Digestive Systems and Nutrition

Nutrients

• Energy in foods is measured in calories.
• Food serves as an essential source of calories.
• Classes of nutrients include:
  • Water: Most vital nutrient.
  • Carbohydrates: 4.1 calories per gram.
  • Lipids: 9.3 calories per gram (approx. twice as many calories as carbohydrates and proteins).
  • Proteins: 4.1 calories per gram.
  • Vitamins: At least 13 essential vitamins.
  • Minerals: Examples include calcium and phosphorus.
  • Trace elements: Minerals needed in very small amounts (e.g., iodine, zinc).

Digestion

• Definition: Breakdown of food into absorbable products to be transported by the bloodstream or lymphatic system.
• Carried out in a digestive tract/system.
• Heterotrophs: Organisms that cannot produce their own food, divided into:
  • Herbivores: Eat plants (e.g., cows, rabbits).
  • Carnivores: Eat meat (e.g., cats, frogs).
  • Omnivores: Eat both plants and meat (e.g., most humans, pigs).
• Autotrophs: Organisms that can make organic molecules used as food.

Types of Digestive Tracts/Systems

• Digestion Types:
  • Intracellular digestion: Occurs within a cell (observed in single-celled organisms and sponges).
  • Extracellular digestion: Takes place within a digestive cavity/tract.
• Incomplete digestive tracts: One opening (mouth) also called a two-way digestive tract.
• Complete digestive tracts: Two openings (mouth and anus) also called a one-way digestive tract.

Vertebrate Digestive Systems

• Composed of a tubular gastrointestinal tract and accessory digestive organs.
• Pathway of Food (Human):
  • Mouth: Teeth shred, tear, grind food (20 in children, 32 in adults).
  • Salivary glands: Produce saliva containing amylase that moistens food.
  • Pharynx: Passageway for swallowing food.
  • Epiglottis: Flap covering the glottis when swallowing.
  • Esophagus: Muscular tube connecting mouth to stomach.
  • Peristalsis: Rhythmic contraction moving contents in the esophagus and stomach.
  • Stomach: Saclike organ churning food with gastric juices (hydrochloric acid and pepsinogen).
  • Chyme: Mixture of partially digested food and gastric juice.
  • Pyloric sphincter: Controls flow of chyme into the small intestine.
  • Small Intestine: Primary location for digestion and absorption (averages 6 meters in length).
  • Three portions: Duodenum, Jejunum, Ileum.
  • Villi: Finger-like projections that increase surface area for absorption, each with microvilli.

Large Intestine (Colon)

• Absorbs water, salts, and some vitamins (no digestion).
• Stores undigestible material until it is eliminated as feces.
• Parts of the Large Intestine:
  • Cecum: Blind or dead end of the large intestine.
  • Colon: Joins the rectum.
  • Rectum: Expels waste (feces).
  • Anus: Opening for waste elimination with two anal sphincters (one involuntary, one voluntary).

Accessory Digestive Organs

• Salivary Glands: Secrete saliva to moisten food.
• Pancreas: Produces pancreatic juice containing enzymes (amylase, lipase, protease) and bicarbonate to neutralize acids.
• Liver: Produces bile to emulsify fats.
• Gallbladder: Stores bile from the liver and sends it to the duodenum via the common bile duct.