Respiration · Contrast
Aerobic vs anaerobic
Two ways to get energy out of the same sugar molecule. One releases far more of it. The other releases it far faster. Almost everything alive uses both, and knowing which is running when is the whole point of this unit.
Start here
One route gets about twenty times more out of the same glucose.
If that were the only thing that mattered, the other route would have disappeared long ago — a twentyfold waste of food is not a small disadvantage. It has not disappeared. Every organism in this unit, from a sprinter to a yeast cell, keeps it available.
So why does anaerobic respiration exist at all?
Because it is faster, and it does not have to wait for a delivery. Aerobic respiration wins on how much energy it gets from each glucose molecule; anaerobic respiration wins on how quickly it can supply energy, and it works when there is no oxygen to be had. Those are two different questions, and confusing them is where most of the trouble in this topic comes from.
At the bench · five situations
Which route is running here?
Commit to an answer before you check it. Three of the five are not the answer most people give first, and in two of them fermentation is supplying nearly all the energy.
The situation
You are sitting still, reading this page.
Which route is supplying the energy?
0 of 5 settled
5 still to settle
Now look back at the five. Did you have the marathon runner?
Nobody marks this but you. Working hard and running short of oxygen are two different questions, and all five turn on the second one.
Side by side
Six differences, and the last one is the one that matters.
- OxygenRequired.Not used at all.
- GlucoseBroken down completely.Broken down only partly — there is energy left in the product.
- ProductsCarbon dioxide and water.Lactic acid in humans; ethanol and carbon dioxide in yeast.
- Energy per glucoseAbout twenty times more.A small fraction of the aerobic yield.
- Where in the cellIn the mitochondria.Outside them, which is why cells with few mitochondria can still do it.
- What it is forEverything, all the time — the default.Energy needed faster than oxygen arrives, or no oxygen available at all.
Key fact
Aerobic respiration uses oxygen, breaks glucose down completely to carbon dioxide and water, and releases far more energy per molecule. Anaerobic respiration uses no oxygen, breaks glucose down only partly, releases much less energy per molecule — and can do it faster, and without waiting.
Think again
“Aerobic respiration is the fast one, because that is the one athletes train for.”
Two different meanings of fast are being run together, and separating them settles most of this topic. Aerobic respiration releases more energy per glucose molecule — about twenty times more — which is a statement about yield. Anaerobic respiration releases energy per second at a higher rate, because it involves fewer steps and does not depend on oxygen arriving from outside the cell. A sprinter needs power now and does not care what it costs in sugar; a marathon runner needs to keep going for two hours and cannot afford waste, so the whole race is run just below the pace where the anaerobic route would take over. Both athletes have the same two systems. What differs is which constraint their event puts them under, and their training moves the line between the two.
“Anaerobic respiration is the emergency backup — something has gone wrong when it happens.”
It is a strategy, not a failure. Yeast in a sealed vat is not in trouble: it is doing the only thing available and doing it deliberately, and we built an industry on the result. Bacteria in a cow's rumen, in a bog, in a sealed jar of sauerkraut and in your own gut live anaerobically as a matter of routine, and some of them are killed by oxygen — for them, air is the emergency. Even in your muscles the anaerobic route is not a fault: it is what lets you sprint for a bus, and the burning that stops you is a control mechanism protecting the muscle, not a breakdown. There is also a historical point worth knowing. Anaerobic respiration is the older process by a long way, because for the first two billion years of life on Earth there was almost no oxygen to respire with — the aerobic route is the newcomer, made possible by photosynthesis filling the atmosphere with a gas that was, at the time, a poison.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Tell them apart
Which statement is true of anaerobic respiration in humans but not of aerobic respiration?
Rung 2 · The one that catches people
Which describes the difference correctly?
Rung 3 · Two athletes
A 100 m sprinter and a marathon runner have the same two respiration systems. Explain why their events depend on them so differently, and why the marathon runner paces the race instead of running as fast as possible.
Rung 4 · Take it somewhere new
Some bacteria are killed by oxygen and live only where there is none — in mud, in a compost heap, in the gut. Explain how an organism can live entirely without oxygen, what it gives up by doing so, and what their existence suggests about the order in which the two kinds of respiration appeared on Earth.
Key note
Aerobic respiration: uses oxygen, glucose broken down completely, products carbon dioxide and water, far more energy per glucose. Anaerobic respiration: no oxygen, glucose broken down partly, lactic acid in humans or ethanol and carbon dioxide in yeast, much less energy per glucose but supplied faster. Organisms use both, and which one dominates depends on how fast energy is needed and whether oxygen can reach the cell.
Going further
Some organisms have never made the switch. Obligate anaerobes are bacteria for which oxygen is toxic, and they are not rare curiosities — they live in the mud of a pond, in the deep layers of a compost heap, in a cow's rumen and in the oxygen-free depths of your own large intestine, where they are part of the community you met in Bacteria in the gut. Their existence is a fossil of the world they evolved in. For roughly the first two billion years of life there was no oxygen worth speaking of, so every living thing respired anaerobically; when photosynthetic bacteria began filling the atmosphere with oxygen, they caused what is arguably the largest mass extinction in Earth's history by poisoning almost everything alive. The survivors either retreated to places the oxygen could not reach, where their descendants still are, or found a way to use the poison — and using it turned out to release about twenty times more energy, which is most of the reason anything larger than a bacterium exists.
Before this lesson
Connects to
At GCSE this becomes
- ATP yields compared numerically, metabolism as a whole, and the biotechnology built on anaerobic organisms.
Where to next
- Next: Food chains and food webs
Ecosystems and interdependence
- Previous: Fermentation and what we use it for
Ask Mr Badmus AI
Want to test a situation that is not one of the five?
"About twenty times more energy" is the usual teaching comparison and comes from the ATP yields met at GCSE; the exact ratio depends on how the accounting is done. The five situations are simplified: in every living example both routes are running at once, and the labels describe which one is supplying most of the energy.
Lesson content © MrBadmusAI.