Why do cells need oxygen?

All cells need energy in order to stay alive and they get it mostly by burning sugars when oxygen is present.  This makes more molecules of ATP than if there is no oxygen.

ATP can then be used to do things in the cell, such as moving molecules around, building molecules to make new cells, or other fancy things like making a muscle contract.

What kind of molecule is haemoglobin?

Haemoglobin is a protein, but quite a complicated one.

All proteins are made by ribosomes in the cytoplasm, which join amino acids into a long chain with peptide bonds between them - see how in this video

But rather than just one single chain, called a polypeptide, haemoglobin has four polypeptides.

Each one has a haem group, which can bind to one molecule of oxygen.

There are millions of haemoglobin molecules inside each of your blood cells!

What do we know about the genes for haemoglobin?

Quite a lot!

The human genes have been sequenced.  If mutations occur to change the DNA instructions, perhaps putting a different amino acid in a particular position in any of the polypeptides, then sometimes the red blood cells behave differently and the haemoglobin does not carry oxygen so efficiently.

This can result in a serious disease called sickle cell anaemia.

The genes for haemoglobin in other animals have been studied too - and they are remarkably similar to our own!