---
title: "Anaerobic Routes and the ATP Yield Compared"
description: "Without oxygen the chain backs up, so cells reduce pyruvate to lactate or to ethanol purely to regenerate NAD. The ATP arithmetic shows how much a cell gives up by doing it."
canonical: https://lightmysky.com/learn/science/anaerobic-routes-and-the-atp-yield-compared-mt_LBZ4laMfqb
source: https://lightmysky.com/learn/science/anaerobic-routes-and-the-atp-yield-compared-mt_LBZ4laMfqb.md
retrieved: 2026-11-12
---

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# Anaerobic Routes and the ATP Yield Compared

Without oxygen the chain backs up, so cells reduce pyruvate to lactate or to ethanol purely to regenerate NAD. The ATP arithmetic shows how much a cell gives up by doing it.

Subject: Science · Area: Organisms & Life Processes · Ages 17 to 18
Page: https://lightmysky.com/learn/science/anaerobic-routes-and-the-atp-yield-compared-mt_LBZ4laMfqb

## Ready when they can

- Explains that the anaerobic pathways exist to regenerate NAD rather than to make ATP.
- Compares the ATP yield per glucose for the aerobic and the anaerobic route.
- Names the anaerobic product in a muscle cell and in a yeast cell.

## Lesson: Running on Fumes

With oxygen, one glucose pays roughly 30 to 38 ATP, depending on the estimate, through the full chain to oxygen. Without oxygen the chain backs up, because reduced NAD, written NADH, has nowhere to unload its electrons. Glycolysis then stalls too, since it needs fresh NAD+, the unloaded form.

Fermentation fixes exactly one problem: it hands NADH's electrons to pyruvate, or in yeast to a molecule made from pyruvate after carbon dioxide is removed, to free NAD+ again. It makes no extra ATP. The anaerobic route nets only 2 ATP per glucose, the same small amount glycolysis gives alone.

*(drawing: Fermentation hands NADH's electrons to pyruvate, or to a molecule made from it, so NAD+ returns to glycolysis. No extra ATP is made.)*

**Example.** Your muscle cells turn pyruvate into lactate during a hard sprint. Yeast turns it into ethanol plus carbon dioxide, which raises bread and fizzes beer. Same problem, two patches, and neither patch makes extra ATP.

**Tip.** When asked why a cell bothers making lactate, answer with NAD+, not ATP. The point is keeping glycolysis alive at the price of most of the ATP yield.

**Recap.** Without oxygen cells ferment pyruvate only to free NAD+, settling for 2 ATP per glucose instead of roughly 30 to 38.

## Practice

8 questions on this page, each with its working shown.

## Needs first

- [Oxidative Phosphorylation and Chemiosmosis](https://lightmysky.com/learn/science/oxidative-phosphorylation-and-chemiosmosis-mt_1wP3-14Y4V)

## Opens up

- [Metabolic Diversity: Choosing an Electron Donor and Acceptor](https://lightmysky.com/learn/science/metabolic-diversity-choosing-an-electron-donor-and-acceptor-mt__2yCr9OSIn)
- [Ecosystem Energy Budgets: GPP, NPP and Transfer Efficiency](https://lightmysky.com/learn/science/ecosystem-energy-budgets-gpp-npp-and-transfer-efficiency-mt_D81CKcOdBI)