Ch 2 · Aerobic Respiration · O Level Biology 5090
Topic 2 of 3 in Respiration — O Level Biology 5090 — 38 lessons.
Introduction
Most of the energy your cells release comes from aerobic respiration, the complete breakdown of glucose using oxygen. This is why you breathe, why your heart pumps blood to every tissue, and why a few minutes without oxygen is fatal. This lesson gives the Cambridge definition and equations, and then looks at the two practical methods examiners favour for measuring how fast an organism takes up oxygen.
Key ideas — Definition and equations
Aerobic respiration is the chemical reactions in cells that use oxygen to break down nutrient molecules to release energy. The key word is oxygen: aerobic means "with air".
🧑🏫 Teacher's explanation
Think of aerobic respiration as the cell's power station. Just as a car engine burns petrol (glucose) with air (oxygen) to move the car (do work), your cells burn glucose with oxygen to generate the energy currency ATP. Without oxygen, the engine sputters and stops; with it, the engine runs smoothly and efficiently.
📖 Definitions
🔍 Worked example — Easy
State the word equation for aerobic respiration and identify the site of the reaction.
🔍 Worked example — Standard
Balance the chemical equation for aerobic respiration: C_6H_12O_6 + O_2 arrow CO_2 + H_2O.
🔍 Worked example — Exam-style
A student writes: Glucose + Oxygen arrow Carbon Dioxide + Water + Energy. Explain why this is incorrect and how to correct it.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Measuring oxygen uptake with hydrogen carbonate indicator
Hydrogen carbonate indicator changes colour with the concentration of dissolved carbon dioxide, which in turn tells you how fast an organism is respiring aerobically:
🧑🏫 Teacher's explanation
Imagine you are in a sealed room with a friend who is breathing heavily. You cannot see the oxygen they are using up, but you can smell the stale air filling the room. In biology, we use a similar trick. We cannot see oxygen disappearing from a sealed tube, but we can see the carbon dioxide being produced. Hydrogen carbonate indicator is our 'smell test' for carbon dioxide. It acts like a chemical mood ring that changes colour depending on how much carbon dioxide is dissolved in the water around it.
📖 Definitions
🔍 Worked example — Easy
A student places a snail in a sealed tube with hydrogen carbonate indicator. After 10 minutes, the indicator is yellow. Explain why the indicator changed colour.
🔍 Worked example — Standard
A student sets up two sealed tubes. Tube A contains germinating seeds and indicator. Tube B contains boiled seeds and indicator. Both start red. After 24 hours, Tube A is yellow and Tube B remains red. Explain the difference in results.
🔍 Worked example — Exam-style
A student places a green leaf in a sealed tube with hydrogen carbonate indicator. The indicator turns purple after 1 hour in bright light. Explain why the indicator turned purple, referring to the rates of respiration and photosynthesis.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Measuring oxygen uptake with a respirometer
A respirometer measures the volume of oxygen absorbed in a known time.
🧑🏫 Teacher's explanation
Imagine you are in a sealed room with a friend who is breathing. If you could somehow remove the carbon dioxide they exhale, the air volume in the room would shrink because they are taking in oxygen. A respirometer works exactly like this: it is a sealed tube where the carbon dioxide produced by the organisms is absorbed by a chemical, so the only change in gas volume is the oxygen being used up.
📖 Definitions
🔍 Worked example — Easy
In a respirometer experiment, the liquid marker moves 15 mm towards the organisms in 10 minutes. Calculate the rate of oxygen uptake in mm/min.
🔍 Worked example — Standard
An experimental tube with germinating peas shows a liquid movement of 20 mm in 10 minutes. A control tube with glass beads shows a movement of 2 mm in the same time due to a slight temperature increase. Calculate the corrected rate of oxygen uptake.
🔍 Worked example — Exam-style
A student investigates the effect of temperature on the rate of respiration in woodlice. At 10^circC, the corrected rate is 0.5 mm/min. At 20^circC, the corrected rate is 1.0 mm/min. At 30^circC, the corrected rate is 1.8 mm/min. Explain the trend observed and why the rate increases with temperature.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(a) Explain why a control tube is necessary in this experiment. (2 marks)
Worked example
🔍 Worked example — Easy
State the word equation for aerobic respiration.
🔍 Worked example — Standard
A student sets up a respirometer with germinating seeds but forgets to add soda lime. The liquid in the manometer does not move. Explain why.
🔍 Worked example — Exam-style
In a respirometer experiment, the liquid in the experimental tube moves 15 mm towards the seeds in 10 minutes. The liquid in the control tube moves 5 mm towards the beads in the same time. Calculate the corrected rate of oxygen uptake due to respiration.
⚠️ Common errors
Common mistakes
Practice
Answers