Anaerobic respiration is ATP production that happens without oxygen, relying on glycolysis (and often fermentation) to keep energy flowing when oxygen is scarce. It makes far less ATP per glucose than aerobic respiration.
Anaerobic respiration is how a cell makes ATP when oxygen isn't available. Oxygen is the final electron acceptor in the electron transport chain, so when it's gone, that chain stalls and the Krebs cycle backs up behind it. The cell falls back on glycolysis, the one ATP-making step that doesn't need oxygen at all.
Glycolysis breaks glucose into pyruvate and makes a small net amount of ATP. The catch is that glycolysis needs a steady supply of NAD+ to keep running. Without oxygen, NADH can't dump its electrons down the electron transport chain to regenerate NAD+. That's where fermentation comes in: it recycles NADH back to NAD+ so glycolysis can keep going. In humans this produces lactic acid; in yeast it makes ethanol and CO2. So anaerobic respiration is really glycolysis kept alive by fermentation, not the full breakdown of glucose you get with oxygen present.
This term sits at the intersection of two CED topics: 3.6 Cellular Respiration and 3.7 Fitness. In 3.6 it's the contrast case that makes the efficiency of aerobic respiration obvious. Aerobic respiration captures roughly 30-32 ATP per glucose, while anaerobic respiration nets only about 2 ATP from glycolysis. That gap is the whole point. In 3.7 it explains why certain organisms (and certain cells, like your muscle cells during a sprint) can survive in low-oxygen conditions, which ties directly into fitness and natural selection. An organism that can make ATP without oxygen can live in places others can't, which is a survival advantage.
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Glycolysis (Unit 3)
Glycolysis is the engine of anaerobic respiration. It's the only ATP-producing step that runs without oxygen, so when oxygen disappears, glycolysis is basically all the cell has left.
Fermentation (Unit 3)
Fermentation is the recycling system that keeps anaerobic respiration going. It regenerates NAD+ from NADH so glycolysis doesn't grind to a halt, even though it makes no extra ATP itself.
Electron Transport Chain (Unit 3)
The ETC is exactly what anaerobic respiration is missing. Without oxygen as the final electron acceptor, the chain shuts down, which is why the cell loses access to the bulk of its ATP yield.
Fitness and Natural Selection (Unit 3)
Being able to make ATP without oxygen is a survival trait. Organisms that can respire anaerobically can colonize low-oxygen habitats like deep-sea vents, giving them a fitness edge competitors lack.
Expect this on multiple-choice questions that ask you to compare energy yields or explain how a cell survives without oxygen. A classic stem describes a microorganism from a deep-sea hydrothermal vent that lacks mitochondria but still makes ATP, and you have to recognize it's using anaerobic pathways like glycolysis and fermentation. Another common stem asks how much ATP anaerobic respiration produces compared to aerobic, where the answer is dramatically less (about 2 versus 30+). Free-response framing has appeared (2017 Short FRQ Q7), so be ready to explain WHY anaerobic respiration yields so little ATP: no oxygen means no functioning electron transport chain, so the cell can't run oxidative phosphorylation.
These overlap but aren't identical. Fermentation is the specific reaction that regenerates NAD+ (making lactic acid or ethanol), while anaerobic respiration is the bigger picture of producing ATP without oxygen. In most organisms anaerobic respiration = glycolysis + fermentation working together, so fermentation is a part of the process, not the whole thing.
Anaerobic respiration makes ATP without oxygen, relying on glycolysis as its main ATP source.
It yields only about 2 ATP per glucose, compared to roughly 30-32 ATP from aerobic respiration.
Fermentation regenerates NAD+ so glycolysis can keep running, but fermentation itself makes no extra ATP.
Without oxygen the electron transport chain shuts down, which is why anaerobic respiration is so much less efficient.
The ability to respire anaerobically is a fitness advantage that lets organisms survive in low-oxygen environments.
It's the process of making ATP without oxygen. The cell relies on glycolysis to produce a small amount of ATP and uses fermentation to recycle NAD+ so glycolysis can keep going.
Far less. Anaerobic respiration nets only about 2 ATP per glucose from glycolysis, while aerobic respiration captures roughly 30-32 ATP because it can use the electron transport chain and oxygen.
Not exactly. Fermentation is the specific step that regenerates NAD+ (producing lactic acid or ethanol), while anaerobic respiration is the broader process of making ATP without oxygen. In most cells, anaerobic respiration combines glycolysis and fermentation.
Because without oxygen, the electron transport chain can't run, so the cell loses oxidative phosphorylation entirely. That leaves only the small ATP yield from glycolysis.
Questions about deep-sea vent microbes that lack mitochondria are testing this: the organism uses anaerobic pathways like glycolysis and fermentation to make ATP, which is also a fitness advantage in low-oxygen habitats.