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Variations on cellular respiration

Fermentation and anaerobic respiration

AP.BIO: ENE‑1 (EU), ENE‑1.K (LO), ENE‑1.K.1 (EK), ENE‑1.K.2 (EK), ENE‑1.L (LO), ENE‑1.L.1 (EK), ENE‑1.L.6 (EK), ENE‑1.L.7 (EK)

How cells extract energy from glucose without oxygen. In yeast, the anaerobic reactions make alcohol, while in your muscles, they make lactic acid.

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Introduction

Ever wonder how yeast ferment barley malt into beer? Or how your muscles keep working when you're exercising so hard that they're very low on oxygen?

Both of these processes can happen thanks to alternative glucose breakdown pathways that occur when normal, oxygen-using (aerobic) cellular respiration is not possible—that is, when oxygen isn't around to act as an acceptor at the end of the electron transport chain. These fermentation pathways consist of glycolysis with some extra reactions tacked on at the end. In yeast, the extra reactions make alcohol, while in your muscles, they make lactic acid.

Fermentation is a widespread pathway, but it is not the only way to get energy from fuels anaerobically (in the absence of oxygen). Some living systems instead use an inorganic molecule other than \text {O}_2O

2

start text, O, end text, start subscript, 2, end subscript, such as sulfate, as a final electron acceptor for an electron transport chain. This process, called anaerobic cellular respiration, is performed by some bacteria and archaea.

In this article, we'll take a closer look at anaerobic cellular respiration and at the different types of fermentation.

Anaerobic cellular respiration

Anaerobic cellular respiration is similar to aerobic cellular respiration in that electrons extracted from a fuel molecule are passed through an electron transport chain, driving \text{ATP}ATPstart text, A, T, P, end text synthesis. Some organisms use sulfate (\text {SO}_4^{2-})(SO

4

2−

)left parenthesis, start text, S, O, end text, start subscript, 4, end subscript, start superscript, 2, minus, end superscript, right parenthesis as the final electron acceptor at the end ot the transport chain, while others use nitrate (\text {NO}_{3}^-)(NO

3