis of Photosynthetic organisms

-Prokaryotes Evolved First

-gave rise to algae

-Some organisms evolved from a second endosymbiotic event

LDR Location

Thylakoid Membrane

LDR inputs

Photons, water

LDR outputs

NADPH, ATP, O2

1st step of LDR

-Photosystem II absorbs light energy

-A photon excites chlorophyll in PSII

Photosystem

a complex of protein & chlorophyll

Light Energy

Different pigments absorb different wavelengths of light

Different organisms produce different pigments

2nd step of LDR

A molecule of H2O is split

H2O is split (photolysis) into: ½O2, 2 H+ & 2 e-

[H+] increases in thylakoid space/lumen

e- replenish chlorophyll; other e- goes to ETC

O2 is produced & exits plant

3rd step of LDR

Electrons are transported through the ETC

Energy from the e- is used to pump H+ into the thylakoid

PSI accepts used e-

PSI absorbs photon re-energizes e-

4th step of LDR

NADP+ is reduced to NADPH

e- from PSI are used to reduce NADP+ to NADPH; H+ also used

NADPH will be used in the Calvin Cycle

Oxidation

loss of electrons

Reduction

gain of electrons

5th step of LDR

ATP is made by ATP Synthase

H+ to flow along gradient through ATP Synthase

Flow is used to add Pi to ADP → ATP (phosphorylation)

ATP will be used in the Calvin Cycle

cyclic electron flow

nonlinear

No NADP+ around to accept e-

PSI e- are recycled

NADP+ is not reduced to NADPH

Calvin Cycle location

Stroma

Calvin Cycle inputs

ATP, NADPH, CO2

Calvin Cycle outputs

NADP+, ADP + Pi, carbohydrates

Phase 1 of the Calvin Cycle

Carbon Fixation

Combines RuBP (5-carbon molecule) & CO2 to make 2x 3-carbon molecules: 3-PGA

RuBisCo

an enzyme that makes CO2 usable

It sometimes confuses Oxygen for Carbon Dioxide

Photorespiration

occurs when RuBisCo fixes O2 instead of CO2

Results in lost energy & carbon

Phase 2 of the Calvin Cycle

Reduction

Energy from ATP & NADPH is used to convert 3-PGA to G3P

Phase 3 of the Calvin Cycle

Regeneration

Most of the G3P made is used to regenerate RuBP.

RuBP regeneration requires ATP.

RuBP regeneration makes this a cycle

C3 photosynthesis

Most Plants

subject to photorespiration

C4 Plants

Reduce photorespiration by fixing CO2 with a different enzyme (also reduce H2O loss)

CAM Plants

Avoid photorespiration by fixing CO2 with another enzyme & separating reactions by time of day