When oxygen runs short, cells can still release a little energy by breaking glucose down only partly. Yeast does it to give alcohol and carbon dioxide, which is how bread rises and beer is brewed. Your muscles do it in a sprint, making lactic acid. The balanced yeast equation and the oxygen debt that follows exercise are Extended content.
Anaerobic respiration is the chemical reactions in cells that break down nutrient molecules to release energy without using oxygen. It releases much less energy per glucose molecule than aerobic respiration. In yeast: glucose → alcohol + carbon dioxide. In muscles during vigorous exercise: glucose → lactic acid. Extended: C6H12O6 → 2C2H5OH + 2CO2; lactic acid builds up in muscles and blood, causing an oxygen debt that is repaid after exercise.
Fig. 1: during a sprint the oxygen taken in falls short of the oxygen needed, so muscles also respire anaerobically; after exercise extra oxygen is taken in to repay the oxygen debt
Section 01
Without oxygen.
Go back to the Respiration tab and choose anaerobic in yeast, then anaerobic in muscles. The mitochondrion disappears, no oxygen arrives, and only two yellow sparks appear instead of a shower.
Aerobic
Anaerobic (yeast)
Anaerobic (muscles)
Oxygen used?
yes
no
no
Products
carbon dioxide + water
alcohol + carbon dioxide
lactic acid
Energy per glucose
a lot
much less
much less
Glucose broken down
completely
only partly
only partly
Definition: anaerobic respiration is the chemical reactions in cells that break down nutrient molecules to release energy without using oxygen.
Much less energy: the products (alcohol, lactic acid) still contain a lot of chemical energy, because glucose is only partly broken down.
Examiner trap
Anaerobic respiration in muscles makes only lactic acid: no carbon dioxide and no alcohol. Only the yeast equation has carbon dioxide. Never write "glucose → lactic acid + carbon dioxide".
Section 02
Yeast, bread and beer.
The yeast in the Yeast tab respires sugar without oxygen, and the carbon dioxide inflates the balloon. The same reaction is used in industry.
Word equation (yeast): glucose → alcohol + carbon dioxide. The alcohol is ethanol.
Extended, balanced: C6H12O6 → 2C2H5OH + 2CO2. Check: 6 C, 12 H and 6 O on each side.
Bread-making: carbon dioxide bubbles make the dough rise; the alcohol evaporates during baking.
Brewing and biofuel: the alcohol is the useful product (beer, wine, bioethanol).
Temperature: as in the sim, fermentation is fastest at a warm optimum (about 35 to 40 °C) and stops if the yeast is overheated.
Examiner trap
The balanced yeast equation has two ethanol and two carbon dioxide, and no oxygen on either side. Ethanol is C2H5OH, not CH3OH.
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
Quick check+10 XP
In the Respiration tab, what changes when you switch from aerobic to anaerobic in muscles?
Quick check+10 XP
Which word equation does the sim show for anaerobic respiration in yeast?
Quick check+10 XP
In the Oxygen debt tab, after you press Stop and rest, what does the heart rate and breathing card show while lactic acid remains?
Quick check+10 XP
In the Oxygen debt tab, where does the yellow lactic acid go after the sprint ends?
Section 03
Sprinting into oxygen debt.
Extended. Press Sprint in the Oxygen debt tab, then Stop and rest. Watch the three lines on the graph (oxygen needed, oxygen taken in, lactic acid), the yellow lactic acid moving from the leg muscle to the liver, and the heart rate and breathing card.
During vigorous exercise: the blood cannot deliver oxygen fast enough, so muscles also respire anaerobically. Lactic acid builds up in the muscles and blood, causing an oxygen debt.
Fast heart rate continues: to transport lactic acid in the blood from the muscles to the liver.
Deeper and faster breathing continues: to supply oxygen for the aerobic respiration of lactic acid.
In the liver: lactic acid is respired aerobically. When it is all gone the debt is repaid and the sim's card reads "back to normal".
Examiner trap
The oxygen debt is the extra oxygen needed after exercise to remove lactic acid, not a shortage of oxygen in the air. Lactic acid is broken down in the liver, and it is carried there by the blood.
Stage 2 · Exam
Exam-style questions
Unlocks once the four checks above are done. Worth more XP, written to IGCSE Paper 2 and Paper 4 standard.
Finish the four checks above to unlock the exam questions
Exam style+20 XP
Which statement about anaerobic respiration is correct?
Exam style+20 XP
Extended: which is the balanced equation for anaerobic respiration in yeast?
Exam style+20 XP
Extended: after a race an athlete keeps breathing deeply and quickly. The main reason is to:
Exam style+20 XP
Why is the dough for bread left in a warm place before baking?
Skill unlocked
Anaerobic respiration, mastered.
This skill is now recorded on your profile. Keep the chain going.