Lecture 15  Photosynthesis - light reactions

1) Photosystems

Photosystem - a light capturing unit found in the thylakoid membrane

a) A photon excites any pigment in the photosystem

b) The energy is transmitted from pigment to pigment by resonance energy transfer

c) The energy is transmitted to the reaction centre where it excites a specific chlorophyll a

d) This chlorophyll a decays by transfer of the electron to a primary acceptor molecule

What happens next?

e) The electron missing in the chlorophyll a is replaced

f) The electron swiped by the primary acceptor enters an electron transport chain

2) Electron transport chains

Several adjacent electron carriers (membrane proteins)

Source of electrons - varies

Terminal electron acceptor - varies

3) Noncyclic electron flow

a) A photon strikes Photosystem II and allows its primary acceptor to gain an e^-

b) The missing electron is replaced by the water-splitting enzyme beneath PSII

      2 H₂O ---> O₂  +  4 H⁺  +  4 e^-

c) The swiped electron enters an ETC

d) A protein in this ETC pumps H⁺ into thylakoid space

Summary:

e) Meanwhile, a photon strikes Photosystem I and allows its primary acceptor to gain an e^-

f) The missing electron is replaced by those exiting the first ETC

g) The swiped electron enters a second ETC

h) A protein in this ETC donates electrons to NADP⁺

      NADP⁺ + H⁺ + 2e^- ---> NADPH

Summary:

4) Using the proton gradient to make ATP

a) What proton gradient?

Noncyclic electron flow generates a massive proton gradient across the thylakoid membrane

b) How is this proton gradient established?

Know Figure 10.18

c) Why is this proton gradient established?

Powers the ATP Synthase

5) Overview

Noncyclic electron flow:

Calvin cycle:

      3 CO₂ + 9 ATP + 6 NADPH ---> sugar

Problem =

6) Cyclic electron flow

Makes ATP without making NADPH

Uses PS I and first ETC

Summary:

What might trigger a switch to cyclic energy flow?

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