(lecture) photosynthesis

photosynthesis overview

1. photon absorbed

2. energy transferred through antenna complex

3. reaction center chlorophyll excited

4. electron passed to primary acceptor

5. electron enters transport chain

electron flow types

Photosystems:

• photosystem II (PSII): contains P680

• photosystem I (PSI): contains P700

Linear flow: H₂O→PSII→ETC→PSI→NADP+

• produces ATP + NADPH + O₂

Cyclic flow: PSI→ETC (cycle)

• produces ATP only

Photophosphorylation

ATP synthesis driven by light energized electron transport

Electron transport builds H+ gradient

ATP synthase converts proton motive force to ATP

three phases of the calvin cycle

1. carbon fixation: CO₂ + RuBP → 3PGA

2. reduction (per molecule of 3PGA): 3PGA + ATP + NADPH → G3P + ADP + NADP⁺

3. regeneration: (per 3 turns): 5G3P + 3ATP→ 3RuBP + 3ADP

key enzyme in Rubisco

catalyzes CO₂ fixation in the calvin cycle

binds O₂ leading to photorespiration

enzyme activity is stimulated by ATP and NADPH produced by light reactions

• central enzyme to global food production

photorespiration

rubisco enzyme can bind CO₂ or O₂

• CO₂→carbon fixation

• O₂→photorespiration

strategies to reduce photorespiration

• C3: rubisco fixes CO₂ directly in mesophyll cells

• C4: CO₂ is released in bundle sheath cells

• CAM: CO₂ is stored as organic acids