VH

Photosynthesis Notes

Photosynthesis

Photosynthesis Overview

  • Photosynthesis consists of two main stages:
    • Light-dependent reactions (photo).
    • Light-independent reactions (synthesis).
  • Oxygen is a byproduct of photosynthesis.
  • Overall process:
    • Carbon dioxide (CO2) + Water (H2O) + Light energy → Glucose (C6H{12}O6) + Oxygen (O2)
  • Location: Chloroplasts
    • Light-dependent reactions occur in the thylakoid membranes.
    • Light-independent reactions (Calvin Cycle) occur in the stroma.

Redox Reactions in Photosynthesis

  • Photosynthesis involves the transfer of energy using electrons (e^-).
  • Electrons are passed between molecules in a process called a redox reaction.
  • Redox Reaction: A chemical reaction involving the transfer of electrons.
  • General Equation: Xe^- + Y → X + Ye^-
  • LEO says GER: A mnemonic to remember redox reactions.
    • LEO: Loss of Electrons is Oxidation (reducing agent).
    • GER: Gain of Electrons is Reduction (oxidizing agent).

Light-Dependent Reactions

  • Occur in: Thylakoid membranes.
  • Stage 1: Capturing solar energy and transferring it to electrons through the splitting of water (H_2O).
  • Stage 2: Using energy to make ATP and transferring electrons to make NADPH.
  • Process: Light energy is absorbed by chlorophyll molecules, generating ATP and NADPH.
  • Key components:
    • Photosystem II (PSII):
      • Absorbs light energy.
      • Splits water molecules (H2O) into oxygen (O2), protons (H^+), and electrons (e^-).
    • Electron Transport Chain:
      • Transports electrons from PSII to Photosystem I (PSI).
      • Generates a proton gradient across the thylakoid membrane.
    • Photosystem I (PSI):
      • Absorbs light energy.
      • Transfers electrons to NADP+, forming NADPH.
    • ATP Synthase:
      • Uses the proton gradient to synthesize ATP from ADP and inorganic phosphate (P_i).
      • This process is called chemiosmosis.
  • Products: ATP, NADPH, and Oxygen (O_2).

Light-Independent Reactions (Calvin Cycle)

  • Occur in: Stroma.
  • Stage 3: Using energy in ATP and high-energy electrons in NADPH to form glucose from carbon dioxide (CO_2).
  • Three Phases:
    • Carbon Fixation (Carboxylation):
      • Carbon dioxide (CO_2) is captured from the atmosphere and enters the stroma through the stomata on leaves
      • Each carbon dioxide molecule is chemically bonded to a 5-carbon molecule called RuBP (ribulose bisphosphate).
      • This reaction is catalyzed by the enzyme RuBisCo (RuBP carboxylase).
      • The resulting 6-carbon compound is unstable and immediately breaks down into two molecules of 3-phosphoglycerate (3-PGA).
      • Each 3-PGA molecule contains 3 carbons.
    • Activation & Reduction of 3-PGA:
      • The 3-carbon compounds (3-PGA) are in a low-energy state and must be converted to a higher-energy state.
      • Each 3-PGA molecule receives a phosphate group from ATP, forming 1,3-bisphosphoglycerate (1,3-BPG).
      • A pair of electrons from NADPH reduces each 1,3-BPG to glyceraldehyde-3-phosphate (G3P), also known as PGAL.
      • For every three molecules of CO_2 that enter the cycle, six molecules of G3P are produced.
      • One G3P molecule exits the cycle and is used to form glucose and other high-energy molecules.
      • Two G3P molecules combine to form one glucose molecule.
    • Regeneration of RuBP:
      • Most of the G3P molecules are used to regenerate RuBP (the CO_2 acceptor molecule).
      • ATP is required to break and reform chemical bonds in this process.
  • For the net synthesis of one G3P molecule (3 carbon molecule), the cycle must take place three times, fixing three molecules of CO_2.
  • To make one glucose molecule would require six cycles and the fixation of six CO_2 molecules!
  • Products: Glucose (C6H{12}O_6), ADP, and NADP+.

Requirements and Products Summary

  • Light-Dependent Reactions:
    • Requirements: Sunlight, Water (H_2O).
    • Products: ATP, NADPH, Oxygen (O_2).
  • Light-Independent Reactions:
    • Requirements: ATP, NADPH, Carbon Dioxide (CO_2), RuBP.
    • Products: G3P.

Overall Stoichiometry for 1 Glucose Molecule

  • 1 Glucose molecule requires:
    • 18 ATP
    • 12 NADPH
    • 6 CO_2
    • 6 RuBP

Summary Equations

  • Light-Dependent Reactions: 6H2O + Sunlight → 6O2 + 18 ATP + 12 NADPH
  • Light-Independent Reactions: 6CO2 + 18 ATP + 12 NADPH → C6H{12}O6

Photosynthesis summary - 1 Glucose

  • Light Reaction
    • 6 H_2O split = photolysis
    • 6 O_2 are released in atmosphere
    • Electrons move along transport chain
    • 18 ADP to 18 ATP in photosystem II from the H^+ gradient
    • 12 NADP^+ reduced to 12 NADPH in photosystem I
  • Light Independent Reaction (Calvin Benson Cycle)
    • 6 molecule of CO_2 combine with 6 molecule of RuBP
    • 12 molecules of PGA formed
    • 12 NADPH are oxidized
    • 18 ATP are consumed
    • 12 molecule of G3P formed
    • 1o molecule of G3P are changed into RuBP
    • 2 G3P molecules are converted into carbohydrates (glucose)

Important Considerations

  • Dark Conditions: The light-independent reactions will continue until the available ATP and NADPH are depleted.
  • Initial Light Exposure: A plant will not immediately produce glucose when placed in the light; it must first produce ATP and NADPH via the light-dependent reactions.
  • Glucose Storage: Plants store glucose in the form of starches (carbohydrates).
  • Nocturnal Glucose Production: Glucose can be synthesized during the night if ATP and NADPH are available, but only until these compounds run out.