S-Cool Revision Summary

S-Cool Revision Summary

We often say that 'germs' cause diseases. What do we mean by this?

We mean that microbes invade our bodies and cause damage. The commonest types of microbes are bacteria and viruses.

Other microbes that rarely cause damage include some microscopic parasites and fungi.

Bacteria are fairly small cells and have a variey of different shapes.

These are:

Speheres known as 'cocci'.

Rods know as 'bacilli'.

Spirals know as 'spirilli'.

Unlike other cells, bacterial cells have no nucleus.Their genetic material (DNA) is free within the cytoplasm. They replicate themselves by dividing into two.

A cell wall surrounds bacterial cells but is not made of cellulose like plant cell walls. This cell wall gives protection to the bacterial cell membrane and shape to the cell.

Some bacteria have a small tail called a flagellum which is thrashed about to propel them. Others have multiple smaller versions of this called cilia.

Harmful bacteria make us ill by either damaging our cells or producing poisonous toxins.

But we are full of nice, friendly bacteria too which help to keep us healthy, for example in our digestive system.

They are not cells, they are much tinier and cannot replicate themselves. Like bacteria, viruses come in all sorts of different shapes.

No matter what shape they are viruses share some common features. They have no nucleus. Instead they have a surrounding protein coat that gives them their unique shape. Inside this is a string of DNA.

The only way viruses can make us ill is to get themselves into our cells.

So viruses can get into our cells in a variety of ways including landing on our cells and injecting their DNA into them. Others break down the cell membranes then sneak inside. Once inside our cells they hijack them and make millions of copies of themselves. Each of these can go off and invade other cells.

Passive Protection

Passive defences are those that are set up to stop bacteria or viruses entering the body. They act as roadblocks. They are found in those places where the invaders are most likely to try to the body.

Invaders try to get inside us via five main areas:

  1. The Skin
  2. The Eye
  3. The Respiratory System
  4. The Reproductive System
  5. The Digestive System

Microbes also use another means of access. They hitch a ride on or inside another organism, called a vector.

Active Protection

Once the body's passive defences are breached there is another line of defence. The active system of cells go and deal with the invaders directly. This is the immune system.

The most important of the cells in this defence system are the white blood cells.

White blood cells are obviously carried around in the blood, but they can also crawl out of blood vessels and get to any cell in the body. They can go anywhere!

The immune system cells act in 3 ways:

  1. Consume the invaders
  2. Produce antibodies
  3. Produce antioxins

Immunisation is giving dead or weakened forms of the disease causing bacteria to a person, usually as an injection. The injection does not cause the disease but the immune system responds and creates antibodies. Then if the person is infected with the live bacteria another time their immune system is ready to kill the bacteria

Antibiotics like penicillin are chemicals that kill bacteria. They do not hurt body cells.

They are very useful for cleaning up infections but they are useless against viruses. So next time you have a cold or 'flu they won't be any use, I'm afraid.