Bacteria and yeast in biotechnology

Last time

Before we start: last time, in Genetic modification and GM crops, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Tiny workers

Bread, yoghurt, insulin and biofuel all depend on microorganisms. Biotechnology uses living organisms, or parts of them, to make useful products. Today: why bacteria are so useful, and how yeast makes bread rise and fuel burn.

Biotechnology

Biotechnology is the use of living organisms, or substances from them, to make useful products or to carry out useful jobs. Microorganisms are ideal: they reproduce very fast, need little space, and can grow on cheap nutrients. Many of them can be grown in a fermenter, which gives them the right conditions.

Why bacteria are useful

Bacteria reproduce quickly, so a large amount of product can be made in a short time. They make complex molecules, such as proteins, that would be difficult or expensive to make in a factory. They have few ethical concerns, compared with using animals, and they have plasmids.

Plasmids

A plasmid is a small circle of DNA in a bacterium, separate from the main chromosome. Genes from other organisms, such as the human insulin gene, can be inserted into a plasmid. The bacterium then makes the protein coded for by that gene. This is genetic modification.

Bacteria in food

Lactic acid bacteria feed on the sugar in milk and make lactic acid by anaerobic respiration. The acid lowers the pH, which makes the milk proteins clump together. This gives yoghurt its thick texture and sour taste. Yoghurt is made at a warm, controlled temperature, so the bacteria grow quickly.

Yeast

Yeast is a single-celled fungus. When there is no oxygen, it respires anaerobically. Glucose changes to ethanol and carbon dioxide, and energy is released. In symbols: C₆H₁₂O₆ makes two C₂H₅OH and two CO₂. The atoms balance.

Bread and biofuel

In bread, the carbon dioxide forms bubbles in the dough, so it rises. The ethanol evaporates during baking. In biofuel, yeast ferments sugar from crops such as sugar cane. The ethanol is used as a fuel. Ethanol is renewable, and its burning gives back only the carbon dioxide the crop absorbed.

What yeast needs

Yeast needs a sugar source, warmth around thirty degrees, and little or no oxygen to ferment. If it is too hot, the enzymes in the yeast are denatured, and fermentation stops. If it is too cold, the enzymes work slowly, so less ethanol and carbon dioxide are made.