What are micro-organisms, how are they grown safely, and how do we use them to make food, drink and medicines?
The main groups of micro-organisms and the structure of bacteria, binary fission and growth conditions, aseptic technique for culturing micro-organisms, and the use of micro-organisms in food, fermentation, antibiotics and biotechnology.
A focused answer to the WJEC GCSE Biology section 2.7 topic on micro-organisms, covering the groups of micro-organisms and bacterial structure, binary fission and growth conditions, aseptic technique for culturing on agar, and the use of micro-organisms in food, fermentation, antibiotics and biotechnology.
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What this dot point is asking
WJEC wants you to know the main groups of micro-organisms and the structure of a bacterial cell, how bacteria reproduce and what affects their growth, how to culture micro-organisms using aseptic technique, and how micro-organisms are used in food, fermentation, antibiotics and biotechnology.
The groups of micro-organisms
Micro-organisms are organisms too small to see with the naked eye. The three main groups are:
- Bacteria, single cells with no nucleus.
- Viruses, much smaller particles that can only reproduce inside living cells.
- Fungi, including single-celled yeast and the moulds.
Some micro-organisms are useful (in food and medicine) while others are harmful pathogens that cause disease.
The structure of a bacterial cell
A bacterial cell is much simpler than an animal or plant cell.
Growth and binary fission
Bacteria reproduce by binary fission: a single cell copies its DNA and splits into two identical cells. In good conditions they can divide very quickly, so the population grows rapidly.
The conditions that affect growth are temperature, nutrients, oxygen and pH. With plenty of food, the right temperature and pH, and the right oxygen level, bacteria multiply fastest.
Culturing micro-organisms safely
To grow micro-organisms in the laboratory they are spread on a jelly called agar in a Petri dish, which provides nutrients. To stop unwanted micro-organisms contaminating the culture (or escaping), we use aseptic technique:
- Sterilise equipment, for example by passing the inoculating loop through a flame.
- Work near a flame and lift the lid as little as possible, so micro-organisms from the air do not fall in.
- Tape the lid on (not sealed all the way round) to keep contaminants out while preventing harmful anaerobic bacteria from growing.
In schools, cultures are incubated at about 25 degrees Celsius. This is warm enough for growth but cool enough to reduce the risk of growing harmful bacteria that thrive at human body temperature.
Using micro-organisms
Micro-organisms have many useful applications.
| Use | Micro-organism | Product |
|---|---|---|
| Yoghurt and cheese | Bacteria | Acid sets the milk |
| Bread and alcohol | Yeast | Carbon dioxide and ethanol |
| Antibiotics | Fungi/bacteria | Penicillin and others |
| Biotechnology | Modified bacteria | Insulin and other substances |
- Fermentation by yeast produces ethanol and carbon dioxide; the carbon dioxide makes bread rise and the ethanol is used in alcoholic drinks.
- Industrial fermenters (bioreactors) control the temperature, pH, oxygen, nutrients and agitation to get the maximum yield of product.
- Genetically modified micro-organisms can be given a gene from another organism, so they make useful substances such as insulin.
Exam-style practice questions
Practice questions written in the style of WJEC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
WJEC style4 marksDescribe the aseptic techniques used when culturing bacteria on an agar plate, and explain why each is needed.Show worked answer →
A 4-mark technique question.
The inoculating loop is heated in a flame to sterilise it and kill any micro-organisms before transferring bacteria, so the culture is not contaminated. The lid is lifted only slightly and the dish is worked near a flame, so micro-organisms from the air do not fall in. After inoculation the lid is taped on (but not sealed all the way round), to stop contaminants entering while still letting some air in and preventing harmful anaerobic bacteria growing.
Markers reward: flame the loop to sterilise it; work near a flame and open the lid as little as possible to avoid airborne contamination; tape the lid to prevent contamination. The reason (prevent contamination) must be linked to the step.
WJEC style3 marksA single bacterium divides every 30 minutes. Calculate how many bacteria there will be after 3 hours, starting from one cell.Show worked answer →
A 3-mark calculation using binary fission.
In 3 hours there are 6 periods of 30 minutes, so the bacterium divides 6 times. The number of bacteria doubles each time, so the number is 2 to the power 6, which is 64. Starting from one cell gives 64 bacteria after 3 hours.
Markers reward: 6 divisions in 3 hours; doubling each time (2 to the power of the number of divisions); final answer 64. Multiplying 1 by 6 instead of doubling is the usual error.
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Sources & how we know this
- WJEC GCSE Biology specification (from 2016) — WJEC (2016)