Photosynthesis and Exam Preparation Notes

Photosynthesis Overview

• Photosystems Involved:

  • Photosystem II and Photosystem I.

Photosystem II (PSII)

1. Photon Interaction: A photon hits a pigment, exciting P680.

2. Electron Transfer: Excited electron is transferred to the primary electron acceptor (pheophytin).

3. P680 as Oxidizing Agent: P680 needs a replacement electron, which it retrieves from H₂O.

4. Water Splitting: H₂O splits, electrons go to P680, and O₂ is released as a by-product.

Electron Transport Chain (ETC)

• ETC Steps from PSII to PSI:

  1. Electrons fall through the chain (Pheophytin ➔ Plastoquinone ➔ Cytochrome Complex ➔ Plastocyanin).

  2. Energy from electron movement drives proton gradient formation across the thylakoid membrane.

  3. H+ diffusion across the membrane synthesizes ATP.

Photosystem I (PSI)

1. Light Energy: Light excites P700, which loses an electron to a primary electron acceptor.

2. Electron Replacement: P700+ accepts an electron from PSII via the ETC, is re-energized and passed downstream.

ATP Synthase and Chemiosmosis

• Function of ATP Synthase: Converts ADP and inorganic phosphate (Pi) to ATP using the energy from H+ ions' kinetic energy.

Second Electron Transport Chain

1. From PS I's primary acceptor (iron-sulfur proteins) to ferredoxin (Fd).

2. Electrons reduce NADP+ to NADPH via NADP+ reductase, to be used in the Calvin cycle.

Cyclic Photophosphorylation

• When Occurs: Under conditions of high NADPH or ATP demand, to protect against excess light.

• Output: Produces ATP only, no O₂ or NADPH.

Cooperation of Light Reactions with the Calvin Cycle

• Light Reactions provide ATP and NADPH needed for the Calvin Cycle, which synthesizes glucose from CO₂.

The Calvin Cycle

• Goal: Use CO₂, NADPH, and ATP to produce glucose.

• Phases:

  1. Carbon Fixation: Rubisco catalyzes the reaction between CO₂ and RuBP.

  2. Reduction Phase: ATP and NADPH convert 3-PGA into G3P.

  3. Regeneration Phase: G3P is recycled to regenerate RuBP, and ATP is consumed.

Detailed Steps of the Calvin Cycle

1. Carbon Fixation: Conversion of CO₂ to organic molecules.

2. Reduction:

   • ATP and NADPH convert 6 molecules of 3-PGA to G3P.

   • ATP converts to ADP; NADPH oxidizes to NADP+.

3. Regeneration: Five G3P molecules recycled to regenerate RuBP; requires ATP.

Summary of key reactions

• G3P Production: One G3P exits the cycle to form glucose and other sugars.

• Three Turns Needed: Each G3P requires three turns of the Calvin cycle.

Photorespiration

• Process: Rubisco may fix O₂ instead of CO₂, complicating carbon fixation.

• Conditions Favoring Photorespiration: High temperature, low CO₂/O₂ ratio (e.g., during stomatal closure).

C3 Plants

• Characteristics: Primary photosynthesis type for ~85% of plants (e.g., rice, wheat).

• Adaptation: Requires moderate temperatures and decent water.

C4 Plants

• Efficiency: Separates CO₂ fixation from Calvin cycle to reduce photorespiration.

• Examples: Corn, sugarcane.

CAM Plants

• Adaptation: Open stomata at night to minimize water loss; perform photosynthesis during the day.

• Examples: Cacti, jade plants.

Winter Adaptations in Deciduous Trees

• Water Loss Management: Trees lose leaves to prevent frost damage.

• Sap Production: Remaining water becomes thick and sugary, harvested as sap.

Conclusion

• Key Questions:

  1. How many times does the Calvin cycle run for one glucose molecule? (6 times)

  2. What is photorespiration? (A less efficient process where Rubisco uses O₂ instead of CO₂).