Animal Nutrition: The Need to Feed

1. Chapter 41 Animal Nutrition

2. 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

3. 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

4. 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

5. Animals feed by four main mechanisms BULK FEEDERS SUSPENSION FEEDERS SUBSTRATE FEEDERS FLUID FEEDERS Figure 41.2

6. 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

7. 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.

8. 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.

9. 100 µm Over nourishment • Results from excessive food intake • Leads to the storage of any excess calories as fat Figure 41.4

10. 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 breastcancer. Obesity is a factor in about 300,000 deaths in US every year.

11. 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.

12. 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 Leptin 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. The hormone PYY, secreted by the small intestine after meals, acts as an appetite suppressant that counters the appetite stimulant ghrelin. PYY Figure 41.5

13. 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.

14. 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

15. 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.

16. 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.

17. A species of birds called petrels • Become obese as chicks due to the need to consume more calories than they burn Figure 41.7

18. 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.

19. 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

20. Herbivorous animals • May suffer mineral deficiencies if they graze on plants in soil lacking key minerals Figure 41.8

21. 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

22. 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

23. Essential amino acids for adults Beansand other legumes Methionine Valine Threonine Phenylalanine Leucine Corn (maize)and other grains Isoleucine Tryptophan Lysine Figure 41.10 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.

24. 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

25. 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 B1, B2, niacin….etc.

26. Table 41.1

27. 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

28. Mineral requirements of humans Table 41.2

29. 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

30. 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

31. 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

32. Mammalian Digestive System • Concept 41.4: Each organ of the mammalian digestive system has specialized food-processing functions

33. The mammalian digestive system consists of the alimentary canal • And various accessory glands that secrete digestive juices through ducts

34. Cardiacorifice Salivaryglands Tongue Oral cavity Mouth Salivaryglands Pharynx Parotid gland Sublingual gland Esophagus Esophagus Submandibular gland Pyloricsphincter Liver Gall-bladder Stomach Stomach Gall-bladder Ascendingportion of large intestine Small intestines Liver Pancreas Pancreas IIeumof small intestine Small intestine Large intestines Duodenum of small intestine Rectum Large intestine Anus Rectum A schematic diagram of the human digestive system Appendix Anus Cecum Figure 41.15

35. 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.

36. 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

37. 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 esophagusconducts food from the pharynx down to the stomach by peristalsis. The muscles at the very top of the esophagus are striated or voluntary.

38. The esophageal sphincter relaxes, allowing the bolus to enter the esophagus. Epiglottisup Bolus of food Tongue Glottisdown and open Epiglottisup Esophageal sphincterrelaxed Pharynx Epiglottis down Esophageal sphinctercontracted Esophageal sphinctercontracted Glottis After the food has entered the esophagus, the larynx moves downward and opens the breathing passage. Larynx Esophagus Trachea To lungs To stomach Glottis upand closed Contractedmuscles Relaxedmuscles Relaxedmuscles The larynx, the upper part of the respiratory tract, moves upward and tips the epiglottis over the glottis, preventing food from entering the trachea. 2 1 6 5 3 4 Waves of muscular contraction (peristalsis) move the bolus down the esophagus to the stomach. 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. The swallowing reflex is triggered when a bolus of food reaches the pharynx. Figure 41.16 Stomach From mouth to stomach

39. 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 pH 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

40. 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 pH 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.

41. 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.

42. Stomach Folds of epithelial tissue Small intestine 2 3 1 1 2 3 Pepsinogen and HCI are secreted into the lumen of the stomach. HCl converts pepsinogen to pepsin. Pepsin then activates more pepsinogen, starting a chain reaction. Pepsin begins the chemical digestion of proteins. Parietal cell Chief cell The lining of the stomach Figure 41.17 • Is coated with mucus, which prevents the gastric juice from destroying the cells Esophagus Cardiac orifice Pyloric sphincter 5 µm Interior surface of stomach. The interior surface of the stomach wall is highly folded and dotted with pits leading into tubular gastric glands. Epithelium Pepsinogen 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. Pepsin (active enzyme) HCl Mucus cells secrete mucus, which lubricates and protects the cells lining the stomach. Chief cells secrete pepsino- gen, an inactive form of the digestive enzyme pepsin. Parietal cells secrete hydrochloric acid (HCl).

43. Bacteria Mucuslayer of stomach 1 µm • Gastric ulcers, lesions in the lining • Are caused mainly by the bacterium Helicobacter pylori Figure 41.18

44. 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.

45. Liver Stomach Gall-bladder Pancreas 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 Bile Acid chyme Intestinaljuice Pancreatic juice Duodenum of small intestine Figure 41.19

46. Pancreas Membrane-boundenteropeptidase Inactivetrypsinogen Trypsin Other inactiveproteases Active proteases Lumen of duodenum • The pancreas produces bicarbonate and proteases which are protein-digesting enzymes • That are activated once they enter the duodenum Figure 41.20

47. Protein digestion Fat digestion Nucleic acid digestion Carbohydrate digestion Oral cavity, pharynx, esophagus Polysaccharides (starch, glycogen) Disaccharides (sucrose, lactose) Salivary amylase Smaller polysaccharides, maltose Proteins Pepsin Stomach Small polypeptides Fat globules (Insoluble in water, fats aggregate as globules.) DNA, RNA Polypeptides Polysaccharides Pancreatic amylases Pancreatic nucleases Lumen of small intes- tine Pancreatic trypsin and chymotrypsin (These proteases cleave bonds adjacent to certain amino acids.) Bile salts Maltose and other disaccharides Fat droplets (A coating of bile salts prevents small drop- lets from coalescing into larger globules, increasing exposure to lipase.) Nucleotides Smaller polypeptides Pancreatic carboxypeptidase Pancreatic lipase Glycerol, fatty acids, glycerides Amino acids Epithelium of small intestine (brush border) Small peptides Nucleotidases Dipeptidases, carboxypeptidase, and aminopeptidase (These proteases split off one amino acid at a time, working from opposite ends of a polypeptide.) Nucleosides Disaccharidases Nucleosidases and phosphatases Nitrogenous bases, sugars, phosphates Monosaccharides Amino acids Enzymatic digestion is completed • As peristalsis moves the mixture of chyme and digestive juices along the small intestine Figure 41.21

48. 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 Entero- gastrone Gall- bladder Gastrin from the stomach recirculates via the bloodstream back to the stomach, where it stimulates the production of gastric juices. Gastrin CCK Stomach 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. Pancreas Secretin Duodenum Secreted by the duodenum, secretin stimulates the pancreas to release sodium bicarbonate, which neutralizes acid chyme from the stomach. CCK Key Stimulation Inhibition Hormones • Hormones help coordinate the secretion of digestive juices into the alimentary canal Figure 41.22

49. 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

50. Microvilli(brush border) Vein carrying blood to hepatic portal vessel Bloodcapillaries Muscle layers Epithelialcells Epithelial cells Largecircularfolds Lacteal Villi Key Lymph vessel Villi Nutrientabsorption Intestinal wall The enormous microvillar surface • Is an adaptation that greatly increases the rate of nutrient absorption Figure 41.23