B7.3

Corals an they have and green algae in their cells benefit each other. So we say that they have a symbiotic relationship.

An orchid attatched to a branch of a tree does no harm to the tree, But the orchid benefits from getting more light. This is an example of commensalism. 

An association between to organisms that benefits one of them ( the parasite ) and harms the other ( the tapeowrm ) is a parasitism. Examples are tape worms and malaria parasites.

parasites have been adapted for finding a host and living on or in them. The evelution of a parasite is believed to be closely linked with the evelution of its host.

tapeworms:

- have suckers to grip the gut wall.        - have a thick cutile to resist enzymes.        - respire anaerobically without oxygen.   - both sex organs to reproduce on own.    - produce lots of eggs to increase chance of finding a new host.

malaria parasites:

- spread to new hosts be mosquitoes.    - Live inside red blood cells so are protected from immune system.    - change surface markers to prevent being recognised by white blood cells.

Parasites cause problems such as human diseases, including malaria. Also reduced food yields.

Sickle-cell anaemia is caused by a faulty recessive allele that codes for Haemoglobin. Some of the red blood cells go out of shape causing these symptoms:

- severe pain, when sickled red blood cells block tiny blood vessels.

- anaemia as damaged cells cant be replaced fast enough.

- tiredness cause by a shortage of oxygen.

However people who carry just one sickle-cell allele are less likely to get malaria. They have an improved chance of survival. So, the frequency of the sickle-cell allele has increased in places where malaria is a problem This is an example of natural selection.