Study Notes on Photosynthesis

PHOTOSYNTHESIS NOTES

INTRODUCTION

  • Photosynthesis is a process that converts light energy into chemical energy, specifically glucose.
  • It primarily occurs in green plants, algae, and some bacteria.

OVERVIEW OF PHOTOSYNTHESIS

  • Photosynthesis involves the utilization of carbon dioxide (CO2) and water (H2O) to produce glucose (C6H12O6) and oxygen (O2).
Diagram 2: Photosynthesis Reactions
  • Photosynthesis can be split into two main stages:
    • Light Reactions
    • Calvin Cycle

LIGHT REACTIONS

  • Occur in the thylakoid membranes of chloroplasts.
  • Convert light energy into chemical energy by producing ATP and NADPH.
Key Components of Light Reactions
  • Photosystems I and II (PSI and PSII):
    • Essential for capturing light energy and producing ATP and NADPH.
  • Electron Transport Chain:
    • Transports electrons and helps create a proton gradient for ATP synthesis.
  • ATP Synthase:
    • Enzyme that synthesizes ATP from ADP and inorganic phosphate (Pi) using the proton gradient.
Events in the Light Reactions
  • Light Absorption:
    • Chlorophyll absorbs light, exciting electrons to higher energy levels.
  • Water Splitting (Photolysis):
    • H2O is split to replace lost electrons in PSII, releasing O2 as a byproduct.
  • Electron Transport:
    • Excited electrons travel through the electron transport chain, down their energy gradient, creating a proton (H+) gradient across the thylakoid membrane.
  • ATP Formation:
    • H+ ions flow back to the stroma through ATP synthase, powering the conversion of ADP + Pi to ATP.
  • NADPH Formation:
    • Electrons reduce NADP+ to NADPH which is used in the Calvin Cycle.
Diagram 6: Structure of Light Reactions
  • Labels:
    • Draw and label:
    • PSI
    • PSII
    • Electron Transport Chain
    • ATP Synthase
    • Pathway of electron transport (from H2O through PSII to PSI and then to NADP+)

CALVIN CYCLE

  • Takes place in the stroma of chloroplasts.
  • Utilizes ATP and NADPH produced in light reactions to convert CO2 into glucose.
Phases of the Calvin Cycle
  1. Carbon Fixation:
    • CO2 is fixed to RuBP by the enzyme Rubisco, forming 3-PGA.
  2. Reduction Phase:
    • ATP and NADPH from light reactions convert 3-PGA into G3P (glyceraldehyde-3-phosphate).
  3. Regeneration Phase:
    • Some G3P is used to regenerate RuBP allowing the cycle to continue.
    • Excess G3P can be utilized to synthesize glucose and other carbohydrates.
Importance of Calvin Cycle
  • Produces organic molecules like glucose that can be used by the plant for energy and growth.
  • CO2 incorporation allows the synthesis of carbohydrates necessary for plant metabolism.
Molecular Structures in the Calvin Cycle (Diagram 11)
  • Transport Molecule:
    • G3P is transported to other parts of the plant to be used for energy or as a building block for other organic compounds.
  • Stored Molecule:
    • Starch is stored for later use as an energy source.

ENERGY TRANSFER AND STORAGE

Energy from Light in G3P
  • Photons capture light energy:
    • This energy excites electrons, which are then used in the production of ATP and NADPH during the light reactions.
  • Energy in the form of ATP and NADPH allows the conversion of G3P during the Calvin Cycle.
Figure Out Path of Electron from Water to G3P
  • Water (H2O) → PSII → Electron Transport Chain → PSI → ATP Synthase → Calvin Cycle → G3P.

PHOTOSYNTHESIS OUTCOMES

  • Photosynthesis results in the production of:
    • Glucose (C6H12O6) for energy and structural use in plants.
    • Oxygen (O2) as a byproduct, essential for the respiration of aerobic organisms.

TEST YOUR KNOWLEDGE

  • Purpose of Photosynthesis:
    • To convert solar energy into chemical energy, produce food (glucose), and release oxygen.
  • Oxygen Production:
    • Occurs when water is split at PSII to replace lost electrons; results in O2 as a waste product.
  • Dogma of Carbon Cycle in Calvin Cycle:
    • The regeneration of G3P and RuBP is fundamental, with 5 out of every 6 G3P molecules being recycled to regenerate RuBP.

PHOTOSYNTHESIS REVIEW QUESTIONS

  1. Which organisms perform photosynthesis?
    • Plants, algae, and some bacteria.
  2. Outline how light energy is converted into chemical energy.
    • Light energy creates ATP and NADPH during light reactions, which are then used to convert CO2 into sugar G3P in the Calvin cycle.