Every cell needs a supply of the substances it uses and a way to remove the substances it makes. Which substances move depends on what the organism needs.
Animals take in oxygen for aerobic respiration and release carbon dioxide as a waste gas. Fish do this at their gills and mammals do it in the lungs. Dissolved food molecules, such as glucose and amino acids, are absorbed from the small intestine into the blood and carried to the cells that use them. Water is taken in by drinking and eating, and it is lost in sweat and urine. Urea is a waste substance made in the liver from excess amino acids. It is carried in the blood to the kidneys and removed in urine.
Plants take in carbon dioxide for photosynthesis and release oxygen as a waste gas from the same process. They take in water from the soil through their roots, together with mineral ions. Nitrate ions are needed to make amino acids and proteins, and magnesium ions are needed to make chlorophyll. Dissolved sugars made in the leaves are transported to the rest of the plant for respiration, growth and storage.