5.1.1Unit 5: Respiration, Internal Environment, Coordination and Gene Technology
Photosynthesis: Overview — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 5 parts Lesson 8 of 36 Unit quiz
Curriculum Equation Key Balanced Photosynthesis EquationFast Exam Solution
6CO₂ + 6H₂O ➔ C₆H₁₂O₆ + 6O₂
Word Equation & Conditions: Carbon Dioxide + Water (+ Light & Chlorophyll) -> Glucose + Oxygen
Part 1 of 5What Photosynthesis Achieves
Photosynthesis is the starting point for the energy in almost every food chain.
Autotrophs
- Producers such as plants and algae make their own organic compounds from simple inorganic substances.
Energy conversion
- Light energy is converted into chemical energy stored in the bonds of organic molecules such as glucose.
Water splitting
- Light energy is used to split water, releasing hydrogen that is later used to reduce carbon dioxide.
Oxygen
- Oxygen is released as a by-product and enters the atmosphere through the stomata.
Fuel store
- Glucose can be respired for ATP, or converted to starch, cellulose, lipids and amino acids.
Part 2 of 5The Overall Equation
The word and symbol equations summarise a long series of reactions.
The Overall Equation
Word equation
- Carbon dioxide and water, in the presence of light and chlorophyll, form glucose and oxygen.
Symbol equation
- 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, which needs light energy and pigments.
Reactants
- Carbon dioxide diffuses into the leaf and water is transported from the roots in the xylem.
Products
- Glucose is used or stored and oxygen leaves, or is used in respiration by the plant.
Part 3 of 5Two Linked Stages
The reactions occur in two connected sets inside the chloroplast.
1
In the light-dependent stage, light is absorbed by pigments in the thylakoid membranes.
2
Light energy is used to make ATP and reduced NADP, and water is split to release oxygen.
3
In the light-independent stage, which occurs in the stroma, carbon dioxide is fixed into organic molecules.
4
ATP and reduced NADP from the first stage supply the energy and hydrogen needed to make carbohydrate.
| Feature | Light-dependent stage | Light-independent stage |
|---|---|---|
| Location | Thylakoid membranes | Stroma |
| Needs light directly? | Yes | No, but depends on its products |
| Inputs | Light, water, ADP, NADP | CO₂, ATP, reduced NADP |
| Outputs | ATP, reduced NADP, O₂ | Triose phosphate, then glucose |
Part 4 of 5Why It Matters
Food chains
- Almost all biomass depends on photosynthesis, directly or indirectly.
Atmosphere
- It keeps the oxygen supply high and removes carbon dioxide from the air.
Respiration
- The glucose made is the substrate that cells later respire to release energy.
Fossil fuels
- The carbon in coal and oil was originally fixed by photosynthesis.
Part 5 of 5Exam Checklist & Tips
5.1.1 – Key Points for the Exam
- Autotroph An organism that makes its own organic molecules from inorganic substances.
- Photosynthesis Conversion of light energy to chemical energy in organic molecules, using carbon dioxide and water.
- Reduced NADP A coenzyme carrying hydrogen from the light-dependent to the light-independent stage.
- Stroma The fluid in the chloroplast where the light-independent reactions occur.
- Thylakoid A flattened membrane sac in the chloroplast containing photosynthetic pigments.
Common Mistakes
- Saying that the light-independent stage happens in the dark only; it simply does not directly need light.
- Writing that the oxygen released comes from carbon dioxide; it comes from water.
- Stating that photosynthesis creates energy; it converts light energy into chemical energy.
- Leaving out the by-product oxygen or leaving the symbol equation unbalanced.
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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.