Haemoglobin

• Globular and water-soluble with 4 polypeptide chains: two α-chains with 141 AA residues, and two β-chains with 146 AA residues
• Each polypeptide chain contains a prosthetic (“haem”) group, contains iron ions (Fe^2^+), makes it the typical red colour - some animals have blue-green blood because they have copper instead of iron
• It binds to oxygen in the lungs, transports it to respiring cells and releases it; each “haem” group will bind to one oxygen molecule (O_2), it can bind to 4 oxygen molecules; haemoglobin is transported in blood in red blood cell
• Blood cells are biconcave structures; they have no organelles, just cytoplasm to make as much room for haemoglob in as possible