Ch 1 · Respiration · O Level Biology 5090
Topic 1 of 3 in Respiration — O Level Biology 5090 — 48 lessons.
Introduction
Ask most students what respiration is and they will say "breathing". In biology the word means something different: respiration is the set of chemical reactions inside every living cell that releases energy from food. Breathing simply delivers the oxygen and removes the carbon dioxide. This lesson establishes the precise Cambridge definition, lists what the released energy is used for, and shows how yeast can be used to investigate the rate of respiration at different temperatures.
Key ideas — What the body uses energy for
Cells need a continuous supply of energy. The syllabus lists seven uses you must be able to state:
🧑🏫 Teacher's explanation
Think of your body as a city that never sleeps. Every building (cell) needs electricity to run its lights, lift, and air conditioning. Respiration is the power plant that generates this electricity (ATP) by burning fuel (glucose). Without a constant supply, the city shuts down. The seven uses of energy are simply the different departments in that city: the transport system (muscles), the factories (protein synthesis), the construction crews (growth/division), the logistics team (active transport), the communication network (nerves), and the heating system (temperature).
📖 Definitions
🔍 Worked example — Easy
State two uses of energy in the body that are NOT related to movement.
🔍 Worked example — Standard
Explain why active transport requires energy, whereas diffusion does not.
🔍 Worked example — Exam-style
A student investigates the effect of temperature on respiration in yeast by measuring the volume of carbon dioxide produced at 10^circC, 20^circC, 30^circC, and 40^circC. The volumes after 10 minutes are 2 cm^3, 8 cm^3, 15 cm^3, and 5 cm^3 respectively. Calculate the percentage increase in respiration rate from 10^circC to 30^circC, and explain the decrease at 40^circC.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(b) List three uses of energy in the body. (3 marks) (c) Explain why the rate of respiration increases with temperature up to a certain point, and then decreases. (3 marks) [8 marks]
Key ideas — The definition of respiration
Respiration is the chemical reactions in cells that break down nutrient molecules and release energy for metabolism. Learn this wording exactly; "describe" questions are marked against it.
🧑🏫 Teacher's explanation
Think of your body as a city that never sleeps. It needs a constant supply of power to keep the lights on, the trains running, and the temperature comfortable. Respiration is the power plant inside every single cell that generates this electricity. It does not create energy from nothing; instead, it burns fuel (like glucose) to release the energy already stored in its chemical bonds. This is why we say respiration 'releases' energy, never 'makes' it.
📖 Definitions
🔍 Worked example — Easy
Define respiration using the exact Cambridge syllabus wording. Distinguish it from breathing.
🔍 Worked example — Standard
List four specific uses of the energy released by respiration in the human body.
🔍 Worked example — Exam-style
An exam question asks: 'Explain why respiration is essential for the growth of a plant seedling.' Use the definition of metabolism to support your answer.
⚠️ Common errors
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🎯 Exam question
(b) State two uses of the energy released by respiration in the body. [2 marks] (c) Explain why plants must carry out respiration even though they can photosynthesize. [2 marks] [6 marks]
Key ideas — Respiration is controlled by enzymes
Respiration is not one reaction but a long series of small steps, and each step is catalysed by a specific enzyme inside the cell. This has an important consequence: the rate of respiration depends on the same factors that affect enzyme activity, especially temperature and pH.
🧑🏫 Teacher's explanation
Think of respiration as a factory assembly line. The raw material (glucose) enters, and a series of machines (enzymes) process it step-by-step to release energy. If one machine breaks or slows down, the whole line stops or slows. Enzymes are biological catalysts that speed up these specific chemical reactions without being used up.
📖 Definitions
🔍 Worked example — Easy
Explain why the rate of respiration increases as temperature rises from 10^circC to 37^circC in human cells.
🔍 Worked example — Standard
A student investigates the effect of temperature on yeast respiration. The rate of CO_2 production is measured at 10^circC, 20^circC, 30^circC, 40^circC, and 50^circC. The rates are 2, 5, 12, 18, and 3 units per minute respectively. Explain the shape of the graph.
🔍 Worked example — Exam-style
A patient has a fever of 42^circC. Explain why this is potentially life-threatening in terms of cellular respiration.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) State the optimum temperature for respiration in this mammal. [1] (b) Explain why the rate of respiration increases between 10^circC and 37^circC. [2] (c) Explain why the rate of respiration decreases at temperatures above 40^circC. [2] [5 marks]
Key ideas — Investigating temperature and respiration in yeast
Yeast is a single-celled fungus that respires glucose and releases carbon dioxide, so the rate of bubble production measures its rate of respiration.
🧑🏫 Teacher's explanation
Imagine a factory where workers (enzymes) assemble products (energy) from raw materials (glucose). If the factory is too cold, the workers move slowly and production is low. If it is too hot, the workers melt and stop working entirely. Yeast respiration follows this pattern: as temperature rises, the chemical reactions speed up, but only until the enzymes denature.
📖 Definitions
🔍 Worked example — Easy
A student sets up a yeast respiration experiment at 20^circC. Explain why a thin layer of oil is placed on the surface of the liquid in the boiling tube.
🔍 Worked example — Standard
In an investigation into the effect of temperature on yeast respiration, a student records the following data for the volume of carbon dioxide produced in 10 minutes: 20^circC: 5 cm^3 30^circC: 15 cm^3 40^circC: 25 cm^3 50^circC: 10 cm^3 60^circC: 0 cm^3 Calculate the rate of respiration (in cm^3 per minute) at 40^circC.
🔍 Worked example — Exam-style
Explain why the rate of respiration in yeast decreases sharply when the temperature is increased from 40^circC to 60^circC, even though higher temperatures usually increase reaction rates.
⚠️ Common errors
🧠 Quick check 1
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🎯 Exam question
(a) State one variable that must be controlled in this experiment. (1 mark) (b) Explain why the oil layer is necessary. (2 marks) (c) The student records the volume of carbon dioxide produced in 10 minutes as follows: 20^circC: 4 cm^3 30^circC: 12 cm^3 40^circC: 20 cm^3 50^circC: 8 cm^3 60^circC: 0 cm^3
Worked example
🔍 Worked example — Easy
State the primary gas produced during anaerobic respiration in yeast and one use of the energy released.
🔍 Worked example — Standard
A student investigates the effect of temperature on yeast respiration. At 30^circC, the rate is 10 bubbles/min. At 50^circC, the rate is 0 bubbles/min. Explain the difference in rates.
🔍 Worked example — Exam-style
A student measured the number of bubbles of gas produced per minute by a yeast and glucose mixture at 20^circC, 40^circC and 60^circC. The results were 6, 24 and 0 bubbles per minute. Explain these results.
⚠️ Common errors
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