Life Without Oxygen: Fermentation and Anaerobic Respiration

Bread, yogurt and a sprint up the stairs

What do a rising loaf of bread, a cup of yogurt, and your burning legs after a sprint up five flights of stairs have in common? In all three, cells are getting energy from glucose without enough oxygen. They are using fermentation.

C3.1 asks you to explain both aerobic and anaerobic pathways. This lesson shows what happens to pyruvate and NADH when oxygen runs out, why the ATP yield is so small, and why fermentation still matters so much, both to living things and to industries across Canada.

The NAD⁺ problem

A cell has only a small supply of NAD⁺. Glycolysis uses 2 NAD⁺ per glucose, turning them into NADH. If oxygen is absent, the ETC can't reoxidize NADH, so within moments all the NAD⁺ is used up and glycolysis stops at step 6. No ATP at all.

Fermentation solves this problem. It adds one or two steps after glycolysis that hand NADH's electrons to an organic molecule made from pyruvate, regenerating NAD⁺. Glycolysis can keep running and keep making its 2 ATP per glucose. Fermentation makes no extra ATP itself; its only job is to recycle NAD⁺.

Two kinds of fermentation

1. Lactate (lactic acid) fermentation

pyruvate + NADH + H⁺ → lactate + NAD⁺ (enzyme: lactate dehydrogenase)

Overall: glucose → 2 lactate, net 2 ATP. No CO₂ is released.

• Where: your skeletal muscle cells during intense exercise, when O₂ delivery can't keep up; red blood cells; and bacteria such as Lactobacillus, used to make yogurt, kefir, sauerkraut, kimchi and many cheeses (the lactate makes them sour and helps preserve them).

2. Alcohol (ethanol) fermentation

Step 1: pyruvate → acetaldehyde + CO₂ (decarboxylation)

Step 2: acetaldehyde + NADH + H⁺ → ethanol + NAD⁺

Overall: C₆H₁₂O₆ → 2 C₂H₅OH + 2 CO₂, net 2 ATP. This matches the curriculum's example: yeast reacts with glucose in the absence of oxygen to produce carbon dioxide and ethanol.

• Where: yeast (a fungus) and some bacteria. In bread, CO₂ bubbles make the dough rise, and the ethanol evaporates in the oven. Yeast fermentation also produces fuel ethanol: Ontario has several plants that ferment corn into ethanol, which is blended into gasoline (many pumps carry a label saying the gasoline may contain ethanol).

Why so little ATP?

In fermentation, the electrons from glucose end up in lactate or ethanol, which are still energy-rich, reduced molecules. (Ethanol burns; that is why it works as a fuel.) Most of glucose's free energy is never released. The final electron acceptor is an organic molecule, not oxygen, so the electrons don't fall very far down the energy staircase.

• Fermentation: 2 ATP per glucose (about 2% efficient).
• Aerobic respiration: about 30 to 32 ATP per glucose (about 34% efficient).

A cell running on fermentation must therefore process glucose about 16 times as fast to make the same ATP. That is why your muscles use up glycogen quickly during a sprint.

What happens to the lactate in your muscles?

During hard exercise, lactate builds up in muscle cells and diffuses into the blood. It does not stay in your muscles for days. Most of it is either used as fuel by other cells (heart muscle, for example, oxidizes lactate back to pyruvate and respires it aerobically) or carried to the liver, where it is converted back to pyruvate and then to glucose. This Cori cycle costs ATP in the liver.

After exercise you keep breathing hard for a while. This extra oxygen use (sometimes called "oxygen debt" or excess post-exercise oxygen consumption) supplies ATP for recycling lactate, restoring ATP and creatine phosphate, and cooling down.

Myth check: the stiff, sore muscles you feel a day or two after a hard workout are not caused by lactate, which is cleared within an hour or so. That delayed soreness is linked to small amounts of muscle damage and the repair process.

Fermentation is not the same as anaerobic respiration

The words are often mixed up, but biologists use them differently:

• Fermentation: glycolysis only; no ETC; the electron acceptor is an organic molecule; 2 ATP per glucose.
• Anaerobic respiration: a full electron transport chain is used, but the final electron acceptor is not O₂. Some bacteria use nitrate (NO₃⁻), producing nitrogen gas; others use sulfate (SO₄²⁻), producing hydrogen sulfide (H₂S), the rotten-egg smell of some marshes and mud. Their ATP yield is more than fermentation but less than aerobic respiration, because these acceptors are weaker oxidizing agents than O₂.

Organisms also differ in how they use oxygen:

• Obligate aerobes need O₂ (humans, most animals and plants).
• Facultative anaerobes can switch (yeast; E. coli).
• Obligate anaerobes are poisoned by O₂ (for example, Clostridium bacteria and the methane-producing archaea in wetlands, sewage digesters and cows' stomachs).

These anaerobic microbes are vital in ecosystems: they decompose matter in waterlogged soils and sediments, and they drive parts of the nitrogen, sulfur and carbon cycles. You will look at their role in Earth's systems in the last lesson of this unit.