How do cells release energy by respiration and how do plants make glucose by photosynthesis?
Aerobic and anaerobic respiration and their word equations, photosynthesis as an endothermic reaction, the limiting factors of light intensity, carbon dioxide concentration and temperature, and the inverse square law relating light intensity to distance.
A focused answer to the OCR Gateway GCSE Combined Science A topic B1 on respiration and photosynthesis, covering aerobic and anaerobic respiration, photosynthesis as an endothermic reaction, limiting factors, and the inverse square relationship between light intensity and distance.
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What this topic is asking
OCR wants you to write the word equations for aerobic and anaerobic respiration, describe photosynthesis as an endothermic reaction that stores energy, explain the limiting factors of photosynthesis, and use the inverse square relationship between light intensity and distance.
Respiration
Aerobic respiration happens in the mitochondria, uses oxygen, and releases a large amount of energy by fully breaking glucose down. Its word equation is:
Anaerobic respiration releases energy without oxygen, but far less per glucose molecule because the glucose is not fully broken down. In human muscle during hard exercise the equation is glucose produces lactic acid; in yeast (used in brewing and baking) it is glucose produces ethanol + carbon dioxide, a process called fermentation. The energy from respiration is used for muscle contraction, keeping warm, building large molecules and active transport. OCR expects you to know that aerobic respiration releases much more energy than anaerobic respiration from the same amount of glucose.
Photosynthesis
Photosynthesis is an endothermic reaction: it takes in energy (from light) rather than releasing it. It happens in chloroplasts, where chlorophyll absorbs light, and its word equation is:
The glucose made is used for respiration, converted to starch for storage, used to make cellulose for cell walls, combined with nitrogen (from nitrate ions) to make amino acids and proteins, or used to make lipids. Because photosynthesis is the source of food and oxygen for almost all life, OCR treats it as a central process linking to ecology and the carbon cycle.
Light intensity and distance
Light intensity falls rapidly as the distance from the lamp increases, following an inverse square law: light intensity , where is the distance from the source. So doubling the distance does not halve the intensity, it quarters it. In the required practical you place a piece of pondweed (or algal balls) at measured distances from a lamp and count the bubbles of oxygen released per minute as a measure of the rate. Plotting the rate against rather than against distance gives a more useful straight-line relationship.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20186 marksA grower measures the rate of photosynthesis of a plant as light intensity is increased. Explain why the rate rises at first and then levels off, and how the grower could increase the maximum rate.Show worked answer →
A Biology Paper 1 six-mark extended response on limiting factors, marked on levels. Reward: at low light intensity, light is the limiting factor, so as it increases the rate of photosynthesis rises in proportion. At higher light intensity the graph levels off because some other factor (carbon dioxide concentration or temperature) has become the limiting factor and light is no longer what holds the rate back. To increase the maximum rate the grower could raise the carbon dioxide concentration or the temperature (up to the optimum), so that factor is no longer limiting. Top answers name the limiting factor in each region, link the plateau to a different factor, and avoid saying "the plant is full".
OCR 20204 marksCompare aerobic and anaerobic respiration in human muscle cells, including the products of each.Show worked answer →
A B1 structured comparison. Reward: both release energy from glucose for the cell. Aerobic respiration uses oxygen, fully breaks down glucose, and releases much more energy per glucose molecule; its products are carbon dioxide and water (word equation: glucose + oxygen produces carbon dioxide + water). Anaerobic respiration happens when oxygen is in short supply (such as hard exercise), does not use oxygen, releases much less energy, and in muscle produces lactic acid (glucose produces lactic acid). Markers credit the role of oxygen, the relative energy yield, and the correct products; a common error is giving carbon dioxide as a product of anaerobic respiration in muscle, which is wrong.
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