The process by which substances are taken into the tissues of organisms is called absorption. It is essential to functions such as digestion, circulation, and respiration.

During digestion, valuable nutrients are absorbed across the epithelial lining of the digestive tract. Absorption occurs largely in the small intestine, which has developed a large surface area for this purpose. The walls of the small intestine contain numerous finger-like projections called villi, which are in turn covered by countless microvilli. Different nutrients are absorbed across the gut epithelium in different ways.

The methods of absorption include active transport, facilitated diffusion, and passive diffusion. Active transport requires energy in the form of adenosine triphosphate (ATP), as well as special carrier molecules that ferry nutrients, (their substrates), across the gut lining. Active transport is involved in the absorption of proteins, which have usually been processed into amino acids or other small peptides. Most...