Chapter 8 pt.2 (March 11, 2025)

Photosynthesis Overview

• Photosynthesis involves the conversion of light energy to chemical energy through a series of reactions occurring in chloroplasts.

High Energy Bonds and Water Splitting

• High energy bonds are formed during the electron transport chain.

• Water splitting complex releases one molecule of oxygen; the electrons produced move through redox reactions.

• These electrons travel from strong electron donors to stronger electron acceptors, eventually filling the reaction center in Photosystem I.

• A carbon source (CO2) is required for the reduction to sugars.

Cyclic vs. Linear Electron Flow

• Summary of cyclic electron flow in Photosystem I was covered.

• Linear electron flow involves both photosystems, producing ATP and NADPH. Cyclic flow does not involve Photosystem II.

• In cyclic flow, electrons cycle through the cytochrome b6 complex, helping produce a proton gradient that drives ATP synthesis.

Photosynthetic Structures

• Photosystems and ATP synthase are spatially separated within thylakoid membranes.

• Photosystem I can use lower energy electrons than Photosystem II.

• This segregation prevents inefficiencies in electron harvesting.

The Calvin Cycle

Phase 1: Carbon Fixation

• Carbon is captured from CO2 as it diffuses into the stroma through the stomata.

• Enzyme Ribulose bisphosphate carboxylase/oxygenase (Rubisco) facilitates the reaction.

• Three CO2 molecules bind to three ribulose biphosphate (RuBP), leading to the formation of 18 carbons (3 x 6 carbon molecules).

Phase 2: Reduction Phase

• Six ATP and six NADPH are used to convert intermediates into G3P (glyceraldehyde-3-phosphate).

• The products of the cycle are reduced sugars, with three turns of the Calvin cycle producing one molecule of G3P.

Phase 3: Regeneration Phase

• The remaining five G3P molecules regenerate RuBP to allow the Calvin cycle to continue.

• Requires more ATP and NADPH to complete the cycle.

Enzyme Efficiency

• The turnover number for Rubisco is low, producing fewer products per second than other enzymes.

• To compensate, plants produce large quantities of Rubisco, making it the most abundant enzyme on Earth.

Water and Stomatal Function

• On hot, dry days, plants close stomata to minimize water loss, impacting CO2 uptake and oxygen release.

• This can lead to increased oxygen concentrations and carbon loss.

Adaptations in Plants

C4 Plants

• C4 plants have evolved to minimize photrespiration by spatially segregating CO2 capture and Calvin cycle processes in different cell types.

• CO2 is initially captured and converted into a four-carbon compound before it is transported to bundle sheath cells to proceed with carbon fixation.

CAM Plants

• CAM plants separate CO2 capture and fixation temporally, capturing CO2 at night to reduce water loss.

• During the day, they utilize the stored CO2 for the Calvin Cycle while keeping stomata closed.

Conclusion

• The intricate mechanisms of photosynthesis highlight the interplay of light energy conversion, carbon fixation, and adaptations in plants to optimize efficiency under varying environmental conditions.