What you'll learn
- Why plants and algae are the main producers of food and biomass.
- The word equation for photosynthesis and why it is an endothermic reaction.
- How temperature, light intensity and carbon dioxide concentration can limit the rate of photosynthesis.
- How to investigate light intensity using pondweed, including Higher Tier inverse square law calculations.
The big idea: producers make biomass
Photosynthesis is one of the most important reactions in biology because it is how plants and algae make food. That food does not just matter to the plant — it supports almost every food chain.
Producer and biomass
A producer is an organism that makes its own food, usually by photosynthesis. Biomass is the mass of living material in an organism, population or area.
Plants and algae are the main producers. They use light energy to make glucose, a sugar. Some of this glucose is used in respiration, and some is converted into substances such as starch, cellulose and other biological molecules. This builds plant or algal biomass.
When animals eat plants, or eat animals that ate plants, that biomass is passed along the food chain.
Why photosynthesis matters
Photosynthesis transfers energy from light into chemical stores in glucose, producing the food and biomass that most organisms depend on.
Where photosynthesis happens
In plants, photosynthesis mainly happens in the leaves. Leaf cells contain chloroplasts, which are small cell structures containing chlorophyll. Chlorophyll is the green pigment that absorbs light energy.
Algae also photosynthesise. Some algae are single-celled, but they still contain chloroplasts and can make glucose using light energy.
The diagram summarises the inputs and outputs of photosynthesis in a plant and in an algal cell.
The photosynthesis reaction
Photosynthesis uses carbon dioxide from the air and water from the soil to make glucose and oxygen.
The word equation is:
carbon dioxide + water → glucose + oxygen
Endothermic reaction
An endothermic reaction takes in energy from its surroundings. Photosynthesis is endothermic because it takes in light energy.
For GCSE Biology, you should know the word equation very confidently. You may also see the balanced symbol equation:
6CO2+6H2O→C6H12O6+6O26\text{CO}_2 + 6\text{H}_2\text{O} \to \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_26CO2+6H2O→C6H12O6+6O2The oxygen is released as a waste product. The glucose is useful because it can be used for respiration or converted into storage and structural molecules.
Photosynthesis does not make energy
Plants do not “make energy”. They transfer light energy into chemical energy stored in glucose.
Rate of photosynthesis
The rate of photosynthesis means how fast photosynthesis is happening.
In experiments, you often estimate the rate by measuring how much oxygen is produced in a certain time. For example, you might count oxygen bubbles from pondweed or collect the oxygen gas in a measuring cylinder.
A useful rate equation is:
rate=volume of oxygen producedtime taken\text{rate} = \frac{\text{volume of oxygen produced}}{\text{time taken}}rate=time takenvolume of oxygen producedCalculating rate of oxygen production
A piece of pondweed produces 6.0 cm³ of oxygen in 4 minutes. Calculate the rate of oxygen production.
- Choose the correct rate calculation:
- Substitute the values:
- Calculate the rate:
Limiting factors
A limiting factor is the factor that is stopping a process from going faster. For photosynthesis, the main limiting factors are:
- light intensity
- carbon dioxide concentration
- temperature
Limiting factor
A limiting factor is the factor in shortest supply that restricts the rate of a reaction or process.
Light intensity
If light intensity is low, the plant cannot absorb much light energy, so photosynthesis is slow. As light intensity increases, the rate of photosynthesis usually increases.
However, eventually the graph levels off. At this point, light is no longer the limiting factor. Something else, such as carbon dioxide concentration or temperature, is limiting the rate.
The graph below shows how the limiting factor changes as light intensity increases.
Identifying the limiting factor
A photosynthesis graph rises steeply at low light intensity, then becomes flat at high light intensity. When extra carbon dioxide is supplied, the flat part rises to a higher rate.
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In the rising section, increasing light intensity increases the rate, so light intensity must be limiting.
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In the flat section, increasing light intensity no longer increases the rate, so light intensity is not the limiting factor.
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Because adding carbon dioxide raises the maximum rate, carbon dioxide concentration was limiting the rate on the original plateau.
Carbon dioxide concentration
Carbon dioxide is a raw material for photosynthesis. If there is not enough carbon dioxide, the plant cannot make glucose quickly, even if there is plenty of light.
In school experiments, sodium hydrogencarbonate solution is often used to provide carbon dioxide for pondweed.
Temperature
Photosynthesis is controlled by enzymes. At low temperatures, enzyme-controlled reactions are slower, so the rate of photosynthesis is lower.
As temperature increases, the rate increases up to an optimum temperature. If the temperature gets too high, enzymes can denature, meaning their active sites change shape and they no longer work properly.
Forgetting enzymes in temperature questions
When explaining temperature, do not just say “particles move faster”. For photosynthesis, link temperature to enzyme-controlled reactions and denaturing at high temperatures.
How limiting factors interact
If you are taking Higher Tier, you also need to explain how the limiting factors interact.
The key idea is that improving one factor only helps if that factor is currently limiting the rate. Once it is no longer limiting, another factor takes over.
For example:
- If light intensity is low, increasing carbon dioxide may not help much because the plant still lacks light energy.
- If carbon dioxide concentration is low, increasing light intensity may not help once the plant has enough light.
- If temperature is too low, both light and carbon dioxide may be available, but enzyme-controlled reactions are still slow.
One factor at a time limits the rate
At any moment, the rate of photosynthesis is controlled by the factor in shortest supply. Increasing a non-limiting factor has little or no effect.
Higher Tier: light intensity and distance
For Higher Tier, you should know that when light is the limiting factor, the rate of photosynthesis is directly proportional to light intensity. This means that if the light intensity doubles, the rate also doubles — but only while light is limiting.
Light intensity decreases as the distance from a lamp increases. For a point light source, light intensity follows the inverse square law:
I∝1d2I \propto \frac{1}{d^2}I∝d21Here, III means light intensity and ddd means distance from the light source.
So if the distance doubles, the light intensity becomes one quarter as large.
Using the inverse square law
A lamp is moved from 20 cm to 40 cm away from pondweed. Compare the new light intensity with the original light intensity.
- Use the inverse square relationship to compare the two intensities:
- Substitute the two distances:
- Simplify the fraction:
The new light intensity is one quarter of the original intensity.
When proportionality stops working
The rate is directly proportional to light intensity only while light is the limiting factor. Once the graph reaches a plateau, another factor is limiting, so increasing light will not increase the rate in the same way.
Core Practical: investigating light intensity
In this required practical, you investigate how changing light intensity affects the rate of photosynthesis in pondweed.
You usually change the distance between a lamp and the pondweed. A closer lamp gives a higher light intensity.
The setup below shows the main apparatus and variables.
Method outline
- Place pondweed in sodium hydrogencarbonate solution to provide carbon dioxide.
- Put a lamp at a measured distance from the pondweed.
- Allow the pondweed a short time to adjust to the light intensity.
- Measure oxygen production for a fixed time, such as counting bubbles per minute or collecting gas volume.
- Repeat at several different distances from the lamp.
- Repeat readings and calculate a mean for each distance.
- Plot rate of photosynthesis against distance or calculated light intensity.
Variables
The independent variable is the variable you change: distance from the lamp, or light intensity.
The dependent variable is the variable you measure: rate of oxygen production.
The control variables are variables kept the same to make the test fair, such as:
- temperature
- carbon dioxide concentration
- length or species of pondweed
- time allowed before measuring
- time period used for each measurement
Improving accuracy
Counting bubbles is simple but not very accurate because bubbles can vary in size. Collecting the volume of oxygen gas is usually more accurate.
Changing temperature accidentally
A lamp can heat the water as well as change light intensity. Use an LED lamp, heat filter, thermometer or water bath so that temperature does not become an uncontrolled variable.
In the exam
- For the reaction, write the full word equation and say photosynthesis is endothermic because it takes in light energy.
- For limiting factors, always state which factor is limiting and explain why increasing it would increase the rate.
- For the core practical, name the independent, dependent and control variables, then describe repeats and calculating a mean.
Check yourself
- Why are plants and algae described as producers?
- What happens to the rate of photosynthesis when light intensity increases but carbon dioxide is limiting?
- In the pondweed practical, how could you stop temperature affecting your results?
