37.1. Digestive Tracts

    A. Heterotrophs take in organic molecules synthesized by other animals.

        1. Digestion provides energy needed to carry out routine metabolic activities and maintain homeostasis.
        2. The digestive tract:
            a. ingests food,
            b. breaks down food into small molecules that can cross plasma membranes.
            c. absorbs these nutrient molecules, and
            d. eliminates nondigestible remains.

    B. Incomplete versus Complete Tracts

        1. Planarians are organisms with an incomplete gut.
            a. It is carnivorous and feeds largely on smaller aquatic animals.
            b. Digestive system contains only a mouth, a pharynx, and an intestine.
            c. To feed, its pharynx extends far beyond mouth to suck up minute quantities at one time.
            d. Digestive enzymes in gastrovascular cavity allow some extracellular digestion.
            e. Digestion is finished intra-cellularly by cells that line the cavity; food diffuses to nearby cells.
            f. Digestive system lacks regions of specialized function.
            g. Tapeworm relatives of planaria lack a digestive system altogether; they absorb food through a body wall
                with modified projections to absorb nutrients from host.
        2. In contrast, earthworm has a complete gut.
            a. Digestive system is composed of a tube with a mouth at one end and an anus at the other end.
            b. Earthworms feed on decayed organic matter in soil.
            c. Different regions have specialized functions (e.g., ingestion, mechanical digestion, etc.).
            d. Muscular pharynx draws in food with sucking action.
            e. Crop is storage area with expansive walls.
            f. Gizzard has thick muscular walls to grind food.
            g. Digestion occurs in intestine, outside of cells.
            h. Surface area for absorption is increased by an intestinal fold called the typhlosole.
            i. Undigested remains exit the body at anus.
        3. Fanworms are relatives of earthworms.
            a. Their feathery tentacles gather fine particles and plankton from water.
            b. Their digestive tract is a simple uniform tube.

    C. Continuous versus Discontinuous Feeders

        1. Clams are continuous feeders, often called a filter feeder.
            a. Water moves into a mantle cavity through an incurrent siphon and deposits particles on gills.
            b. Cilia move particles to labial palps which direct them into mouth and to stomach.
            c. Digestive enzymes from a digestive gland help amoeboid cells in the tract complete digestion.
        2. Marine fanworms are sessile filter feeders; only small particles are consumed.
        3. Baleen whales are active filter feeders; baleen (fringe) filters small krill from water.
        4. Squids are an example of discontinuous feeders
            a. Head of a squid has ten arms; two arms seize prey and bring it to the squid's mouth.
            b. Beak-like jaws and a radula (toothy tongue) reduce food to pieces.
            c. Esophagus leads to a stomach that holds food until digestion is complete.
            d. Discontinuous feeders require a storage region in their gut.

    D. Adaptation to Diet

        1. Animals are herbivores (eat plants) or carnivores (eat animals) or omnivores (eat both).
        2. Mammal dentition differs according to mode of nutrition.
            a. Omnivores, including humans, have dentition that accommodate a vegetable diet and a meat diet.
            b. Omnivore teeth include incisors (shearing), canines (tearing), premolars (grinding), and molars (crushing).
            c. Herbivores have large, flat premolars and molars for grinding plant matter.
            d. Grazers (e.g., horses) also have sharp incisors for clipping off grass and leaves.
            e. Hard to digest plant material requires extensive grinding to disrupt plant cell walls.
            f. Animals that feed on plants may have long and complex digestive tracts and bacteria in their digestive
                tracts that can digest cellulose, producing nutrients that an animal can use.
            g. Some grazers have a rumen to digest chewed grasses; partially digested cud is rechewed.
            h. Carnivores' pointed incisors and canines tear off pieces small enough to swallow
            i. Meat is rich in protein and fatty acids and is easier to digest than plant material.
            j. Carnivores have fewer molars for grinding and a shorter digestive tract with less specialization.

37.2. Human Digestive Tract

    A. Human Digestion

        1. Human digestive tract is complete and complex.
        2. Each part of the digestive system has a specific function.
        3. Digestion of food in humans is an extracellular process.
        4. Enzymes are secreted into the digestive tract by nearby glands which never contain food themselves.
        5. Digestion requires a cooperative effort by production of hormones and actions of nervous system.

        B. Mouth

        1. Human dentition has many specializations because humans are omnivores.
        2. Food is chewed in the mouth and mixed with saliva.
            a. Three pairs of salivary glands secrete saliva by way of ducts into the mouth.
            b. Salivary amylase is enzyme that begins starch digestion; maltose is common end product.

Salivary amylase
starch + H2O    maltose

            c. Food is manipulated by a muscular tongue with touch and pressure receptors.
            d. Taste buds are located primarily on tongue but also on the surface of the mouth; chemical receptors
                are stimulated by chemical composition of food.
            e. Food is chewed and mixed with saliva to form a bolus in preparation for swallowing.

    C. The Pharynx and the Esophagus

        1. Digestive and respiratory passages come together in pharynx, then separate.
            a. During swallowing, path of air to lungs could be blocked if food entered trachea.
            b. Epiglottis covers opening into trachea as muscles move bolus through pharynx into esophagus.
        2. Esophagus is a muscular tube that moves swallowed food to stomach by peristalsis.

    D. Stomach

        1. Stomach stores a liters of partially digested food freeing humans from continual eating.
        2. Pioneer work by Dr. William Beaumont revealed much of the stomach's functions in the mid-1800s.
            a. French-Canadian fur trapper had an opening (fistula); Dr. Beaumont could observe his stomach.
            b. He collected gastric juice produced by cells of gastric glands.
            c. Walls of the stomach contract vigorously and mix food with juices secreted when food enters.
            d. He found gastric juice contains hydrochloric acid and another digestive substance, pepsin.
            e. He discovered gastric juices are produced independently of protective mucous secretions.
            f. His careful work pioneered study of the physiology of digestion.
        3. Hydrochloric acid (HCl) lowers pH of the gastric contents to about 2.
            a. Epithelial lining of the stomach has millions of gastric pits leading to gastric glands.
            b. This acid kills most bacteria and other microorganisms.
            c. Low pH also stops activity of salivary amylase and promotes activity of pepsin.
        4. Pepsin is a hydrolytic enzyme that acts on proteins to produce peptides.

pepsin
protein + H2O     peptides

        5. A thick layer of mucus protects wall of the stomach and first part of duodenum from HCl and pepsin.
        6. Ulcers develop when lining is exposed to digestive action; recent research indicates this is usually due to
            infection by Helicobacter pylori bacteria.
        7. Stomach contents, a thick, soupy mixture, are called chyme.
        8. At base of the stomach is a narrow opening controlled by a sphincter (a circular muscle valve).
            a. When the sphincter relaxes, chyme enters duodenum; a neural reflex causes the sphincter to contract
                closing off the opening.
            b. Duodenum is first part of the small intestine.
            c. The sphincter relaxes and allows more chyme to enter the duodenum.
            d. The slow, rhythmic pace with which chyme exits the stomach allows thorough digestion.

    E. Small Intestine

        1. Human small intestine is a coiled muscular tube about three meters long.
        2. Mucous membrane lining has ridges and furrows; surfaces are covered by villi.
            a. Villi are finger-like projections whose surface cells are covered by microvilli.
            b. Microvilli are minute projections, a brush border, of surface cells of intestinal villi.
            c. Ridges, furrows, villi, and microvilli greatly increase effective surface area of small intestine.
        3. As chyme enters duodenum, proteins and carbohydrates are partly digested; no fat digestion occurs.
        4. Additional digestion is aided by secretions from liver and pancreas.
            a. Bile is a secretion of liver temporarily stored in gallbladder before sent to duodenum.
            b. Bile emulsifies fat; bile is a green byproduct of the breakdown of hemoglobin.
            c. Bile contains bile salts that help in emulsification of fat.
                1) Emulsification breaks fat globules into microscopic droplets.

bile salts
fat      fat droplets

                2) This increases fat digestion by increasing surface area of fat globules exposed to enzymes.
            d. Pancreatic juice secreted by pancreas and contains the following:
                1) sodium bicarbonate [NaCO3] that neutralizes acidity of chyme; pH of small intestine is slightly basic;
                2) pancreatic amylase that digests starch to maltose

pancreatic amylase
starch + H2O     maltose:

                3) trypsin and other enzymes that digest protein to peptides;

trypsin
protein + H2O     peptides :

                4) lipase that digests fat droplets to glycerol and fatty acids. (Table 43.2)

lipase
fat droplets + H2O      glycerol + fatty acids

            e. Epithelial cells of villi produce intestinal enzymes attached to plasma membrane of microvilli.
            f. Intestinal secretions complete digestion of peptides and sugars; peptides are digested by peptidases to amino acids;

peptidases
peptides + H2O    amino acids

                and maltose from the first step in starch digestion is converted by maltase to glucose.

maltase
maltose + H2O    glucose

        5. Large molecules of carbohydrates, proteins and fats are broken into small molecules absorbed by villi.

    F. Absorption by Villi

        1. Small intestine is specialized for absorption by the huge number of villi that line the intestinal wall.
        2. If a smooth tube, intestine would have to be 500-600 m long to have a comparable surface area.
        3. Each villus contains blood vessels and a lymphatic lacteal.
        4. Lacteal is lymphatic vessel in an intestinal villus that aids in absorption of fats.
        5. Sugars and amino acids enter villi cells and are absorbed into bloodstream.
        6. Glycerol and fatty acids enter villi cells; reassembled into fat molecules, they move into lacteals.
        7. Absorption involves diffusion and active transport requiring expenditure of cellular energy.

    G. Control of Digestive Juices  

        1. Gastrin is produced by cells in gastric glands of stomach wall; stimulates gastric glands and increases
            gastric motility; its secretion is stimulated by a meal rich in protein.
        2. Secretin is produced by cells in duodenal wall; stimulates pancreas to secrete fluids rich in NaCO3 into
            duodenum; secretion is stimulated by acid chyme.
        3. Cholecystokinin (CCK) produced by duodenal wall stimulates pancreas to increase pancreatic juice and
            liver to increase output of bile; causes gallbladder to release bile; secretion is stimulated by fats.
        4. Gastric inhibitory peptide (GIP) from duodenal wall inhibits gastric gland secretion and stomach motility.

    H. Accessory Organs

        1. Pancreas
            a. Pancreas lies deep within abdominal cavity, just below stomach, and rests on posterior abdominal wall.
            b. It is an elongated and somewhat flattened organ.
            c. As an endocrine gland, it secretes glucogon and insulin hormone into bloodstream.
            d. As an exocrine gland, it secretes pancreatic juice.
                1) Pancreatic juice contains sodium bicarbonate that neutralizes acidic chyme.
                2) Digestive enzymes digest carbohydrates, fats and proteins. (Table 43.2)
        2. Liver is a large glandular organ that fills the top of abdominal cavity, just below diaphragm.
            a. Liver has numerous functions:
                1) It detoxifies blood by removing and metabolizing poisonous substances.
                2) It makes plasma proteins including albumin and fibrinogen.
                3) Liver destroys old red blood cells; converts hemoglobin to bilirubin and biliverdin in bile.
                4) It produces bile stored in gallbladder before entering duodenum to emulsify fats.
                5) It stores glucose as glycogen; breaks down glycogen to maintain constant blood glucose concentration.
                6) Liver produces urea from amino groups and ammonia.
            b. Blood vessels from large and small intestines lead to liver as hepatic portal vein.
            c. Liver maintains blood glucose level at 0.1% by removing glucose from hepatic portal vein to store as
                glycogen; when needed, glycogen is broken down and glucose enters hepatic vein.
            d. Amino acids can be converted to glucose but deamination (removal of amino groups) must occur.
            e. Using complex metabolic pathway, liver converts amino groups to urea.
            f. Urea is most common human nitrogenous waste it is transported by blood to kidneys.
        3. Liver Disorders
            a. Jaundice is a symptom: yellowish skin due to a large amount of bilirubin in blood.
            b. In hemolytic jaundice, RBCs are broken down in abnormally large amounts.
            c. In obstructive jaundice, there is blockage of the bile ducts (gall stones) or damage to liver cells.
            d. Viral hepatitis is liver infection.
                1) Hepatitis A results from eating contaminated food.
                2) Hepatitis B and C are spread by blood transfusions, kidney dialysis, and unsterile needle use.
                3) All three can be caused from sexual contact.
            e. Cirrhosis is a chronic disease where liver tissue is replaced by fatty and then scar tissue; alcoholics
                provide too much alcohol for liver to break down.

    I. Large Intestine

        1. Large intestine is region following the small intestine.
        2. It has four parts: cecum, colon, rectum, and anal canal.
        3. Appendix
            a. This is finger-like projection extending from cecum, a blind sac at junction of small and large intestine.
            b. It may play a role in fighting infections.
            c. If infected appendix bursts, it results in general abdominal infection (peritonitis).
        4. 1.5 liters of water enter digestive tract daily from drinking; another 8.5 liters enter from various secretions.
            a. About 95% of this total liquid is reabsorbed by small intestine; remainder by cells of colon.
            b. If water is not reabsorbed, it causes diarrhea which can cause serious dehydration and ion loss.
        5. Large intestine functions in ion regulation, absorbing salts plus vitamin K produced by intestinal bacteria.
        6. Large intestine that terminates at the anus, an external opening.
        7. Feces
            a. Feces consists of 75% water and 25% solid matter.
            b. One-third of the solid matter is intestinal bacteria.
            c. Remainder is undigested wastes, fats, organic material, mucus, and dead cells from intestinal lining.
        8. Intestinal polyps are small growths arising from epithelial lining.
            a. Whether they are benign or cancerous, they can be removed surgically.
            b. Low-fat, high-fiber diet promotes regularity and is protection against mutagenic agents.

37.3. Nutrition

    A. Balanced diet, required for good health, includes a properly proportioned variety of foods.  

    B. Vitamins

        1. Vitamins are essential organic compounds the body cannot make but requires for metabolic activities.
        2. Many are portions of coenzymes: niacin is part of NAD+ and riboflavin is a part of FAD.
        3. Coenzymes are needed in small amounts because they are used over again and again.
        4. Vitamin A is not a coenzyme but a precursor for visual pigment that prevents night blindness.
        5. Lack of vitamins results in vitamin deficiencies.
        6. 13 vitamins are divided into those that are fat soluble and those that are water soluble.

    C. Antioxidants

        1. Cell metabolism generates free radicals, unstable molecules with an extra electron; O3- is common.
        2. Free radicals stabilize by eventually donating electrons to another molecule; this damages cellular molecules.
        3. Free radicles damage DNA, proteins, etc. by donating an electron; this may cause cancer or plaque in arteries.
        4. Vitamins C, E, and A—abundant in fruits—are antioxidants that defend against free radicals.
        5. Supplements do not replace fruits and vegetables that contain many other beneficial compounds.

    D. Vitamin D

        1. Skin cells contain a precursor cholesterol molecule converted to vitamin D by UV light exposure.
        2. Only a small amount of sunlight is needed.
        3. Vitamin D leaves skin and is modified in kidneys and then in liver until it becomes calcitriol.
        4. Calcitriol circulates throughout body regulating calcium uptake and metabolism.
        5. Calcitriol promotes absorption of calcium by intestines.
        6. Lack of vitamin D leads to rickets in children; poor mineralization of skeleton causes bowing of legs.
        7. Most milk is fortified with vitamin D to prevent rickets.

    E. Minerals  

        1. We require macrominerals (e.g., calcium, phosphorus) in amounts of over 100 mg per day. (Table 43.4)
            a. They are constituents of cells and body fluids and structural components of tissues.
            b. Calcium is needed to build bones and teeth and for nerve conduction and muscle contraction.
        2. Microminerals are elements (e.g., zinc, iron) recommended in amounts less than 20 mg per day.
            a. These microminerals are more likely to have very specific functions.
            b. Iron is needed to produce hemoglobin; adult females need more due to menstrual loss of blood.
            c. Iodine is used to produce thyroxin, a hormone of the thyroid glands.
            d. Minute amounts of molybdenum, selenium, chromium, nickel, vanadium, silicon, and arsenic are essential.
            e. Some individuals may not receive enough calcium, magnesium (stress), or zinc (vegetarian).

    F. Calcium

        1. Calcium supplements counteract osteoporosis that afflicts 25% of older men, 50% of older women.
        2. Porous bones break easily due to lack of calcium.
        3. After menopause, bone-eating cells called osteoclasts are more active than bone-forming osteoblasts.
        4. Calcium supplements have been shown to slow bone loss in the elderly.
        5. Intake of 1,000-1,500 mg calcium/day is recommended; supplemental calcium is usually necessary.
        6. Exercise is effective in building bone mass.

    G. Sodium

        1. Recommended daily intake of sodium is 400-3,300 mg; average American intake is 4,000-4,700 mg.
        2. High sodium intake has been linked to hypertension in some people.
        3. One third of our sodium intake is found naturally in foods; another third is added in processing.
        4. We add one-third of our salt intake in cooking or as table salt.