How do plants convert light energy into chemical energy?
Explain how photosynthesis uses light energy to convert carbon dioxide and water into glucose
Photosynthesis in chloroplasts uses light to convert carbon dioxide and water into glucose and oxygen, via light-dependent and light-independent reactions, limited by light, CO2 and temperature.
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What this dot point is asking
You need to explain the purpose of photosynthesis, write its overall equation, describe the two stages and where they occur, and explain the factors that limit its rate.
Why photosynthesis matters
Photosynthesis is how plants, algae and some bacteria capture light energy and store it as chemical energy in glucose. It is the foundation of almost all food chains and the source of atmospheric oxygen. The overall equation is:
carbon dioxide + water gives glucose + oxygen (using light energy)
It takes place in chloroplasts, where the green pigment chlorophyll absorbs light. Chloroplasts have internal membranes (thylakoids stacked into grana) surrounded by a fluid stroma.
The two stages
Light-dependent reactions
These occur in the thylakoid membranes and require light:
- Chlorophyll absorbs light energy.
- Water is split (photolysis), releasing oxygen as a by-product.
- The energy is used to produce ATP and NADPH (energy-carrying molecules).
Light-independent reactions (Calvin cycle)
These occur in the stroma and do not directly need light:
- The ATP and NADPH from the light-dependent stage provide energy.
- Carbon dioxide is fixed and combined to build glucose.
The two stages are linked: the light-dependent stage supplies the energy carriers that the light-independent stage spends.
Limiting factors
The rate of photosynthesis is controlled by whichever factor is in shortest supply, the limiting factor:
- Light intensity. More light increases the rate until another factor becomes limiting; in darkness, photosynthesis stops.
- Carbon dioxide concentration. More carbon dioxide increases the rate up to a point.
- Temperature. The reactions are enzyme-controlled, so the rate rises with temperature to an optimum, then falls as enzymes denature.
Exam-style practice questions
Practice questions written in the style of SACE Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
2018 SACE Stage 22 marksStudents exposed spinach leaves to light of increasing intensity, resulting in an increased concentration of oxygen in the air around the leaves. Explain why increasing the light intensity resulted in an increased concentration of oxygen.Show worked answer →
For 2 marks, link light to the rate of photosynthesis, then to oxygen.
Light provides the energy for the light-dependent reactions of photosynthesis. Increasing light intensity increases the rate of photosynthesis (until another factor becomes limiting).
Photosynthesis produces oxygen as a by-product (from the splitting of water), so a faster rate of photosynthesis releases more oxygen, increasing the oxygen concentration in the air around the leaves.
2018 SACE Stage 22 marksUsing graphs of photosynthesis and respiration rate against light intensity, explain why the concentration of oxygen in the air surrounding the leaves remains constant at a particular light intensity.Show worked answer →
For 2 marks, identify the balance point.
At this light intensity the rate of photosynthesis equals the rate of respiration (the compensation point).
The oxygen produced by photosynthesis is used up at the same rate by respiration, so there is no net gain or loss of oxygen, and the oxygen concentration in the surrounding air stays constant.
2019 SACE Stage 22 marksSpecies D is a bacterium that cannot grow without oxygen (an obligate aerobe). Cyanobacteria are able to photosynthesise. Explain why it is unlikely that species D is a cyanobacterium.Show worked answer →
For 2 marks, connect photosynthesis to oxygen.
Cyanobacteria photosynthesise, and photosynthesis produces oxygen, so a cyanobacterium can generate its own oxygen and would be expected to grow even at low oxygen concentrations.
Species D, however, only grows when oxygen is supplied from outside (it is an obligate aerobe and shows no growth at 0% oxygen), which is inconsistent with an organism that can make its own oxygen by photosynthesis. Therefore species D is unlikely to be a cyanobacterium.