Photosynthesis: Light-Dependent and Light-Independent Reactions

Objectives:
- Distinguish between the two stages of photosynthesis: light-dependent and light-independent reactions.
- Describe the energy transformations occurring during photosynthesis.

The thylakoid membrane is pivotal for light-dependent reactions, contributing to the green pigment in leaves due to chlorophyll.
- Forms stacked structures known as grana (singular: granum).
- The internal compartment of the thylakoid is designated as the thylakoid lumen.

Three main steps occur:
    1. Excitation of Photosystems by Light Energy
    2. Production of ATP via an Electron Transport Chain
    3. Reduction of NADP+ and Photolysis of Water

Photosystems: Composed of pigment groups located in the thylakoid membrane.
    - Named for their maximum light absorption wavelengths:
        - Photosystem I (PS I): Absorbs at 700 nm.
        - Photosystem II (PS II): Absorbs at 680 nm.
When light energy is absorbed, it energizes electrons in the pigments, causing them to become excited and transfer to carrier molecules in the thylakoid membrane, moving towards the thylakoid stroma.

The process involves several key actions:
    1. Excited electrons are relayed from PS II (P680 molecule-pair) to an electron transport chain in the thylakoid membrane.
    2. As electrons are passed along the chain, energy is released and utilized to pump H+ ions into the thylakoid lumen, creating an electrochemical gradient.
    3. H+ ions migrate back to the stroma through ATP synthase via chemiosmosis, triggering ATP synthesis—a process known as photophosphorylation.
    4. De-energised electrons are transferred from PS II to PS I.

The b6-f complex aids in pumping protons across the thylakoid membrane, contributing to the hydrogen ion concentration gradient.

Electrons from PS I combine with a carrier molecule, reducing NADP+ to form NADPH (essential for light-independent reactions).
    - Water molecules are split (photolysis), yielding:
        - H+ ions (used for chemiosmosis).
        - O2 (released as a by-product).
Electrons in PS I are replenished by those from PS II, emphasizing the connection between both photosystems.

Occur in the stroma of chloroplasts, utilizing energy from light-dependent reactions to produce organic molecules.

Carbon Fixation:
- Involves the enzyme RuBP carboxylase oxygenase (Rubisco) attaching CO2 to ribulose bisphosphate (RuBP), resulting in an unstable six-carbon compound that splits into two molecules of 3-phosphoglycerate (3-PGA).
Reduction of Glycerate-3-Phosphate:
- 3-PGA is converted to glyceraldehyde 3-phosphate (G3P) using NADPH and ATP.
- Out of 6 G3P produced, only 1 can be utilized for biomass while the rest continue in the cycle.
Regeneration of Ribulose Bisphosphate (RuBP):
- Remaining 5 G3P molecules convert back to regenerate RuBP, requiring energy from ATP.
- It emphasizes that 6 cycles are necessary to synthesize one molecule of glucose from 3 CO2 input, as only 2 out of 12 G3P molecules contribute to glucose production.

$6 CO_2 + 18 ATP + 12 NADPH + H_2O <br>ightarrow 2 G3P + 16 P_i + 18 ADP + 12 NADP^+$

Rubisco: Most abundant protein on Earth, catalyzing the first reaction of the Calvin cycle.
- It plays a pivotal role in converting 100 billion tonnes of CO2 into carbohydrates annually.
- Constitutes over 50% of the total protein found in plant leaves.

Links light-dependent reactions and Calvin Cycle, with an emphasis on the interchange of reactants and products essential for energetic and metabolic processes in cells.