light-dependent reactions

Q: What are light-dependent reactions?

A: They are reactions where light is needed to harvest energy.

Q: Which photosystems are used by photoautotrophs?

A: Both Photosystem I and Photosystem II are light-capturing complexes used by photoautotrophs.

Q: What happens first in Photosystem II?

A: A photon of light strikes the antenna complex in Photosystem II.

Absorbed by P680

P680 becomes “excited” and oxidized

• Electron jumps to a higher energy level → becomes P680+

• Electron accepted by primary electron acceptor

• P680+ is now positively charged

Photosystem II Step-2

• P680+ is electronegative → pulls an electron from water

  • Water is oxidized and splits

• Primary acceptor transfers electron to Plastoquinone (PQ)

  • PQ moves it to cytochrome complex

• PQ also accepts protons from stroma and releases them into lumen

Q: Photosystem II – Step 3

• Electrons picked up by plastocyanin (electron carrier)

• Electrons transferred to Photosystem I

• Buildup of protons in lumen → proton gradient forms

Q: Photosystem I – Step 4

• Photon of light strikes antenna complex in Photosystem I

  • Absorbed by P700

• P700 gets “excited” and oxidized

  • Transfers electron to primary electron acceptor → becomes P700+

• P700+ can act as electron acceptor or be reduced back to P700

Q: Photosystem I – Step 5

• Electron carrier ferredoxin (iron-sulfur protein) transports electrons

• Oxidizing ferredoxin causes NADP+ → NADP reduction

Q: Photosystem I – Step 6

A:

• NADP receives another electron and proton from ferredoxin →

  • forms NADPH

  • H+ moves into lumen

• Proton gradient used to produce ATP

Chemiosmosis Q: Where is the proton gradient created?

A: In the thylakoid membrane.

Q: What are the three mechanisms that occur?

1) Protons move into the lumen by PQ.
2⃣ Proton concentration increases by 2 for every water molecule split in the lumen.
3⃣ Removal of one proton from NADPH formation decreases proton concentration in the stroma.

Q: What happens after this?

A: Protons move through ATP synthase to produce ATP.

Q: What is this process identical to?

A: Cellular respiration in humans.

Q: What do both cellular respiration and photosynthesis have in common?

A: Electrons are moved along a chain driven by a proton gradient.

Q: What is the energy difference between them?

• Cellular respiration: Starts with high energy (NADH) → ends with low energy (H₂O).

• Photosynthesis: Starts with low energy (H₂O) → ends with high energy (NADPH).

Q: Why are two photosystems needed in photosynthesis?

A: To establish a proton gradient.

Q: How many water molecules must be oxidized to generate 4 electrons?

A: Two.

Q: How many photons are needed to move one electron down the chain?

A: Two photons.

Q: How many photons are absorbed for 4 electrons?

A: Eight photons of light.

Q: Can Photosystem I work independently from Photosystem II?

A: Yes, through cyclic electron transport.

Q: What happens in cyclic electron transport?

A: Reduced ferredoxin donates electrons to PQ.

Q: What does PQ do?

A: It is constantly reduced and oxidized, moving protons into the lumen.

Q: What does this create?

A: A strong proton gradient.

Q: Is water oxidation or NADP+ reduction needed?

A: No.

Q: Why is this method used?

A: It is more effective.