PhotosynthesisAQA GCSE Biology: Revision notes
Section 1
What are the equations for photosynthesis?
Photosynthesis converts light energy into chemical energy stored in glucose. You must be able to write and interpret both the word equation and the balanced symbol equation.
Word equation: Carbon dioxide + Water → Glucose + Oxygen (in the presence of light and chlorophyll)
Balanced symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (in the presence of light energy)
Key points:
- The equation shows the inputs (reactants) and outputs (products)
- Light energy and chlorophyll are essential catalysts, written above or below the arrow
- The equation is endothermic — it requires energy input from light to occur
- This is the reverse of aerobic respiration in terms of inputs and outputs
Examiners expect you to balance the equation correctly. Remember: 6 carbons on the left (6CO₂) must equal 6 carbons on the right (C₆H₁₂O₆). Check oxygen and hydrogen atoms match too.
Students often forget to balance the equation or write 'light' as a reactant instead of showing it above the arrow. Light is energy, not a chemical reactant.
Section 2
What are the limiting factors of photosynthesis?
The rate of photosynthesis is limited by three main factors. At any given time, whichever factor is in shortest supply (the limiting factor) controls the overall rate.
Light intensity:
- As light intensity increases, the rate of photosynthesis increases proportionally (linear relationship)
- At very high light intensities, light is no longer the limiting factor; the rate plateaus
- Other factors (CO₂ or temperature) then become limiting
Carbon dioxide (CO₂) concentration:
- As CO₂ concentration increases, the rate of photosynthesis increases
- Rate plateaus when another factor becomes limiting
- Low CO₂ is often the limiting factor in normal atmospheric conditions
Temperature:
- Photosynthesis rate increases as temperature rises (up to an optimum, usually 25-35°C)
- The rate is controlled by enzyme activity in photosynthetic reactions
- Beyond the optimum, enzymes denature and the rate falls sharply
- Below freezing, photosynthesis stops because enzymes cannot function
How to identify the limiting factor from a graph:
- If the line is steep and rising: that factor is limiting
- If the line is flat/plateau: that factor is no longer limiting; something else is
When interpreting graphs, always describe the relationship: say whether the rate increases, stays constant, or decreases, and explain why in terms of limiting factors.
A limiting factor works like the narrowest point in a funnel — no matter how much water you pour in, the narrowest point controls how fast it flows through.
Section 3
How does light intensity affect photosynthesis (including inverse square law)?
Light intensity and photosynthesis rate:
- Rate of photosynthesis is directly proportional to light intensity (at low to moderate intensities)
- Doubling light intensity doubles the rate of photosynthesis
- At very high intensities, the rate plateaus because another factor becomes limiting
The inverse square law (Higher Tier): Light intensity is inversely proportional to the square of the distance from the light source.
Formula: I = 1/d²
Where:
- I = light intensity
- d = distance from the light source
Practical application:
- If you double the distance from a lamp, light intensity becomes ¼ (1/2²) of the original
- If you triple the distance, light intensity becomes 1/9 (1/3²) of the original
- To double light intensity, you must reduce distance to 1/√2 ≈ 0.71 times the original distance
Why this matters in experiments:
- In investigations measuring photosynthesis rate at different light intensities, controlling distance from the light source is crucial
- Small changes in distance cause large changes in light intensity reaching the plant
A lamp is 10 cm from a plant. If you move it to 20 cm away, the light intensity becomes 1/(2²) = 1/4 of the original. If the original rate was 100 units, the new rate would be approximately 25 units (assuming light is the limiting factor).
Higher Tier students must be able to calculate light intensity changes using the inverse square law formula and explain why distance control is essential in photosynthesis experiments.
Section 4
What are the uses of glucose produced in photosynthesis?
Glucose is the product of photosynthesis and is used for various purposes in plants:
| Use of Glucose | Explanation |
|---|---|
| Respiration | Glucose is broken down via respiration to release energy (ATP) for all plant activities |
| Cellulose synthesis | Glucose molecules are joined together to form cellulose, the structural component of plant cell walls |
| Starch storage | Glucose is converted to starch and stored in plastids (e.g. amyloplasts) in leaves, roots, and seeds for later use |
| Lipid (oil) storage | Glucose is converted into fats and oils for energy storage, particularly in seeds |
| Amino acid synthesis | Glucose is used to synthesise amino acids (combined with nitrogen from nitrate ions) for making proteins |
Key points:
- Not all glucose is used immediately; excess is stored as starch or oils
- The distribution of glucose between these uses depends on the plant's needs at that time
- During the day, some glucose is stored; at night, stored glucose and starch are used for respiration
- Cellulose is needed constantly for growth and structural support
Examiners test understanding of glucose distribution. Be specific: say which use applies in which situation (e.g., 'starch is stored when light is available but used during darkness').
Section 5
What are the light-dependent and light-independent reactions? (Higher Tier)
Photosynthesis consists of two main stages occurring in different parts of the chloroplast:
Light-dependent reactions (light reactions):
- Location: Thylakoid membranes in the chloroplast
- Requires: Light energy
- Process:
- Light energy is absorbed by chlorophyll
- Water is split (photolysis) into hydrogen ions, electrons, and oxygen
- Electrons are excited and move along the electron transport chain
- Energy from electrons is used to synthesise ATP (energy carrier)
- Electrons reduce NADP⁺ to form reduced NADP (electron carrier)
- Oxygen is released as a byproduct
- Products: ATP and reduced NADP (both used in the light-independent reactions)
Light-independent reactions (Calvin cycle):
- Location: Stroma of the chloroplast
- Requires: No direct light (but depends on ATP and reduced NADP from light reactions)
- Process:
- CO₂ is fixed by the enzyme RuBisCO to ribulose bisphosphate (RuBP), forming an unstable intermediate
- This breaks down into 3-phosphoglycerate (3-PG)
- ATP and reduced NADP from light reactions are used to reduce 3-PG to glyceraldehyde-3-phosphate (G3P)
- Most G3P is recycled to regenerate RuBP (using ATP)
- One G3P exits the cycle for every 6 CO₂ fixed; this is used to synthesise glucose
- Products: Glucose (and regeneration of RuBP)
Key relationship:
- Light reactions produce ATP and reduced NADP, which drive the Calvin cycle
- Light reactions release O₂; Calvin cycle consumes CO₂
- Increasing light intensity increases ATP and reduced NADP production, speeding up the Calvin cycle and CO₂ fixation
Higher Tier: Be precise about locations — light reactions in thylakoid membranes, Calvin cycle in stroma. Explain how the products of one stage are the reactants of the other.
Light reactions are like a solar panel charging a battery (producing ATP and reduced NADP), while the Calvin cycle uses that stored energy to build glucose from CO₂.
Section 6
How do you investigate photosynthesis experimentally?
Investigating the effect of light intensity on photosynthesis:
Method:
- Use an aquatic plant (e.g. Elodea) and a light source
- Vary the distance between the light source and the plant (e.g. 10 cm, 20 cm, 30 cm)
- Use the inverse square law: as you increase distance, light intensity decreases according to I = 1/d²
- Measure the rate of photosynthesis by counting oxygen bubbles released per unit time (or collect oxygen in an inverted test tube)
- Plot results on a graph: light intensity (or distance) on x-axis, rate of photosynthesis on y-axis
- Use a thermostatically controlled water bath to keep temperature constant
Investigating the effect of carbon dioxide concentration on photosynthesis:
Method:
- Use an aquatic plant in a sealed container
- Vary CO₂ concentration (e.g. by adding sodium bicarbonate in different amounts, or using different concentrations of sodium hydrogen carbonate solution)
- Keep light intensity and temperature constant
- Measure rate of photosynthesis by counting oxygen bubbles or measuring gas volume
- Plot results: CO₂ concentration on x-axis, rate of photosynthesis on y-axis
Variables to control:
- Temperature: Use a water bath to maintain constant temperature
- Light wavelength: Use the same light source for all trials
- Plant species and size: Use the same plant or plants of similar size
- pH and mineral nutrients: Use the same solution throughout
- Time: Measure over the same time period for each trial
Safety and practical considerations:
- Handle electrical equipment carefully (lights near water)
- Wear eye protection when using chemical solutions
- Ensure the setup allows gas to escape safely (sealed containers need venting)
- Use a datalogger or graduated measuring equipment for accuracy
When writing about experimental design, state which variable you are changing (independent), which you are measuring (dependent), and list all variables you are keeping constant. Examiners expect precision.
Students often forget to control temperature in light intensity experiments, but temperature affects enzyme activity in the Calvin cycle. This will affect results unless temperature is held constant.
Must Know
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Photosynthesis equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (with light and chlorophyll). It is an endothermic reaction that converts light energy into chemical energy in glucose.
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Three limiting factors: Light intensity (rate ∝ light up to a plateau), CO₂ concentration (rate ∝ CO₂ until plateau), and temperature (enzyme-controlled; optimal range is ~25-35°C). The factor in shortest supply limits the overall rate.
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Inverse square law (HT): Light intensity I = 1/d². Doubling distance reduces intensity to ¼; tripling distance reduces it to 1/9. This is why distance from the light source must be controlled in experiments.
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Glucose uses: Respiration (energy), cellulose (cell walls), starch/oils (storage), amino acids (proteins). Plants distribute glucose based on their immediate needs.
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Light-dependent reactions (HT): Occur in thylakoid membranes; split water, release O₂, and produce ATP and reduced NADP using light energy.
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Light-independent reactions/Calvin cycle (HT): Occur in stroma; use ATP and reduced NADP to fix CO₂ and synthesise glucose. The two stages are interdependent: light reactions fuel the Calvin cycle.
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Experimental design: Control all variables except the one you are testing. Measure rate of photosynthesis by oxygen bubbles or gas volume. Plot graphs and explain relationships in terms of limiting factors.
That's the notes covered.
Carry on to the next subtopic.