A plant cannot walk towards food, catch prey or eat a meal. Instead, it builds energy-containing organic compounds from simple substances around it. Carbon dioxide comes from the air, water comes mainly from the soil, and light provides the energy needed to bring the process together.
This is photosynthesis. The word literally refers to building substances using light, but there is an important energy idea behind the definition: light energy does not disappear. It is converted into chemical energy associated with the carbohydrates produced.
Once you understand where the raw materials come from, how the leaf is adapted to receive them, and why changing environmental conditions changes the rate, most photosynthesis questions become much easier to reason through.
Try these five questions before reading the chapter. They check the basic ideas you will need before tackling limiting factors and experimental questions.
- 5 diagnostic questions
- Core photosynthesis concepts
- Instant explanation for every answer option
What is photosynthesis?
Photosynthesis is the process by which green plants manufacture carbohydrates from carbon dioxide and water, using light energy absorbed by chlorophyll.
The important word here is energy. Carbon dioxide and water are relatively simple raw materials. The plant uses light energy to form energy-containing organic compounds. In this way, light energy entering the leaf becomes chemical energy associated with the carbohydrates that the plant produces.
Energy change in photosynthesis:
light energy → chemical energy stored in organic compounds
The word and chemical equations
Word equation
carbon dioxide + water ⟶ glucose + oxygen
Light energy and chlorophyll are required.
Balanced chemical equation
6CO₂ + 6H₂O ⟶ C₆H₁₂O₆ + 6O₂
Light energy and chlorophyll are required.
Notice that light energy and chlorophyll are not written as reactants. They are required for photosynthesis, but they are not raw materials consumed in the same sense as carbon dioxide and water.
Raw materials: carbon dioxide and water
Required for the process: light energy and chlorophyll
Products shown in the overall equation: glucose and oxygen
What exactly does chlorophyll do?
Chlorophyll is the green pigment found in chloroplasts. Its key role is to absorb light energy. This energy is then converted into chemical energy that can be used in the formation of carbohydrates.
This distinction matters in examinations. Saying only that chlorophyll “makes the plant green” describes its colour but not its biological role. Likewise, saying that chlorophyll is “food for the plant” is incorrect. Chlorophyll helps the plant capture the energy needed to manufacture organic compounds.
Exam-ready explanation: Chlorophyll absorbs light energy and enables it to be converted into chemical energy for the formation of carbohydrates.
Why is photosynthesis so important?
Photosynthesis matters far beyond an individual leaf. It introduces energy into most biological communities by converting light energy into chemical energy stored in organic compounds.
Photosynthesis supplies organic food
A green plant can use the carbohydrates it manufactures to build new tissues and to supply substrates for respiration. Animals cannot manufacture their own organic food from carbon dioxide and water, so they obtain organic matter by feeding on plants or on organisms that ultimately obtained their food from plants.
This means that the chemical energy passing along most food chains was originally captured from light by photosynthetic organisms.
Photosynthesis releases oxygen
Oxygen is released during photosynthesis. Most organisms use oxygen for aerobic respiration, so photosynthesis also helps maintain the oxygen supply on which aerobic organisms depend.
The big biological link: photosynthesis produces organic compounds and releases oxygen; organisms can then use organic compounds and oxygen in aerobic respiration to release usable energy.
How is a leaf adapted for photosynthesis?
You may already have studied the transverse section of a dicotyledonous leaf in Plant Structure. For photosynthesis, the important question is not simply “What is each layer called?” but “How does its structure help the leaf capture light, obtain carbon dioxide or receive water?”
| Leaf feature | How it helps photosynthesis |
|---|---|
| Broad, flat lamina | Provides a large surface area for receiving light. |
| Thin leaf | Creates short diffusion distances between the atmosphere and internal photosynthesising cells. |
| Transparent upper epidermis | Allows light to pass through to the photosynthetic tissues beneath it. |
| Palisade mesophyll near the upper surface | Receives strong light and contains many chloroplasts, making it a major site of photosynthesis. |
| Spongy mesophyll with interconnected air spaces | Provides internal pathways through which carbon dioxide and oxygen can diffuse. |
| Stomata | Allow gases to move between the atmosphere and the internal air spaces of the leaf. |
| Xylem in vascular bundles | Supplies water to the leaf. |
| Phloem in vascular bundles | Transports organic solutes such as sucrose away from sources such as photosynthesising leaves. |
The distribution of chloroplasts is especially important. Palisade mesophyll cells generally contain more chloroplasts than spongy mesophyll cells because they are positioned where light intensity is usually greatest.
How does carbon dioxide reach the mesophyll cells?
Carbon dioxide is present in the atmosphere, but the chloroplasts that need it are inside the leaf. The gas therefore has to move from the surrounding air to the photosynthesising cells.
Follow the pathway:
atmosphere → stomata → internal air spaces → moist mesophyll cell surfaces → mesophyll cells
During photosynthesis, carbon dioxide is being used by mesophyll cells. Its concentration inside the leaf can therefore become lower than its concentration in the surrounding atmosphere.
Carbon dioxide then moves down this concentration gradient by diffusion. It enters through the stomata and diffuses through the interconnected air spaces of the spongy mesophyll. It dissolves in the thin film of moisture on cell surfaces before diffusing into the mesophyll cells, where it can be used in photosynthesis.
Oxygen produced during photosynthesis can diffuse in the opposite overall direction, moving from the photosynthesising tissues into the air spaces and eventually out through the stomata when its concentration is higher inside the leaf.
Remember: carbon dioxide is not pumped actively through the stomata. Its movement into the leaf is explained mainly by diffusion down a concentration gradient.
What happens to the carbohydrates made by photosynthesis?
Making carbohydrate is not the end of the story. The plant can use the carbohydrate in several different ways depending on its needs.
| Use | Why it matters to the plant |
|---|---|
| Respiration | Glucose can be used as a respiratory substrate to release usable energy for cellular activities. |
| Storage as starch | Carbohydrate can be converted to starch for storage. |
| Transport as sucrose | Sucrose can be transported through the phloem from sources to other parts of the plant. |
| Formation of cellulose | Cellulose is used to build plant cell walls. |
| Formation of fats and oils | Carbohydrate can contribute to the formation of energy-rich storage compounds. |
| Formation of amino acids and proteins | Carbon-containing compounds can combine with a nitrogen source obtained from mineral ions to form amino acids, which are then used to make proteins. |
This is why it is misleading to imagine that all glucose produced by a leaf simply remains there. Photosynthetic products enter a network of reactions and transport processes throughout the plant.
What is a limiting factor?
A limiting factor is a factor that is restricting the rate of a process. If that factor is increased while the other relevant conditions remain suitable, the rate increases.
Photosynthesis depends on several conditions at the same time. Three particularly important limiting factors are:
- light intensity
- carbon dioxide concentration
- temperature
The crucial idea is that the limiting factor can change. Increasing one factor may increase the rate at first, but eventually another factor may become limiting.
Light intensity
At low light intensity, increasing light intensity usually increases the rate of photosynthesis because more light energy becomes available.
Eventually the graph may level off. At this point, further increases in light intensity do not increase the rate because light is no longer the main limiting factor. Carbon dioxide concentration, temperature or another factor may now be restricting the process.
Carbon dioxide concentration
Carbon dioxide is a raw material for photosynthesis. When carbon dioxide concentration is low, increasing it can increase the rate because more carbon dioxide becomes available to the photosynthesising cells.
Again, the increase does not continue indefinitely. Once another factor becomes limiting, adding still more carbon dioxide has little or no further effect on the rate.
Temperature
Photosynthesis involves enzyme-controlled reactions. At low temperatures, molecules have less kinetic energy and the relevant enzyme-controlled reactions proceed more slowly.
As temperature rises towards a suitable range, the rate generally increases. Beyond an optimum, however, the rate can decrease because the enzymes involved become less effective and may eventually be denatured at sufficiently high temperatures.
Light intensity: usually increases rate until another factor becomes limiting.
Carbon dioxide concentration: usually increases rate until another factor becomes limiting.
Temperature: rate rises towards an optimum, then falls when high temperatures disrupt enzyme function.
How to read a limiting-factor graph
Graph questions are easier if you avoid trying to memorise where a particular factor “should” be limiting. Instead, ask one practical question:
If I increase this factor from the point shown, does the rate increase?
If the answer is yes, that factor is limiting the rate at that point.
Example: light intensity at two carbon dioxide concentrations
At X, increasing light intensity alone would have little effect because the lower curve has already levelled off. However, the higher-CO₂ curve reaches a greater rate. This shows that increasing carbon dioxide concentration can raise the rate under these conditions, so carbon dioxide concentration is limiting at X.
Be careful with the word plateau. A plateau does not mean photosynthesis has stopped. It means increasing the factor shown on the horizontal axis no longer increases the rate under those conditions.
Also remember that biology is not always restricted to one limiting factor at a time. Under some conditions, changing more than one factor could increase the rate.
How can the rate of photosynthesis be investigated?
A common investigation uses a submerged aquatic plant. As the plant photosynthesises, oxygen is released and can be used to estimate the rate of photosynthesis.
A suitable basic method
- Place a similar length of aquatic plant in water containing a suitable source of carbon dioxide.
- Provide a light source.
- Change one factor such as light intensity, carbon dioxide concentration or temperature.
- Keep the other important variables constant.
- Allow the plant time to adjust to the new condition before taking measurements.
- Measure the amount of oxygen produced during a fixed period.
- Repeat the measurement and calculate a mean.
- Compare the rate of oxygen production under the different conditions.
Rate of oxygen production:
rate = volume of oxygen produced ÷ time
Changing light intensity
Light intensity can be changed by changing the position of the light source relative to the plant or by using a controllable light source. Temperature and carbon dioxide availability should be kept as constant as possible.
A lamp may also heat the water. If the temperature rises while the lamp is moved closer, you have changed two variables instead of one. A water bath, heat filter or suitable low-heat light source can help control temperature.
Changing carbon dioxide concentration
Carbon dioxide availability can be changed by using different concentrations of a suitable carbon dioxide source such as sodium hydrogencarbonate solution. Light intensity and temperature should then be controlled.
Changing temperature
Temperature can be varied using a controlled water bath. Light intensity and carbon dioxide availability should be kept constant so that any change in measured rate can be attributed more confidently to temperature.
Why measuring oxygen volume is better than simply counting bubbles
Counting bubbles is convenient, but bubbles are not always the same size. Ten large bubbles may contain much more gas than ten small bubbles.
Measuring the volume of oxygen produced per unit time using suitable gas-collection apparatus therefore provides a more quantitative estimate of the rate. Repeated readings also improve reliability.
| If you change... | Useful dependent variable | Important controls |
|---|---|---|
| Light intensity | Volume of oxygen produced per unit time | Temperature, carbon dioxide availability, plant size/species, measurement time |
| Carbon dioxide concentration | Volume of oxygen produced per unit time | Light intensity, temperature, plant size/species, measurement time |
| Temperature | Volume of oxygen produced per unit time | Light intensity, carbon dioxide availability, plant size/species, measurement time |
Common Mistakes
| Common mistake | Why it is wrong | Better approach |
|---|---|---|
| Writing light and chlorophyll as reactants | Carbon dioxide and water are the raw materials. Light supplies energy and chlorophyll absorbs that light energy. | Write carbon dioxide + water → glucose + oxygen, then state that light energy and chlorophyll are required. |
| Saying a plant obtains its food from the soil | Roots absorb water and mineral ions, but the plant manufactures carbohydrates through photosynthesis. | Distinguish mineral nutrition from the manufacture of organic compounds. |
| Saying carbon dioxide travels from the roots to the leaves through xylem | Most carbon dioxide used by a leaf comes from the atmosphere and diffuses through stomata. | Use the pathway atmosphere → stomata → air spaces → mesophyll cells. |
| Saying more light always means a faster rate of photosynthesis | Once light is no longer limiting, another factor restricts the rate and the curve can level off. | Look for whether increasing light intensity actually increases the rate at the point shown. |
| Saying a higher temperature always increases photosynthesis | Photosynthesis involves enzymes. Above an optimum, high temperatures can reduce enzyme activity and eventually denature enzymes. | Describe a rise towards an optimum followed by a decrease at excessively high temperatures. |
| Assuming a plateau means photosynthesis has stopped | The rate can still be substantial on a plateau. | State that increasing the factor on the horizontal axis no longer increases the rate. |
| Assuming there can only ever be one limiting factor | Under some conditions, changing more than one environmental factor can affect the rate. | Use the data given rather than assuming that exactly one factor must be limiting. |
| Treating bubble count as an exact measurement of oxygen production | Bubbles can differ in volume. | Where possible, measure gas volume per unit time and repeat measurements. |
What You Should Be Able to Do
Define photosynthesis
Explain that green plants manufacture carbohydrates from carbon dioxide and water using light energy absorbed by chlorophyll.
Write both equations
State the word equation and the balanced symbol equation 6CO₂ + 6H₂O ⟶ C₆H₁₂O₆ + 6O₂.
Explain the role of chlorophyll
State that chlorophyll absorbs light energy, which is converted into chemical energy for the formation of carbohydrates.
Explain how the leaf is adapted
Link chloroplast distribution, stomata, air spaces and vascular bundles to photosynthesis and gaseous exchange.
Trace carbon dioxide into the leaf
Describe diffusion from the atmosphere through stomata and internal air spaces to mesophyll cells.
Explain the uses of carbohydrates
Link photosynthetic products to respiration, storage, transport and the formation of other biological molecules and structures.
Interpret limiting-factor graphs
Decide whether light intensity, carbon dioxide concentration or temperature is limiting by asking whether changing it would increase the rate.
Plan a photosynthesis investigation
Identify the independent, dependent and controlled variables, measure oxygen production appropriately and improve reliability through repeats.
These ten questions move beyond basic recall. Use equations, leaf adaptations, experimental reasoning and limiting-factor ideas to decide each answer.
- 10 mastery questions
- Application, graph and experimental reasoning
- Instant explanation for every answer option
Frequently asked questions
Photosynthesis: questions students often ask
Most photosynthesis mistakes come from mixing up raw materials with conditions, confusing photosynthesis with respiration, or treating limiting-factor graphs as rules to memorise instead of evidence to interpret.
Are light and chlorophyll reactants in photosynthesis?
No. The raw materials in the overall equation are carbon dioxide and water. Light supplies energy, while chlorophyll absorbs that light energy. They are therefore required for photosynthesis but should not be treated as ordinary reactants that are used up in the reaction equation.
Why do plants need respiration if they already photosynthesise?
Photosynthesis stores energy in organic compounds. Respiration releases usable energy from respiratory substrates so cells can carry out activities such as active transport, synthesis and growth. Plants therefore both photosynthesise and respire.
Do plants photosynthesise at night?
Photosynthesis requires light energy, so under normal outdoor conditions it stops when there is no usable light. Respiration, however, continues in living plant cells during both day and night.
Does carbon dioxide enter a plant through its roots?
Most carbon dioxide used for photosynthesis in leaves comes directly from the atmosphere. It diffuses through the stomata, through the internal air spaces and into the mesophyll cells.
Why are palisade mesophyll cells especially important for photosynthesis?
Palisade mesophyll cells are positioned near the upper surface of the leaf, where light is relatively strong, and they contain many chloroplasts. These features make them a major photosynthetic tissue.
Why does a photosynthesis graph eventually level off as light intensity increases?
At first, light may be the limiting factor, so increasing it raises the rate. Eventually light is available in sufficient amount and another factor, such as carbon dioxide concentration or temperature, becomes limiting. Further increases in light then produce little or no increase in rate.
Can more than one factor limit photosynthesis at the same time?
Yes. Biological systems do not always have one exclusive limiting factor. Experimental data may show that changing more than one factor could increase the rate. Use the evidence in the graph or experiment rather than assuming there must be exactly one limiting factor.
Why does high temperature eventually reduce the rate of photosynthesis?
Photosynthesis involves enzyme-controlled reactions. Moderate warming can increase reaction rates, but excessive temperatures disrupt enzyme structure and function. At sufficiently high temperatures enzymes may become denatured, causing the rate to fall.
Is counting oxygen bubbles a good way to measure photosynthesis?
It can provide a rough estimate, but bubbles are not necessarily equal in size. Measuring the volume of oxygen produced per unit time is more quantitative. Repeating measurements and calculating a mean also improves reliability.
Does all the glucose made during photosynthesis become starch?
No. Some carbohydrate may be stored as starch, but it can also be used in respiration, converted to sucrose for transport, used to make cellulose or converted into other organic compounds such as fats and amino acids.
Does a plateau on a photosynthesis graph mean the plant has stopped photosynthesising?
No. A plateau means the rate is no longer increasing as the factor on the horizontal axis increases. Photosynthesis may still be occurring rapidly; another factor is preventing a further increase.
What is the best way to identify a limiting factor in an exam graph?
Ask what would happen if the factor were increased from the point shown. If increasing it would increase the rate of photosynthesis, it is limiting the rate there. If increasing it produces no further increase, look for another factor that could be restricting the process.
Photosynthesis becomes much easier when you see it as an energy-and-materials story. Carbon dioxide diffuses in from the atmosphere, water arrives through the xylem, chlorophyll captures light energy, and the plant uses that energy to form carbohydrates. Those carbohydrates can then be respired, stored, transported or used to build new biological material. From there, limiting-factor questions are simply about asking what the plant needs next — and whether increasing that factor would allow photosynthesis to proceed faster.