Photosynthesis Overview

Photosynthesis

• Photosynthesis is a process where simple organic substances like glucose are synthesized from simple molecules (water and carbon dioxide) by green plants.
• This process requires energy, supplied by sunlight.
• The overall process is: 6H₂O + 6CO₂ → C₆H₁₂O₆ + 6O₂
• Photosynthesis has three main steps:
  • Capturing energy from sunlight.
  • Converting light energy into chemical energy (ATP and NADPH).
  • Using the chemical energy to synthesize organic molecules (e.g., carbohydrates) from carbon dioxide.

Chloroplast Structure

• Chloroplasts contain stacks of flattened organelles called thylakoids.
• One stack of thylakoids is called a granum.
• Grana float within a cytoplasm-like fluid in the chloroplast called stroma.

Chlorophyll

• Chlorophyll a and b are present in higher plants.
• Chlorophyll a is the primary pigment for light reactions.
• Chlorophyll b transfers absorbed energy to chlorophyll a.

Photosynthesis Process

• Photosynthesis consists of two sets of reactions:
  • Light-dependent reactions (in the thylakoid membrane).
  • Light-independent reactions (Calvin cycle) in the stroma.
• CO₂ + H₂O + Energy → O₂ + C₆H₁₂O₆

Light-Dependent Reactions

• Photosynthetic membrane (thylakoid membrane) captures light.
• Water molecules are split: 2H₂O → 2H⁺ + ½O₂ + 2e⁻
• Electrons move through electron carriers.
• Energy from electron flow produces ATP and NADPH.

Light-Independent Reactions (Calvin Cycle)

• Occurs in the stroma.
• Three stages:
  • Carbon fixation (CO₂ combines with RuBP, forming 3-PGA).
  • Reduction (3-PGA is reduced to G3P using NADPH and ATP).
  • Regeneration (RuBP is regenerated to prepare for more CO₂ fixation).
• The cycle produces glucose.

Respiration

• Respiration is the breakdown of complex molecules (sugars) to simpler molecules and releases energy in the form of ATP and electron carriers (NADPH and FADH₂).
• Plants use photosynthesis to store light energy in sugars and respiration to break these sugars down, transferring energy into ATP.

Phases of Cellular Respiration

1. Glycolysis (anaerobic respiration)
2. Formation of acetyl Coenzyme A.
3. Krebs cycle (tricarboxylic or citric acid cycle).
4. Oxidative phosphorylation: electron transport and chemiosmosis

• Starts with glucose and ends with ATP (energy).

Glycolysis

• Occurs in the absence of oxygen (anaerobic).
• Splits glucose (C₆H₁₂O₆) into two pyruvate (C₃H₄O₃) molecules.
• Produces four ATP and two NADH.

Formation of Acetyl Coenzyme A

• Pyruvic acid is converted to Acetyl CoA.
• Release of CO₂ and NADH is involved.

The Citric Acid Cycle

• A series of chemical reactions.
• Input is Acetyl-CoA, output is NADH, FADH₂, 2ATP, CO₂

Oxidative Phosphorylation

• Involves electron transport and chemiosmosis.
• Uses energy from electron transport chain to produce ATP through chemiosmosis.

Respiration as a Source of Metabolites

• Respiration is a critical source for various metabolites in the cell.
• Breakdown of molecules provides building blocks for crucial cellular components.

Day vs. Night (Plant Metabolism)

• Daytime: Plants primarily perform photosynthesis, consuming carbon dioxide and releasing oxygen.
• Nighttime: Plants primarily engage in cellular respiration, consuming oxygen and releasing carbon dioxide.

Quiz Questions (Summary)

• Photosynthesis takes place inside the mesophyll of leaves in plants.
• Photosynthesis is the process where plants utilize energy from sunlight to create sugars (glucose).
• Glycolysis is the enzymatic pathway for breaking down sugars into pyruvate molecules in plants.