Lecture 19: Animal Nutrition

 

1) Introduction

•         As a group, animals exhibit a great variety of nutritional adaptations.

•         For example, the snowshoe hare of the northern forests, obtains all their nutrients from plants alone.

•         Hares and rabbits have a large intestinal pouch housing prokaryotes and protists that digest cellulose.

•         For any animal, a nutritionally adequate diet is essential for homeostasis, a steady-state balance in body functions.

•         A balanced diet provides fuel for cellular work and the materials needed to construct organic molecules.

•         A nutritionally adequate diet satisfies three needs:

•         fuel (chemical energy) for all the cellular work of the body;

•         the organic raw materials animals use in biosynthesis (carbon skeletons to make many of their own molecules);

•         essential nutrients, substances that the animals cannot make for itself from any raw material and therefore must obtain in food in prefabricated form.

•         The flow of food energy into and out of an animal can be viewed as a “budget,” with the production of ATP accounting for the largest fraction by far of the energy budget of most animals.

•         ATP powers basal or resting metabolism, as well as activity, and, in endothermic animals, temperature regulation.

•         Nearly all ATP is derived from oxidation of organic fuel molecules - carbohydrates, proteins, and fats - in cellular respiration.

•         The monomers of any of these substances can be used as fuel, though priority is usually given to carbohydrates and fats.

•         Fats are especially rich in energy, liberating about twice the energy liberated from an equal amount of carbohydrate or protein during oxidation.

•         When an animal takes in more calories than it needs to produce ATP, the excess can be used for biosynthesis.

•         This biosynthesis can be used to grow in size or for reproduction, or can be stored in energy depots.

•         In humans, the liver and muscle cells store energy as glycogen, a polymer made up of many glucose units.

•         Glucose is a major fuel molecule for cells, and its metabolism, regulated by hormone action, is an important aspect of homeostasis.

•         If glycogen stores are full and caloric intake still exceeds caloric expenditure, the excess is usually stored as fat.

•         The human body regulates the use and storage of glucose, a major cellular fuel.

(1) When glucose levels rise above a set point,

(2) the pancreas secretes insulin into the blood.

(3) Insulin enhances the transport of glucose into body cells and stimulates the liver and muscle cells to store glucose as glycogen, dropping blood glucose levels.

(4) When glucose levels drop below a set point, (5) the pancreas secretes glucagon into the blood.

(6) Glucagon promotes the breakdown of glycogen and the release of glucose into the blood, increasing the blood glucose levels.

•         In mammals, a hormone called leptin, produced by adipose cells, is a key player in a complex feedback mechanism regulating fat storage and use.

•         A high leptin level cues the brain to depress appetite and to increase energy-consuming muscular activity and body-heat production.

•         Conversely, loss of body fat decreases leptin levels in the blood, signaling the brain to increase appetite and weight gain.

•         These feedback mechanisms regulate body weight around a fairly rigid set point in some individuals and over a relatively wide range in others.

•         Animals require 20 amino acids to make proteins.

•         Most animals can synthesize half of these if their diet includes organic nitrogen.

•         Essential amino acids must be obtained from food in prefabricated form.

•         Eight amino acids are essential in the adult human with a ninth, histidine, essential for infants.

•         The same amino acids are essential for most animals.

•         Vitamins are organic molecules required in the diet in quantities that are quite small compared with the relatively large quantities of essential amino acids and fatty acids animals need.

•         While vitamins are required in tiny amounts - from about 0.01 mg to 100 mg per day - depending on the vitamin, vitamin deficiency (or overdose in some cases) can cause serious problems.

•         Minerals are simple inorganic nutrients, usually required in small amounts - from less than 1 mg to about 2,500 mg per day.

•         Iron is a component of the cytochromes that function in cellular respiration and of hemoglobin, the oxygen binding protein of red blood cells. 

•         While sodium, potassium, and chloride have a major influence on the osmotic balance between cells and the interstitial fluids, excess consumption of salt (sodium chloride) is harmful.

•         The average U.S. citizen eats enough salt to provide about 20 times the required amount of sodium.

•         Excess consumption of salt or several other minerals can upset homeostatic balance and cause toxic side effects.

•         For example, too much sodium is associated with high blood pressure, and excess iron causes liver damage. 

 

2) Mammalian digestion

•         The general principles of food processing are similar for a diversity of animals, including the mammalian system which we will use as a representative example.

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

•         Peristalsis, rhythmic waves of contraction by smooth muscles in the walls of the canal, push food along.

•         Sphincters, muscular ringlike valves, regulate the passage of material between specialized chambers of the canal.

•         The accessory glands include the salivary glands, the pancreas, the liver, and the gallbladder.

•         The stomach’s second line of defense against self-digestion is a coating of mucus, secreted by epithelial cells, that protects the stomach lining.

•         Still, the epithelium is continually eroded, and the epithelium is completely replaced by mitosis every three days.

•         Gastric ulcers, lesions in the stomach lining, are caused by the acid-tolerant bacterium Heliobacter pylori.

•         Ulcers are often treated with antibiotics.

•         The liver performs a wide variety of important functions in the body, including the production of bile.

•         Bile is stored in the gallbladder until needed.

•         It contains 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.

•         Specific enzymes from the pancreas and the duodenal wall have specific roles in digesting macromolecules.

•         The digestion of starch and glycogen, begun by salivary amylase in the oral cavity, continues in the small intestine.

•         Pancreatic amylases hydrolyze starch, glycogen, and smaller polysaccharides into disaccharides.

•         A family of disaccharidases hydrolyze each disaccharide into monomers.

•         Maltase splits maltose into two glucose molecules.

•         Sucrase splits sucrose, a sugar found in milk, into glucose and fructose.

•         These enzymes are built into the membranes and extracellular matrix of the intestinal epithelium which is also the site of sugar absorption.

•         Digestion of proteins in the small intestine completes the process begun by pepsin.

•         Several enzymes in the duodenum dismantle polypeptides into their amino acids or into small peptides that in turn are attacked by other enzymes.

•         Trypsin and chymotrypsin attack peptide bonds adjacent to specific amino acids, breaking larger polypeptides into shorter chains.

•         Dipeptidase, attached to the intestinal lining, split smaller chains.

•         Carboxypeptidases and aminopeptidase split off one amino acid from the carboxyl or amino end of a peptide, respectively.

•         Most digestion occurs in the duodenum.

•         The other two sections of the small intestine, the jejunum and ileum, function mainly in the absorption of nutrients and water.

•         To enter the body, nutrients in the lumen must pass the lining of the digestive tract.

•         The small intestine has a huge surface area - 300 m2, roughly the size of a tennis court.

•         Hormones released by the wall of the stomach and duodenum help ensure that digestive secretions are present only when needed.

•         When we see, smell, or taste food, impulses from the brain initiate the secretion of gastric juice.

•         Certain substances in food stimulate the stomach wall to release the hormone gastrin into the circulatory system.

•         As it recirculates, gastrin stimulates further secretion of gastric juice.

•         If the pH of the stomach contents becomes too low, the acid will inhibit the release of gastrin.

•         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 in the the small intestine, the rest in the colon.

•         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.

•         Diarrhea results if insufficient water is absorbed and constipation if too much water is absorbed.

•         Living in the large intestine is a rich flora of mostly harmless bacteria.

•         One of the most common inhabitants of the human colon is Escherichia coli, a favorite research organism.

•         As a byproduct of their metabolism, many colon bacteria generate gases, including methane and hydrogen sulfide.

•         Some bacteria produce vitamins, including biotin, folic acid, vitamin K, and several B vitamins, which supplement our dietary intake of vitamins.

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3) Evolutionary considerations

•         Dentition, an animal’s assortment of teeth, is one example of structural variation reflecting diet.

•         Particularly in mammals, evolutionary adaptation of teeth for processing different kinds of food is one of the major reasons that mammals have been so successful.

•         Nonmammalian vertebrates generally have less specialized dentition, but there are exceptions.

•         For example, poisonous snakes, like rattlesnakes, have fangs, modified teeth that inject venom into prey.

•         Some snakes have hollow fangs, like syringes, other drip poison along grooves in the tooth surface.

•         All snakes have another important anatomic adaptation for feeding, the ability swallow large prey whole.

•         The lower jaw is loosely hinged to the skull by an elastic ligament that permits the mouth and throat to open very wide for swallowing.

•         The length of the vertebrate digestive system is also correlated with diet.

•         In general, herbivores and omnivores have longer alimentary canals relative to their body sizes than to
carnivores, providing more time for digestion and more surface areas for absorption of nutrients.

•         Vegetation is more difficult to digest than meat because it contains cells walls.

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