Functions of proteins
Proteins perform many tasks in the cell. Structural: they are part of membranes and non-membrane organelles. Catalytic: enzymes are biological catalysts of protein nature; each usually acts on one reaction or one kind of substance (specificity) and speeds the reaction up thousands or even millions of times; they work best at normal temperature and pH. Transport: haemoglobin carries oxygen from the lungs to the tissues. Movement: contractile proteins such as actin and myosin enable muscle contraction and movement of cilia and flagella. Signal: receptor proteins in the membrane receive external signals. Defence: white blood cells make antibodies that neutralise foreign proteins and microorganisms; vaccination is based on this mechanism. Reserve: milk, egg and seed proteins. Hormonal: insulin regulates the glucose level in the blood. Energy: 1 g of protein gives about 17.6 kJ when fully broken down, but energy is mainly obtained from carbohydrates and fats.
“Protein professions”. Each group presents one function (enzyme, haemoglobin, antibody, insulin, actin) as a “profession” and gives the class an example.