Photosynthesis in C3 Plants: Light-Dependent and Light-Independent Stages

1. Overview of Photosynthesis

• Photosynthesis: Process by which plants (autotrophs) convert light energy into chemical energy in the form of sugars (e.g., glucose).
• Overall Equation:
  $$6CO_2 + 6H_2O + \text{Light Energy} \rightarrow C_6H_{12}O_6 + 6O_2$$
• Location: Chloroplasts (organelles within plant cells).

KEY TAKEAWAY: Photosynthesis transforms light energy into chemical energy stored in glucose.

2. Chloroplast Structure

• Outer and Inner Membranes: Create a double membrane around the chloroplast.
• Stroma: Fluid-filled space inside the chloroplast, surrounding the thylakoids.
• Thylakoids: Flattened, sac-like membranes arranged in stacks called grana.
• Grana (singular: granum): Stacks of thylakoids.
• Thylakoid Lumen: Space inside the thylakoid.

3. Stages of Photosynthesis

3.1. Light-Dependent Stage (Light Reactions)

• Location: Thylakoid membranes of the chloroplast.
• Inputs:
  • Water ($H_2O$)
  • Light energy
  • ADP (Adenosine Diphosphate)
  • $P_i$ (Inorganic Phosphate)
  • NADP$^+$ (Nicotinamide Adenine Dinucleotide Phosphate)
• Outputs:
  • Oxygen ($O_2$)
  • ATP (Adenosine Triphosphate)
  • NADPH (Nicotinamide Adenine Dinucleotide Phosphate + H$^+$)
• Process:
  1. Light energy is absorbed by chlorophyll and other pigments in the thylakoid membranes.
  2. Water molecules are split (photolysis), releasing electrons, protons ($H^+$), and oxygen. Oxygen is released as a byproduct.
  3. Electrons are passed along an electron transport chain (ETC), releasing energy that is used to pump protons into the thylakoid lumen, creating a proton gradient.
  4. Protons flow down the concentration gradient through ATP synthase, driving the synthesis of ATP (photophosphorylation).
  5. Electrons and protons are used to reduce NADP$^+$ to NADPH.

3.2. Light-Independent Stage (Calvin Cycle)

• Also known as: Carbon Fixation, Dark Reactions
• Location: Stroma of the chloroplast.
• Inputs:
  • Carbon Dioxide ($CO_2$)
  • ATP
  • NADPH
  • RuBP (Ribulose-1,5-bisphosphate)
• Outputs:
  • Glucose (or other sugars/carbohydrates)
  • ADP
  • $P_i$ (Inorganic Phosphate)
  • NADP$^+$
  • RuBP (Ribulose-1,5-bisphosphate) - regenerated
• Process:
  1. Carbon Fixation: $CO_2$ is incorporated into an organic molecule, RuBP, with the help of the enzyme RuBisCO (Ribulose-1,5-bisphosphate carboxylase/oxygenase).
  2. Reduction: The resulting molecule is reduced using ATP and NADPH, producing a three-carbon sugar (G3P or Glyceraldehyde-3-phosphate).
  3. Regeneration: RuBP is regenerated from G3P, allowing the cycle to continue. Glucose (or other carbohydrates) are produced from G3P.

COMMON MISTAKE: Light is not an input molecule of the Calvin Cycle. ATP and NADPH, produced in the light-dependent stage, are the energy sources for the Calvin Cycle.

4. Summary Table

EXAM TIP: Understand the specific inputs and outputs of each stage. VCAA often tests this directly. Use the table above as a study aid.

5. C3 Plants

• C3 Photosynthesis: The most common pathway for carbon fixation. The first stable compound formed in the Calvin cycle is a 3-carbon molecule (3-phosphoglycerate).
• RuBisCO: The enzyme responsible for carbon fixation in C3 plants. It can also bind to $O_2$ in a process called photorespiration, which reduces the efficiency of photosynthesis, especially in hot, dry conditions.

VCAA FOCUS: VCAA frequently asks about the inputs, outputs, and locations, so make sure you know them well. They also like to test your knowledge of the role of water.