Two Sets of Reactions:

1. Light Dependent Reactions

   • Occur in the thylakoid membrane

   • Require light

   • Produce ATP and NADPH

2. Calvin Cycle (Light Independent)

   • Occurs in the stroma

   • Process where enzymes reduce CO₂ into carbohydrates

   • Requires ATP, NADPH, and CO₂

Key Terms:

• NADP+: Molecule that accepts and delivers 2 electrons and 1 hydrogen ion (H⁺).

• Reduction: The process of gaining electrons.

• Oxidation: The process of losing electrons.

ATP Cycle:

• ATP (Adenosine Triphosphate): High energy molecule with 3 phosphate groups.

• ADP (Adenosine Diphosphate): Lower energy molecule with 2 phosphate groups.

Light Dependent Reactions:

1. Water Splitting (Photolysis)

   • Water is split at PSII, donating electrons to PSII.

   • Oxygen atoms are released as O₂.

   • Hydrogen ions (H⁺) build up in concentration in the lumen.

2. Light Absorption

   • Light is absorbed by pigments in PSII.

   • Energy transfers to electrons in the reaction center, energizing them.

3. Electron Transport Chain (ETC)

   • Electrons move down the ETC.

4. Photosystem I (PSI)

   • PSI accepts electrons from the ETC.

   • Light is absorbed by pigments, reenergizing the electrons.

Electron Transport Chain: a series of protein complexes and other molecules that transfer electrons through a membrane within mitochondria (in cellular respiration) or thylakoids (in photosynthesis). This process is critical for generating ATP, the energy currency of the cell.

Photosynthesis Notes

1. NADP Reductase:

   • Electrons pass through the membrane protein called NADP reductase.

2. Lumen Function:

   • The lumen acts as a hydrogen ion reservoir.

   • Hydrogen ions (H⁺) move through ATP synthase.

3. ATP Synthase:

   • ATP synthase catalyzes the bonding of ADP and an inorganic phosphate (Pi) to produce ATP.

   • Chemiosmosis: Movement of ions down a chemical gradient.

4. Final Products:

   • The main products of the light-dependent reactions are ATP and NADPH.

5. RuBisCO:

   • RuBisCO is the enzyme that fixes carbon into ribulose bisphosphate (RuBP).

6. Carbon Reduction:

   • ATP and NADPH, produced from the light-dependent reactions, are used in the Calvin Cycle.

   • 3PGA (3-phosphoglycerate) undergoes reduction to form PGAL (phosphoglyceraldehyde) and G3P (glyceraldehyde-3-phosphate). These are essentially the same molecule.

7. Organic Molecule Production:

   • The 3PGA molecules are starting materials for many organic molecules.

   • They help produce starch, cellulose, sucrose, and plant oils.

   • Two G3P molecules combine to form one glucose molecule.

Photorespiration and Calvin Cycle Notes

1. Photorespiration:

   • Definition: Occurs when Rubisco fixes O₂ instead of CO₂, leading to a loss of CO₂.

   • Conditions: More likely to happen when CO₂ levels are low and O₂ levels are high.

   • Impact on Plants: Not beneficial, as it reduces the efficiency of photosynthesis and wastes energy.

2. Key Molecules:

   • RUBP: Ribulose bisphosphate, the substrate for Rubisco.

   • 3-PGA: 3-Phosphoglycerate, a product formed in the Calvin Cycle.

   • Triose Phosphate: Includes molecules like G3P (Glyceraldehyde-3-phosphate).

   • 2-PG: 2-Phosphoglycolate, a byproduct of photorespiration.

3. Calvin Cycle Overview:

   • Main Steps:

     • CO₂ is fixed by Rubisco into RUBP.

     • Forms 3-PGA, which is then converted into triose phosphates using ATP and NADPH.

   • Outputs: Sugars and other organic molecules.

4. Evolutionary Context:

   • Photorespiration evolved when atmospheric O₂ levels were lower and did not significantly harm plants.

   • As O₂ levels increased, the inefficiency of photorespiration became more problematic for plants.

   • Cyanobacteria evolved processes that produced sugars and oxygen, contributing to the rise in atmospheric O₂ levels.

5. Significance of Oxygen Increase:

   • The increase in O₂ allowed for the evolution of complex life forms and aerobic organisms.

   • Anaerobic life forms declined as a result of rising oxygen levels.