Photosynthesis and Respiration.
Notes
Photosynthesis
Endothermic reaction in chloroplasts, using light energy.
- Word equation: carbon dioxide + water → glucose + oxygen
- Symbol equation: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
Glucose is used for respiration, converted to starch for storage, to cellulose for cell walls, to lipids for seeds, and to amino acids (with nitrate ions) for protein.
Limiting factors
The factor in shortest supply sets the rate. For photosynthesis: light intensity, carbon dioxide concentration, temperature (and amount of chlorophyll).
- On a graph, the rate rises then levels off. The plateau means another factor is now limiting.
- Inverse square law (higher tier): light intensity ∝ 1/d². Doubling the distance from the lamp cuts intensity to ¼.
Respiration
Exothermic, occurs in all living cells.
- Aerobic: glucose + oxygen → carbon dioxide + water (mitochondria). C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O
- Anaerobic in animals: glucose → lactic acid
- Anaerobic in yeast (fermentation): glucose → ethanol + carbon dioxide
- Anaerobic respiration releases much less energy per glucose than aerobic.
Exercise and oxygen debt
During vigorous exercise the muscles respire anaerobically, producing lactic acid. Afterwards the extra oxygen needed to break lactic acid down is the oxygen debt. This is why breathing stays fast after exercise.
Metabolism
The sum of all the chemical reactions in a cell or the body, including building molecules (such as glucose to starch) and breaking them down.
Required practical: effect of light intensity
Move a lamp to different distances from pondweed and count bubbles of oxygen per minute. Control temperature using a water bath or heat shield.
Common mistakes
- Saying plants photosynthesise only in the day and respire only at night. They respire all the time.
- Describing respiration as "breathing".
- Saying a plateau means the reaction has stopped. It means something else is limiting.
Exam tips
- State which limiting factor applies to each part of a graph.
- For inverse square questions, show the ratio of distances squared.
Worksheet
Total: 27 marks.
- Write the balanced symbol equation for photosynthesis, and state whether the reaction is endothermic or exothermic. (3)
- The light intensity 10 cm from a lamp is 80 units. Use the inverse square law to calculate the light intensity at 20 cm. (3)
- A graph shows the rate of photosynthesis rising as light intensity increases, then levelling off.
(a) Explain why the graph levels off. (2)
(b) Name two factors that could be limiting at the plateau. (2) - Glucose is made in the leaves. Describe three ways the plant uses glucose. (3)
- Compare aerobic and anaerobic respiration in humans. Include the word equations and the amount of energy released. (5)
- Explain why a person continues to breathe quickly and deeply after a sprint has finished. (4)
- Write the word equation for anaerobic respiration in yeast, and name one industry that uses it. (3)
- Explain why a tropical greenhouse owner might raise the carbon dioxide concentration but not the light intensity on a sunny day. (2) (Challenge)
Show mark scheme and answers (tutor copy)
Try the worksheet first, then check your work here.
- 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (B1 correct reactants and products, B1 correctly balanced). The reaction is endothermic, taking in energy from light. (B1)
- Intensity ∝ 1/d² (M1). The distance doubles, so intensity falls to 1/4 (M1). 80 ÷ 4 = 20 units. (A1)
- (a) Another factor has become limiting (B1), so increasing light no longer increases the rate (B1).
(b) Any two of: carbon dioxide concentration, temperature, chlorophyll amount. (B1 each) - Any three: respiration (release energy); converted to starch for storage; converted to cellulose for cell walls; converted to lipids for storage in seeds; combined with nitrate ions to make amino acids and then proteins. (B1 each)
- Aerobic: glucose + oxygen → carbon dioxide + water (B1). Anaerobic: glucose → lactic acid (B1). Aerobic uses oxygen, anaerobic does not (B1). Aerobic releases much more energy per glucose (B1). Anaerobic is incomplete breakdown of glucose (B1).
- Muscles respired anaerobically during the sprint, producing lactic acid (B1). Extra oxygen is needed to break down the lactic acid (B1). This extra oxygen is the oxygen debt (B1). Fast, deep breathing and a fast heart rate deliver the oxygen until the debt is repaid (B1).
- Glucose → ethanol + carbon dioxide (B1, B1). Industry: brewing or baking. (B1)
- On a sunny day, light is already plentiful, so extra light will not increase the rate (B1). Carbon dioxide is more likely to be the limiting factor, so raising it raises the rate (B1).