Most animals including humans feed by a process called holozoic nutrition. This means that the animal has a gut tube (alimentary canal) that runs through its body and the animal has a mouth at one end (this is where the food goes in), an anus at the other end where undigested food (faeces) passes out.
Ingestion: the first stage of feeding involves food being taken up into the mouth (not too complicated to understand I hope…)
Most food is made up of very large molecules (macromolecules) such as starch, proteins and lipids. These molecules are too large to be absorbed from the gut tube into the blood. So the second stage of feeding involves chemically breaking down these large food molecules into smaller solubles molecules that can be absorbed.
Digestion: the chemical breakdown of large insoluble food molecules (e.g. proteins/starch/lipids) into smaller soluble molecules (amino acids/sugars/fatty acids and glycerol) that can be absorbed. Digestion is a chemical process and every digestive reaction is catalysed by a specific enzyme. Almost all these digestive enzymes are secreted into the gut tube and mixed in with the food. Please see a later post on Digestion to get a full understanding of these reactions.
Absorption: the small products of digestion are then absorbed into the blood stream. This process occurs almost exclusively in a region of the small intestine called the ileum. The structure of the ileum is beautifully adapted for efficient absorption – see a later post on Absorption for full details!
Assimilation: this is the stage of processing food that is sometimes left out. This is because it does not happen in the alimentary canal but instead in the body cells of the animal. The small soluble products of digestion (glucose, amino acids, fatty acids etc.) are taken up into cells and used to build the animal’s own macromolecules – proteins/lipids/glycogen etc. Assimilation is a term for how small molecules are built up and used in the cells of the animal.
Egestion: this is the final stage and involves undigested food, mostly cellulose in humans, being passed out of the anus as faeces at the end of the alimentary canal.
Don’t confuse egestion with excretion….. Egestion is simply passing out undigested food from the end of the large intestine. If you think of your body like a tube of polos (you know I do…) then the stuff that comes out of the large intestine has never actually been inside your cells. It is mostly cellulose and other plant fibres that went into the mouth a day or two ago, haven’t been digested nor absorbed and so come out the other end. Excretion on the other hand is the process of removing waste molecules that have been made inside cells. So the lungs removing carbon dioxide, the kidneys taking urea out of the blood to make urine – these are examples of excretion.
A slightly lavatorial way to end the post but important nonetheless: if you are talking about urine production, this is excretion, but “number twos” or “Richards” (look up cockney-rhyming slang for EAL speakers) are not excretion, they are egestion….