Anaerobic Respiration in Humans vs Yeast

In both humans and yeast, anaerobic respiration releases energy from glucose without oxygen. Both use glycolysis to make a small amount of ATP, but they process the resulting pyruvate differently: human cells produce lactate, while yeast cells produce ethanol and carbon dioxide.

What happens in both?

Glycolysis takes place in the cytoplasm. It breaks down glucose into pyruvate and produces a net gain of $2$ ATP molecules per glucose molecule. It also reduces NAD, a molecule that carries hydrogen. For glycolysis to continue, the reduced NAD must be converted back to NAD.

Anaerobic means oxygen is not needed for these reactions. ATP transfers energy to processes such as muscle contraction. This shared first stage explains why both organisms can continue releasing some energy when oxygen is unavailable, although glycolysis yields much less ATP per glucose than complete aerobic respiration.

What is different?

In human cells, reduced NAD transfers hydrogen to pyruvate, forming lactate. This regenerates NAD so glycolysis can continue. Carbon dioxide is not released in this step.

In yeast cells, pyruvate first loses carbon dioxide and forms ethanal. Reduced NAD then transfers hydrogen to ethanal, forming ethanol and regenerating NAD. This process is sometimes called alcoholic fermentation.

In both cases, reduced NAD is oxidised when it passes on hydrogen. Without regenerated NAD to accept more hydrogen during glycolysis, this pathway would stop, so the cells would no longer make ATP from glucose in this way.

For example, if a human muscle cell has too little oxygen to meet its demand during intense exercise, it can keep making ATP through glycolysis by forming lactate. Yeast can keep glycolysis going without oxygen by forming ethanol instead.

Exam mistake to avoid: the reactions that form lactate or ethanol do not provide the net ATP gain. The ATP comes from glycolysis; those later reactions regenerate NAD so glycolysis can carry on.