Bioenergetics · GCSE Biology
Anaerobic respiration
Compare anaerobic respiration for GCSE Biology: lactic acid in animal cells, ethanol and carbon dioxide in yeast and plants, incomplete energy transfer, and why it is used only for short bursts or fermentation.
Anaerobic means without oxygen. Animals: glucose → lactic acid. Yeast and plants: glucose → ethanol + carbon dioxide. Fast but much less energy than aerobic, and it leaves a problem to clear up.
The important bits
What you need to know
- 1
Anaerobic respiration is the incomplete breakdown of glucose without oxygen. It transfers far less energy than aerobic respiration.
- 2
In animal cells (for example muscle): glucose → lactic acid. No carbon dioxide in this equation. Lactic acid causes muscle fatigue (pain, reduced contraction).
- 3
In yeast and some plant cells: glucose → ethanol + carbon dioxide. This is fermentation. Yeast is used in bread (CO₂ makes dough rise) and in brewing and wine-making (ethanol).
- 4
Plants may respire anaerobically in waterlogged roots, producing ethanol, which can be toxic if it accumulates.
- 5
Anaerobic respiration is useful for a short burst when oxygen cannot be delivered fast enough (sprinting, lifting). It cannot support endurance because energy yield is low and lactic acid builds up.
- 6
Aerobic: mitochondria, oxygen, CO₂ + water, high energy yield. Anaerobic in animals: cytoplasm, no oxygen, lactic acid, low yield. Do not put lactic acid in yeast or ethanol in human muscle.
- 7
Microorganisms in biogas generators and in decay can respire anaerobically, producing methane or other products, linking this page to ecology.
- 8
After anaerobic exercise in humans, extra oxygen is needed to process lactic acid — the oxygen debt (oxygen recovery). That is the next topic; do not ignore it in a sprint story.
Quotations worth analysing
Short evidence. Real method.
“glucose → lactic acid”
No oxygen, no carbon dioxide in the equation. Pair with muscle fatigue and a later oxygen debt.
“glucose → ethanol + carbon dioxide”
This is fermentation. Bread uses the CO₂; alcoholic drinks use the ethanol. Do not write lactic acid for yeast.
“Anaerobic respiration transfers much less energy than aerobic respiration.”
Incomplete breakdown of glucose is the reason. “It is weaker” is not enough.
Go deeper
Which equation do I write for which organism?
Read the stem. Human muscle, sprinter, athlete: glucose → lactic acid. Yeast, brewing, bread dough, fermenter: glucose → ethanol + carbon dioxide. Waterlogged plant root: the yeast-style equation, ethanol + CO₂. Aerobic always needs oxygen and produces CO₂ + water in animals, plants and yeast when oxygen is present. Yeast is facultative: with oxygen it respires aerobically (more energy, more cells); without oxygen it ferments. Bakers want some CO₂; they also want the yeast alive, so conditions are controlled. In the exam, a student who writes lactic acid for beer loses the product mark. A student who writes ethanol for a 100 m runner loses the animal-equation mark. Learn the two anaerobic equations as a locked pair.
Go deeper
Why use a low-yield process at all?
It is faster to start and does not wait for the heart and lungs to catch up. In a 100 m sprint, oxygen delivery cannot match demand in time, so muscle cells switch to anaerobic respiration, transfer enough energy for a few seconds of contraction, and accept lactic acid as the cost. Endurance athletes stay mostly aerobic: they have more mitochondria, better oxygen delivery, and they use glycogen steadily. If you ran a marathon on lactic acid alone you would stop very quickly. Yeast in a sealed brewery has no choice: no oxygen, so fermentation. The low yield means lots of glucose must be used for a modest energy transfer, which is why anaerobic cells consume glucose faster — a point that appears on some data questions as a steeper drop in glucose concentration.
Go deeper
How do bread and brewing questions earn method marks?
Bread: yeast respires anaerobically in dough, CO₂ bubbles are trapped in gluten, the dough rises; ethanol evaporates in the oven. Warmth speeds the enzyme-controlled reaction; boiling denatures the enzymes and kills the yeast. Brewing: anaerobic conditions, sugar from grain or fruit, ethanol accumulates until it inhibits the yeast or sugar runs out. Temperature control matters because enzymes have an optimum. These are applications of the yeast equation, not a different kind of respiration. Link back to enzymes: fermentation rate versus temperature is an enzyme graph in disguise. Do not write that the yeast “breathes out” alcohol.
See the idea in action
A student sprints 200 m. Muscle cells cannot get oxygen fast enough, so they respire anaerobically: glucose → lactic acid. Little energy is transferred compared with aerobic respiration, but it is enough for a short burst. Lactic acid builds up and muscles fatigue. Yeast in bread dough would instead produce ethanol + carbon dioxide; the carbon dioxide makes the loaf rise. Writing the yeast equation for the sprinter would be wrong.
Exam technique
Turn knowledge into marks
Match the equation to the organism before you write. Animals: lactic acid. Yeast and plants: ethanol + CO₂. Always say much less energy than aerobic. Mention cytoplasm rather than mitochondria for the anaerobic pathway at GCSE.
Common mistakes
Do not give these marks away
- 01
Writing the yeast equation for animal muscle, or putting lactic acid in yeast.
- 02
Saying anaerobic respiration produces more energy, or that it occurs in mitochondria in the same way as aerobic.
- 03
Forgetting carbon dioxide in fermentation, or adding oxygen to an anaerobic equation.
What are the products of anaerobic respiration in yeast?
ALactic acid only
BEthanol and carbon dioxide
CCarbon dioxide and water
DGlucose and oxygen
Show the answer
Ethanol and carbon dioxide. Yeast ferments glucose to ethanol and carbon dioxide. Lactic acid is the product in animal cells. Carbon dioxide and water are the products of aerobic respiration.
Quick questions
If this is the bit you searched
What is the equation for anaerobic respiration in animals?
Glucose → lactic acid. It happens when oxygen is in short supply, for example in sprinting muscle.
What is the equation for anaerobic respiration in yeast?
Glucose → ethanol + carbon dioxide. This fermentation is used in bread-making and brewing.
Why does anaerobic respiration transfer less energy?
Glucose is only partly broken down. Aerobic respiration oxidises glucose fully to carbon dioxide and water and transfers much more energy.
Does anaerobic respiration happen in plants?
Yes, for example in waterlogged roots: glucose → ethanol + carbon dioxide. Ethanol can damage tissues if it builds up.