Chapter 41 Animal Nutrition

The mouth and oral cavity

1) Adapted to receive food, sense food, and begin preparing it for digestion. The mouth is the opening to the alimentary canal. 2) The muscular tongue contains taste buds and aids in handling food. The tongue mixes food with saliva and moves food in discrete lumps (boli, singular: bolus) to the pharynx. 3) The teeth: teeth and the tongue are used to grind up food (and kill prey) and mix it with saliva. This grinding increases the effective surface area of the food. The surface of the teeth, the enamel, is one of the hardest materials produced by living things. 4) Salivary glands: saliva contains mucus (a glycoprotein called mucin) to aid the movement of food, cleanses the mouth (buffers the teeth from chemical decay) and protects the epithelia from abrasion, binds up food boli, and contains salivary amylase (which begins carbohydrate digestion). Saliva may also contain compounds that kill bacteria.

The stomach

1) Distensible organ for storing food (in humans it holds about 2 liters of food). 2) Protein digestion begins in the stomach, but only a little absorption occurs here (water, alcohol). Most other materials are not chemically digested here. 3) The churning activity of the stomach mixes the food with digestive enzymes and acid (HCl). This churning produces the semiliquid chyme. The pH of the chyme is about 2 (very acidic). A squirt of chyme at a time is released to the intestines. It takes about 2 to 6 hours after a meal for the stomach to empty. 4) The pyloric sphincter is a band of smooth muscle separating the stomach from the small intestine. 5) Gastric secretions from gastric glands in the mucosa (which is very thick in the stomach), it contains inactive pepsinogen (converted into active protease, pepsin), hydrochloric acid (HCl). Mucus is made by goblet cells, chief cells secrete pepsinogen, and parietal cells secrete hydrochloric acid / HCl. The presence of food causes the release of secretions. The acids and proteases kill off organisms in the gut and start protein digestion. The protease has to be released in its inactive form so it does not digest the cell that makes it. The stomach also secretes an alkaline secretion with the inner lining to neutralize the pepsin, so that it does not digest the proteins of the stomach lining itself. 6) The stomach's second line of defense against self-digestion is a coating of mucus, secreted by epithelial cells, that partially protects the stomach lining. The epithelium is continually eroded, and the epithelium is completely replaced by mitosis every three days. 7) Gastrin is a peptide hormone produced by the stomach that stimulates mucosa cell division (replace damaged cells), and stimulates gastric glands to secrete more gastric juices.

Extracellular digestion versus intracellular digestion

1) In most animals, at least some hydrolysis occurs by extracellular digestion, the breakdown of food outside cells. 2) Extracellular digestion occurs within compartments that are continuous with the outside of the animal's body. 3) Extracellular digestion allows organisms to devour much larger prey than can be ingested by phagocytosis and digested intracellularly.

The pancreas

1) Produces pancreatic juice, which is secreted into pancreatic duct. 2) Thepancreaticductleadstothesmallintestine(duodenum). 3) Pancreatic juice contains both bicarbonates (which neutralizes the acid chyme) and a wide variety of hydrolytic enzymes. 4) Regulation of pancreatic secretion A) Secretin stimulates release of pancreatic juice with high bicarbonate concentrations, neutralizes chyme and promotes activity of digestive enzymes of small intestine. B) Cholecystokinin stimulates release of pancreatic juice high in enzymes and the release of bile from the gallbladder.

The large intestine

1) The large intestine functions to reabsorb water and electrolytes and to store and form feces. The main branch of the human colon is 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 water reabsorption occurs in the small intestine, but the rest is absorbed in the colon). Many electrolyte ions are absorbed in the colon as well. There are no enzymes secreted by the colon, but there is enzymatic hydrolysis occurring by the activities of bacteria. 2) The large intestine, or colon, is connected to the small intestine at the pouch-like cecum (where another sphincter controls the movement of materials). The relatively small cecum of humans has a fingerlike extension, the appendix, which is considered a vestigial organ in humans. The colon has several sections: ascending, transverse, descending, and sigmoid. The sigmoidal part of the colon is connected to the rectum, leading to the anus 3) A lot of mucus is secreted by goblet cells in the large intestine. The large intestine lacks villi. The mucus protects against abrasion, binds particles into feces, and neutralizes excess acids. 4) Many billions of bacteria are found in the colon; they form about 40% of the fecal mass. They help to create certain vitamins, especially vitamin K and some B vitamins, which can be absorbed by the colon. Various gases are formed by digestive activities of bacteria, including hydrogen, carbon dioxide, and methane gas (these gases typically pass out the anus). 5) 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. Peristalsis occurs in the large intestine, defecation occurs about 2 to 3 times a day. 6) 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. 7) Between the rectum and the anus are two sphincters, one involuntary and one voluntary. One or more times each day, strong contractions of the colon create an urge to defecate.

The liver

1) The liver has many important functions: A) metabolism of carbohydrates (especially the conversion of glycogen to glucose and vice versa), lipids and proteins (deaminate amino acids for metabolism), B) storage of food nutrients (glycogen and lipids and proteins), vitamins and minerals, C) synthesize albumins globulins, prothrombin, and other proteins in the blood, D) filtering of blood (removal of worn out red blood cells, white blood cells, foreign matter and bacteria via phagocytic cells called reticuloendothelial cells (Kupffer's cells), E) destruction / storage of toxic or poisonous compounds (including alcohol), F) remove/alter thyroid hormones and steroid hormones, G) the creation and secretion of bile, H) convert ammonia (from the deamination of amino acids) into urea (the less toxic nitrogen waste). The urea cycle basically combines carbon dioxide and ammonia to form urea. 2) The liver produces bile, which contains bile salts, bile pigments (from the breakdown of hemoglobin) and electrolytes. The bile comes from the bilirubin of worn out red blood cells in the blood. Bile salts emulsify fats (greatly increases the surface area so that lipases can work effectively) and help to absorb some vitamins, cholesterol, phospholipids and triglycerides by the small intestine. 3) The gallbladder stores bile between meals. It stores 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. The release of bile from the gallbladder is stimulated by cholecystokinin from the small intestine, which is stimulated by the presence of fatty chyme in the small intestine.

The pharynx and esophagus serve as passageways for food (and for air via the pharynx).

1) The pharynx 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 ensures that a bolus will be guided into the entrance of the esophagus and not down the windpipe. 2) 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 (peristalsis) by smooth muscles in the rest of the esophagus then takes over. (Heartburn and acid reflux occurs when stomach acids are splashed back into these sensitive membranes via a partially open esophageal (cardiac) sphincter (ring of smooth muscle fibers that normally seal off the stomach from the esophagus). The esophageal linings are not adapted to acidic secretions like the lining of the stomach.

The small intestine

1) The small intestine completes chemical digestion, absorbs the products of digestion, and transports the rest of the undigested material to the large intestine. 2) The small intestine is about 5 to 6 meters long, consists of three sections: the first 25 cm is the duodenum (anterior portion), and then the jejunum and finally the ileum (largest portion, at the posterior end). Mesenteries (peritoneum membranes) suspend these structures from the posterior wall of the abdomen. The small intestine has a huge surface area: about 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. The longest part of the gastrointestinal tract. 3) The wall of the small intestine is lined with many villi which aid in absorption by increasing surface area. Small cytoplasmic extensions off the columnar epithelial cells are called microvilli also increasing the effective surface area. 4) Intestinal glands secrete watery fluid that aids absorption, and a thick alkaline mucus to neutralize acids. 5) In addition to the pancreatic enzymes, some enzymes produced by the intestinal mucosal cells are found on the cell membranes of these cells in the microvilli. These enzymes split disaccharides (disaccharidases) into monosaccharides and dipeptides into single amino acids (dipeptidases). 6) The simple biomolecules then are absorbed across the mucosa cells of the villi. Many of the molecules enter the capillary beds in the mesenteries around the small intestine, by diffusion or by active transport. Lipids are reformed as triglycerides inside the cell and are packaged as chylomicrons, which are picked up primarily by the lymphatic vessels. 7) The contents of the small intestine are moved by peristaltic waves. Diarrhea is formed by a strong peristaltic waves pushing the liquid contents of the small intestine more rapidly into the large intestine, and then this is defecated. Final digestion and nutrient absorption occur in the small intestine over a period of 5 to 6 hours. In 12 to 24 hours, undigested material passes through the large intestine, and feces are expelled through the anus. 8) The ileocecal sphincter joins the small intestine with the large intestine. It prevents the backflow of food from the large intestine to the small intestine. Near this site, a blind tube called the appendix (vermiform appendix) is located. In some herbivores with an appendix this organ is specialized for fermentation. Immune cells are present in the appendix. 9) The capillaries and veins that drain the nutrients from the villi of the small intestine converge to form the hepatic portal vessel (hepatic portal system), which leads to the liver. There, the liver filters and collects and processes the incoming nutrient-rich blood. The hepatic vein then joins the vena cava

Digestion occurs in specialized compartments

1) To avoid digesting their own cells and tissues, most organisms conduct digestion in specialized compartments. 2) The simplest digestive compartments are food vacuoles, organelles in which hydrolytic enzymes (from lysosomes) break down food without digesting the cell's own cytoplasm via process called intracellular digestion. This occurs in the protists, sponges and other animals.

4 main stages of food processing

1. Ingestion 2. Digestion 3. Absorption 4. Elimination

Complete digestive tracts (alimentary canals)

A) In contrast to cnidarians and flatworms, most higher animals (annelids, molluscs, arthropods, echinoderms and chordates) 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. B) Food ingested through the mouth and pharynx passes through an esophagus that leads to a crop, gizzard, or stomach, depending on the species. C) Crops and stomachs usually serve as food storage organs, although some digestion occurs there too. Gizzards grind and fragment food. D) In the intestines, digestive enzymes hydrolyze the food molecules, and nutrients are absorbed across the lining of the tube into the blood. Most digestion occurs extracellularly. E) Undigested wastes are eliminated through the anus. F) The alimentary canal enables organisms to ingest additional food before earlier meals are completely digested.

Incomplete digestive tracts (=gastrovascular cavities)

A) Many animals with simple body plans, such as cnidarians and flatworms, have digestive sacs with single openings. There is no separate anus to an incomplete digestive tract. B) Prey are stuffed through the mouth into the gastrovascular cavity, then the prey are partially digested by enzymes secreted by gastrodermal cells. C) Chemical digestion starts extracellularly first, with gastrodermal cells subsequently absorbing food particles (via phagocytosis). D) After absorption, most of the actual hydrolysis of macromolecules occurs intracellularly. E) Undigested materials are eliminated through the mouth.

Assimilation or absorption

After the food is digested, the animal's cells take up small molecules such as amino acids and simple sugars from the digestive cavity.

Food is packaged in bulk form (as other organisms) and contains very complex arrays of molecules, including large polymers, or various substances that may be difficult to process (lignin, chitin) or may even be toxic (alkaloids, terpenes, and so on)

Animals cannot directly use macromolecules like proteins, fats, and polysaccharides.

The Mammalian Digestive System

Consists of the alimentary canal and various accessory glands that secrete digestive juices into the canal through ducts.

Chemical Digestion cleaves macromolecules into their component monomers, which the animal then uses to make its own molecules or as fuel for ATP production.

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.

Polymers are too large to pass through membranes and enter the cells of the animal.

Macromolecules that make up an animal are usually not identical to those of its food. In building their macromolecules, however, all organisms use common monomers. For example, soybeans, fungi, fruit flies, and humans all assemble their proteins from the same 20 amino acids.

Chemical digestion is usually preceded by mechanical fragmentation of the food (by chewing or macerating food).

Stomachs and crops and gizzards tend to grind and mix food. Breaking food into smaller pieces increases the surface area exposed to the hydrolytic enzymes found in most digestive juices.

Ingestion

The act of eating, is the first stage of food processing

Digestion

The second stage of food processing, is the process of breaking food down into molecules small enough for the body to absorb

Egestion or Elimination

Undigested material passes out of the digestive cavity