The Digestive System

Overview: The Need to Feed

  • Nutrition:
    • Ingestion
    • Digestion
    • Fermentation
    • Absorption
  • In general, animals fall into three categories:
    • Herbivores
    • Carnivores
    • Omnivores
  • Most animals are also opportunistic feeders

Diet and Essential Nutrients

  • An animal’s diet must supply:
    • Chemical energy
    • Organic molecules
    • Essential nutrients, which are required by cells and must be obtained from dietary sources

Essential Nutrients

  • Must be obtained because organisms can’t make them or in enough quantity
  • Vitamins:
    • Humans, apes, monkeys, and guinea pigs have lost the ability to synthesize ascorbic acid (vitamin C).
  • Amino acids
    • Humans require 9.
  • Long-chain unsaturated fatty acids:
    • Vertebrates can synthesize cholesterol, a key component of steroid hormones
  • Minerals

Major Vitamins Required by Humans

  • Vitamin A (retinol)
    • Function: Used in making visual pigments, maintaining epithelial tissues
    • Source: Green vegetables, milk products, liver
    • Deficiency Symptoms: Night blindness, flaky skin
  • B-complex vitamins
    • B1
      • Function: Coenzyme in CO2CO_2 removal during cellular respiration
      • Source: Meat, grains, legumes
      • Deficiency Symptoms: Beriberi, weakening of heart, edema
    • B2 (riboflavin)
      • Function: Part of coenzymes FAD and FMN, which play metabolic roles
      • Source: Many different kinds of foods
      • Deficiency Symptoms: Inflammation and breakdown of skin, eye irritation
    • B3 (niacin)
      • Function: Part of coenzymes NAD* and NADP+
      • Source: Liver, lean meats, grains
      • Deficiency Symptoms: Pellagra, inflammation of nerves, mental disorders
    • B5 (pantothenic acid)
      • Function: Part of coenzyme-A, a key connection between carbohydrate and fat metabolism
      • Source: Many different kinds of foods
      • Deficiency Symptoms: Rare: fatigue, loss of coordination
    • B6 (pyridoxine)
      • Function: Coenzyme in many phases of amino acid metabolism
      • Source: Cereals, vegetables, meats
      • Deficiency Symptoms: Anemia, convulsions, irritability
    • B12 (cyanocobalamin)
      • Function: Coenzyme in the production of nucleic acids
      • Source: Red meats, dairy products
      • Deficiency Symptoms: Pernicious anemia
    • Biotin
      • Function: Coenzyme in fat synthesis and amino acid metabolism
      • Source: Meat, vegetables
      • Deficiency Symptoms: Rare: depression, nausea
    • Folic acid
      • Function: Coenzyme in amino acid and nucleic acid metabolism
      • Source: Green vegetables
      • Deficiency Symptoms: Anemia, diarrhea
  • Vitamin C
    • Function: Important in forming collagen, cementum of bone, teeth, connective tissue of blood vessels; may help maintain resistance to infection
    • Source: Fruit, green leafy vegetables
    • Deficiency Symptoms: Scurvy, breakdown of skin, blood vessels
  • Vitamin D (calciferol)
    • Function: Increases absorption of calcium and promotes bone formation
    • Source: Dairy products, cod liver oil
    • Deficiency Symptoms: Rickets, bone deformities
  • Vitamin E (tocopherol)
    • Function: Protects fatty acids and cell membranes from oxidation
    • Source: Margarine, seeds, green leafy vegetables
    • Deficiency Symptoms: Rare
  • Vitamin K
    • Function: Essential to blood clotting
    • Source: Green leafy vegetables
    • Deficiency Symptoms: Severe bleeding

The Main Stages of Food Processing

  1. Ingestion
  2. Digestion
    • Mechanical digestion
    • Chemical digestion (enzymatic hydrolysis)
  3. Absorption
  4. Elimination
    • Undigested material
    • Nutrient molecules enter body cells

Digestion

  • Breaking food down into molecules small enough to absorb
    • Mechanical digestion
    • Chemical digestion è enzymatic hydrolysis splits bonds in molecules with the addition of water.
    • Absorption
    • Elimination

Digestive Compartments

  • Most animals process food in specialized compartments.
  • These compartments reduce the risk of an animal digesting its own cells and tissues.

Intracellular Digestion

  • Food particles are engulfed by phagocytosis.
  • Digestion in a hydra
  • Food vacuoles, containing food, fuse with lysosomes containing hydrolytic enzymes.
    1. Digestive enzymes released
    2. Food particles broken down
    3. Food particles engulfed and digested

Extracellular Digestion

  • Breakdown of food particles outside of cells.
  • It occurs in compartments that are continuous with the outside of the animal’s body.
  • Animals with simple body plans have a gastrovascular cavity that functions in both digestion and distribution of nutrients.

Complete Digestive Tract

  • More complex animals have a digestive tube with two openings, a mouth and an anus.
    • complete digestive tract = alimentary canal
  • It can have specialized regions that carry out digestion and absorption in a stepwise fashion.

Mammalian Digestive System

  • Organs specialized for sequential stages of food processing form the mammalian digestive system
  • Alimentary canal and accessory glands that secrete digestive juices through ducts:
    • Salivary glands
    • Pancreas
    • Liver
    • Gallbladder
  • The human digestive system
  • Peristalsis: rhythmic contractions

The Oral Cavity, Pharynx, and Esophagus

  1. Oral cavity
    • Salivary glands deliver saliva to lubricate food.
    • Teeth chew food into smaller particles that are exposed to salivary amylase, initiating breakdown of glucose polymers.
    • Saliva also contains mucus, a viscous mixture of water, salts, cells, and glycoproteins.
    • The tongue shapes food into a bolus and provides help with swallowing.
  2. The throat, or pharynx, is the junction that opens to both the esophagus and the trachea.
  3. The esophagus connects to the stomach. The trachea (windpipe) leads to the lungs.
    • The esophagus conducts food from the pharynx down to the stomach by peristalsis.

Swallowing Reflex and Esophageal Peristalsis

  • From mouth to stomach: the swallowing reflex and esophageal peristalsis

Chemical Digestion in the Stomach

  • The stomach stores food and secretes gastric juice, which converts a meal to acid chyme.
  • Gastric juice has a low pH of about 2, which kills bacteria and denatures proteins.
  • Gastric juice is made up of hydrochloric acid (HCl) and pepsin.
  • Pepsin is a protease, or protein-digesting enzyme, that cleaves proteins into smaller peptides.
  • Parietal cells secrete hydrogen and chloride ions separately into the lumen (cavity) of the stomach.
  • Chief cells secrete inactive pepsinogen, which is activated to pepsin when mixed with hydrochloric acid in the stomach.
  • Mucus protects the stomach lining from gastric juice.

The Stomach and its Secretions

  • Gastric pits on interior surface of stomach
  • Folds of epithelial tissue

Stomach Dynamics

  • Coordinated contraction and relaxation of stomach muscle churn the stomach’s contents.
  • Sphincters prevent chyme from entering the esophagus and regulate its entry into the small intestine.

Digestion in the Small Intestine

  • The small intestine is the longest section of the alimentary canal.
  • It is the major organ of digestion and absorption.

Chemical Digestion in the Human Digestive System

  • Oral cavity, pharynx, esophagus:
    • Carbohydrate digestion: Polysaccharides to Disaccharides via Salivary amylase
  • Stomach:
    • Protein digestion: Proteins to Small polypeptides via Pepsin
  • Small intestine (enzymes from pancreas):
    • Carbohydrate digestion: Polysaccharides to Disaccharides via Pancreatic amylases
    • Protein digestion: Small polypeptides to Smaller polypeptides via Pancreatic trypsin and chymotrypsin; Smaller polypeptides to Small peptides via Pancreatic carboxypeptidase
    • Nucleic acid digestion: DNA, RNA to Nucleotides via Pancreatic nucleases
    • Fat digestion: Fat (triglycerides) to Glycerol, fatty acids, monoglycerides via Pancreatic lipase
  • Small intestine (enzymes from epithelium):
    • Carbohydrate digestion: Disaccharides to Monosaccharides via Disaccharidases
    • Protein digestion: Small peptides to Amino acids via Dipeptidases, carboxypeptidase, and aminopeptidase
    • Nucleic acid digestion: Nucleotides to Nucleosides via Nucleotidases; Nucleosides to Nitrogenous bases, sugars, phosphates via Nucleosidases and phosphatases

Pancreatic Secretions

  • The pancreas produces proteases trypsin and chymotrypsin that are activated in the lumen of the duodenum.
  • Its solution is alkaline and neutralizes the acidic chyme.

Bile Production by the Liver

  • In the small intestine, bile aids in digestion and absorption of fats
  • Bile is made in the liver and stored in the gallbladder
  • Bile also destroys nonfunctional red blood cells

Secretions of the Small Intestine

  • The epithelial lining of the duodenum produces several digestive enzymes
  • Enzymatic digestion is completed as peristalsis moves the chyme and digestive juices along the small intestine
  • Most digestion occurs in the duodenum; the jejunum and ileum function mainly in absorption of nutrients and water

Absorption in the Small Intestine

  • The small intestine has a huge surface area, due to villi and microvilli that are exposed to the intestinal lumen
  • The enormous microvillar surface creates a brush border that greatly increases the rate of nutrient absorption
  • Transport across the epithelial cells can be passive or active depending on the nutrient

Nutrient Absorption in the Small Intestine

  • Villi
  • Microvilli (brush border) at apical (lumenal) surface
  • Epithelial cells

Absorption in the Large Intestine

  • The colon of the large intestine is connected to the small intestine
  • The cecum aids in the fermentation of plant material and connects where the small and large intestines meet
  • The human cecum has an extension called the appendix, which plays a very minor role in immunity

Function of the Colon

  • A major function of the colon is to recover water that has entered the alimentary canal
  • The colon houses bacteria (e.g., Escherichia coli) that live on unabsorbed organic material; some produce vitamins
  • Feces, including undigested material and bacteria, become more solid as they move through the colon
  • Feces are stored in the rectum until they can be eliminated through the anus
  • Two sphincters between the rectum and anus control bowel movements

The Digestive System

  • Breaks Down Food to Small Molecules for Absorption
  • The large intestine (aka colon)
    • Reabsorbs water
    • Consolidates food particles into feces
  • Rabbit cecotropes Rabbit feces