Biology 102 - General Biology

Animal Structure and Function

The Digestive System

All living things require nutrients or food. Nutrients, broadly defined, are those molecules that are required to build all the biomolecules of the organism's body and to provide the energy to power its energy requiring metabolic processes (replication, transcription, translation, etc.). Some organisms, the primary producers in the food chain, have great synthetic powers. These photo autotrophs and chemoautotrophs require only the "small" molecules such as CO₂, H₂O, H₂S, nitrates, phosphates, sulfates, etc. as nutrients. From these simple (inorganic) molecules they can produce all the organic molecules of which they are composed. They are specialized to use sunlight or inorganic substances as a source of energy. We, the consumers, rely on them as the ultimate source of our nutrients.

We will look in more depth at the nutrient acquisition of animals. Animals require complex biomolecules as nutrients. However, these molecules, if they are macromolecules, must be broken down into their subunits before we can utilize them. If we eat cows, pigs, chickens, broccoli, bananas, eggs, and milk, we do not become a cow, pig, chicken, broccoli, etc. We disassemble the proteins, nucleic acids, polysaccharides, lipids of our food into their subunits and reassemble them into our proteins, DNA, RNA, polysaccharides, and lipids.

We, and the other animals beginning with the round worms (nematodes) have a one-way digestive tract. It can be likened to a factory line where each section performs a specialized function. The digestive tract includes, in order, the mouth, pharynx, esophagus, stomach, small intestine, large intestine (with caecum or appendix), rectum and anus. Food travels in one direction, unlike in the Cnidaria and flatworms which have only one opening into and out of their digestive chamber. The more primitive animals release digestive enzymes (from their lysosomes) into the digestive cavity and then absorb the subunits into the cells lining the cavity. Fungi and bacteria also secrete digestive enzymes into the surrounding environment absorb the smaller sized nutrients that result.

Digestion is both mechanical and chemical. Mechanical digestion refers to breaking down the food into smaller more manageable pieces. Our hands, tongue, teeth, (smooth) muscles of the digestive tract, H⁺ ions (hydrochloric acid) in the stomach, and bile are all involved in the breakdown of food to smaller pieces which can then be attacked by the digestive enzymes. Birds have no teeth so they grind up their food in their gizzard which is a muscular organ. They swallow sand or small stones which go to the gizzard and their food is ground up in the gizzard, another example of mechanical digestion. Bile is produced by the liver and stored in the gall bladder. It acts to emulsify fats much like a detergent providing a larger surface area on which the lipases can work. The action of bile is another example of mechanical digestion. When the fat particles are small enough, lipases (enzymes) will break the covalent bonds of the complex lipids releasing glycerol and fatty acids. This is chemical digestion.

Chemical digestion is the breaking of covalent bonds between the subunits of the macromolecules one ingests. The enzymes that break down carbohydrates into sugars are called amylases. The amylase produced by the salivary glands begins the process of chemical digestion. You may notice a sweet taste if you hold a soda cracker in your mouth for a little while. The "sweet" taste is from the smaller sugars released from the starch polymer in the cracker. This amylase is the only digestive enzyme released in the mouth. The pancreas and small intestine also release amylases into the small intestine. The intestinal lining produces disaccharidases.

Saliva and mucus secreted by cells along the digestive tract help the food slide past the pharynx and down the esophagus and into the stomach. The mucus coats the lining of the digestive tract and protects it from digesting itself! The timing of the release of the various digestive enzymes is controlled by hormones and normally occurs only if food is around. This control of enzyme release also protects the digestive tract when there is no food in it. The esophagus has both circular and longitudinal muscles that (usually) propel the food down toward the stomach. In babies, these muscles are not well coordinated and so they often "spit up" or even engage in "projectile" vomiting.

The muscles of the stomach participate in mechanical digestion by churning the food "bolus." Protein digestion begins in the stomach with pepsin, a protease, which works best at a low pH (which you might expect since there is so much acid produced by the cells lining the stomach). Pepsin is the only enzyme produced in the stomach. The acid begins the breakdown of bones and denatures the proteins, opening them up for action by the proteases.

Other proteases are produced in the pancreas and small intestine. They have specific names such as pepsin, trypsin, chymotrypsin, and some are called amino or carboxy peptidases depending on which end of the molecule they "chew." Incidentally, papain is a protease found in pineapple and papaya! The reason you must use cooked pineapple in jello is because the cooking denatures the enzyme. If fresh pineapple is used, the jello, which is primarily protein, will be broken down by the papain.

The principal site of (chemical) digestion and absorption of nutrients is the small intestine. It is there that the bile from the liver and the pancreatic digestive enzymes are received. In addition, the cells lining the small intestine also make digestive enzymes. The nutrients are absorbed by the cells lining the small intestine. Lipids, if small enough, can be absorbed directly through the cell membranes of the cells lining the intestine. This is possible because, as you remember, the cell membranes are primarily lipid in composition.

An increase of absorptive area of the small intestine is accomplished by throwing the tissue lining into folds called villi and to add to the absorptive area, the cells of the villi have microvilli (supported by the intracellular microfilaments composed of actin) on the surface facing the gut.

The unabsorbed food is passed on to the large intestine whose job it is to reabsorb water and ions and concentrate the waste material. Diarrhea results when the food passes too quickly through the large intestine preventing water reabsorption. The waste, or feces, is eliminated from the rectum and anus. The feces contain bile and the remains of undigested food which has never been in the system. This is different from excretion which is the release of a fluid, urine, formed after the blood is filtered by the kidney. The waste products of urine are metabolic wastes.

The digestive enzymes are produced by the epithelial cells of exocrine glands or exocrine cells. Exocrine glands have ducts that carry the gland cell products to the site where they are utilized. The salivary gland, cells of the stomach that make pepsin, the pancreas, and some cells lining the small intestine are exocrine in function. The pancreas also makes the hormones, insulin and glucagon, which control glucose uptake by the cells of the body. Hormones are made by endocrine glands or endocrine cells. They are ductless glands which release their products into the bloodstream. In general, only those cells with membrane receptors for the specific hormone are stimulated to respond.