Skip to main content

Photosynthesis

Subject: Biology
Topic: 4
Cambridge Code: 0610 / 0970 / 5090

Overview​

Photosynthesis - Process of converting light energy into chemical energy

6CO2+6H2O+light energy→C6H12O6+6O26CO_2 + 6H_2O + \text{light energy} \rightarrow C_6H_{12}O_6 + 6O_2

Location: Chloroplast (in plant cells)

Importance:

  • Produces glucose (food)
  • Releases oxygen
  • Basis of food chains

Chloroplast Structure​

Parts​

Outer membrane: Permeable, controls entry

Inner membrane: Selectively permeable

Thylakoids: Disc-shaped sacs

  • Stacked in grana
  • Site of light reactions
  • Contains chlorophyll

Stroma: Liquid-filled space

  • Site of light-independent reactions
  • Contains enzymes

Granum: Stack of thylakoids

Light-Dependent Reactions​

Location: Thylakoid membranes

Occurs: In presence of light

Process​

  1. Light absorption - Chlorophyll absorbs photons
  2. Photolysis of water - Light splits H₂O: 2H2O→4H++4e−+O22H_2O \rightarrow 4H^+ + 4e^- + O_2
  3. Electron transfer - Electrons move through thylakoid
  4. ATP production - Energy used to produce ATP
  5. Reduction of NADP - NADP⁺ reduced to NADPH

Products​

  • ATP - Energy carrier
  • NADPH - Reducing agent
  • Oxygen - Released (byproduct)

Light Intensity Effect​

  • Increases from 0 → Rate increases
  • Plateaus at high intensity (other factors limit)

Light-Independent Reactions (Calvin Cycle)​

Location: Stroma

Occurs: Day and night (requires products from light reactions)

Three Stages​

1. Carbon Fixation

  • CO₂ combines with RuBP (5 carbons)
  • Enzyme: RuBisCO
  • Forms 3-phosphoglycerate (3 carbons × 2)

2. Reduction

  • 3-phosphoglycerate reduced to triose phosphate
  • Uses ATP and NADPH
  • Some triose phosphate exits cycle

3. Regeneration

  • Remaining triose phosphate regenerates RuBP
  • Uses ATP

Products​

  • Glucose - Built from triose phosphate
  • Other compounds - Amino acids, lipids

Factors Affecting Photosynthesis​

Light Intensity​

  • Limiting factor at low light
  • Rate increases linearly
  • Plateaus when other factors limit

Carbon Dioxide Concentration​

  • Limiting factor at low CO₂
  • Rate increases with CO₂
  • Plateaus at high CO₂

Temperature​

  • Increases enzyme activity (up to optimum ~25°C)
  • Further increase denatures enzymes
  • Enzyme activity decreases

Chlorophyll Content​

  • More chlorophyll = more light absorption
  • Can be limiting factor

Limiting Factors​

Limiting factor - Factor in lowest relative amount

At any time, ONE factor limits rate:

  • Low light? → Light is limiting
  • Low CO₂? → CO₂ is limiting
  • Low temperature? → Temperature is limiting

Practical Example​

Plant grown under:

  • Limiting light: Increase light → Rate increases
  • Non-limiting light: Increase light → No change

Adaptation to Light Conditions​

Shade Plants​

  • Larger leaves with more surface area
  • More chlorophyll (darker green)
  • Thicker palisade layer
  • Better at low light

Sun Plants​

  • Smaller leaves
  • Less chlorophyll (lighter green)
  • Thinner leaves with air spaces
  • Efficient at high light

Key Points​

  1. Photosynthesis converts light to chemical energy
  2. Light reactions produce ATP and NADPH
  3. Calvin cycle uses these to fix CO₂
  4. Light, CO₂, temperature affect rate
  5. One factor limits at any time
  6. Chloroplast structure relates to function

Practice Questions​

  1. Write equation for photosynthesis
  2. Describe light-dependent reactions
  3. Explain Calvin cycle stages
  4. Graph photosynthesis rate vs light intensity
  5. Identify limiting factors
  6. Compare shade and sun plants

Revision Tips​

  • Learn overall equation
  • Understand two reaction stages
  • Know limiting factors
  • Practice rate graphs
  • Relate structure to function
  • Know optimum temperature