Respiration and Photosynthesis
Respiration and photosynthesis are the two linked biological processes that living things use to store and release energy. Photosynthesis happens in the chloroplasts of green plants and algae, using light energy to turn carbon dioxide and water into glucose and oxygen, while respiration in every living cell's mitochondria releases energy from glucose.
Method
- Learn and be able to write the word equations for photosynthesis and for aerobic respiration.
- Learn the balanced symbol equations and check they balance, with equal atoms of each element on both sides.
- Compare aerobic and anaerobic respiration in humans and in yeast, including their products.
- Learn the required practical steps for testing a leaf for starch and interpreting the colour change.
- Practise interpreting rate graphs linking light intensity, carbon dioxide concentration and temperature to the rate of photosynthesis.
- Understand limiting factors and how they explain the shape of rate graphs.
Worked example
A group of students counts bubbles of oxygen released by pondweed at three distances from a lamp: 5cm, 10cm and 20cm, giving rates of 48, 12 and 3 bubbles per minute. Show that the rate of photosynthesis is inversely proportional to the square of the distance for this data.
- Square each distance: 5 squared = 25, 10 squared = 100, 20 squared = 400.
- Multiply the first rate by its distance squared: 48 x 25 = 1200.
- Repeat for the other two rows: 12 x 100 = 1200 and 3 x 400 = 1200.
- Compare the three values: all three equal 1200.
- State the conclusion: since rate x distance squared is constant, rate is inversely proportional to distance squared.
Practice questions
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Q1State the gas absorbed by a plant from the air for use in photosynthesis.Show answer
Answer: Carbon dioxide
Q2Name the organelle where most aerobic respiration reactions take place in a cell.Show answer
Answer: Mitochondria
Q3State one product of anaerobic respiration in yeast, other than energy.Show answer
Answer: Ethanol (or carbon dioxide)
Q4The equation for photosynthesis is 6CO2 + 6H2O -> C6H12O6 + 6O2. Calculate the total number of oxygen atoms on the reactant (left-hand) side of this equation.Show answer
Answer: 18 (6CO2 has 6 x 2 = 12 oxygen atoms; 6H2O has 6 x 1 = 6 oxygen atoms; 12 + 6 = 18)
Q5A pupil counts bubbles from pondweed over three repeats: 22, 24 and 35 bubbles per minute. One result is anomalous. Calculate the mean of the two remaining results.Show answer
Answer: 23 bubbles per minute (35 is anomalous; (22+24)/2 = 23)
Q6At CO2 concentrations of 500 ppm, 900 ppm and 1300 ppm, the rate of photosynthesis of a plant was 8, 15 and 15.5 (arbitrary units), with light and temperature kept constant at optimal levels. Explain why the rate barely changes between 900 ppm and 1300 ppm.Show answer
Answer: CO2 is no longer the limiting factor above about 900 ppm; another factor (e.g. light intensity or temperature) is limiting the rate instead.
Exam-style questions
Written in the style of a KS3 Science exam paper, with a full mark scheme.
Explain, in terms of chloroplasts and mitochondria, why plant cells (unlike animal cells) can carry out both photosynthesis and respiration.
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A student measures the rate of oxygen bubble production from pondweed at three light intensities: at intensity 1 the rate is 6 bubbles/min, at intensity 4 the rate is 24 bubbles/min, and at intensity 9 the rate is 54 bubbles/min. Show that the rate is directly proportional to light intensity for this data, and use your result to predict the rate at intensity 12.
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A commercial grower cultivates tomato plants in a glasshouse. On cold, dull days in winter she uses electric heaters and grow-lights, and also releases extra carbon dioxide gas. Explain how light intensity, temperature and carbon dioxide concentration can each affect the rate of photosynthesis in the tomato plants, and explain why increasing all three at once will not increase the rate of photosynthesis indefinitely.
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Free printable worksheet
Want more practice on paper? Download the respiration and photosynthesis worksheet pack - 18 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
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