The Digestive Systems



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

completed.



DIGESTIVE ENZYMES

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.



THE GUT

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 meal is ejected, another meal

cannot be consumed. As a result, such animals seldom exceed a dry weight