1. Light Reactions

2. Bozmeman Science
“Photosynthesis & Respiration”

3. Light Reactions Summary:
Light energy splits H2O to O2 releasing high energy electrons (e-)
Movement of e- used to generate ATP
Electrons end up on NADP+, reducing it to NADPH

4. Photosystem: reaction center & light-harvesting complexes (pigment + protein)

5. Photosystems of photosynthesis
2 photosystems in thylakoid membrane
collections of chlorophyll molecules
act as light-gathering molecules
Photosystem II
chlorophyll a
P680 = absorbs 680nm wavelength red light
Photosystem I
chlorophyll b
P700 = absorbs 700nm wavelength red light
reaction center
Photons are absorbed by clusters of pigment molecules (antenna molecules) in the thylakoid membrane.
When any antenna molecule absorbs a photon, it is transmitted from molecule to molecule until it reaches a particular chlorophyll a molecule = the reaction center.
At the reaction center is a primary electron acceptor which removes an excited electron from the reaction center chlorophyll a.
This starts the light reactions.
Don’t compete with each other, work synergistically using different wavelengths.
antenna pigments

6. What’s Happening?
Photons are absorbed by clusters of pigment molecules (antenna molecules) in the thylakoid membrane.
When any antenna molecule absorbs a photon, it is transmitted from molecule to molecule until it reaches a particular chlorophyll a molecule = the reaction center.
At the reaction center is a primary electron acceptor which removes an excited electron from the reaction center chlorophyll a.
This starts the light reactions.
Don’t compete with each other, work synergistically using different wavelengths

7. Electrons in chlorophyll molecules are excited by absorption of light

8. Electron Flow Two routes for electron flow:
Linear (noncyclic) electron flow
Cyclic electron flow

9. Light Reaction (Linear electron flow)
Chlorophyll excited by light absorption
E passed to reaction center of Photosystem II (protein + chlorophyll a)
e- captured by primary electron acceptor
Redox reaction  e- transfer
e- prevented from losing energy (drop to ground state)
H2O is split to replace e-  O2 formed

10. Light Reactions Cont’d.
e- passed to Photosystem I via ETC
energy transfer pumps H+ to thylakoid space
ATP produced by photophosphorylation
e- moves from PS 1’s primary electron acceptor to 2nd ETC
NADP+ reduced to NADPH
MAIN IDEA: Use solar E to generate ATP & NADPH to provide E for Calvin cycle

11. Turn & Talk: EXPLAIN what is happening
Reaction Center = Chlorophyll a

12. Turn & Talk: EXPLAIN what is happening

13. Turn & Talk: EXPLAIN what is happening

14. Reaction Center = Chlorophyll b

16. Mechanical analogy for the light reactions

17. Cyclic Electron Flow: uses PS I only; produces ATP for Calvin Cycle (no O2 or NADPH produced)

18. Both respiration and photosynthesis use chemiosmosis to generate ATP

19. Proton motive force generated by:
H+ from water
H+ pumped across by cytochrome
Removal of H+ from stroma when NADP+ is reduced