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Animal Nutrition

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Chapter 37 – Animal Nutrition


  1.  An 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); and essential nutrients (substances the animal cannot make for itself from any raw material)
  2.  Animals obtain the fuel that powers the work of their body cells from the oxidation of organic molecules and the monomers are used to generate ATP by cellular respiration.
  3.   Animals must meet their basal metabolic rate to sustain life.
  4.   Taking in more calories that it consumes causes the body to store the excess calories.  For example, the liver and muscle cells store energy in the form of glycogen.  If the glycogen stores are full, the excess calories are stored as fat. 
  5.   An undernourished person is one whose diet is deficient in calories.  When starvation for calories begins, the body breaks down its own proteins for fuel, muscles decrease in size and the brain becomes deficient and often the damage is irreversible.
  6.   A common feedback mechanism regulates fate storage and use in mammals.  A hormone called leptin produced by the adipose cells.  An increase in adipose tissue increases leptin levels in the brain and this cues the brain to depress appetite and 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.
  7.   Essential nutrients
  8.         A.  Heterotrophs do not always produce the raw materials it needs to continue growth and development so essential nutrients must be obtained.
    1. One essential nutrient is vitamin C or ascorbic acid.
    2. An animal lacking these essential nutrients is said to be malnourished instead of undernourished.
    3. There are four classes of essential nutrients
      1. Essential amino acids
        1. Must be obtained from food in prefabricated form.
        2. Eight amino acids are essential in the human diet
        3. One that lacks one or more essential amino acid results in a protein deficiency and often causes physical and mental development problems.
        4. Most of these come from eggs, cheese and animal products.
      2. Essential Fatty acids
        1. Usually unsaturated fatty acids
        2. Linoleic acid is an example
        3. Required to make some of the phospholipid found in membranes.
        4. Very rare
      3. Vitamins
        1. Organic molecules required in small amounts.
        2. 13 vitamins are essential
          1. Water-soluble include B complex for coenzyme activity, Vitamin C for synthesis of connective tissue.
            1. Excess are excreted through the urine.
          2. Fat-soluble vitamins
            1. Examples are vitamins A, D, E, and K.
              1. A is used in the visual pigments of the eyes
              2. D aids in calcium absorption and bone formation
              3. E is not fully understood but seems to protect the phospholipids from oxidation.
              4. K is required for blood clotting
              5. Excess is not excreted but are deposited in body fat.
      4. Minerals
        1. Inorganic nutrients required in very small amounts.
        2. Calcium is needed for functioning of nerves, muscles and formation of bones.
        3. Phosphorus is used in the formation of ATP and nucleic acids.
        4. Iron is used in the Cytochromes involved in cellular respiration and hemoglobin.
        5. Magnesium, zinc, copper, manganese, selenium, and molybdenum are cofactors for enzymes.
  9. Four main stages of food processing
    1. Ingestion is the act of eating
    2. Digestion is the process of breaking down food molecules into bits small enough to absorb.  Macromolecules cannot be used directly because they are too large to pass through the membranes.  This is catalyzed by hydrolytic enzymes and is usually preceded by mechanical fragmentation of food.
    3. Absorption is the taking up of small molecules like amino acids from the digestive compartment.
    4. Elimination is the removal of undigested material out of the digestive compartment.
  10. Digestion occurs in specialized compartments
    1. Intracellular digestion
      1. Food vacuoles contain enzymes that break down food without digesting the cell’s own cytoplasm.
      2. Protists digest their meals in food vacuoles usually after phagocytosis or pinocytosis.
      3. The food vacuoles fuse with lysosomes to have this occur.
    2. Extracellular digest
      1. In most animals, at least some hydrolysis occurs by extracellular digest, which occurs in compartments that are continuous with the outside of the animals body.
      2. Simple body plans have digestive sacs with signal openings called gastrovascular cavities that function in both digestion and distribution of nutrients.
      3. Complete digestive tracts or alimentary canals have two openings, a mouth and an anus.
        1. Food moves in one direction and it can be organized into specialized regions.
        2. Food ingested through the mouth and pharynx passes through an esophagus that leads to a crop, gizzard, or stomach.  Crops and stomachs normally store the food and gizzards tend to grind it.
  11. The Mammalian Digestive System
    1. Consists of an alimentary canal with various accessory glands that secrete digestive juices into canals through ducts.
    2. Peristalsis is the rhythmic waves of contraction by smooth muscles in the wall of the canal that pushes the food along the tract.
    3. Sphincters are used to close off the tube like drawstrings and regulate the passage of material.
    4. Accessory glands are three pairs of salivary glands, the pancreas, the liver, and the gall bladder.
  12. Digestion begins in the oral cavity
    1. Physical and chemical digestion takes place here. 
    2. Macerating or chewing the food is the mechanical breakdown of the food making it easier to swallow and increases the surface area.
    3. The presence of food in the oral cavity triggers a nervous reflex for the salivary glands to deliver saliva through ducts in the oral cavity.
      1. Mucin is dissolved in the saliva, which protects the soft lining of the mouth from abrasion and lubricates the food.
      2. It also contains buffers to prevent tooth decay and antibacterial agents may kill much of the bacteria that enters.
      3. Salivary amylase is also release that begins the digestion of carbohydrates by breaking down starch and glycogen.
    4. The tongue tastes the food and manipulates it during chewing helping form the food into a ball called a bolus, which is then pushed to the pharynx.
  13. The pharynx
    1. This is the intersection of the esophagus and trachea.
    2. Swallowing causes the top of the trachea to move up so that its opening the glottis is covered by a cartilaginous flap called the epiglottis and forces the food to move into the esophagus
  14. The Esophagus
    1. Conducts food from the pharynx to the stomach.
    2. Peristalsis squeezes the bolus along.  (It is possible to eat while standing on your head because of these reactions.  Reverse peristalsis is regurgitation.)
    3. The muscles at the top are striated and voluntary.
    4. Salivary amylase continues its digestion of starch.
    5. A sphincter is used to control movement into the stomach.
  15. The Stomach
    1. Located on the left side of the abdominal cavity, just below the diaphragm.
    2. Has a very elastic wall and can accommodate about 2L of food and water.
    3. The epithelium secretes gastric juice that mixes with food and has a pH of 2.
      1. This disrupts the extracellular matrix that binds cells together and kills any other bacteria present.
    4. Pepsin is an enzyme that begins the breakdown of proteins by breaking the peptide bonds adjacent to specific amino acids and works best in a strong acidic environment.
    5. Chief cells located in gastric pits synthesize and secrete pepsin in an inactive form called pepsinogen, which is converted by HCl to pepsin.  (This is a positive feedback)
    6. A coating of mucus secreted by the epithelial cells protects the lining from being destroyed and the epithelium is constantly replacing the cells involved.
    7. A gastric ulcer is caused by bacteria and can be treated using antibiotics but may worsen if the lining is destroyed faster than it can be regenerated.
    8. The contents of the stomach are mixed every 20 seconds by the smooth muscles and becomes known as chyme. 
    9. The backflow of chyme into the esophagus is called heartburn.
    10. At the junction of the small intestine and the stomach is the pyloric sphincter.
  16. The Small Intestine
    1. 6m in length
    2. Most digestion takes place here.
    3. Also responsible for absorption
    4. The pancreas, liver, and gall bladder contribute to the digestion.
    5. The first 25cm is called the duodenum
      1. The pancreas produces several enzymes as well as bicarbonate, which act as a buffer to neutralize the acid from the stomach.
      2. The liver produces bile, which is a mixture, stored in the gallbladder until needed.
        1. Bile contains no enzymes but bile salts that act as detergents and aid in the digestion and absorption of fats.
  17. Carbohydrate digestion
    1. Begins in the oral cavity by amylase, which breaks down polysaccharides. 
    2. Continues in the small intestine using pancreatic amylases.
    3. Disaccharidases break down the disaccharides into monosaccharides in the epithelium of the small intestine and are then absorbed into the blood stream
  18. Protein digestion
    1. Begins in the stomach with the conversion of pepsinogen to pepsin due to HCl.
    2. In the lumen of the small intestine, trypsin, chymotrypsin, carboxypeptidase and aminopeptidase digest the polypeptides into amino acids
    3. Dipeptidases digest the small peptides in the epithelium of the intestine into amino acids.
  19. Nucleic acid digestion
    1. Begins in the lumen of the small intestine where DNA and RNA are catalyzed by nucleases into nucleotides
    2. This process is completed in the epithelium of the small intestine by nucleotidases.
  20. Fat digestion
    1. Takes place in the lumen of the small intestine.
    2. Bile salts turn fat globules into fat droplets.
    3. Lipases then digest the fat droplets into glycerol, fatty acids, and glycerides.
  21. Absorption of Nutrients
    1. Nutrients must cross the lining of the digestive tract
    2. Large circular folds bear fingerlike projections that contain villi and each of the epithelial cells of a villus contains microvilli.
    3. Penetrating the core of each villus is a net of capillaries and a small vessel of the lymphatic system called a lacteal.
    4. Nutrients are absorbed into the capillaries or lacteals.
    5. Often the transport is passive like with fructose but amino acids, peptides, vitamins, and glucose must be actively transported.
    6. The capillaries and veins drain nutrients into the hepatic portal vessel, which leads to the liver, which has access to amino acids and sugars first.
    7. The blood that leaves the liver may have a very different balance of nutrients than it did initially.
  22. Hormones help regulate digestion
    1. Hormones ensure that digestive secretions are present only when needed.
    2. The hormone gastrin recirculates the blood stream back to the stomach wall and stimulates the secretin of gastric juice.
    3. Enterogastones are secreted by the wall of the duodenum.
      1. The acidic pH of the chyme stimulates the release of secretin, which causes the pancreas to secrete the bicarbonate to buffer the acid.
      2. Cholecytokinin (CCK) is secreted in response to amino acid and fat presence and causes the gallbladder to release bile and pancreatic enzymes.
  23. The Large Intestine
    1. Also known as the colon
    2. Its main function is to reabsorb water that has entered the canal.
    3. The waste or feces becomes more solid as it moves through the colon by peristalsis.
    4. Too much water reabsorbed causes constipation.
    5. Not enough water absorbed causes diarrhea.