Animal Nutrition

• Animals have two basic requirements: chemical energy for synthesizing ATP, and carbon-containing compounds for building molecules.
• Animals are heterotrophs - they obtain the energy and nutrients they need from other organisms.
• Four processes are needed to obtain energy from food:

  • Ingestion is the process of bringing food into the digestive tract (also known as alimentary canal or gastrointestinal tract)
  • Digestion is the breakdown of food
  • Absorption is the uptake of specific ions and molecules across the epithelium of the digestive tract
  • Waste has to be eliminated
• The mount of energy provided by foods is measured in kilocalories.
• Essential Nutrients

  • Those that cannot be synthesized and must be obtained from diet
  • Essential amino acids
  • Essential fatty acids
  • Vitamins are organic compounds that are vital for health but are required only in minute amounts - have multiple functions including coenzyme activity
  • Minerals are inorganic substances used as components of enzyme cofactors, they are structural materials, and their ions are the major electrolytes in the body
• Vitamin D

  • Bodies can make vitamin D from cholesterol molecules in series of steps
  • One reaction is catalyzed by ultraviolet light
  • Light strikes a molecule of 7- dehydrocholesterol and changes its shape so that it becomes a form of vitamin D called D3
  • Enzymes can then complete the process to make the active form of the vitamin
  • People have to be exposed to sunlight to perform all the steps to make their own vitamin D, otherwise they must eat it in foods
  • Needed for our bodies to absorb calcium in our small intestine
• Vitamin D acts in the same way as a steroid hormone:

  • Enters cells and binds to a protein called the vitamin D receptor (VDR) and turns on certain genes.
  • One of the genes it activates makes the calcium transport protein calbindin.
  • The DNA region that the vitamin D–VDR complex binds to is called the vitamin D response element, or VDRE - at least a thousand different genes have VDREs
• Vitamin D Deficiency

  • In children, this causes a condition called rickets in which the weakened leg bones become deformed
  • In adults, it causes osteomalacia (softening of the bones) or osteoporosis (fragile bones)
  • Health Canada recommends that Canadians receive at least 600 IU of vitamin D a day to maintain adequate levels of calcium in our bodies
  • In Canada, vitamin D is added to our milk
• Digestion

  • Key process in animals because unlike plants and unicellular organisms, animals do not acquire nutrients as individual molecules
  • Breakdown of food into small enough pieces to allow nutrients to be extracted, and waste materials must then be eliminated
• The Digestive Tract

  • Incomplete digestive tracts
  • Have one opening for both ingestion of
  • food and elimination of wastes
  • The mouth opens into the gastrovascular cavity, where digestion takes place
  • Complete digestive tracts
  • Have two openings—the mouth for ingestion and the anus for elimination of wastes
  • The interior of this tube communicates directly with the external environment via these openings
• Overview

  • In the mouth, enzymes in the saliva begin the chemical breakdown of carbohydrates and lipids
  • Chemical digestion of protein begins in the acidic environment of the stomach
  • Chemical processing of carbohydrates, proteins, and lipids is completed in the small intestine
  • The small molecules that result are absorbed in the small intestine along with water, vitamins, and ions
  • In the large intestine, more water is absorbed, and the material remaining in the large intestine is feces that are eventually excreted
• Mouth

  • Digestion starts here
  • In mammals, digestion begins with chewing
  • Chemical digestion begins in the mouth
  • Salivary amylase cleaves bonds to release maltose from starch and glycogen
  • Cells in the tongue synthesize and secrete lingual lipase, which begins the digestion of lipids
  • Breaking down triglycerides into diglycerides and fatty acids
  • Salivary glands in the mouth produce a slimy substance called mucus
  • Makes food soft and slippery enough to be swallowed
• Esophagus

  • Once food is swallowed, it enters the esophagus, which connects the mouth and stomach
  • The food is propelled to the stomach by a wave of muscle contractions called peristalsis
  • In response to nerve signals, the smooth muscles in the esophagus contract and relax in a coordinated fashion
  • The system is a reflex - an automatic reaction to a stimulus - that is stimulated by the act of swallowing
• Stomach

  • The stomach is a tough, muscular pouch bracketed on both ends by ringlike muscles called sphincters, which control the passage of material
  • When food enters the stomach, muscular contractions result in churning that mixes and breaks down the food mechanically
  • Stomach also aids in partial digestion of proteins
• Protein Digestion (Stomach)

  • The lumen of the stomach is highly acidic; the predominant acid in the stomach is hydrochloric acid (HCl)
  • The enzyme in the stomach responsible for digesting proteins is pepsin
  • Specialized stomach cells called chief cells contain a pepsin precursor called pepsinogen that is converted to pepsin in the presence of the HCl
  • Secretion of a protein-digesting enzyme in inactive form is important
  • It prevents destruction of proteins in the cells where the enzyme is synthesized
• Production of Stomach Acid

  • Parietal cells are located in the pits of the stomach lining and are the source of HCl in gastric juice
  • Gastric juice can have a pH as low as 1.5
  • This acid condition helps to denature the proteins, making it easier for pepsin to digest them
• Production of Stomach Acid

  • Mucous cells secrete mucus, which lines the gastric epithelium and protects the stomach from damage by HCl
  • Parietal cells have a high concentration of the enzyme carbonic anhydrase and mitochondria
  • Carbonic anhydrase catalyzes the formation of carbonic acid (H2CO3), which immediately dissociates into the bicarbonate ion (HCO3−) and a proton
• Small Intestine (Part 1)

  • Peristalsis in the stomach moves small amounts of material through the sphincter at the base of the stomach and into the small intestine
  • The food mixes with secretions from the pancreas and liver and begins to move through the tube
  • At the end of the small intestine, digestion is complete and most nutrients - along with water - have been absorbed
• Small Intestine (Part 2)

  • The small intestine has an enormous surface area for absorption of nutrients and water due to the epithelial tissue being covered with projections called villi
  • The surface area increases the rate of nutrient absorption
  • Each villus contains blood vessels and a lymphatic vessel called a lacteal, nutrients pass quickly from epithelial cells into the body’s transport systems
• Water

  • When solutes from digested material are absorbed into the epithelium of the small intestine, water follows passively by osmosis
• Protein Processing by Pancreatic Enzymes

  • Due to the highly acidic nature of the stomach, the secondary and tertiary structures of proteins are broken down
  • Pepsin breaks down the peptide bonds between amino acids of proteins, breaking them into small chains of amino acids
  • In the small intestine, protein digestion is complete, yielding individual amino acids that can enter the bloodstream and be transported to cells in the body
• Protein Processing by Pancreatic Enzymes

  • Proteases are enzymes that are released from the pancreas into the small intestine and digest polypeptides to monomers
  • There are many types of proteases, each specific for a different kind of polypeptide
  • Proteases are synthesized in inactive form by the pancreas, transferred through the pancreatic duct to the small intestine, and activated there
  • Pancreatic enzymes are activated by another enzyme known as enteropeptidase/enterokinase
• Protein Processing by Pancreatic Enzymes

  • Enteropeptidase/enterokinase activates a pancreatic enzyme called trypsinogen by removing a short section from the N-terminus of trypsinogen, resulting in the active enzyme trypsin
  • Trypsin, in turn, activates other enzymes that are synthesized by the pancreas and secreted in an inactive form
  • Each enzyme begins cleaving specific peptide bonds
  • Eventually, polypeptides are broken up into amino acid monomers
• Large Intestine

  • By this point, most water and virtually al available nutrients have been absorbed
  • Primary function is to form feces by absorbing extra water and compacting remaining waste
  • These processes occur in the colon, the main section of the large intestine
  • Feces are held in the rectum (final part of the large intestine) until they can be eliminated
  • Human colon also contains symbiotic micro-organisms that digest cellulose
  • These bacteria produce several important nutrients such as vitamin K
• Insulin and Homeostasis

  • Insulin is produced in the pancreas when blood glucose levels are high
  • Binds to receptors on cells and causes them to increase their rate of glucose uptake and processing
  • Specifically, insulin stimulates cells in the liver and skeletal muscle to import glucose from the blood and synthesize glycogen from glucose monomers - as a result, blood glucose levels decline
• Insulin and Homeostasis (Part 2)

  • If blood glucose levels fall too much, cells in the pancreas secrete a hormone called glucagon
  • In response to glucagon, cells in the liver catabolize glycogen and produce glucose via gluconeogenesis - the synthesis of glucose from noncarbohydrate compounds
  • As a result, glucose levels in the blood rise
  • Insulin and glucagon form a negative feedback system
• Digesting Lipids: Bile and Transport

  • The pancreatic secretions include digestive enzymes that act on fats as well as proteins and carbohydrates
  • Pancreatic lipase breaks certain bonds present in complex fats – Results in the release of fatty acids and other small lipids
  • Hydrophobic fats tend to enter the small intestine in large globules that must be broken up by a process known as emulsification before digestion can begin
• Digesting Lipids: Bile and Transport

  • Emulsification results from the action of small molecules called bile salts; they are synthesized in the liver and secreted in a complex solution called bile, which is stored in the gallbladder
  • When bile enters the small intestine, it raises the pH and emulsifies the fat into small globules, thus increasing the surface area
• Digesting Lipids: Bile and Transport

  • Pancreatic lipase breaks the globules and digests them
  • After the monoglycerides and fatty acids are released by lipase activity, they enter the small intestine epithelial cells by simple diffusion
  • Once inside the cell, they are processed into protein-coated globules called chylomicrons
  • Which diffuse into lacteals by the process of exocytosis
  • The lacteals merge with larger lymph vessels that merge with veins, and in this way fats enter the bloodstream without clogging small blood vessels