Chapter 33 Animal nutrition

• Nutrient


  Any substance consumed by an animal that is needed for survival, growth, development, tissue repair, or reproduction
• Ingested organic macromolecules


  Used for two general purposes: to provide energy and to make new molecules (biosynthesis)
• All animals require the same fundamental organic macromolecules
• How animals eat


  1. Ingestion

  2. Chemical digestion via enzymes

  3. Mechanical digestion

  4. Digestion

  5. Absorption

  6. Elimination
• Types of Digestive Systems

  • Single-celled organisms and sponges digest their food intracellularly
  • Other multicellular animals digest their food extracellularly within a digestive cavity
  • Cnidarians and flatworms have a gastrovascular cavity with only one opening and no specialized regions
• Vertebrate Digestive Systems


  • Gastrointestinal tract is layered: Mucosa, Submucosa, Muscularis, Serosa
• The Stomach


  • Saclike portion of tract
  • Convoluted surface allows expansion
  • Contains 3 layers of smooth muscles for mixing food with gastric juice
  • 3 kinds of secretory cells: Mucus-secreting, Parietal (secrete HCl and intrinsic factor), Chief (secrete pepsinogen)
• Herbivore Stomachs


  • Herbivores must digest cellulose but lack cellulase, so they rely on microbes to digest cellulose
  • Simple stomach uses cecum at connection between small and large intestine
  • Complex stomach has several chambers (e.g. ruminant)
• Small Intestine


  • Nearly all digestion of food, and absorption of food and water
  • Hydrolytic enzymes found on luminal surface or secreted by pancreas
  • Specialized for increased surface area (600x) through folding, villi, and microvilli
• Pancreas


  • Secretes pancreatic fluid into the duodenum, containing enzymes (trypsin, chymotrypsin, amylase, lipase) and bicarbonate
  • Both exocrine and endocrine gland
• Liver


  • Secretes bile, which contains bile pigments and bile salts for emulsification of fats
  • Gallbladder stores and concentrates bile
• Absorption


  1. Protein, amino acids and carbohydrates transported through epithelial cells to blood

  2. Fatty acids and monoglycerides diffuse into epithelial cells, reassembled into triglycerides and chylomicrons, enter lymphatic system

  3. Almost all fluid reabsorbed in small intestine
• Large Intestine (Colon)


  • Much shorter than small intestine, but has larger diameter
  • No digestion occurs, function is to reabsorb water, electrolytes, and vitamin K
  • Prepare waste for expulsion
• Regulation of Digestion


  • Nervous system affects control of muscular and glandular activity
  • Hormones secreted by cells in stomach and small intestine target cells in pancreas and gallbladder
• Regulation of the Digestive Tract

  1. Nervous system stimulates salivary and gastric secretions

  2. Proteins stimulate release of gastrin, which triggers HCl and pepsinogen secretion

  3. Enterogastrones (duodenal hormones) inhibit stomach contractions and prevent additional chyme from entering duodenum

  4. CCK, secretin, and GIP inhibit gastric motility and secretions, CCK also stimulates gallbladder contraction and pancreatic enzyme secretion, secretin stimulates pancreatic bicarbonate secretion
• The digestive system is responsible for breaking down food into smaller molecules that can be absorbed by the body.
• Digestion involves mechanical breakdown (chewing) and chemical breakdown (enzymatic reactions).
• Enzymes are proteins that catalyze biochemical reactions, including those involved in digestion.
• Mechanical digestion involves physical processes such as chewing, grinding, mixing, and propelling food through the digestive tract.
• Chemical digestion breaks down large organic molecules into simpler substances using enzymes produced by various organs.
• Enzymes are proteins that catalyze chemical reactions without being altered or consumed themselves.
• Stomach


  A saclike portion of the digestive tract responsible for breaking down food into smaller particles. It has a convoluted surface and three layers of smooth muscle for mixing food with gastric juice. There are three kinds of secretory cells in the stomach: mucus-secreting cells, parietal cells (which secrete hydrochloric acid and intrinsic factor), and chief cells (which secrete pepsinogen).
• Large intestine


  The last part of the digestive tract responsible for absorbing water and electrolytes from the remaining undigested food and preparing it for elimination from the body. It has a much smoother surface than the stomach and is much longer, divided into several regions including the cecum, colon, rectum, and anus. The walls of the large intestine contain a layer of muscle that helps to move the contents of the intestine along.