key stage 3

Module 15 - Microbes & Disease - Revision notes

Here are the key ideas from Module 15. Make sure that you also look at your written notes and the sites on the links page.

• What are microorganisms?
• Bacteria
• Viruses
• Fungi
• The uses of microorganisms
• How do harmful microorganisms spread?
• How can we protect ourselves against infectious disease?
• Antibiotics
• Immunisation (Vaccination)

What are microorganisms?

Microorganisms (sometimes called microbes) are the smallest living things. They are so small that they cannot be seen without the help of a microscope. There are three types of microorganisms you need to know about:

• Bacteria
• Viruses
• Fungi

Bacteria

Bacteria are simple organisms that contain only one cell (they are single-celled). Their cell is different to animal and plant cells because it contains no nucleus. It is also much smaller than an animal or plant cell. Bacteria can be harmful, in which case they are often known as germs. Harmful bacteria can cause food poisoning or very serious diseases like tuberculosis (TB), tetanus or meningitis. However, many bacteria are harmless to humans; human skin is covered in bacteria and the human digestive system contains many bacteria that cause no problems at all. Bacteria need a food supply to allow them to grow and multiply. Many bacteria are involved in the decomposition (rotting) of dead plants and animals.

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Viruses

Viruses do not have any of the structures found in normal cells. They do not need food. They are made of a protein coat that contains a strand of DNA. They need to enter a cell of another living thing to reproduce. They then use the cell to make new copies of the virus to spread to other organisms. Viruses are even smaller than bacteria. Diseases caused by viruses include the common cold, flu, chicken pox, measles and AIDS.

It is a point of argument whether viruses are actually alive, because they don’t grow, respire, excrete etc.

Fungi

Not all fungi are microorganisms; mushrooms, for example, are fungi that can be seen without a microscope. Fungi that are microorganisms include yeast, penicillin and the organism that causes Athlete’s Foot.

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The uses of microorganisms

It is a mistake to think that all microorganisms are harmful; often, humans use microorganisms for more useful purposes:

Yeast (a fungus) respires when sugars are present to form two substances; carbon dioxide and alcohol. This process is called fermentation. We can use the carbon dioxide when baking bread to make the dough rise, so yeast is used in bread-making. We can use the alcohol to make alcoholic drinks, so yeast is used in the manufacture of beer, wines or spirits.

Penicillin (another fungus) makes a form of antibiotic (see below) for fighting against bacteria.

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How do harmful microorganisms spread?

There are many ways that harmful microorganisms can spread:

• person-to person - by blood (e.g. by sharing needles),saliva (e.g. by kissing) or, for very infectious diseases, through the air (e.g. by coughing or sneezing)
• by food: bacteria may survive in food that is not cooked properly or that is reheated and give you food poisoning that makes you vomit.
• by water: contaminated water may spread diseases like typhoid or cholera.
• by insects: many insects are spread by insects. The plague (Black Death) for example, was carried by fleas living on rats.

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How can we protect ourselves against infectious disease?

Your body has many natural defences against harmful microorganisms entering and causing harm:

The skin acts as a barrier against microorganisms entering.

Hairs in your nose and in the passageways leading down to the lungs are coated in mucus that traps invaders. The hairs can then push the mucus up and out when you cough, sneeze or blow your nose.

Enzymes in tears and saliva break down bacteria to stop them entering your body.

If you eat or drink something that contains bacteria, they are often destroyed by the acid in your stomach.

Your blood contains white blood cells and platelets (as well as red blood cells for carrying oxygen):

• White blood cells can ingest (eat) bacteria in your blood or from between your body’s cells. Some white blood cells can produce special chemicals called antibodies that destroy microorganisms. If an area of the body becomes infected, many white blood cells will rush to that area to attack the microorganisms that have invaded. Pus is the remains of thousands of white blood cells that have died defending your body from infection!
• Platelets help your blood to clot (scab) if your skin is cut. This stops bacteria from invading using the open wound and, of course, stops the bleeding.

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Antibiotics

Antibiotics are drugs that doctors prescribe against bacterial infections. They do not work against diseases caused by viruses, which is why doctors will not give them to patients with a common cold or flu. The most famous antibiotic is called penicillin. It was discovered by a Scottish scientist called Alexander Fleming.

• How Fleming discovered antibiotics (Adobe Acrobat file - 45k)

Immunisation (Vaccination)

Immunisation is the process of injecting people with dead or inactive versions of microorganisms (vaccination). Once a person has been vaccinated, their body starts to make antibodies that destroy the microorganism. Because the microorganisms in the vaccination are not dangerous, the person does not get ill. However, if they later come into contact with a live (dangerous) microorganism, then they already have the antibodies they need to destroy it before it multiplies and makes them ill.

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