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3.3Unit 3 — Life on Earth

Photosynthesis

5 min read 4 parts Lesson 14 of 31

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Section 1 of 4The two stages of photosynthesis

pasted 1782543971044 0 Two Stages of Photosynthesis
Effect of temperature on rate of respiration (graph)
photosynthesis chloroplast
Photosynthesis equation (icons)
The ATP–ADP Cycle
enzyme reaction cycle infographic
ATP ADP cycle
stomata carbon fixation landscape
Photosynthesis summary: carbon dioxide, water, light to glucose and oxygen
Photosynthesis: sources of reactants and uses of products

Carbon Dioxide + Water → (Light Energy & Chlorophyll) → Glucose + Oxygen

1
Stage 1
Light Reactions (Photolysis)
  • Light energy from the Sun is trapped by the green pigment chlorophyll in chloroplasts.
  • Light energy is converted into chemical energy: generates ATP from ADP and Pi.
  • Water is split into hydrogen and oxygen (photolysis):

    Water → Hydrogen + Oxygen

  • Oxygen diffuses out of leaf cells through stomata as a waste by-product.
  • Hydrogen and ATP are passed on to Stage 2.
2
Stage 2
Carbon Fixation
  • A cycle of enzyme-controlled biochemical reactions (does not require light directly).
  • Hydrogen and ATP (from Stage 1) are combined with Carbon Dioxide from the air to produce Glucose.
  • Because it is enzyme-controlled, it is highly sensitive to temperature changes.
3
Fates of synthesized glucose
  1. Used immediately as a substrate in cellular respiration to release ATP.
  2. Stored as the insoluble carbohydrate starch for later energy needs.
  3. Converted into the structural carbohydrate cellulose to build plant cell walls.

Section 2 of 4Limiting factors of photosynthesis

Graph of carbon dioxide concentration against plant biomass
Effect of Limiting Factors on the Rate of Photosynthesis — 2D Simulation
pasted 1782544101824 0
Effect of Light Intensity on Rate of Photosynthesis — 2D Simulation
Effect of carbon dioxide concentration on rate of photosynthesis (graph)
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r049-enzyme-temperature
Light intensity limiting photosynthesis (graph)
Effect of light intensity on rate of photosynthesis (graph)
Carbon dioxide concentration limiting photosynthesis (graph)
Photosynthesis
Photosynthesis — Full 2D Simulation
1Limiting factor
A limiting factor is an environmental condition in shortest supply that restricts the rate of photosynthesis.
2The three key limiting factors
  • Light intensity
    • Provides energy for the light reactions;
      • as light increases, rate increases until another factor becomes limiting.
  • Carbon dioxide concentration
    • Provides essential carbon atoms for carbon fixation;
      • rate levels off when light or temperature limits.
  • Temperature
    • Affects the rate of enzyme action in carbon fixation;
      • rate increases with temperature up to the optimum (~25–30 °C), then drops sharply due to enzyme denaturation.

Section 3 of 4Key terms

TermMeaning
ChlorophyllThe green pigment located in chloroplasts that absorbs light energy for photosynthesis.
PhotolysisThe splitting of water molecules into hydrogen and oxygen driven by absorbed light energy.
Carbon fixationThe enzymatic stage of photosynthesis combining hydrogen and CO2 to form glucose using ATP.
Limiting factorA variable that limits the rate of a biological process when present in sub-optimal quantity.

Section 4 of 4Exam technique

Avoid these mistakes
Stating oxygen is passed to Stage 2 — oxygen is a waste gas that diffuses out; only hydrogen and ATP pass to Stage 2.
Omitting temperature as an enzyme factor — carbon fixation is controlled by enzymes, so temperatures above optimum denature enzymes and halt photosynthesis.
Biology 0610 & 9700 Exam Model Limiting Factors Master

Photosynthesis: Limiting Factors & Graph Plateau

6CO2 + 6H2O → C6H12O6 + 6O2 (Light & Chlorophyll required)

  • Limiting Factor Definition: Something present in the environment in such short supply that it restricts life processes.
  • Graph Plateau: When rate levels off as light intensity increases, light is no longer the limiting factor; CO2 concentration or temperature has become limiting.
  • High Temperature Drop: Beyond optimum temp (~40°C), enzymes (e.g. RuBisCO) denature, permanently decreasing rate.