Unciellular organisms obtain the nutrients they need via diffusion or endocytosis across the plasma membrane. The smaller multicellular organisms may also obtain their nutrients through this means. However, diffusion and endocytosis is very inefficient when it comes to serving the needs of large multicellular organisms. These processes do not occur fast enough and provide enough quantities of nutrients for the needs of especially the cells in the internal tissues of the organism. These larger organisms obtain food via the digestive system. Food enters the body through a mouth, passes through a tube which breaks the macromoleucles down into their subunits (amino acids, monosaccharides, fatty acids, etc....).

The nutrients only truly enter the body after digestion takes place. The subunit molecules, amino acids, monosaccharides, fatty acids, nucleic acids, etc... diffuse from higher concentration in the digestive cavity to lower concentrations in the blood. The process where the subunits pass from the digestive tract to the blood is known as absorption. The bulk of nutrient absorption occurs in the portion of the small intestine which contains numeorus, small finger-like projection of the small intestine wall known as villi. The villi essentially increase the surface area of the small intestine wall, so diffusion occurs faster. They also increase the network of capillaries present at the interface between the digestive and circulatory systems, which allows for faster absorption.

As the nutrient subunits travel to the cells of the various body tissues, they either diffuse from high to low concentration into the cells, or the cells use ATP to actively transport and concentrate them inside the cell. The process where food nutrients become part of the tissues of the body is known as assimilation.