a. Animal cells depend on the animal's digestive system for nutrients.

b. Digestion basically involves hydrolyzing food polymers and then absorbing the resulting monomeric units into the body.

c. Primitive animals digest their food in a blind pouch gastrovascular cavity, with a single opening for both ingestion and the expulsion of undigested materials (Figure 47.1).

d. Most animals have a digestive tube–a gastrointestinal (GI) tract or alimentary canal–running through the body from the mouth to the anus.

e. Digestive enzymes are secreted by glands in the intestinal walls and by accessory glands.

f. The monomers that result from digestion are selectively absorbed through the walls of the tract into the circulatory system, and indigestible materials are expelled through the anus.

g. The GI tract lumen is actually outside the body; materials only enter the body when they move across the intestinal walls.

h. Absorbed substances are moved throughout the body to be metabolized for their energy or assimilated into the body's structure.

i. Vertebrates digest their food with secretions of three major glands–salivary glands, pancreas, and liver–and of gland cells that line the GI tract.

a. Each animal species is adapted to consuming a limited range of foods that will supply all its essential nutrients.

b. Animals can feed in many ways, and have a variety of specialized structures for capturing and ingesting food (Figure 47.2).

c. Filter feeders strain small particles of food out of the water, often with modified structures such as antennae, tentacles, combs, gills, rakes, or bristles (Figure 47.3).

d. The mouths of various animals are designed for biting, chewing, gobbling, tearing, or sometimes forcing whole organisms, or pieces of them, into the digestive system (Figure 47.4).

e. Animals thus divide food resources in an ecosystem in part by acquiring feeding apparatuses of different sizes and shapes.

a. Most animals tear food into pieces or grind or chew it to begin the digestion process.

b. Breaking food down mechanically increases the surface/volume ratio, exposing it more quickly to more digestive enzymes.

c. Figure 47.5 illustrates the GI tract of a bird, which includes a crop for storage and a gizzard, which grinds the food with the aid of small stones.

d. Similar small grinding stones, called gastroliths, have been found with dinosaur skeletons.

e. A crop or other internal storage compartment for food allows animals to eat only occasionally and remain safe from predators more often.

f. Vertebrates commonly break up their food with teeth; the teeth of snakes and some fish also serve to trap and hold prey (Figure 47.6).

g. Mammals have four kinds of teeth for cutting (incisors), tearing (canines), and chewing (premolars and molars); see Figure 47.7.

h. The tongue moves food around, mixes it with digestive enzymes, and shapes it into a bolus that can be swallowed.

i. Ruminants stuff food into the first pouch of a stomach and then regurgitate and chew it as cud; this also allows them to eat when they can and digest later.

a. The mammalian GI tract is made of four tissue layers; from outside to inside they are the mucosa, submucosa, muscularis, and serosa (Figure 47.8).

b. The mucosa is made of columnar epithelial cells, which absorb digested food in some regions, and cells that secrete digestive enzymes, hormones, and the mucus that covers most of the tract.

c. The submucosa is a connective tissue layer that includes blood vessels and glands.

d. The muscularis is made of smooth muscle in an inner circular layer and an outer longitudinal layer; it propels materials down the tract.

e. The serosa is made of connective tissue and squamous epithelium.

f. The bolus formed in the mouth is swallowed and is pushed via peristalsis (Figure 47.10) through the esophagus to the stomach (Figure 47.9).

g. The esophageal sphincter at the upper end and the pyloric sphincter at the lower end of the stomach close the stomach off and control the intake and expulsion of food.

h. In the stomach, food is mixed with juices and enzymes to produce a thick, souplike chyme.

i. Mucus secreted by the mucosal stomach lining shields the surface against digesting itself.

j. Food in the stomach stimulates peristaltic contractions that run from one end to another and assist in the mixing and digestion of the food.

k. After about 30—60 minutes of digestion in the stomach, the pyloric sphincter opens with each peristaltic contraction and bits of chyme are squirted into the small intestine.

l. Additional enzymes are secreted in the small intestine from the pancreas and intestinal gland cells; bile from the liver is also added.

m. Indigestible components of food pass into the large intestine, or colon, which regulates the amount of water and ions absorbed into the bloodstream from there.

n. The contents of the colon eventually reach the rectum, where they are compacted and stored for elimination.

o. About half the solid mass of the feces consists of bacteria that inhabit the colon; these are essential for good colon function, since they provide bulk and also supply some vitamins.

a. Food commonly includes starch from plant cells and glycogen from animal cells, both of which are glucose polymers that are easily digested.

b. In the mouth, food is mixed with amylase from the salivary glands (Figure 47.11).

c. Amylase leaves maltose and dextrins after breaking glucose residue bonds; it is inactivated in the stomach.

d. Amylase is also excreted by the pancreas for release into the small intestine; glands in the mucosa of the small intestine produce maltase, sucrase, and lactase.

e. Plant biomass is mostly cellulose and other polysaccharides that we cannot digest.

f. A few species of microorganisms produce cellulases, and some animals get nourishment from cellulose by harboring these organisms in their intestines.

a. Most proteolytic enzymes, or proteases, are extremely specific in the bonds they cut; proteases from different glands supplement one another to thoroughly digest proteins (Figure 47.12).

b. Protein digestion begins in the stomach, where pits of mucosa contain chief cells that secrete pepsinogen and parietal cells that secrete hydrochloric acid (Figure 47.13).

c. Concepts 47.1 addresses the variety of cell types in the digestive system.

d. Protein digestion also occurs in the small intestine through proteases secreted by the pancreas: trypsin, chymotrypsin, aminopeptidase, and carboxypeptidase.

e. The pancreas also secretes a bicarbonate solution that neutralized the acidic contents of the stomach and creates a basic pH that is optimal for enzyme action.

f. To keep active proteases from digesting one another and the cells that make them, gland cells synthesize and store proteolytic enzymes in an inactive proenzyme, or zymogen, form.

a. The enteric nervous system (ENS) consists of the neural connections of the digestive tract.

b. The ENS is centered in nerve plexuses, which are like webs of neurons lying within the walls of the gut and extending along the length of the GI tract in the submucosa and muscularis.

c. The submucosal plexus regulates glands and smooth muscle in the mucosa.

d. The myenteric plexus, in the muscularis, regulates gut movements such as peristalsis and segmentation.

e. Gut movements are controlled locally by very short reflex arcs, as sensory neurons, interneurons, and motor neurons all exist there.

f. A sequence of hormones superimposed on the ENS carries signals between parts of the GI tract (Figure 47.14).

g. The ENS uses acetylcholine, epinephrine, dopamine, ATP, serotonin, and several other neurotransmitters.

h. The GI tract can be stimulated by the sight, smell, or even the thought of food.

i. The peptides and amino acids that result from protein digestion stimulate stomach endocrine cells to secrete gastrin, which stimulates further secretion by chief and parietal cells (Figure 47.15).

j. This is a positive feedback circuit that is terminated by an accumulation of acid that lowers the stomach pH.

a. Acidic chyme from the stomach enters the small intestine and triggers the release of secretin from gland cells; this stimulates the duct cells of the pancreas to release alkaline fluid into the intestine (Figure 47.16).

b. Amino acids and lipids stimulate other gland cells to secrete cholecystokinin (CCK), which stimulates the acinar cells of the pancreas to secrete digestive enzymes.

c. A gastric inhibitory peptide inhibits motility of the stomach and ensures that the stomach processes food slowly enough for the intestine to keep pace.

a. Concepts 47.2 describes liver functions.

b. The liver produces bile, which is essential for the assimilation of fat molecules in food.

c. Bile salts act as detergent molecules that emulsify fats.

d. Bile is stored in the gallbladder lying under the liver.

e. Cholecystokinin, secreted by the small intestine, stimulates the release of bile into the small intestine and secretin stimulates the liver to produce more bile.

f. Bile components are circulated back to the liver and reused in a cycle known as enterohepatic circulation (Figure 47.17).

a. The vertebrate GI shows the importance of a large surface/volume ration (Figure 47.18).

b. The small intestine surface area is increased by large folds, which are covered by 4—5 million villi, each about a millimeter long.

c. The epithelial cells of each villus are covered by microvilli (Figure 47.19).

d. Each villus contains a bed of capillaries that collect most of the material absorbed through the mucosa.

e. Each villus also has a lacteal, an extension of the lymphatic system that is more permeable than the capillaries and absorbs lipids.

a. Veins from the villi converge to form the hepatic portal system (Figure 47.20), which transports absorbed food molecules to the liver.

b. A portal circulatory system is a venous system that runs between two capillary beds.

c. During and after a meal, an animal is in the absorptive phase nutritionally, with usually more than enough glucose.

d. Considerable glucose is converted into triglycerides and stored as fat in adipose tissue (Table 47.1).

e. During the absorptive phase, all tissues can synthesize new proteins with the abundant amino acids derived from food proteins.

a. In the intestine, neutral fats and phospholipids from foods are digested into fatty acids, glycerol, and the phospholipid bases.

b. Lipids can only be transported in blood plasma as soluble lipoproteins, attached to carrier proteins that mask their hydrophobic regions.

c. Chylomicrons are large lipoproteins synthesized by the intestinal mucosa (Figure 47.21).

d. Other lipoproteins are called high-density (HDL), low-density (LDL), and very-low-density (VLDL), those with the most lipid being the most dense (Figure 47.22).

e. Atherosclerosis (see Section 45.9) is due to the abnormal deposition of cholesterol in lumpy plaques on the inner walls of blood vessels; it is most common in people with high LDL levels and less common in people with high HDL levels.

f. The HDL level is increased by aerobic exercise.

g. A diet high in fatty acids and cholesterol produces high LDL levels and is a major factor in cardiovascular disease.

h. LDL levels are elevated in people with familial hypercholesterolemia, an autosomal dominant condition.

a. Well-adapted animal bodies deal with times of more or less food by means of hormones.

b. After absorbing all the food in its intestine, an animal goes into the postabsorptive phase, or fasting phase, in which some absorptive phase processes are reversed (Figure 47.23).

c. After glycogen stores in muscle and liver cells are exhausted, adipose tissue cells respond to glucagon and epinephrine and start lipolysis to break down triglycerides.

d. The resulting fatty acids are released into the bloodstream and carried to other tissues by serum albumin.

e. As cells continue to depend on fatty acids, they begin converting their acetyl-CoA products into ketone bodies: acetone (later discharged from the lungs), and acetoacetate and beta-hydroxybutyrate, which are used for energy.

f. In most people, cardiac muscle uses fatty acids as a preferable energy source, but in the Sherpas of the Himalayas, the preferential energy source is glucose, giving them greater endurance.

g. After absorbing a meal, the animal body goes into a highly adaptive metabolic mode, preparing the possibility of fasting before another meal is available.

h. In this phase, the metabolism slows down, body temperature falls, some protein is broken down, and the amino acids are catabolized for energy.

a. Figure 47.24 summarizes several of the previously discussed points and also addresses the regulation of plasma glucose.

b. The glucose pool in the plasma exchanges with liver and muscle glycogen stores, with fat, and with protein.

c. A factor in insulin metabolism is the level of dietary chromium; glucose tolerance factor is a combination of chromium ion and the coenzyme NAD+ that is necessary for the action of insulin.

d. Besides insulin and glucagon, epinephrine (adrenaline) is another hormone that is released when glucose levels fall; it breaks down glycogen in the liver.

e. Growth hormone, or somatotropin, is also released when glucose levels fall; it inhibits the uptake of glucose to ensure that spare amounts are available for the brain.

f. Somatotropin stimulates growth in all tissues that grow, and it inhibits the breakdown of proteins.

g. Cortisol, a principal hormone of the adrenal cortex, promotes the breakdown of protein to amino acids and their conversion to glucose.

a. Homeostatic mechanisms in mammals maintain constant levels of various ions.

b. The kidneys regulate the bulk of the ions in the body; calcitonin and parathyroid hormone (PTH) keep the calcium level constant.

c. Calcitonin and PTH operate via a simple negative feedback system to keep the plasma calcium pool at a constant level (Figure 47.25).

d. When the plasma level rises above 10 mg per 100 ml, thyroid gland cells produce calcitonin (thyrocalcitonin), which stimulates osteoblasts to deposit more calcium phosphate as bone.

e. Calcitonin also stimulates the kidney to excrete calcium and phosphate and the intestine to reduce the absorption of calcium ions.

f. If the plasma calcium level falls below 7 mg per 100 ml, the parathyroid glands produce parathyroid hormone, which produces the opposite effects, including stimulating the osteoclasts to dissolve calcium phosphate and release it into the blood.

g. Calcium nutrition depends on a healthy exposure to sunlight.

h. The absorption of calcium in the intestine requires a calcium-binding protein whose synthesis is induced by the hormone dihydroxy-vitamin D3.

i. This hormone is also a vitamin that can be obtained from food, and its synthesis is induced by ultraviolet light (Figure 47.26).

a. Iron is an enzyme activator and a component of the electron-transport proteins, such as cytochrome, and of hemoglobin.

b. Iron circulates in the blood combined with a protein, transferrin (Figure 47.27).

c. Iron is stored in the liver, spleen, and bone marrow, and is used in the bone marrow by maturing erythrocytes.

d. Iron—ferritin complexes sometimes form large bodies called siderosomes.

e. As old erythrocytes are digested, the heme groups in their hemoglobin are converted into green and orange pigments (biliverdin and bilirubin), which are removed by the liver and excreted into the intestine.

f. These pigments give bile its green color and feces its brown color; they also contribute to the yellow color of urine, since they are partially excreted through the kidneys.

g. In jaundice, a symptom of liver disease, the skin and eyeballs are yellowish due to excess bilirubin.

h. Erythrocyte destruction in humans releases large amounts of iron that is mostly recycled, except for that lost in sweat, sloughed off cells, urine, and feces.

i. Normal iron metabolism requires trace elements such as copper and molybdenum.

a. The tissues of an organism are in a state of constant turnover.

b. It has been shown that cellular components break down and are removed from the cell at characteristic rates (Table 47.2).

c. It has been estimated that 25 percent of the calcium in human blood exchanges with bone reservoirs every day.

d. Overall, most cells in the human body are being continually replaced by new ones.

e. Dynamic identity is an aspect of being alive, though a body's structures may change dramatically.

a. Animals have lost the ability to synthesize several essential amino acids; these are methionine, threonine, leucine, isoleucine, valine, lysine, tryptophan, and phenylalanine for adult humans.

b. Histine is also essential for children and possibly for adults, and infants require arginine.

c. People beset by cultural, emotional, or economic restrictions may have difficulty getting the proper mixture of amino acids from plant proteins.

d. However, several cultures have devised cooking methods that release useful amino acids from certain foods that would not otherwise supply them through normal digestion.

e. Every animal needs enough bulk protein to maintain a proper nitrogen balance (Figure 47.28), which means taking in more than is being excreted, for a net synthesis of protein.

f. Most amino acids are metabolized to glucose or fatty acids; their amino groups are removed and eliminated primarily as urea.

g. Unless amino groups are replaced, there is not enough for protein synthesis.

h. Kwashiokor is a dietary protein deficiency characterized by edema, apathy, and modification of skin and hair, and is particularly prevalent in malnourished young children.

a. Though we still have the metabolic apparatus that evolved over millions of years to process a certain diet, we do not eat as our ancestors did (Table 47.3).

b. Our changing diets place great strains on our health.

c. Modern European-Americans eat much less protein and much more fat than their remote ancestors did, and the fat is more likely to be saturated than polyunsaturated.

d. Diets containing more saturated fats are associated with hypercholersterolemia and atherosclerosis.

e. Simple sugar, which is more commonly in our diets today, enters the bloodstream more quickly than the complex starch that made up diets of the past.

f. Our consumption of dietary fiber has fallen, and excess salt strains renal mechanisms.

a. The road to personal health lies partly in understanding complex nutritional interactions.