Topic 8.3: PHOTOSynTHESIS - BioNinja

[Pages:1]Topic 8.3: PHOTOSynTHESIS

Stages of Photosynthesis

Photosynthesis is a two-step process: ? The light dependent reactions occur in the thylakoids and convert light energy into chemical energy (ATP + NADPH) ? The light independent reactions occur in the stroma and use this chemical energy to make organic compounds

Light Dependent Reactions

Non-Cyclic Photophosphorylation ? Chlorophyll in Photosystems I and II absorb light, which

triggers the release of energised electrons (photoactivation) ? The electrons from PS I reduce NADP (forms NADPH) ? The electrons from PS II move through an electron

transport chain before replacing the electrons from PS I ? The transport chain produces ATP (photophosphorylation) ? Electrons lost from PS II are replaced by water (photolysis)

Cyclic Photophosphorylation ? Only chlorophyll in Photosystem I are activated by light ? The electrons move through an electron transport chain

before returning to their original location (i.e. cyclic) ? The transport chain produces ATP (photophosphorylation) ? Cyclic photophosphorylation does not produce NADPH ? Hence, while cyclic photophosphorylation can produce

usable energy (ATP), it cannot produce organic molecules

STROMA Light

e-

NADPH

Light

ATP

NADP

e-

STROMA

Light

ATP

e-

e-

e-

H2O

H+ O2

THYLAKOID

Photosystem II (P680)

Photosystem I (P700)

e-

H+ THYLAKOID

Photosystem I (P700)

Light Independent Reactions

Step 1: Carbon Fixation ? Rubisco catalyses the carboxylation of RuBP (requires CO2) ? This forms two 3C compounds called GP

Step 2: Reduction of GP ? GP is phosphorylated by ATP and reduced by NADPH ? This converts each GP molecule into a TP molecule

Step 3: Regeneration of RuBP ? One molecule of TP is used to form half a sugar

(two complete cycles are needed to form a glucose molecule) ? The remaining TP molecules are used to reform RuBP

3 ? CO2

Step 1:

3 ? RuBP Carbon

GP ? 6

Fixation ATP

ATP

Step 3:

Step 2:

ADP

ADP

Regeneration Reduction

+ Pi

of RUBP

of GP

+ Pi

NADPH

1 ? TP ? ? sugar

TP

NADP+

? 6

+ H+

Lollipop Experiment

The light independent reactions were elucidated by Melvin Calvin (i.e. Calvin cycle) using a `lollipop'-shaped apparatus ? Radioactive CO2 was incorporated to identify the different carbon compounds (involves chromatography and autoradiography)

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