Photosynthesis and limiting factors: GCSE biology guide

Photosynthesis is the process plants use to convert light energy into glucose. It is one of the most important topics in GCSE Biology and appears in questions on equations, cell structure, limiting factors and required practicals.

If you understand the reaction itself and the three factors that limit its rate, you can handle the vast majority of photosynthesis questions. This guide covers everything you need – from the basic equation to graph interpretation and the required practical.

Photosynthesis is an endothermic reaction that takes place in the chloroplasts of plant cells and algae. During photosynthesis, light energy is absorbed by chlorophyll (the green pigment in chloroplasts) and used to convert carbon dioxide and water into glucose and oxygen.

The word equation is:

Carbon dioxide + water → glucose + oxygen

The balanced symbol equation is:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

You need to know both equations. The symbol equation is higher-demand and often appears in six-mark questions. Notice that the reactants are simple, widely available molecules – carbon dioxide from the air and water from the soil – while the products are glucose (used by the plant for energy and building materials) and oxygen (released as a by-product).

Photosynthesis takes place in the chloroplasts. These are organelles found in the cells of leaves and green stems. Chloroplasts contain chlorophyll, which absorbs light energy – primarily from the red and blue parts of the visible spectrum. Green light is reflected, which is why leaves appear green.

Leaves are adapted for photosynthesis in several ways. They are broad and flat to maximise the surface area for absorbing light. They are thin so that carbon dioxide can diffuse quickly to the cells where photosynthesis occurs. The upper epidermis is transparent to let light through to the palisade mesophyll cells beneath, which are packed with chloroplasts. Stomata on the lower surface allow carbon dioxide in and oxygen out, and the air spaces in the spongy mesophyll layer help gases diffuse through the leaf.

Plants use the glucose produced by photosynthesis in several ways. Some is used immediately in respiration to release energy for life processes. Some is converted into insoluble starch for storage – this is why the iodine test for starch is used to show that photosynthesis has occurred. Glucose is also converted into cellulose to build cell walls, combined with nitrate ions from the soil to make amino acids and proteins, or stored as lipids (fats and oils) in seeds.

Understanding what happens to glucose is important because exam questions often ask you to link photosynthesis to other biological processes.

A limiting factor is the factor that is in shortest supply at any given time, and therefore restricts the rate of photosynthesis. Even if other conditions are ideal, the rate cannot increase beyond what the limiting factor allows. There are three limiting factors you need to know: Light intensity, carbon dioxide concentration and temperature.

Light provides the energy for photosynthesis. As light intensity increases, the rate of photosynthesis increases – up to a point. Eventually the graph levels off and forms a plateau. At this point, light is no longer the limiting factor. Something else, such as CO₂ concentration or temperature, is now restricting the rate.

On a graph, you see a steep rise at low light intensities followed by a flat line. The flat section tells you that increasing the light further has no effect because another factor has become limiting.

In the exam, you may be asked to sketch or interpret this type of graph. The key thing to identify is where the line stops rising – that is the point at which light stops being the limiting factor.

In the required practical, you vary the distance between a lamp and a piece of pondweed to change the light intensity. The relationship between distance and light intensity follows the inverse square law:

Light intensity ∝ 1 / d²

This means that if you double the distance from the lamp, the light intensity drops to a quarter of its original value. If you triple the distance, the light intensity drops to a ninth.

You may be asked to calculate relative light intensity using this formula. For example, at a distance of 10 cm, the relative light intensity is 1 / 10² = 1 / 100 = 0.01. At 20 cm, it is 1 / 400 = 0.0025. This confirms that moving the lamp further away has a dramatic effect on how much light reaches the plant.

Carbon dioxide is one of the raw materials for photosynthesis. As CO₂ concentration increases, the rate of photosynthesis increases – again, up to a point. The graph looks similar to the one for light intensity: A steep rise followed by a plateau.

At the plateau, CO₂ is no longer limiting. Either light intensity or temperature is now the factor holding back the rate. In commercial greenhouses, farmers pump additional CO₂ into the air to push this plateau higher and increase crop yields.

The normal concentration of CO₂ in the atmosphere is about 0.04%. In greenhouse conditions, this is often raised to 0.1% or higher to maximise the rate of photosynthesis.

Temperature affects the rate of photosynthesis differently from the other two factors. As temperature increases, the rate increases because the enzymes involved work faster – molecules have more kinetic energy and collide more frequently. However, above an optimum temperature (around 40–45 °C for most plants), the enzymes begin to denature. Their active sites change shape and they can no longer catalyse the reaction. The rate drops sharply.

The graph for temperature is therefore not a plateau curve. It rises to a peak and then falls steeply. This makes it easy to distinguish from the light and CO₂ graphs in an exam.

Exam questions often show a graph with two or three curves at different conditions – for example, rate of photosynthesis against light intensity, with separate curves for low CO₂ and high CO₂. You need to be able to explain what is happening at each stage.

Where the line is rising, the factor on the x-axis is limiting. Where the line plateaus, that factor is no longer limiting – another factor has taken over. If a second curve at a higher CO₂ concentration plateaus at a higher rate, it tells you that CO₂ was the limiting factor at the plateau of the first curve.

When describing these graphs, always state three things: Which factor is limiting in the rising section, which factor becomes limiting at the plateau, and what evidence from the graph supports your answer. This structure is what examiners look for in longer-answer questions.

In this practical, you use a piece of aquatic plant – usually Elodea (pondweed) – placed in a beaker of water. A lamp is positioned at measured distances from the plant. As the plant photosynthesises, it produces oxygen, which is released as bubbles. You count the number of bubbles produced in a set time at each distance.

The independent variable is the distance between the lamp and the plant (which you use to calculate light intensity via the inverse square law). The dependent variable is the rate of oxygen production, measured by counting bubbles per minute. Control variables include the temperature of the water, the concentration of CO₂, the length of the pondweed and the time for each reading.

Counting individual bubbles is not very accurate because bubbles vary in size. A more precise method is to use a gas syringe to collect the oxygen and measure its volume. You should also allow the plant to acclimatise for two minutes at each new distance before you start counting, so the rate has time to stabilise.

Repeat readings at each distance and calculate a mean to improve reliability. Carrying out the experiment in a darkened room (or using a screen) reduces the effect of background light, which would otherwise act as an uncontrolled variable.