Digestion is the mechanical and chemical breaking down
of food into smaller components, to a form that can be
absorbed, for instance, by a blood stream. Digestion is a
form of catabolism.
In mammals, food enters the mouth, being chewed
by teeth, and broken down by the saliva from the glands.
Then it travels down the oesophagus into the stomach.
Acids break down most of the food. The "leftovers" go
through the small intestine, through the large intestine,
and are excreted during defecation.
Other organisms use different mechanisms to digest
Digestive systems take many forms from simply secret-
ing biotoxins and digestives into the extracellular envir-
onment prior to ingestion. Once potential nutrients or
food is inside the organism, digestion can be conducted
in the cytoplasm, in a vesicle or a sac-like structure,
through a tube, or through several specialized organs
aimed at making the absorption of nutrients more
Viruses digest through the invasion of cells to access
food held within vacuoles. Specific binding sites
between viral capsid proteins and receptors on host cell
surfaces are used to fuse the membranes of the virus and
its target cell. The cell membrane is then (1) punctured
and an opening is established, (2) the host cell is induced
to endocytose the virus, and the resulting vacuole is
either punctured or digested, or (3) some portion of the
host plasma membrane, cell wall or capsule is diges-
ted. The viral capsid or genome is injected into the
host cell’s cytoplasm.
Bacteria use several systems to obtain nutrients from
other organisms in the environment.
Channel transport system
In a channel transport system several proteins form a
contiguous channel traversing the inner and outer
membranes of the bacteria. It is a simple system, which
consists of only three protein subunits: the ABC protein,
membrane fusion protein (MFP), and outer membrane
protein (OMP). This secretion system transports various