The digestive systems works on nutrients taken from the environment, breaking them down into simpler
products, and then absorbing the products together with water and salts so they can be used in metabolism.

The individuals of all species contain many different protiens or carbohydrates in the cell walls of
bacteria, chitin in the external skeletons of arthropods, and so on and foreign proteins or carbohydrates are
rarely incorporated unchanged. They are usually first broken down to their constituents before being built
up again as the proteins or carbohydrates belonging to the organism.

In such organisms that are not cellular as amoebae, digestion occurs inside the cell. Intracelular
digestion is also found in some higher animals such as mussels and sponges. Muscles, for instance, filter
algae and other tiny organisms from the water and digest them within the cells of a special digestive gland.
Intracellular digestion occurs even in some animals that consume large pieces of food; the prey captured by
Hydra, for example, is partly digested in the coelenteron, a gut-like cavity, and the residue is completely
digested in cells of the body wall. Scientists believe that intracellular digestion in organisms such as Hydra
has evolved from such protozoans as amoebae and paramecia.

In most higher animals digestion is completed not in the cell in the cavity of a digestive tract (the
stomach and intestine). Animals with this type of digestion include crustaceans, insects, cephalopods,
tunicates, and all vertebrates. A few animals with digestive tracts also partially digest their food before
eating it. Some spiders, for example, pierce their prey with fangs that pump digestive fluids into the victim.
This liquefies the softer parts, which the spider then sucks into its stomach and intestine, where digestion is

In digestion large molecules are split into smaller ones by enzyme hydrolosis, so named because
water is taken up in the process. The enzymes that hydrolyze proteins, fats, and carbohydrates are called,
respectively, proteases, lipases, and carbohydrases, or amylases.

Some insects, birds, and herbivorous animals can digest substances that most other animals
cannot. The clothes moth digests hair and wool, termite digests wood; and herbivores digest the big,
fibrous cellulose of plants that is completely indigestible to other animals. However, none of these unusual
organisms produce the needed digestive enzymes, which are furnished instead by bacteria or protozoans,
harbored in special parts of the digestive tract. Each termite species, for example, carries protozoans
peculiar to it that attack wood and change it into dihestible substances. The newborn termite is infected
with its digestive aids by feeding from older termites. Eat as they may, they will die of starvation if
isolated before this infection occurs. Herbivorous mollusks like the snail produce their own cellulose-
digesting enzymes, as does the shipworm, a wood-boring mollusk.

No vertebrate produces cellulose-digesting cellulases. Thus, the initial stages of digestion in the
cow and other ruminants are carried out by enzymes secreted by bacteria in the rumen, a large sac that
precedes the true stomach. Plant fiber enters the rumen, is attacked by the bacterial enzymes, and is then
returned to the mouth for further chewing; the food so returned is called the cud. This chewing increases
the surface area exposed to the bacteria.
The presence of digestive bacteria in ruminants results in a specialized metabolism of proteins and
carbohydrates. Most of the ruminant's protein needs are actually supplied by the rumen bacteria, which use
such simple substances as urea and inorganic sulfates to manufacture proteins. Ruminants digest that
protein and do not require in their diets certain amino acids (the basic building blocks of proteins) that are
indispensable in the diets of other animals.

All kinds of organic materials can be digested and used by organisms. Most of the digestion products,
however, are the same: simple sugars from the hydrolysis of carbohydrates; fatty acids from the hydrolysis
of fats; and amino acids from proteins. These substances yield the energy and body-building material
needed by conventional animals.

In amoebae a vacuole serves the same purpose as the intestine, or gut, in the higher animals. Some parasites
need no gut but absorb material from the host through their external surfaces. Organisms below the
evolutionary level of the flatworms have a single gut opening that connects with the environment. The
single opening has one major disadvantage, however: until the residue of one