Cells need a steady supply of some substances and must remove others. Which substances move, and in which direction, depends on what the organism does.
Animals take in oxygen from the air or water for aerobic respiration in their cells, and they release carbon dioxide, a waste product of respiration. Dissolved food molecules such as glucose and amino acids are absorbed from the digestive system into the blood and carried to cells. Glucose is used in respiration to release energy, and amino acids are used to build proteins. Urea is a waste product made in the liver when excess amino acids are broken down. It is carried in the blood to the kidneys and removed from the body in urine.
Plants take in carbon dioxide through their leaves for photosynthesis and release oxygen as a waste product of it. They also take in oxygen and release carbon dioxide for respiration. They absorb water from the soil through their roots to use in photosynthesis, to keep their cells firm and to carry substances. They absorb mineral ions, such as nitrate, which is needed to make amino acids and so proteins. Sugars made in the leaves are transported to the parts of the plant that need them.