Aerobic respiration uses oxygen and releases a lot of energy. Anaerobic respiration works without oxygen and releases less. The same starting substance, glucose, leads to different products depending on the conditions and the organism.
This lesson follows distinguishing gas exchange from respiration and leads into experiment data on respiration.
What are the models?
The equations below are word equations. Energy is written in brackets because it is released, not a substance that is made.
| Type | Word equation |
|---|---|
| Aerobic | glucose + oxygen → carbon dioxide + water (+ energy released) |
| Anaerobic in muscle | glucose → lactic acid (+ a smaller amount of energy released) |
| Anaerobic in yeast | glucose → ethanol + carbon dioxide (+ a smaller amount of energy released) |
The balanced symbol equation for the aerobic model is C6H12O6 + 6O2 → 6CO2 + 6H2O. Count the atoms on each side to check: 6 C, 12 H and 18 O on both sides.
What does “a smaller amount” mean?
Aerobic respiration breaks glucose down completely, so nearly all the chemical energy stored in it is released. In anaerobic respiration the glucose is only partly broken down, so a lot of the energy remains stored in the lactic acid or ethanol. Less energy is released from each glucose molecule, but it can be released quickly without oxygen.
Worked example
A student sprints 100 m, then jogs for 20 minutes. Explain which type of respiration dominates in each case, using the models. This is a simplified model of the body.
Step 1, the sprint: the muscles need energy very fast and the oxygen supply cannot keep up with the demand. So some respiration in the muscle cells is anaerobic: glucose → lactic acid, with energy released.
Step 2, the jog: the demand is lower and steady, and breathing and circulation can supply enough oxygen. So most respiration is aerobic: glucose + oxygen → carbon dioxide + water, with energy released.
Step 3, compare the energy: aerobic respiration releases more energy per glucose, so it supports a longer period of activity.
Step 4, after the sprint: the student keeps breathing hard for a while. The extra oxygen helps break down the lactic acid that built up, and some syllabuses call this the oxygen debt. Check your syllabus for the exact term and depth.
Step 5, yeast contrast: in a sealed flask of yeast and sugar solution, the yeast respires anaerobically. The gas that bubbles off is carbon dioxide and the liquid contains ethanol.
The mistake to watch for
A plausible mistake is to say that anaerobic respiration “makes no energy” or that muscle anaerobic respiration makes ethanol.
Mistaken answer: “Anaerobic respiration does not make energy, and in muscle it produces ethanol and carbon dioxide.”
Both claims are wrong: energy is released in smaller amounts, and muscle makes lactic acid.
The correction: anaerobic respiration releases less energy, not none, and the products depend on the organism. Keep a three-row table and check both the substance and the organism before writing the product.
Check yourself
1. Write the word equation for anaerobic respiration in yeast.
Show answer
glucose → ethanol + carbon dioxide (+ energy released)
2. Give two differences between aerobic and anaerobic respiration in muscle.
Show answer
Aerobic respiration uses oxygen, and anaerobic does not. Aerobic forms carbon dioxide and water, while anaerobic forms lactic acid. Aerobic releases much more energy per glucose.
3. Why is yeast fermentation useful in bread making?
Show answer
The yeast respires anaerobically in the dough and makes carbon dioxide. The gas forms bubbles that make the dough rise, and the ethanol evaporates during baking.
Where this leads next
Now use these models on real numbers in interpreting a respiration experiment from supplied data. The mixed practice set also includes a question on each equation.
Students who know the equations but blank on an unfamiliar context can rehearse with a teacher in online one-to-one Biology tuition.