Photosynthesis and Limiting Factors

Almost all life on Earth depends, directly or indirectly, on photosynthesis — the process by which plants and algae use light energy to make their own food. This lesson, part of Topic B1 of OCR Gateway Science A, covers photosynthesis as an endothermic process, the word equation you must know, the limiting factors that control its rate (light intensity, carbon dioxide concentration and temperature), the required practical investigating the effect of light, and — for Higher tier — the inverse-square law linking light intensity to distance. It is the natural partner to respiration: photosynthesis stores energy, respiration releases it.

By the end you should be able to state that photosynthesis is endothermic, write its word equation, explain how the three limiting factors affect the rate, describe the practical, and (Higher) use the inverse-square law for light.

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What Is Photosynthesis?

Photosynthesis is the process in which plants and algae use light energy (absorbed by chlorophyll in the chloroplasts) to convert carbon dioxide and water into glucose and oxygen. Two key points:

• Photosynthesis is endothermic — it takes in energy from the surroundings (light energy), the opposite of exothermic respiration.
• It happens in chloroplasts, which contain the green pigment chlorophyll that absorbs the light.

The word equation is essential for all tiers:

$\text{carbon dioxide} + \text{water} \xrightarrow{\text{light}} \text{glucose} + \text{oxygen}$carbon dioxide+waterlight​glucose+oxygen

(The Higher-tier symbol equation is $6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2$6CO2​+6H2​O→C6​H12​O6​+6O2​, with light energy absorbed by chlorophyll.)

flowchart LR
  A["Carbon dioxide"] --> C["Photosynthesis<br/>(in chloroplasts, needs light + chlorophyll)"]
  B["Water"] --> C
  L["Light energy"] -.-> C
  C --> D["Glucose"]
  C --> E["Oxygen"]

Exam Tip: Photosynthesis and respiration are near-opposites. Photosynthesis: endothermic, in chloroplasts, makes glucose + oxygen. Respiration: exothermic, mainly in mitochondria, uses glucose + oxygen. Don't muddle the reactants and products — read whether the question is about making food or releasing energy.

The glucose made in photosynthesis is the plant's food, and it is put to several uses:

Use of glucose	Why the plant needs it
Respiration	Released energy powers the plant's life processes
Converted to starch for storage	Starch is insoluble, so it can be stored without affecting water movement; it is a compact energy store
Used to make cellulose	Strengthens the cell walls
Used to make lipids (oils) for storage	Energy store, especially in seeds
Combined with nitrate ions to make amino acids	Amino acids are joined into proteins

This is why a healthy, well-lit plant can build its entire body — leaves, stem, roots, flowers and seeds — ultimately from carbon dioxide, water and a few mineral ions, using light as the energy source. Starch storage is especially important: testing a leaf for starch (by removing its chlorophyll in hot ethanol and adding iodine, which turns blue-black where starch is present) is a classic way to show that a leaf has been photosynthesising.

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The Rate of Photosynthesis and Limiting Factors

The rate of photosynthesis can be sped up or slowed down by environmental conditions. A limiting factor is the factor that, at a given moment, is in shortest supply and therefore holds back the rate — increase that factor and the rate rises; increase a different one and nothing changes. The three limiting factors are:

1. Light intensity
2. Carbon dioxide concentration
3. Temperature

Light intensity

Light provides the energy for photosynthesis. As light intensity increases, the rate of photosynthesis increases — until some other factor becomes limiting, at which point the graph levels off. In darkness, photosynthesis stops.

Carbon dioxide concentration

Carbon dioxide is a raw material. As carbon dioxide concentration increases, the rate increases, again until another factor (often light or temperature) becomes limiting and the rate plateaus.

Temperature

Photosynthesis relies on enzymes, so temperature matters. As temperature increases, the rate increases up to an optimum. Above the optimum, the enzymes controlling photosynthesis denature (their active sites change shape), so the rate falls — just like the enzyme behaviour you met earlier.

Limiting factor	Effect as it increases	What happens at the plateau / top
Light intensity	Rate increases	Levels off when light is no longer limiting
Carbon dioxide	Rate increases	Levels off when CO₂ is no longer limiting
Temperature	Rate increases to an optimum	Falls above the optimum as enzymes denature

Exam Tip: The signature of a limiting factor on a graph is a line that rises and then flattens. Where it flattens, that factor is no longer the one holding the rate back — something else has become limiting. Temperature is the exception: too high and the rate falls (denaturation), it does not just plateau.

How the factors interact

Only one factor limits the rate at any moment — the one in shortest supply. Imagine a plant on a dull morning: light intensity is low, so light is the limiting factor and the plant photosynthesises slowly even though there is plenty of carbon dioxide and the temperature is fine. As the sun rises and light increases, the rate climbs — until light is no longer in short supply. Now a different factor (perhaps carbon dioxide) becomes limiting, and increasing the light further makes no difference; the graph levels off. To speed photosynthesis up again you would have to increase whichever factor is now limiting.

This idea has real economic importance in commercial greenhouses. Growers can increase yields by removing limiting factors: providing artificial light so plants can photosynthesise for longer, burning fuels or adding CO₂ to raise the carbon dioxide concentration, and using heaters to keep the temperature near the optimum. They must balance the cost of doing this against the extra value of the crop — a neat application of the limiting-factor idea that OCR may ask you to discuss.

Reading a limiting-factor graph

A classic exam graph plots the rate of photosynthesis against light intensity at two different carbon dioxide concentrations. Both lines rise steeply at low light and then level off, but the line at the higher CO₂ concentration levels off at a higher rate. You should be able to explain every part of this:

• The steep rising part: here light is the limiting factor — increasing light increases the rate because more light energy is available.
• The levelling off: here light is no longer limiting; something else (CO₂ concentration or temperature) has become the limiting factor, so adding more light makes no difference.
• The difference between the two lines: the line with more CO₂ reaches a higher plateau, because with more carbon dioxide available the plant can photosynthesise faster before CO₂ becomes limiting.

Being able to point to a specific part of a graph and name which factor is limiting there is exactly the skill examiners test, and it scores well at the top band.

Photosynthesis and Respiration Together