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 of a few grams. To support a large body and maintain an active life, energy is needed at a rate that can only be met by a complicated digestive system: a tubular gut with two openings, mouth and anus, which allows both feeding at will and the specialization of tissue to carry out the sequential stages of digestion. DIGESTIVE GLANDS In animals with a tubular gut digestive enzymes are produced and secreted by special glands. Such secretions perform a wide variety of functions.
The glands near the mouth may supply the means of paralyzing the prey, as in spiders and some snakes; maintaining the diet of vertebrate blood in a liquid state, as in leeches and some insects; binding and lubricating the food bolus, as in insects and many vertebrates; and keeping the mouth clean and beginning starch digestion, as in mammals. In many species of invertebrates the middle portion of the gut has two large digestive glands that produce secretions effecting the digestion of carbohydrates, proteins, and lipids. These glands serve functions similar to those performed by the exocrine pancreas in mammals. The liver in vertebrates produces bile, which aids in the absorption of fats in the gut. THE ABSORPTIVE SYSTEM To attain sufficient rates of food absorption and energy production, a large animal must have a correspondingly large surface area of digestive tract. Increased surface is attained by the coiling of a long gut within the body cavity; by the formation of folds of the epithelium, the lining or inner layer of the gut; and by fingerlike protuberances, or villi, on the folds. The total absorptive area in humans exceeds the area of a tennis court.