Ch 3 · Anaerobic Respiration · O Level Biology 5090
Topic 3 of 3 in Respiration — O Level Biology 5090 — 38 lessons.
Introduction
A sprinter's leg muscles use oxygen faster than the blood can deliver it, yet they keep contracting. Bread dough rises and beer ferments in sealed containers with no air. Both depend on anaerobic respiration: releasing energy from glucose without oxygen. It is a poor substitute for aerobic respiration, releasing only a fraction of the energy, but it keeps cells going when oxygen runs short. candidates must also explain why you keep panting after the race is over.
Key ideas — Definition and word equations
Anaerobic respiration is the chemical reactions in cells that break down nutrient molecules to release energy without using oxygen. The products depend on the organism.
🧑🏫 Teacher's explanation
Imagine you are running a marathon. Your muscles need energy to keep moving, but the oxygen you breathe in cannot get to your muscle cells fast enough. Your body has a backup plan: it breaks down glucose without using oxygen. This is anaerobic respiration. It is like using a small, inefficient battery instead of a large, efficient power plant. It works, but it produces waste products and much less energy.
📖 Definitions
🔍 Worked example — Easy
State the word equation for anaerobic respiration in yeast.
🔍 Worked example — Standard
Explain why a sprinter feels a 'burning' sensation in their legs after a 100m race, but a marathon runner does not feel this immediately after finishing.
🔍 Worked example — Exam-style
Calculate the number of carbon atoms in the products of the balanced equation for anaerobic respiration in yeast: C_6H_12O_6 arrow 2C_2H_5OH + 2CO_2.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
(b) Explain why a person's breathing rate remains elevated for several minutes after finishing a sprint. [3 marks] [5 marks]
Key ideas — Much less energy per glucose molecule
Anaerobic respiration releases much less energy per glucose molecule than aerobic respiration. In aerobic respiration glucose is completely oxidised to carbon dioxide and water, so nearly all its chemical energy is released. In anaerobic respiration the breakdown is incomplete: lactic acid and ethanol are still energy-rich molecules, so most of the energy stays locked inside them.
🧑🏫 Teacher's explanation
Think of a glucose molecule as a full tank of petrol. Aerobic respiration is like a highly efficient engine that burns the petrol completely, extracting almost all the energy to power the car for a long journey. Anaerobic respiration is like a faulty engine that only burns a tiny fraction of the petrol; most of the fuel remains unburned and is dumped as waste products (lactic acid or ethanol). Because so little energy is extracted per molecule, the cell must burn through glucose much faster to keep up with demand.
📖 Definitions
🔍 Worked example — Easy
State the word equation for anaerobic respiration in yeast.
🔍 Worked example — Standard
Explain why a sprinter's breathing rate remains high for several minutes after finishing a race.
🔍 Worked example — Exam-style
Aerobic respiration releases approximately 38 ATP molecules per glucose, while anaerobic respiration releases 2 ATP molecules per glucose. Calculate the percentage of energy released by anaerobic respiration compared to aerobic respiration.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Key ideas — Oxygen debt and recovery
the oxygen debt. During vigorous exercise, the heart and lungs cannot supply oxygen to the muscles as fast as it is needed, so the muscles respire anaerobically as well as aerobically. Lactic acid builds up in the muscle cells and diffuses into the blood, lowering blood pH and contributing to muscle fatigue. The lactic acid must be removed, and this needs oxygen. The oxygen debt, also called Excess Post-exercise Oxygen Consumption (EPOC), is the extra oxygen taken in after exercise to break down the accumulated lactic acid.
🧑🏫 Teacher's explanation
Imagine you are running a marathon. Your muscles are like engines that need a steady supply of fuel (glucose) and air (oxygen) to run smoothly. When you sprint, the demand for energy spikes so high that the 'air supply' (oxygen from your lungs) cannot keep up. The engine switches to a backup mode: anaerobic respiration. This mode works without oxygen but is inefficient and produces a waste product called lactic acid.
📖 Definitions
🔍 Worked example — Easy
State the word equation for anaerobic respiration in human muscle cells during vigorous exercise.
🔍 Worked example — Standard
Explain why the breathing rate remains high after vigorous exercise has stopped.
🔍 Worked example — Exam-style
A student runs a 100m sprint. Their heart rate is 180 bpm during the sprint and remains at 150 bpm for 5 minutes afterwards. Explain the physiological reason for the sustained high heart rate during the recovery period.
⚠️ Common errors
🧠 Quick check 1
🧠 Quick check 2
🎯 Exam question
Worked example
🔍 Worked example — Easy
State the word equation for anaerobic respiration in yeast cells.
🔍 Worked example — Standard
Compare the energy yield of anaerobic respiration in muscles with aerobic respiration. Why is anaerobic respiration less efficient?
🔍 Worked example — Exam-style
A student runs 400 m as fast as possible. Her breathing rate and heart rate remain above resting values for several minutes after she stops. Explain why.
⚠️ Common errors
Common mistakes
Practice
Answers