Chapter 41 Animal Nutrition Biology Seventh Edition Neil

• Overview: The Need to Feed • Every mealtime is a reminder that we are heterotrophs i. e Dependent on a regular supply of food Figure 41. 1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• In general, animals fall into one of three dietary categories – Herbivores eat mainly autotrophs (plants and algae) – Carnivores eat other animals – Omnivores regularly consume animals as well as plants or algal matter Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Regardless of what an animal eats, an adequate diet must satisfy three nutritional needs – Fuel for all cellular work – The organic raw materials for biosynthesis – Essential nutrients, substances such as vitamins that the animal cannot make for itself Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Animals feed by four main mechanisms BULK FEEDERS SUSPENSION FEEDERS SUBSTRATE FEEDERS FLUID FEEDERS

• Concept 41. 1: Homeostatic mechanisms manage an animal’s energy budget • Nearly all of an animal’s ATP generation – Is based on the oxidation of energy-rich molecules: carbohydrates, proteins, and fats Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Glucose Regulation as an Example of Homeostasis • Animals store excess calories – As glycogen in the liver and muscles and as fat – When low calories are taken, fuel is taken out from storage deposits and gets oxidized. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Caloric Imbalance • Undernourishment – Occurs in animals when their diets are chronically deficient in calories – Can have detrimental effects on an animal as the body starts breaking down its own proteins for fuel. Muscles began to decrease in size and brain may have protein deficiency. If this condition persists for long time, death usually happens. – Anorexia nervosa is another cause of under nutrition. In this case sufferers (mainly females) compulsively starves themselves to lose weight. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Over nourishment – Results from excessive food intake – Leads to the storage of

Obesity as a Human Health Problem • The World Health Organization – Now recognizes obesity as a major global health problem – In US the percentage of obese (very overweight) people has doubled to 30% of population in last two decades and the just overweight percentage has become 35% i. e 65% of population are overweight. – Obesity is mainly due to the increase in consumption of fattening food and the sedentary life style. • Obesity contributes to a number of health problems, including – Diabetes, cardiovascular disease, and colon and breast cancer. Obesity is a factor in about 300, 000 deaths in US every year. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Researchers have discovered – Several of the mechanisms that help regulate body weight • Over the long term, homeostatic mechanisms – Are feedback circuits that control the body’s storage and metabolism of fat • Several chemical signals called hormones – Regulate both long-term and short-term appetite by affecting a “satiety center” in the brain. Figure 41 -5. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Secreted by the stomach wall, ghrelin is one of the signals that triggers feelings of hunger as mealtimes approach. In dieters who lose weight, ghrelin levels increase, which may be one reason it’s so hard to stay on a diet. Produced by adipose (fat) tissue, leptin suppresses appetite as its level increases. When body fat decreases, leptin levels fall, and appetite increases. Ghrelin Insulin The hormone PYY, secreted by the small intestine after meals, acts as an appetite suppressant that counters the appetite stimulant ghrelin. Leptin Figure 41. 5 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings PYY A rise in blood sugar level after a meal stimulates the pancreas to secrete insulin (see Figure 41. 3). In addition to its other functions, insulin suppresses appetite by acting on the brain.

• Inheritance is a major factor in obesity – Most of the weight regulating hormones are proteins that are encoded by certain genes – This hereditary concept explains why some people struggle to keep their weights under control while other can eat what ever they want without gaining weight. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• The complexity of weight control in humans – Is evident from studies of the hormone leptin • Mice that inherit a defect in the gene for leptin – Become very obese Figure 41. 6 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• The discovery of the leptin deficiency mutation in mice made front-page news thinking that might apply to humans too. • Unfortunately, it appears that lots of obese people does not have a problem in leptin, in fact some of them have abnormally high leptin levels yet they do not lose weight. • For some reason the brain satiety centers does not respond to high leptin levels in some of the obese people thus, it might not prevent the weight gain in those people. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Obesity and Evolution • The problem of maintaining weight partly stems from our evolutionary past – When fat hoarding was a means of survival when there was no continues supply of fatty foods for the year. – In this case nature favored survival of people who can hoard fat (engorge rich fats) when they are available. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

A species of birds called petrels – Become obese as chicks due to the

• Concept 41. 2: An animal’s diet must supply carbon skeletons and essential nutrients • To build the complex molecules it needs to grow, maintain itself, and reproduce – An animal must obtain organic precursors (carbon skeletons) from its food • Given a source of carbon like sugar and a source of nitrogen from protein – animals can fabricate a great variety of organic molecules. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Besides fuel and carbon skeletons – An animal’s diet must also supply essential nutrients in preassembled form as the animal cells can not make them from precursors. Example is vitamin C • An animal that is malnourished – Is missing one or more essential nutrients in its diet • Malnutrition is more common than undernutrition. – Some obese people could be malnourished Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Herbivorous animals – May suffer mineral deficiencies if they graze on plants in soil

Essential Amino Acids • Animals require 20 amino acids – And can synthesize about half of them from the other molecules they obtain from their diet • The remaining amino acids, the essential amino acids – Must be obtained from food in preassembled form Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• A diet that provides insufficient amounts of one or more essential amino acids – Causes a form of malnutrition called protein deficiency. This child have a malnutrition disorder called kwashirkor in which he eats enough calories but NOT enough proteins. Figure 41. 9 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Most plant proteins are incomplete in amino acid makeup • So individuals who must eat only plant proteins need to eat a variety to ensure that they get all the essential amino acids. However, the most relible sources of essential a. a is meat, eggs, cheese and other animal products. Essential amino acids for adults Methionine Valine Threonine Phenylalanine Corn (maize) and other grains Leucine Isoleucine Tryptophan Figure 41. 10 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Lysine Beans and other legumes

Essential Fatty Acids • Animals can synthesize most of the fatty acids they need • The essential fatty acids – Are certain unsaturated fatty acids • Deficiencies in fatty acids are rare Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Vitamins • Vitamins are organic molecules – Required in the diet in small amounts, however, their deficiency can cause sever problems. • To date, 13 vitamins essential to humans – Have been identified • Vitamins are grouped into two categories – Fat-soluble , A, D, E, K – Water-soluble; such as B 1, B 2, niacin…. etc. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Table 41. 1 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Minerals • Minerals are simple inorganic nutrients – Usually required in small amounts – Calcium and phosphorus are needed the most as they are needed for building the body bones – Iron is also needed in high amounts for the hemoglobin production. Excess iron can lead to liver damage – Potassium, sodium and chloride are need for nerve functioning and osmotic balance between cells and interstitial fluid Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Mineral requirements of humans Table 41. 2 Copyright © 2005 Pearson Education, Inc. publishing

• Concept 41. 3: The main stages of food processing are ingestion, digestion, absorption, and elimination • Ingestion, the act of eating – Is the first stage of food processing Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Digestion, the second stage of food processing – Is the process of breaking food down into molecules small enough to absorb – Involves enzymatic hydrolysis of polymers into their monomers Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Absorption, the third stage of food processing – Is the uptake of nutrients by body cells • Elimination, the fourth stage of food processing – Occurs as undigested material passes out of the digestive compartment Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Mammalian Digestive System • Concept 41. 4: Each organ of the mammalian digestive system has specialized foodprocessing functions Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• The mammalian digestive system consists of the alimentary canal – And various accessory glands that secrete digestive juices through ducts Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Cardiac orifice Tongue Salivary glands Oral cavity Parotid gland Sublingual gland Esophagus Pyloric sphinct er Liver Ascending portion of large intestine Mouth Pharynx Esophagus Submandibular gland Stomach Gallbladder Liver Pancreas IIeum of small intestine Small intestine Rectum Appendix Stomach Small intestines Pancreas Large intestine Figure 41. 15 Salivary glands Anus Cecum Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Duodenum of small intestine Large intestines Rectum Anus A schematic diagram of the human digestive system

• Food is pushed along the digestive tract by peristalsis – Rhythmic waves of contraction of smooth muscles in the wall of the canal – At sum junctions, the muscle layer is modified into ring like valves called sphincters. These sphincters regulate the passage of materials through the canal. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

The Oral Cavity, Pharynx, and Esophagus • In the oral cavity, food is lubricated and digestion begins – And teeth chew food into smaller particles that are exposed to salivary amylase, initiating the breakdown of glucose polymers such as starch and glycogen. – Saliva contain slippery glycoprotein called mucin that lubricates the food – The tongue tastes the food and shapes it into a ball called bolus that is easy to swallow Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• The region we call our throat is the pharynx – A junction that opens to both esophagus and trachea (windpipe). When we swallow, the top of the trachea moves up and the glottis is blocked by a cartilaginous flap called the epiglottis so that the bolus moves into the entrance of the esophagus. • The esophagus conducts food from the pharynx down to the stomach by peristalsis. The muscles at the very top of the esophagus are striated or voluntary. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

From mouth to stomach The esophageal sphincter relaxes, allowing the bolus to enter the esophagus. 4 Bolus of food Tongue Glottis down and open Epiglottis up Pharynx Esophageal Epiglottis sphincter contracted down Glottis Larynx Trachea Esophageal sphincter relaxed 5 After the food To stomach 1 When a person is not swallowing, the esophageal sphincter muscle is contracted, the epiglottis is up, and the glottis is open, allowing air to flow through the trachea to the lungs. Glottis up and closed 2 The swallowing reflex is triggered when a bolus of food reaches the pharynx. Esophageal sphincter contracted has entered the esophagus, the larynx moves downward and opens the breathing passage. Esophagus To lungs Figure 41. 16 Epiglottis up Relaxed muscles Contracted muscles 3 The larynx, the upper part of the respiratory tract, 6 Waves of muscular contraction moves upward and (peristalsis) tips the epiglottis move the bolus over the glottis, preventing food down the esophagus to the stomach. from entering the Relaxed muscles trachea. Stomach Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

The Stomach • It is located in the upper abdominal cavity just below the diaphragm. It can stretch to accommodate about 2 L of food and fluid. • It stores food so we do not have to eat constantly. • It secretes digestive fluid called the gastric juice from the epithelium lining. Pepsin is present in these juices • With a high concentration of HCl, gastric acid has a p. H of 2 which is acidic enough to dissolve iron nails. – The acid is secreted to disrupt the extracellular matrix that binds cells together in meat and plants. – The acid kills most of the bacteria inters with food Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Present with the gastric juice is the pepsin which is an enzyme that begins the digestion of proteins by breaking the peptide bonds. This enzyme works strongly in acidic environment. • The acidic p. H denatures the food thus increasing the surface area that exposed to digestive enzymes. • Question; what prevents the pepsin from destroying the cells of the stomach wall? • Pepsin is secreted in an inactive form called pepsinogen by specialized cells called the chief cells located in gastric pits. – Cells called parietal cells secrete the HCl. – The acid converts the pepsiongen to the active form, pepsin. – Because different cells secrete pepsinogen and HCl , they do not mix until they inter the lumen of the stomach. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• The stomach is coated with mucus secreted by epithelial cells that help the stomach lining. When these epithelial cells got eroded, they are regenerated by mitosis. • Epithelial cells completely regenerate the stomach lining every 3 days. – Gastric ulcers, lesions in the stomach are mainly caused by a bacterium called Helicobacter pylori and are treated with antibiotics. Figure 41 -18 – As a result of mixing and enzyme action the stomach content become nutrient rich broth called acid chyme. – Stomach is normally closed at both ends. The cardiac orifice opens only when there is a bolus arriving. The occasional backflow of broth cases the heartburn which may cause an ulcer if the problem persists. – The pyloric sphincter, is the opining of the stomach to the intestine. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

The lining of the stomach Figure 41. 17 • Is coated with mucus, which prevents the gastric juice from destroying the cells Esophagus Cardiac orifice Stomach 5 µm Pyloric sphincter Interior surface of stomach. The interior surface of the stomach wall is highly folded and dotted with pits leading into tubular gastric glands. Gastric gland. The gastric glands have three types of cells that secrete different components of the gastric juice: mucus cells, chief cells, and parietal cells. Folds of epithelial tissue Small intestine Epithelium 3 Pepsinogen 2 HCl Pepsinogen and HCI Pepsin (active enzyme) are secreted into the lumen of the stomach. 1 1 2 Mucus cells secrete mucus, which lubricates and protects the cells lining the stomach. 3 Chief cells secrete pepsinogen, an inactive form of the digestive enzyme pepsin. Parietal cells secrete hydrochloric acid (HCl). Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chief cell Parietal cell HCl converts pepsinogen to pepsin. Pepsin then activates more pepsinogen, starting a chain reaction. Pepsin begins the chemical digestion of proteins.

• Gastric ulcers, lesions in the lining – Are caused mainly by the bacterium Helicobacter pylori Bacteria Figure 41. 18 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 1 µm Mucus layer of stomach

The Small Intestine – Is the longest section of the alimentary canal, around 6 meters – Is the major organ of digestion and absorption – First 25 cm is called duodenum; acid chyme mixes with the digestive juices from pancreas, liver, gallbladder, and the glands of intestinal wall itself. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Enzymatic Action in the Small Intestine • The first portion of the small intestine is the duodenum – Where acid chyme from the stomach mixes with digestive juices from the pancreas, liver, gallbladder, and intestine itself Liver Bile Gallbladder Stomach Intestinal juice Acid chyme Pancreatic juice Figure 41. 19 Duodenum of small intestine Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Pancreas

• The pancreas produces bicarbonate and proteases which are protein-digesting enzymes – That are activated once they enter the duodenum Pancreas Membrane-bound enteropeptidase Inactive trypsinogen Other inactive proteases Figure 41. 20 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Lumen of duodenum Trypsin Active proteases

Enzymatic digestion is completed – As peristalsis moves the mixture of chyme and digestive juices along the small intestine Oral cavity, pharynx, esophagus Protein digestion Carbohydrate digestion Nucleic acid digestion Fat digestion Polysaccharides Disaccharides (starch, glycogen) (sucrose, lactose) Salivary amylase Smaller polysaccharides, maltose Proteins Pepsin Small polypeptides Stomach Lumen of small intestine Polysaccharides Pancreatic amylases Maltose and other disaccharides Polypeptides DNA, RNA Pancreatic nucleases Pancreatic trypsin and chymotrypsin (These proteases cleave bonds adjacent to certain amino acids. ) Smaller polypeptides Nucleotides Pancreatic carboxypeptidase Figure 41. 21 Small peptides Disaccharidases Dipeptidases, carboxypeptidase, and aminopeptidase (These proteases split off one amino acid at a time, working from opposite ends of a polypeptide. ) Monosaccharides Amino acids Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Bile salts Fat droplets (A coating of bile salts prevents small droplets from coalescing into larger globules, increasing exposure to lipase. ) Pancreatic lipase Glycerol, fatty acids, glycerides Amino acids Epithelium of small intestine (brush border) Fat globules (Insoluble in water, fats aggregate as globules. ) Nucleotidases Nucleosidases and phosphatases Nitrogenous bases, sugars, phosphates

Hormones • Hormones help coordinate the secretion of digestive juices into the alimentary canal Enterogastrone secreted by the duodenum inhibits peristalsis and acid secretion by the stomach, thereby slowing digestion when acid chyme rich in fats enters the duodenum. Liver Enterogastrone Gallbladder Gastrin CCK Amino acids or fatty acids in the duodenum trigger the release of cholecystokinin (CCK), which stimulates the release of digestive enzymes from the pancreas and bile from the gallbladder. Stomach Pancreas Gastrin from the stomach recirculates via the bloodstream back to the stomach, where it stimulates the production of gastric juices. Secretin Duodenum CCK Key Stimulation Figure 41. 22 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Inhibition Secreted by the duodenum, secretin stimulates the pancreas to release sodium bicarbonate, which neutralizes acid chyme from the stomach.

Absorption of Nutrients • The small intestine has a huge surface area – Due to the presence of villi and microvilli that are exposed to the intestinal lumen Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

The enormous microvillar surface • Is an adaptation that greatly increases the rate of nutrient absorption Microvilli (brush border) Vein carrying blood to hepatic portal vessel Figure 41. 23 Blood capillaries Epithelial cells Muscle layers Villi Epithelial cells Large circular folds Lacteal Key Nutrient absorption Intestinal wall Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Villi Lymph vessel

• The core of each villus – Contains a network of microscopic blood vessels and a small vessel of the lymphatic system called a lacteal Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Amino acids and sugars – simple sugars such as fructose moves by diffusion down its concentration gradient from lumen to the epithelial cells. (Passive) – Amino acids, small peptides, vitamins glucose and several other simple sugars are pumped against concentration gradient. (Active) • After glycerol and fatty acids are absorbed by epithelial cells – They are recombined into fats within these cells Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• These fats are then mixed with cholesterol and coated with proteins – Forming small molecules called chylomicrons, which are transported into lacteals – Chylomicrons gets out of the epithelium cells by exocytosis. – The lymph containing chylomicrons eventually drains from lymphatic system into large veins that return blood to the heart. • Hepatic portal vessel – Capillaries and veins that drain nutrients away from villi all converge into a single circulatory channel go to the liver. – Therefore, liver has first access to sugars and amino acids absorbed after a meal is digested and when the nutrients leave their composition will be changed. Then, they leave and go to all parts of the body. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Digestion and absorption of fats Fat globule 1 Large fat globules are emulsified by bile salts in the duodenum. Bile salts Digestion of fat by the pancreatic enzyme lipase yields free fatty acids and monoglycerides, which then form micelles. 2 Fat droplets coated with bile salts Micelles made up of fatty acids, monoglycerides, and bile salts 3 Fatty acids and monoglycerides leave micelles and enter epithelial cells by diffusion. 4 Figure 41. 24 Epithelial cells of small intestine Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 2 Lacteal Chylomicrons containing fatty substances are transported out of the epithelial cells and into lacteals, where they are carried away from the intestine by lymph.

The Large Intestine (colon) – Is connected to the small intestine at a T-shape junction (one arm is the appendix) with a sphincter controls the movement of material. Figure 41. 25 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• A major function of the colon – Is to recover water that has entered the alimentary canal – About 7 L of fluid is secreted into the lumen which is more than people drink each day – This comes from the recycling of water – Together, the small and the large intestine absorbs 90% of the water that inters the alimentary canal. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• The wastes of the digestive tract, the feces – Become more solid as they move through the colon – Pass through the rectum and exit via the anus – Takes 12 -24 h to travel through the digestive tract. – When there is a bacterial or viral infection that interferes with the water absorption e. g cholera, then less water is absorbed and diarrhea occurs. – When there is a constipation due to the slow peristalsis, then more water get absorbed and thus the feces become hard and hard to exist the rectum and the anus. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Intestinal bacteria generate gases like H 2 S and methane while other bacteria produce some vitamins including biotin, folic acid, vitamin K, and other B vitamins. • Feces also contain masses of bacteria mainly E. coli, up to 40% of its weight and contain cellulose which helps the regular movement of the feces • The terminal portion of the colon is called rectum where the feces stored until they can be eliminated. • Presence of E. coli in drinking water or any food indicates fecal contamination. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

End of chapter Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Concept 41. 5: Evolutionary adaptations of vertebrate digestive systems are often associated

Some Dental Adaptations • Dentition, an animal’s assortment of teeth – Is one example

• Mammals have specialized dentition – That best enables them to ingest their usual diet Incisors Canines (a) Carnivore (b) Herbivore Figure 41. 26 a–c (c) Omnivore Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Molars Premolars

• Some animals have adaptations – That help them through periods when their bodies demand extraordinary amounts of protein Figure 41. 11 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Stomach and Intestinal Adaptations • Herbivores generally have longer alimentary canals than carnivores – Reflecting the longer time needed to digest vegetation Small intestine Stomach Cecum Colon (large intestine) Figure 41. 27 Carnivore Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Herbivore

Symbiotic Adaptations • Many herbivorous animals have fermentation chambers – Where symbiotic microorganisms digest

• The most elaborate adaptations for an herbivorous diet – Have evolved in the animals called ruminants 1 Rumen. When the cow first chews and swallows a mouthful of grass, boluses (green arrows) enter the rumen. 2 Reticulum. Some boluses also enter the reticulum. In both the rumen and the reticulum, symbiotic prokaryotes and protists (mainly ciliates) go to work on the cellulose-rich meal. As by-products of their metabolism, the microorganisms secrete fatty acids. The cow periodically regurgitates and rechews the cud (red arrows), which further breaks down the fibers, making them more accessible to further microbial action. Intestine Esophagus Figure 41. 28 4 Abomasum. The cud, containing great numbers of microorganisms, finally passes to the abomasum for digestion by the cow‘s own enzymes (black arrows). Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 3 Omasum. The cow then reswallows the cud (blue arrows), which moves to the omasum, where water is removed.

• The four stages of food processing Small molecules Pieces of food Mechanical digestion Chemical digestion (enzymatic hydrolysis) Nutrient molecules enter body cells Undigested material Food 1 INGESTION 2 DIGESTION 3 ABSORPTION Figure 41. 12 Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 4 ELIMINATION

Digestive Compartments • Most animals process food – In specialized compartments Copyright © 2005

Intracellular Digestion • In intracellular digestion – Food particles are engulfed by endocytosis and digested within food vacuoles Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Extracellular Digestion • Extracellular digestion – Is the breakdown of food particles outside cells

• Animals with simple body plans – Have a gastrovascular cavity that functions in both digestion and distribution of nutrients Tentacles Mouth Food Gastrovascular cavity Epidermis Mesenchyme Gastrodermis Nutritive muscular cells Flagella Gland cells Food vacuoles Figure 41. 13 Mesenchyme Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• Animals with a more complex body plan – Have a digestive tube with two openings, a mouth and an anus • This digestive tube – Is called a complete digestive tract or an alimentary canal Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

• The digestive tube can be organized into specialized regions – That carry out digestion and nutrient absorption in a stepwise fashion (a) (b) Earthworm. The digestive tract of an earthworm includes a muscular pharynx that sucks food in through the mouth. Food passes through the esophagus and is stored and moistened in the crop. The muscular gizzard, which contains small bits of sand gravel, pulverizes the food. Digestion and absorption occur in the intestine, which has a dorsal fold, the typhlosole, that increases the surface area for nutrient absorption. Grasshopper. A grasshopper has several digestive chambers grouped into three main regions: a foregut, with an esophagus and crop; a midgut; and a hindgut. Food is moistened and stored in the crop, but most digestion occurs in the midgut. Gastric ceca, pouches extending from the midgut, absorb nutrients. Esophagus Crop Gizzard Intestine Pharynx Anus Mouth Typhlosole Lumen of intestine Foregut Midgut Hindgut Esophagus Rectum Anus Mouth Crop Gastric ceca Esophagus (c) Figure 41. 14 a–c Bird. Many birds have three separate chambers— the crop, stomach, and gizzard—where food is pulverized and churned before passing into the intestine. A bird’s crop and gizzard function very much like those of an earthworm. In most birds, chemical digestion and absorption of nutrients occur in the intestine. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Gizzard Mouth Intestine Crop Stomach Anus

Glucose is a major fuel for cells • Its metabolism, regulated by hormone action, is an important example of homeostasis 1 When blood glucose level rises, a gland called the pancreas secretes insulin, a hormone, into the blood. 2 Insulin enhances the transport of glucose into body cells and stimulates the liver and muscle cells to store glucose as glycogen. As a result, blood glucose level drops. STIMULUS: Blood glucose level rises after eating. Homeostasis: 90 mg glucose/ 100 m. L blood Figure 41. 3 4 Glucagon promotes the breakdown of glycogen in the liver and the release of glucose into the blood, increasing blood glucose level. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings STIMULUS: Blood glucose level drops below set point. 3 When blood glucose level drops, the pancreas secretes the hormone glucagon, which opposes the effect of insulin.