Calvin Cycle and Light Reactions in Photosynthesis

Overview of the light reactions of photosynthesis.
The reactions can occur through two paths:
- Linear Electron Flow
- Cyclic Electron Flow

There are two types of photosystems involved:
- Photosystem II (PSII): Represented as the beige mass in the membrane.
- Photosystem I (PSI): Represented as the pink mass in the membrane.
Composed of various proteins and pigment molecules essential for energy transformations.
Light energy captured by pigment molecules leads to:
- Excitation of electrons to a higher energy state.
- Resulting in their oxidation in PSII.

After oxidation in PSII, electrons flow through an electron transport system:
- Different from mitochondrial electron transport systems.
- Components include plastoquinone (PQ) and ferredoxin (Fd).
- Similar strategies but distinct machinery compared to mitochondria.

NADP+ Reduction
- Electrons ultimately reduce NADP+ into NADPH:
- Electrons are transferred to NADP+ along with protons (H+).
- Generates NADPH used in later stages of photosynthesis.

Energy from electron flow creates a proton (H+) gradient across the thylakoid membrane:
- High concentration of H+ in the thylakoid lumen.
- ATP synthase enzyme facilitates the conversion of ADP + P_i into ATP as H+ ions flow through it.

Oxygen (O2) is generated through the oxidation of water in PSII:
- Water (H2O) donates electrons to replenish PSII.
- This oxidation reaction also produces oxygen gas.

An alternative path to generate ATP only:
- Does not produce NADPH or oxygen.
- Involves only PSI and recycles electrons back.
Enhances ATP production when ATP is low relative to NADPH.
Helps maintain balance for Calvin Cycle requirements.

The Calvin Cycle relies on products from the light reactions:
- ATP: 18 molecules needed for complete carbon transformation.
- NADPH: 12 molecules needed for converting carbon into sugars.
Cycle generates a three-carbon carbohydrate, Glyceraldehyde-3-Phosphate (G3P)

Carbon Fixation:
- Carbon dioxide is captured and converted into a stable molecule by the enzyme RuBisCO.
- CO2 binds to Ribulose bisphosphate (RuBP), forming a six-carbon intermediate that quickly splits into two three-carbon molecules.
Reduction and Carbohydrate Production:
- Carbohydrates are formed through the reduction of 3-phosphoglycerate (3-PGA) using NADPH and ATP.
- Generates G3P, some of which exits the cycle for glucose synthesis or other organic molecules.
Regeneration of RuBP:
- Converts remaining G3P back into RuBP, allowing the cycle to continue.
- This also requires the use of ATP.

The Calvin Cycle was elucidated through extensive experiments, including:
- Using carbon-14 labelled CO2 to trace carbon atoms.
- Chromatography methods to identify molecules formed during various time intervals.

C3 Plants: Majority of plants that conduct the Calvin Cycle as described.
C4 Plants:
- Initially capture CO2 into a four-carbon compound.
- Utilizes a different enzyme that preferentially binds CO2 to avoid photorespiration in hot/dry climates.
CAM Plants:
- Separate carbon fixation and the Calvin cycle temporally (day vs night).
- Stomata open at night to capture CO2, closed during the day to reduce water loss.

Occurs when RuBisCO binds O2 instead of CO2, often under stress conditions:
- Results in a loss of previously fixed carbon and reduced photosynthetic efficiency.

Plant adaptation strategies are crucial for survival in varying environments, balancing water conservation with photosynthetic efficiency.
Different strategies are employed based on environmental conditions.