Every living cell needs to take in some substances and get rid of others. In a large organism these substances also have to be carried between different parts of the body. The main substances are oxygen, carbon dioxide, water, dissolved food molecules, mineral ions and urea.
Cells need oxygen for respiration, which releases energy. In animals, oxygen passes from the lungs into the blood and is carried to the cells. Carbon dioxide is a waste product of respiration. It passes from the cells into the blood and is carried to the lungs to be breathed out. Water is needed for chemical reactions in cells and is taken in by drinking, carried in the blood and lost in sweat and urine.
Dissolved food molecules such as glucose and amino acids are absorbed in the small intestine and carried by the blood to every cell. Mineral ions are also absorbed from food and carried in the blood. For example, iron ions are needed to make haemoglobin. Urea is a waste product made in the liver from excess amino acids. It is carried in the blood plasma to the kidneys and removed from the body in urine.
Plants have the same needs. Roots take in water and mineral ions from the soil, and the water is carried up the plant in the xylem. Carbon dioxide diffuses into the leaves for photosynthesis, oxygen leaves, and the sugars made in the leaves are transported to the rest of the plant.