Question

1. why are both cyclic and non - cyclic pathways needed for the calvin cycle (light - independent reaction) to operate?
2. based on your drawing from the previous page, what is the source of replacement electrons for those released from photosystem i?
3. based on your drawing from the previous page, what is the source of replacement electrons for those released from photosystem ii?
4. based on your drawing, is carbon dioxide involved in the light - dependent reaction?
5. briefly explain the mechanism by which atp is synthesized during the light - dependent reaction (hint: it is very similar to the way atp is synthesized during cellular respiration).

Explanation:

1. The cyclic pathway mainly generates ATP, while the non - cyclic pathway provides both ATP and NADPH. The Calvin cycle needs both ATP for energy and NADPH for reducing power, so both are required.
2. The replacement electrons for those released from photosystem I come from photosystem II via the electron - transport chain.
3. The source of replacement electrons for those released from photosystem II is water. Water is split during the light - dependent reactions, releasing electrons, protons, and oxygen.
4. Carbon dioxide is not involved in the light - dependent reaction. The light - dependent reactions focus on capturing light energy, generating ATP and NADPH, and splitting water. Carbon dioxide is used in the Calvin cycle (light - independent reaction).
5. During the light - dependent reaction, a proton gradient is established across the thylakoid membrane. Protons flow back through ATP synthase (chemiosmosis), and the energy from this flow is used to phosphorylate ADP to ATP, similar to the process in cellular respiration where a proton gradient across the inner mitochondrial membrane drives ATP synthesis.

Answer:

1. The cyclic pathway generates ATP and the non - cyclic pathway provides ATP and NADPH, both of which are needed for the Calvin cycle.
2. Photosystem II via the electron - transport chain.
3. Water.
4. No.
5. A proton gradient is established across the thylakoid membrane, and protons flow through ATP synthase (chemiosmosis) to phosphorylate ADP to ATP.