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Photosynthesis uses light energy to build glucose from carbon dioxide and water, releasing oxygen. Chlorophyll in chloroplasts absorbs light. The reaction is endothermic overall because it transfers energy into the products. Glucose can be used in respiration, converted into starch for storage, incorporated into cellulose, converted to lipids or combined with mineral-derived nitrogen to make amino acids. Photosynthesis is therefore connected to growth as well as gas exchange.
Video by Cognito. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Describe the photosynthesis reaction
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: Which gas is released by photosynthesis?
Oxygen. Water and CO2 are used to form carbohydrate and oxygen.
After watching 2: What does chlorophyll absorb?
Light energy. It enables the light-dependent energy input.
What you'll be able to do
Photosynthesis uses light energy to build glucose from carbon dioxide and water, releasing oxygen. Chlorophyll in chloroplasts absorbs light. The reaction is endothermic overall because it transfers energy into the products. Glucose can be used in respiration, converted into starch for storage, incorporated into cellulose, converted to lipids or combined with mineral-derived nitrogen to make amino acids. Photosynthesis is therefore connected to growth as well as gas exchange.
Light intensity, carbon dioxide concentration and temperature can limit photosynthesis. Raising one factor increases rate only until another factor becomes limiting. Very high temperature can damage enzymes and lower rate. In a greenhouse, adding light or carbon dioxide costs money, so maximum possible rate need not maximise profit. Use the graph’s axes and plateau to identify which conclusion the data support, rather than assuming the same factor limits every point.
Aquatic plants can show oxygen production, but counting bubbles is approximate because bubble sizes vary. Collecting gas volume over a stated time can improve measurement. Control temperature and carbon dioxide availability while changing light intensity. Allow the plant to acclimatise, repeat readings and consider respiration occurring at the same time. Gas output measures net production, so no bubbles do not necessarily prove that no photosynthesis is occurring.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: Give two uses of glucose besides immediate respiration.
Starch storage and cellulose synthesis.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.