Photosynthesis Notes

Overview of Photosynthesis

• Definition: Photosynthesis is the process by which plants, some bacteria, and protistans utilize sunlight energy to convert carbon dioxide and water into glucose and oxygen.

• Word Equation:

  • Carbon Dioxide + Water ➞ Glucose + Oxygen

• Chlorophyll: The green pigment crucial for capturing sunlight.

  • Contains a lipid-soluble hydrocarbon tail and a hydrophilic head with a magnesium ion.

  • Accessory pigments (chlorophyll b, xanthophylls, carotenoids) assist by absorbing different light wavelengths.

  • Chlorophyll a primarily absorbs violet-blue and reddish-orange light, with minimal absorption in the green spectrum.

Leaf Structure and Function

• Function: Leaves act as solar energy collectors filled with photosynthetic cells.

• Photosynthesis Process:

  • Water from roots travels to leaves via xylem vessels.

  • Carbon dioxide enters and products (sugar and oxygen) exit through stomata, protected by guard cells.

  • Water Loss: High transpiration rates, e.g., cottonwood trees lose 100 gallons/hour in hot conditions.

Chloroplast Structure

• Thylakoids: Stackable discs (grana) where photosynthesis occurs; surrounded by stroma in eukaryotic chloroplasts.

• Membranes: Three membranes in chloroplasts, while mitochondria have two.

Stages of Photosynthesis

1. Light-dependent Reactions:

   • Occur in grana.

   • Chlorophyll absorbs light, exciting electrons and causing photoionization.

   • Splitting of water (photolysis) into O2, H+, and electrons.

   • Generates ATP and NADPH through:

     • Photophosphorylation: ATP formation.

     • Reduction of NADP+ to NADPH.

2. Light-independent Reactions (Calvin Cycle):

   • Occur in the stroma using ATP and NADPH.

   • Carbon dioxide is fixed into organic molecules (RuBP ➞ GP ➞ GALP).

   • One GALP is converted into glucose, while others regenerate RuBP.

Light-dependent Reactions Detailed

• Photoionnization: Excited electrons transferred to an electron acceptor, leading to water splitting.

• Photosystems:

  • PSII (P680): Absorbs light first, energizing electrons.

  • PSI (P700): Follows and also adds energy to electrons.

  • Z Scheme: Energy flow drawn in a Z shape, representing energy transitions through photosystems and the electron transport chain.

ATP Synthesis

• From ADP:

  • Phosphorylation occurs via condensation reaction, forming ATP.

• Chemiosmosis:

  • H+ ions pumped into the thylakoid lumen create a gradient.

  • Diffusion of H+ back to the stroma drives ATP synthesis.

Cyclic vs Non-cyclic Phosphorylation

• Non-cyclic: Produces both ATP and NADPH; involves both PSII and PSI.

• Cyclic: Only involves PSI, generates ATP by recycling electrons without forming NADPH.

Calvin Cycle (Light-independent)

• Carbon Fixation: Carbon dioxide combines with RuBP forming unstable six-carbon sugar, which splits into GP.

• Reduction Phase: GP is phosphorylated and reduced to GALP using ATP and NADPH.

• Regeneration of RuBP: Some GALP is converted to glucose; others regenerate RuBP allowing the cycle to continue.

Factors Affecting Photosynthesis

1. Light Intensity: Increases photosynthesis rate up to a saturation point.

2. Carbon Dioxide Concentration: Higher CO2 levels enhance photosynthesis until limited by other factors.

3. Temperature: Enzyme catalysis rate for reactions—optimum temperature enhances rates, but extreme heat declines efficacy.

Summary of Key Processes

• Photosynthesis consists of two main stages: light-dependent (producing ATP and NADPH) and light-independent (Calvin Cycle transforming fixed carbon into glucose).

• The entire process is vital for energy conversion from sunlight into chemical energy, fueling various life processes.