1. Photosynthesis Overview

• Definition: Photosynthesis is the process where plants, some bacteria, and protistans convert sunlight into chemical energy in the form of glucose, using carbon dioxide (CO2) and water (H2O).

• Overall Equation:

  • CO2 + H2O → C6H12O6 + O2

• Chlorophyll:

  • Main pigment involved, allowing plants to absorb sunlight.

  • Variants include chlorophyll a (the primary pigment) and accessory pigments like chlorophyll b, carotenoids, xanthophylls.

• Absorption Spectrum:

  • Chlorophyll a absorbs violet-blue and reddish-orange light, reflecting green light.

2. Leaf Structure and Function

• Leaves: Act as solar collectors filled with photosynthetic cells.

• Gas Exchange:

  • Stomata: Small openings on the leaf surface for CO2 intake and O2 release.

  • Guard Cells: Regulate the opening and closing of stomata to control water loss and gas exchange.

• Water Transport:

  • Water absorbed by roots travels to leaves through xylem vessels.

• Photosynthesis Process: Raw materials enter cells, products (sugar, oxygen) exit.

3. Chloroplast Structure

• Thylakoids:

  • Disk-like structures where light-dependent reactions occur, organized into stacks (grana).

• Stroma: Fluid-filled space surrounding grana, where light-independent reactions take place.

• Membranes: Chloroplasts have three membranes creating distinct compartments for photosynthesis.

4. Stages of Photosynthesis

4.1. Light-Dependent Reactions

• Occur in the thylakoid membranes.

• Key Processes:

  • Photoactivation: Light energy excites electrons in chlorophyll.

  • Photolysis: Water splitting releases electrons, protons, and oxygen.

  • ATP & NADPH Production: Excited electrons produce ATP (via photophosphorylation) and reduce NADP+ to NADPH.

4.2. Light-Independent Reactions (Calvin Cycle)

• Occur in the stroma, use ATP and NADPH to convert CO2 into carbohydrates.

• Carbon Fixation:

  • CO2 combines with ribulose bisphosphate (RuBP) to form 3-phosphoglycerate (3-PGA).

  • ATP and NADPH convert 3-PGA into glyceraldehyde 3-phosphate (G3P).

• G3P Products:

  • 1 G3P is used to form glucose, other carbohydrates, or lipids.

  • 1 G3P is converted back to RuBP to continue the cycle.

5. Non-Cyclic vs. Cyclic Phosphorylation

5.1. Non-Cyclic Phosphorylation (Z Scheme)

• Involves both Photosystem I (PSI) and Photosystem II (PSII).

• Excited electrons from PSII move through the electron transport chain, aiding ATP synthesis and reducing NADP+ to NADPH.

5.2. Cyclic Phosphorylation

• Involves only PSI when additional ATP is required.

• Produces ATP without generating NADPH by cycling electrons back to PSI.

6. Chemiosmosis and ATP Synthesis

• Mechanism: H+ ions pumped across the thylakoid membrane establish a proton gradient.

• H+ ions diffuse back into the stroma through ATP synthase, generating ATP in the process.

7. Factors Affecting Photosynthesis

7.1. Light Intensity

• Higher light intensity increases the rate of photosynthesis until limited by another factor.

• Different photosystems have optimal light absorption wavelengths (PSI: 700 nm, PSII: 680 nm).

7.2. Carbon Dioxide Concentration

• Increased CO2 concentration boosts the light-independent reaction rate until limited by other factors.

7.3. Temperature

• Photosynthesis is enzyme-catalyzed; rates increase with temperature until an optimum is reached, after which enzymatic activity declines.