Animal Nutrition

ANIMAL NUTRITION Introduction As a group, animals exhibit a great variety of nutritional adaptations. For example, the snowshoe hare of the northern forests obtains all its nutrients from plants alone. Hares and rabbits have a large intestinal pouch housing prokaryotes and protists that digest cellulose. For any animal, a nutritionally adequate diet is essential for homeostasis, a steady-state balance in body functions. A balanced diet provides fuel for cellular work and the materials needed to construct organic molecules. A. Nutritional Requirements 1. Animals are heterotrophs that require food for fuel, carbon skeletons, and essential nutrients: an overview A nutritionally adequate diet satisfies three needs: Fuel (chemical energy) for all the cellular work of the body. The organic raw materials animals use in biosynthesis (carbon skeletons to make many of their own molecules). Essential nutrients, substances that the animals cannot make for itself from any raw material and therefore must obtain in food in prefabricated form. 2. Homeostatic mechanisms manage an animal’s fuel The flow of food energy into and out of an animal can be viewed as a “budget,” with the production of ATP accounting for the largest fraction by far of the energy budget of most animals. ATP powers basal or resting metabolism, as well as activity, and, in endothermic animals, temperature regulation. Nearly all ATP is derived from oxidation of organic fuel molecules— carbohydrates, proteins, and fats—in cellular respiration. The monomers of any of these substances can be used as fuel, though priority is usually given to carbohydrates and fats. Fats are especially rich in energy, liberating about twice the energy liberated from an equal amount of carbohydrate or protein during oxidation. When an animal takes in more calories than it needs to produce ATP, the excess can be used for biosynthesis. This biosynthesis can be used to grow in size or for reproduction, or can be stored in energy depots. In humans, the liver and muscle cells store energy as glycogen, a polymer made up of many glucose units. Glucose is a major fuel molecule for cells, and its metabolism, regulated by hormone action, is an important aspect of homeostasis. If glycogen stores are full and caloric intake still exceeds caloric expenditure, the excess is usually stored as fat. The human body regulates the use and storage of glucose, a major cellular fuel. • (1) When glucose levels rise above a set point, (2) the pancreas secretes insulin into the blood. • (3) Insulin enhances the transport of glucose into body cells and stimulates the liver and muscle cells to store glucose as glycogen, dropping blood glucose levels. • (4) When glucose levels drop below a set point, (5) the pancreas secretes glucagon into the blood. • (6) Glucagon promotes the breakdown of glycogen and the release of glucose into the blood, increasing the blood glucose levels. The pancreas uses the hormones insulin and glucagon to signal distant cells to take up or release glucose to regulate levels in the blood. When fewer calories are taken in than are expended, fuel is taken out of storage depots and oxidized. The human body generally expends liver glycogen first, and then draws on muscle glycogen and fat. Most healthy people—even if they are not obese—have enough stored fat to sustain them through several weeks of starvation. The average human’s energy needs can be fueled by the oxidation of only 0.3 kg of fat per day. Severe problems occur if the energy budget remains out of balance for long periods. If the diet of a person or other animal is chronically deficient in calories, undernourishment results. The stores of glycogen and fat are used up, the body begins breaking down its own proteins for fuel, muscles begin to decrease in size, and the brain can become protein-deficient. If energy intake remains less than energy expenditure, death will eventually result, and even if a seriously undernourished person survives, some damage may be irreversible. Because a diet of a single staple such as rice or corn can often provide sufficient calories, undernourishment is generally common only where drought, war, or some other crisis has severely disrupted the food supply. Another cause of undernourishment is anorexia nervosa, an eating disorder associated with a compulsive aversion to body fat. Overnourishment, or obesity, the result from excessive food intake, is a common problem in the United States and other affluent nations. The human body tends to store any excess fat molecules obtained from food instead of using them for fuel. In contrast, when we eat an excess of carbohydrates, the body tends to increase its rate of carbohydrate oxidation. While fat hoarding can be a liability today, it probably provided a fitness advantage for our hunting/gathering ancestors, enabling individuals with genes promoting the storage of high-energy molecules during feasts to survive the eventual famines. Despite its propensity to store fat, the human body seems to impose limits on weight gain (or loss). Some people remain at a more-or-less constant weight no matter how much they eat. Most dieters return to their former weight soon after they stop dieting. In mammals, a hormone called leptin, produced by adipose cells, is a key player in a complex feedback mechanism regulating fat storage and use. A high leptin level cues the brain to depress appetite and to increase energy-consuming muscular activity and body-heat production. Conversely, loss of body fat decreases leptin levels in the blood, signaling the brain to increase appetite and weight gain. These feedback mechanisms regulate body weight around a fairly rigid set point in some individuals and over a relatively wide range in others. Researchers have identified some of the genes involved in fat homeostasis and several chemical signals that underlie the brain’s regulatory role. Some of the signals and signal antagonists are under development as potential medications for obesity. Obesity may be beneficial in certain species. Small seabirds called petrels fly long distances to find food, which is rich in lipids. By bringing lipid-rich food to their chicks, the parents minimize the weight of food that they must carry. However, because these foods are low in protein, young petrels have to consume more calories than they burn in metabolism—and consequently they become obese. In some petrel species, chicks at the end of the growth period weigh much more their parents and are too heavy to fly and they need to starve for several days to fly. The fat reserves do help growing chicks to survive periods when parents are unable to find food. 3. An animal’s diet must supply essential nutrients and carbon skeletons for biosynthesis In addition to fuel for ATP production, an animal’s diet must supply all the raw materials for biosynthesis. This requires organic precursors (carbon skeletons) from its food. Given a source of organic carbon (such as sugar) and a source of organic nitrogen (usually in amino acids from the digestion of proteins), animals can fabricate a great variety of organic molecules - carbohydrates, proteins, and lipids. Besides fuel and carbon skeletons, an animal’s diet must also supply essential nutrients. These are materials that must be obtained in preassembled form because the animal’s cells cannot make them from any raw material. Some materials are essential for all animals, but others are needed only by certain species. For example, ascorbic acid (vitamin C) is an essential nutrient for humans and other primates, guinea pigs, and some birds and snakes, but not for most other animals. An animal whose diet is missing one or more essential nutrients is said to be malnourished. For example, many herbivores living where soils and plants are deficient in phosphorus eat bones to obtain this essential nutrient. Malnutrition is much more common than undernourishment in human populations, and it is even possible for an overnourished individual to be malnourished. Animals require 20 amino acids to make proteins. Most animals can synthesize half of these if their diet includes organic nitrogen. Essential amino acids must be obtained from food in prefabricated form. Eight amino acids are essential in the adult human with a ninth, histidine, essential for infants. The same amino acids are essential for most animals. A diet that provides insufficient amounts of one or more essential amino acids causes a form of malnutrition known as protein deficiency. This is the most common type of malnutrition among humans. The victims are usually children, who, if they survive infancy, are likely to be retarded in physical and perhaps mental development. The proteins in animals products, such as meat, eggs, and cheese, are “complete,” which means that they provide all the essential amino acids in their proper proportion. Most plant proteins are “incomplete,” being deficient in one or more essential amino acid. For example, corn is deficient in the amino acid lysine. Individuals who are forced by economic necessity or other circumstances to obtain nearly all their calories from corn would show symptoms of protein deficiency. This is true from any diet limited to a single plant source, including rice, wheat, or potatoes. Protein deficiency from a vegetarian diet can be avoided by eating a combination of plant foods that complement each other to supply all essential amino acids. For example, beans supply the lysine that is missing in corn, and corn provides the methionine which is deficient in beans. Because the body cannot easily store amino acids, a diet with all essential amino acids must be eaten each day, otherwise protein synthesis is retarded. Some animals have special adaptations that get them through periods where their bodies demand extraordinary amounts of protein. For example, penguins use their muscle proteins as a source of amino acids to make new proteins during molting. While animals can synthesize most of the fatty acids they need, they cannot synthesize essential fatty acids. These are certain unsaturated fatty acids, including linoleic acids, that are required by humans. Most diets furnish ample quantities of essential fatty acids, and thus deficiencies are rare. Vitamins are organic molecules required in the diet in quantities that are quite small compared with the relatively large quantities of essential amino acids and fatty acids animals need. While vitamins are required in tiny amounts—from about 0.01 mg to 100 mg per day—depending on the vitamin, vitamin deficiency (or overdose in some cases) can cause serious problems. So far 13 vitamins essential to humans have been identified. These can be grouped into water-soluble vitamins and fat-soluble vitamins, with extremely diverse physiological functions. The subject of vitamin dosage has aroused heated scientific and popular debate. Some believe that it is sufficient to meet recommended daily allowances (RDAs), the nutrient intakes proposed by nutritionists to maintain health. Others argue that RDAs are set too low for some vitamins, and a fraction of these people believe, probably mistakenly, that massive doses of vitamins confer health benefits. While research is ongoing, all that can be said with any certainty is that people who eat a balanced diet are not likely to develop symptoms of vitamin deficiency. Minerals are simple inorganic nutrients, usually required in small amounts - from less than 1 mg to about 2,500 mg per day. Mineral requirements vary with animal species. Humans and other vertebrates require relatively large quantities of calcium and phosphorus for the construction and maintenance of bone among other uses. Iron is a component of the cytochromes that function in cellular respiration and of hemoglobin, the oxygen binding protein of red blood cells. While sodium, potassium, and chloride have a major influence on the osmotic balance between cells and the interstitial fluids, excess consumption of salt (sodium chloride) is harmful. The average U.S. citizen eats enough salt to provide about 20 times the required amount of sodium. Excess consumption of salt or several other minerals can upset homeostatic balance and cause toxic side effects. For example, too much sodium is associated with high blood pressure, and excess iron causes liver damage. B. Food Types and Feeding Mechanisms 1. Most animals are opportunistic feeders All animals eat other organisms—dead or alive, whole or by the piece (including parasites). In general, animals fit into one of three dietary categories. Herbivores, such as gorillas, cows, hares, and many snails, eat mainly autotrophs (plants, algae). Carnivores, such as sharks, hawks, spiders, and snakes, eat other animals. Omnivores, such as cockroaches, bears, raccoons, and humans, consume animal and plant or algal matter. Humans evolved as hunters, scavengers, and gatherers. While the terms herbivore, carnivore, and omnivore represent the kinds of food that an animal usually eats, most animals are opportunistic, eating foods that are outside their main dietary category when these foods are available. For example, cattle and deer, which are herbivores, may occasionally eat small animals or bird eggs. Most carnivores obtain some nutrients from plant materials that remain in the digestive tract of the prey that they eat. All animals consume bacteria along with other types of food. 2. Diverse feeding adaptations have evolved among animals The mechanisms by which animals ingest food are highly variable but fall into four main groups. Many aquatic animals, such as clams, are suspension-feeders that sift small food particles from the water. Baleen whales, the largest animals ever to live, swim with their mouths agape, straining millions of small animals from huge volumes of water forced through screenlike plates (baleen) attached to their jaws. Deposit-feeders, like earthworms, eat their way through dirt or sediments and extract partially decayed organic material consumed along with the soil or sediments. Substrate-feeders live in or on their food source, eating their way through the food. For example, maggots burrow into animal carcasses and leaf miners tunnel through the interior of leaves. Fluid-feeders make their living sucking nutrient-rich fluids from a living host and are considered parasites. Mosquitoes and leaches suck blood from animals. Aphids tap the phloem sap of plants. In contrast, hummingbirds and bees are fluid-feeders that aid their host plants, transferring pollen as they move from flower to flower to obtain nectar. Most animals are bulk-feeders that eat relatively large pieces of food. Their adaptations include such diverse utensils as tentacles, pincers, claws, poisonous fangs, and jaws and teeth that kill their prey or tear off pieces of meat or vegetation. C. Overview of Food Processing 1. The four main stages of food processing are ingestion, digestion, absorption, and elimination Ingestion, the act of eating, is only the first stage of food processing. Food is “packaged” in bulk form and contains very complex arrays of molecules, including large polymers and various substances that may be difficult to process or may even be toxic. Animals cannot use macromolecules like proteins, fats, and carbohydrates in the form of starch or other polysaccharides. First, polymers are too large to pass through membranes and enter the cells of the animal. Second, the macromolecules that make up an animal are not identical to those of its food. In building their macromolecules, however, all organisms use common monomers. For example, soybeans, fruit flies, and humans all assemble their proteins from the same 20 amino acids. Digestion, the second stage of food processing, is the process of breaking food down into molecules small enough for the body to absorb. Digestion cleaves macromolecules into their component monomers, which the animal then uses to make its own molecules or as fuel for ATP production. Polysaccharides and disaccharides are split into simple sugars. Fats are digested to glycerol and fatty acids. Proteins are broken down into amino acids. Nucleic acids are cleaved into nucleotides. Digestion reverses the process that a cell uses to link together monomers to form macromolecules. Rather than removing a molecule of water for each new covalent bond formed, digestion breaks bonds with the addition of water via enzymatic hydrolysis. A variety of hydrolytic enzymes catalyze the digestion of each of the classes of macromolecules found in food. Chemical digestion is usually preceded by mechanical fragmentation of the food—by chewing, for instance. Breaking food into smaller pieces increases the surface area exposed to digestive juices containing hydrolytic enzymes. After the food is digested, the animal’s cells take up small molecules such as amino acids and simple sugars from the digestive compartment, a process called absorption. During elimination, undigested material passes out of the digestive compartment. 2. Digestion occurs in specialized compartments To avoid digesting their own cells and tissues, most organisms conduct digestion in specialized compartments. The simplest digestive compartments are food vacuoles, organelles in which hydrolytic enzymes break down food without digesting the cell’s own cytoplasm, a process termed intracellular digestion. This is the sole digestive strategy in heterotrophic protists and in sponges, the only animal that digests their food this way. • (1) Heterotrophic protists engulf their food by phagocytosis or pinocytosis and (2) digest their meals in food vacuoles. • (3) Newly formed vacuoles are carried around the cell (4) until they fuse with lysosomes, which are organelles containing hydrolytic enzymes. • (5) Later, the vacuole fuses with an anal pore and its contents are eliminated. In most animals, at least some hydrolysis occurs by extracellular digestion, the breakdown of food outside cells. Extracellular digestion occurs within compartments that are continuous with the outside of the animal’s body. This enables organisms to devour much larger prey than can be ingested by phagocytosis and digested intracellularly. Many animals with simple body plans, such as cnidarians and flatworms, have digestive sacs with single openings, called gastrovascular cavities. For example, a hydra captures its prey with nematocysts and stuffs the prey through the mouth into the gastrovascular cavity. The prey is then partially digested by enzymes secreted by gastrodermal cells. These cells absorb food particles and most of the actual hydrolysis of macromolecules occurs intracellularly. Undigested materials are eliminated through the mouth. In contrast to cnidarians and flatworms, most animals have complete digestive tracts or alimentary canals with a mouth, digestive tube, and an anus. Because food moves in one direction, the tube can be organized into special regions that carry out digestion and nutrient absorption in a stepwise fashion. Food ingested through the mouth and pharynx passes through an esophagus that leads to a crop, gizzard, or stomach, depending on the species. Crops and stomachs usually serve as food storage organs, although some digestion occurs there too. Gizzards grind and fragment food. In the intestine, digestive enzymes hydrolyze the food molecules, and nutrients are absorbed across the lining of the tube into the blood. Undigested wastes are eliminated through the anus. This system enables organisms to ingest additional food before earlier meals are completely digested. D. The Mammalian Digestive System The general principles of food processing are similar for a diversity of animals, including the mammalian system which we will use as a representative example. The mammalian digestive system consists of the alimentary canal and various accessory glands that secrete digestive juices into the canal through ducts. Peristalsis, rhythmic waves of contraction by smooth muscles in the walls of the canal, push food along. Sphincters, muscular ringlike valves, regulate the passage of material between specialized chambers of the canal. The accessory glands include the salivary glands, the pancreas, the liver, and the gallbladder. After chewing and swallowing, it takes 5 to 10 seconds for food to pass down the esophagus to the stomach, where it spends 2 to 6 hours being partially digested. Final digestion and nutrient absorption occur in the small intestine over a period of 5 to 6 hours. In 12 to 24 hours, any undigested material passes through the large intestine, and feces are expelled through the anus. 1. The oral cavity, pharynx, and esophagus initiate food processing Both physical and chemical digestion of food begins in the mouth. During chewing, teeth of various shapes cut, smash, and grind food, making it easier to swallow and increasing its surface area. The presence of food in the oral cavity triggers a nervous reflex that causes the salivary glands to deliver saliva through ducts to the oral cavity. Salivation may occur in anticipation because of learned associations between eating and the time of day, cooking odors, or other stimuli. Saliva contains a slippery glycoprotein called mucin, which protects the soft lining of the mouth from abrasion and lubricates the food for easier swallowing. Saliva also contains buffers that help prevent tooth decay by neutralizing acid in the mouth. Antibacterial agents in saliva kill many bacteria that enter the mouth with food. Chemical digestion of carbohydrates, a main source of chemical energy, begins in the oral cavity. Saliva contains salivary amylase, an enzyme that hydrolyzes starch and glycogen into smaller polysaccharides and the disaccharide maltose. The tongue tastes food, manipulates it during chewing, and helps shape the food into a ball called a bolus. During swallowing, the tongue pushes a bolus back into the oral cavity and into the pharynx. The pharynx, also called the throat, is a junction that opens to both the esophagus and the trachea (windpipe). When we swallow, the top of the windpipe moves up such that its opening, the glottis, is blocked by a cartilaginous flap, the epiglottis. This mechanism normally ensures that a bolus will be guided into the entrance of the esophagus and not directed down the windpipe. • (1) When not swallowing, the esophageal sphincter muscles are contracted, the epiglottis is up, and the glottis is open, allowing airflow to the lungs. • (2) When a food bolus reaches the pharynx, (3) the larynx moves upward and the epiglottis tips over the glottis, closing off the trachea. • (4) The esophageal sphincter relaxes and the bolus enters the esophagus. • (5) In the meantime, the larynx moves downward and the trachea is opened, (6) and peristalsis moves the bolus down the esophagus to the stomach. The esophagus conducts food from the pharynx down to the stomach by peristalsis. The muscles at the very top of the esophagus are striated and therefore under voluntary control. Involuntary waves of contraction by smooth muscles in the rest of the esophagus then takes over. 2. The stomach stores food and performs preliminary digestion The stomach is located in the upper abdominal cavity, just below the diaphragm. With accordionlike folds and a very elastic wall, the stomach can stretch to accommodate about 2 L of food and fluid, storing an entire meal. The stomach also secretes a digestive fluid called gastric juice and mixes this secretion with the food by the churning action of the smooth muscles in the stomach wall. Gastric juice is secreted by the epithelium lining numerous deep pits in the stomach wall. With a high concentration of hydrochloric acid, the pH of the gastric juice is about 2—acidic enough to digest iron nails. This acid disrupts the extracellular matrix that binds cells together. It kills most bacteria that are swallowed with food. Also present in gastric juice is pepsin, an enzyme that begins the hydrolysis of proteins. Pepsin, which works well in strongly acidic environments, breaks peptide bonds adjacent to specific amino acids, producing smaller polypeptides. Pepsin is secreted in an inactive form, called pepsinogen by specialized chief cells in gastric pits. Parietal cells, also in the pits, secrete hydrochloric acid which converts pepsinogen to the active pepsin only when both reach the lumen of the stomach, minimizing self-digestion. Also, in a positive-feedback system, activated pepsin can activate more pepsinogen molecules. The stomach’s second line of defense against self-digestion is a coating of mucus, secreted by epithelial cells, that protects the stomach lining. Still, the epithelium is continually eroded, and the epithelium is completely replaced by mitosis every three days. Gastric ulcers, lesions in the stomach lining, are caused by the acid-tolerant bacterium Heliobacter pylori. Ulcers are often treated with antibiotics. About every 20 seconds, the stomach contents are mixed by the churning action of smooth muscles. As a result of mixing and enzyme action, what begins in the stomach as a recently swallowed meal becomes a nutrient-rich broth known as acid chyme. Most of the time the stomach is closed off at either end. The opening from the esophagus to the stomach, the cardiac orifice, normally dilates only when a bolus driven by peristalsis arrives. The occasional backflow of acid chyme from the stomach into the lower esophagus causes heartburn. At the opening from the stomach to the small intestine is the pyloric sphincter, which helps regulate the passage of chyme into the intestine. A squirt at a time, it takes about 2 to 6 hours after a meal for the stomach to empty. 3. The small intestine is the major organ of digestion and absorption With a length of over 6 m in humans, the small intestine is the longest section of the alimentary canal. Most of the enzymatic hydrolysis of food macromolecules and most of the absorption of nutrients into the blood occurs in the small intestine. In the first 25 cm or so of the small intestine, the duodenum, acid chyme from the stomach mixes with digestive juices from the pancreas, liver, gall bladder, and gland cells of the intestinal wall. The pancreas produces several hydrolytic enzymes and an alkaline solution rich in bicarbonate which buffers the acidity of the chyme from the stomach. The liver performs a wide variety of important functions in the body, including the production of bile. Bile is stored in the gallbladder until needed. It contains bile salts which act as detergents that aid in the digestion and absorption of fats. Bile also contains pigments that are by-products of red blood cell destruction in the liver. These bile pigments are eliminated from the body with the feces. Specific enzymes from the pancreas and the duodenal wall have specific roles in digesting macromolecules.0 The digestion of starch and glycogen, begun by salivary amylase in the oral cavity, continues in the small intestine. Pancreatic amylases hydrolyze starch, glycogen, and smaller polysaccharides into disaccharides. A family of disaccharidases hydrolyzes each disaccharide into monomers. Maltase splits maltose into two glucose molecules. Sucrase splits sucrose, a sugar found in milk, into glucose and fructose. These enzymes are built into the membranes and extracellular matrix of the intestinal epithelium which is also the site of sugar absorption. Digestion of proteins in the small intestine completes the process begun by pepsin. Several enzymes in the duodenum dismantle polypeptides into their amino acids or into small peptides that in turn are attacked by other enzymes. Trypsin and chymotrypsin attack peptide bonds adjacent to specific amino acids, breaking larger polypeptides into shorter chains. Dipeptidases, attached to the intestinal lining, split smaller chains. Carboxypeptidase and aminopeptidase split off one amino acid from the carboxyl or amino end of a peptide, respectively. Many of the protein-digesting enzymes, such as aminopeptidase, are secreted by the intestinal epithelium, but trypsin, chymotrypsin, and carboxypeptidase are secreted in inactive form by the pancreas. Another intestinal enzyme, enteropeptidase, converts inactive trypsinogen into active trypsin. Active trypsin then activates the other two. The digestion of nucleic acids involves a hydrolytic assault similar to that mounted on proteins. A team of enzymes called nucleases hydrolyzes DNA and RNA into their component nucleotides. Other hydrolytic enzymes then break nucleotides down further into nucleosides, nitrogenous bases, sugars, and phosphates. Nearly all the fat in a meal reaches the small intestine undigested. Normally fat molecules are insoluble in water, but bile salts, secreted by the gallbladder into the duodenum, coat tiny fats droplets and keep them from coalescing, a process known as emulsification. The large surface area of these small droplets is exposed to lipase, an enzyme that hydrolyzes fat molecules into glycerol, fatty acids, and glycerides. Most digestion occurs in the duodenum. The other two sections of the small intestine, the jejunum and ileum, function mainly in the absorption of nutrients and water. To enter the body, nutrients in the lumen must pass the lining of the digestive tract. The small intestine has a huge surface area - 300 m2, roughly the size of a tennis court. The enormous surface of the small intestine is an adaptation that greatly increases the rate of nutrient absorption. Large circular folds in the lining bear fingerlike projections called villi, and each epithelial cell of a villus has many microscopic appendages called microvilli that are exposed to the intestinal lumen. Penetrating the core of each villus is a net of microscopic blood vessels (capillaries) and a single vessel of the lymphatic system called a lacteal. Nutrients are absorbed across the intestinal epithelium and then across the unicellular epithelium of capillaries or lacteals. Only these two single layers of epithelial cells separate nutrients in the lumen of the intestine from the bloodstream. In some cases, such as fructose, transport of nutrients across the epithelial cells is passive, as molecules move down their concentration gradients from the lumen of the intestine into the epithelial cells, and then into capillaries. Other nutrients, including amino acids, small peptides, vitamins, and glucose, are pumped against concentration gradients by epithelial membranes. This active transport allows the intestine to absorb a much higher proportion of the nutrients in the intestine than would be possible with passive diffusion. In some cases, transport of nutrients across the epithelial cells is passive. Compounds like frustose move down their concentration gradients from the lumen of the intestine into the epithelial cells, and then into capillaries. Most are transported by exocytosis out of epithelial cells and into lacteals. The lacteals converge into the larger vessels of the lymphatic system, eventually draining into large veins that return blood to the heart. In contrast, glycerol and fatty acids absorbed by epithelial cells are recombined into fats. The fats are mixed with cholesterol and coated with special proteins to form small globules called chylomicrons. The capillaries and veins that drain nutrients away from the villi converge into the hepatic portal vessel, which leads directly to the liver. Therefore, the liver—which has the metabolic versatility to interconvert various organic molecules—has first access to amino acids and sugars absorbed after a meal is digested. The liver modifies and regulates this varied mix before releasing materials back into the blood stream. For example, the liver helps regulate the levels of glucose in the blood, ensuring that blood exiting the liver usually has a glucose concentration very close to 0.1%, regardless of carbohydrate content of the meal. The digestive and absorptive processes are very effective in obtaining energy and nutrients. People eating the typical diets consumed in developed countries usually absorb 80 to 90 percent of the organic material in their food. Much of the undigestible material is cellulose from plant cell walls. The active mechanisms of digestion, including peristalsis, enzyme secretion, and active transport, may require that an animal expend an amount of energy equal to between 3% and 30% of the chemical energy contained in the meal. 4. Hormones help regulate digestion Hormones released by the wall of the stomach and duodenum help ensure that digestive secretions are present only when needed. When we see, smell, or taste food, impulses from the brain initiate the secretion of gastric juice. Certain substances in food stimulate the stomach wall to release the hormone gastrin into the circulatory system. As it recirculates, gastrin stimulates further secretion of gastric juice. If the pH of the stomach contents becomes too low, the acid will inhibit the release of gastrin. Other hormones, collectively called enterogastrones, are secreted by the walls of the duodenum. The acidic pH of the chyme entering the duodenum stimulates epidermal cells to release the hormone secretin which signals the pancreas to release bicarbonate to neutralize the chyme. Cholecystokinin (CCK), secreted in response to the presence of amino acids or fatty acids, causes the gallbladder to contract and release bile into the small intestine and triggers the release of pancreatic enzymes. The chyme, particularly if rich in fats, causes the duodenum to release other enterogastrones that inhibit peristalsis by the stomach, slowing entry of food. 5. Reclaiming water is a major function of the large intestine The large intestine, or colon, is connected to the small intestine at a T-shaped junction where a sphincter controls the movement of materials. One arm of the T is a pouch called the cecum. The relatively small cecum of humans has a fingerlike extension, the appendix, that makes a minor contribution to body defense. The main branch of the human colon is shaped like an upside-down U about 1.5 m long. A major function of the colon is to recover water that has entered the alimentary canal as the solvent to various digestive juices. About 7 L of fluid are secreted into the lumen of the digestive tract of a person each day. Over 90% of the water is reabsorbed, most in the small intestine, the rest in the colon. Digestive wastes, the feces, become more solid as they are moved along the colon by peristalsis. Movement in the colon is sluggish, requiring 12 to 24 hours for material to travel the length of the organ. Diarrhea results if insufficient water is absorbed and constipation if too much water is absorbed. Living in the large intestine is a rich flora of mostly harmless bacteria. One of the most common inhabitants of the human colon is Escherichia coli, a favorite research organism. As a byproduct of their metabolism, many colon bacteria generate gases, including methane and hydrogen sulfide. Some bacteria produce vitamins, including biotin, folic acid, vitamin K, and several B vitamins, which supplement our dietary intake of vitamins. Feces contain masses of bacteria and undigested materials including cellulose. Although cellulose fibers have no caloric value to humans, their presence in the diet helps move food along the digestive tract. The feces may also contain excess salts that are excreted into the lumen of the colon. The terminal portion of the colon is called the rectum, where feces are stored until they can be eliminated. Between the rectum and the anus are two sphincters, one involuntary and one voluntary. Once or more each day, strong contractions of the colon create an urge to defecate. E. Evolutionary Adaptations of Vertebrate Digestive Systems 1. Structural adaptations of digestive systems are often associated with diet The digestive systems of mammals and other vertebrates are variations on a common plan. However, there are many intriguing variations, often associated with the animal’s diet. Dentition, an animal’s assortment of teeth, is one example of structural variation reflecting diet. Particularly in mammals, evolutionary adaptation of teeth for processing different kinds of food is one of the major reasons that mammals have been so successful. Nonmammalian vertebrates generally have less specialized dentition, but there are exceptions. For example, poisonous snakes, like rattlesnakes, have fangs, modified teeth that inject venom into prey. Some snakes have hollow fangs, like syringes, other drip poison along grooves in the tooth surface. All snakes have another important anatomic adaptation for feeding, the ability swallow large prey whole. The lower jaw is loosely hinged to the skull by an elastic ligament that permits the mouth and throat to open very wide for swallowing. Large, expandable stomachs are common in carnivores, which may go for a long time between meals and therefore must eat as much as they can when they do catch prey. For example, a 200-kg African lion can consume 40 kg of meat in one meal. The length of the vertebrate digestive system is also correlated with diet. In general, herbivores and omnivores have longer alimentary canals relative to their body sizes than to carnivores, providing more time for digestion and more surface areas for absorption of nutrients. Vegetation is more difficult to digest than meat because it contains cells walls. 2. Symbiotic microorganisms help nourish many vertebrates Much of the chemical energy in the diet of herbivorous animals is contained in the cellulose of plant cell walls. However, animals do not produce enzymes that hydrolyze cellulose. Many vertebrates (and termites) solve this problem by housing large populations of symbiotic bacteria and protists in special fermentation chambers in their alimentary canals. These microorganisms do have enzymes that can digest cellulose to simple sugars that the animal can absorb. The location of symbiotic microbes in herbivores’ digestive tracts varies depending on the species. The hoatzin, an herbivorous bird that lives in South American rain forests, has a large, muscular crop that houses symbiotic microorganisms. Many herbivorous mammals, including horses, house symbiotic microorganisms in a large cecum, the pouch where the small and large intestines connect. The symbiotic bacteria of rabbits and some rodents live in the large intestine and cecum. Since most nutrients are absorbed in the small intestine, these organisms recover nutrients from fermentation in the large intestine by eating some of their feces and passing food through a second time. The koala also has an enlarged cecum, where symbiotic bacteria ferment finely shredded eucalyptus leaves. The most elaborate adaptations for a herbivorous diet have evolved in the ruminants, which include deer, cattle, and sheep. • (1) When the cow first chews and swallows a mouthful of grass, boluses enter the rumen and (2) the reticulum. Symbiotic bacteria and protists digest this cellulose-rich meal, secreting fatty acids. Periodically, the cow regurgitates and rechews the cud, which further breaks down the cellulose fibers. • (3) The cow then reswallows the cud, which moves to the omasum, where water is removed. • (4) The cud, with many microorganisms, passes to the abomasum for digestion by the cow’s enzymes.