 Another great example of the interplay between cell structure and function are red blood cells. Blood is made up of water, salts, nutrients, wastes, proteins and three types of blood cells. Red blood cells, white blood cells and platelets. The white blood cells, also called leukocytes, are responsible for defence and immunity. Platelets are cell fragments which work to clot the blood when needed, like if you cut yourself. Red blood cells, also known as erythrocytes, carry oxygen. Red blood cells have evolved to be very efficient carriers of oxygen. They have no nuclei and no mitochondria. They only do anaerobic cell respiration. Most of the space in the cell is devoted to carrying hemoglobin, a protein that can hold up to four oxygen molecules at a time. A typical red blood cell has about 250 million hemoglobin, so each full cell coming from the lungs can have about one billion molecules of oxygen.