AQA GCSE Biology Unit B3 Chapter 3

• Sterilise the inoculating loop (used to transfer microorganisms to the agar) by heating it to red hot in a Bunsen flame. Leave it to cool.
• Dip the loop in the bacteria you want to grow and make zigzag streaks across the surface of the agar. Close the lid quickly to aviod contamination
• Secure the lid of the Petri dish with short pieces of tape to stop contamination from microorganisms in the air and stop the microbes in the dish escaping. Don't seal all the way around the edge (the microorganisms need oxygen to respire)

If you need an uncontaminated culture

• Petri dishes, innoculating loops and the culture should be sterilised
• The lid must be held in place with tape to stop any microorganisms getting in or out

Cultures in schools are incubated at 25 degrees C to stop human pathogens growing.

In industry, higher temperatures can be used to grow microbes more quickly.

Yeast cells arre simple single-celled organisms. When oxygen is present, they respire aerobically and reproduce quickly. If there isn't oxygen, they respire anaerobically and produce ethanol and carbon dioxide. This process is called fermentation.

We use yeast in brewing beer. Firstly, the starch in barley grains is broken down by enzymes into sugars. When these sugars have been fermented, hops are added for flavour.

When wine is made the grapes contain natural sugars which the yeast use for energy.

Bacteria are used to make cheese and yoghurt.

In yoghurt production bacteria are added to warm milk. The lactose in the milk is fermented by the bacteria, producing lactic acid. The lactic acid causes the milk to clot and yoghurt is formed.

In cheese making, curds are formed by the action of bacteria on the milk. Then the final flavour and texture depends on other bacteria added later.

Industrial fermenters are large vessels used to grow microorganisms.

• There is an air supply to provide oxygen for respiration
• there is a stirrer used to keep the microbes spread out and to keep the temperature the same everywhere in the vessel
• There is a water-cooled jacket around the outside, as the respiring microbes release heat and a constant temperature needs to be maintained
• There are sensors to monitor pH and temperture

Penicillin is made from the mould penicillium in a fermenter

The solution inside the vessel contains sugar and nutrients. Penicillium only starts to reproduce penicillin when most of the nutrients are used up.

Plants and waste material (containing carbohydrate) can be broken down by microbes anaerobically to produce biogas. The gas is mainly methane.

This can be done large-scale with waste from sewage works/sugar factories or on a small scale (especially in developing countries).

Sugar cane juices and glucose from maize starch can be fermented to produce ethanol. Microorganisms repire anaerobically in this process.

The ethanol must be distilled before it's used as a fuel in motor vehicles

Ethanol could replace fossil fuels in the future. It is carbon friendly.