Chapter 50: Nutrition, Digestion, and Absorption Notes

Chapter 50: Nutrition, Digestion, and Absorption

What Living Forms Require from Food?

  • Energy needs are met by ingestion, digestion, and assimilation of food.
  • Heterotrophs obtain nutrition by consuming other organisms and depend on autotrophs.
  • Autotrophs synthesize their own food using solar or chemical energy.
  • Food consists of atoms and bonds forming biological macromolecules (carbohydrates, fats, proteins, nucleic acids), which store energy.
  • The digestive system converts food into energy and other necessary molecules at the cellular level.

Food Storage Between Meals

  • Carbohydrates are stored as glycogen in the liver and muscle cells.
  • Fats store more energy per gram and require little water, making them compact for storage. They are primarily stored beneath the skin and around major organs.
  • Proteins are not typically used for storage but can be metabolized as a last resort.

Effects of Insufficient Food Intake

  • If an animal consumes too little food, the body first uses glycogen and fat reserves.
  • If energy reserves are depleted, proteins are metabolized, starting with blood plasma proteins.
  • Kwashiorkor syndrome results from malnutrition (lack of protein in the diet), causing the body to break down blood proteins and muscle tissue for fuel.
  • This condition affects fluid distribution, leading to edema (swelling).

Effects of Excess Food Intake

  • If an animal consumes more food than needed, the excess is stored as glycogen and body fat, increasing body mass.
  • Amino acids, the building blocks of proteins, include essential amino acids that cannot be synthesized by the animal and must be obtained from the diet (8 essential amino acids for humans).

Essential Nutrients

  • Humans require essential fatty acids, macronutrients, micronutrients, and vitamins from their diet.
  • Macronutrients: elements required in large amounts (e.g., sodium, potassium, calcium).

Micronutrients

  • Micronutrients: elements required in tiny amounts (e.g., iron, zinc).
Table 51.1: Mineral Elements Required by Animals (Part 2)
  • Chromium (Cr)
    • Source: Meat, dairy, whole grains, legumes, yeast
    • Function: Glucose metabolism
  • Cobalt (Co)
    • Source: Meat, tap water
    • Function: Found in vitamin B₁₂, formation of red blood cells
  • Copper (Cu)
    • Source: Liver, meat, fish, shellfish, legumes, whole grains, nuts
    • Function: Found in active site of many redox enzymes and electron carriers; production of hemoglobin; bone formation
  • Fluorine (F)
    • Source: Most water supplies
    • Function: Found in teeth; helps prevent tooth decay.
  • Iodine (I)
    • Source: Fish, shellfish, iodized salt
    • Function: Found in thyroid hormones
  • Iron (Fe)
    • Source: Liver, meat, green vegetables, eggs, whole grains, legumes, nuts
    • Function: Found in active sites of many redox enzymes and electron carriers, hemoglobin, and myoglobin
  • Manganese (Mn)
    • Source: Organ meats, whole grains, legumes, nuts, tea, coffee
    • Function: Activates many enzymes
  • Molybdenum (Mo)
    • Source: Organ meats, dairy, whole grains, green vegetables, legumes
    • Function: Found in some enzymes
  • Selenium (Se)
    • Source: Meat, seafood, whole grains, eggs, milk, garlic
    • Function: Fat metabolism
  • Zinc (Zn)
    • Source: Liver, fish, shellfish, and many other foods
    • Function: Found in some enzymes and some transcription factors; insulin physiology

Vitamins

  • Humans require 13 vitamins, which are carbon compounds needed for growth and metabolism.
  • Most vitamins act as coenzymes, aiding enzymes in catalyzing reactions.
  • Vitamins can be water-soluble (excess is eliminated in urine, e.g., vitamin C) or fat-soluble (can accumulate to toxic levels in body fat and liver).

The Digestive System

  • The digestive system mechanically (smashing) and chemically (enzymes) reduces food from polymers to monomers: fatty acids, sugars, amino acids, nucleotides.
  • It synthesizes its own polymers based on the body’s needs (e.g., more actin or myosin for building bigger muscles).

Human Digestion

  • Digestion occurs through hollow organs that connect to form the gastrointestinal tract (GI).
  • The GI is a long (9 m), continuous, muscular digestive tube from the mouth to the anus, protected by an epithelial layer for selective passage of materials.

Gastrointestinal (GI) Tract Structure

  • In vertebrates, the GI tract has a layered plan:
    • Lumen: Gut cavity.
    • Mucosal epithelium: Secretes mucus, digestive enzymes, and hormones; contains microvilli for nutrient absorption.
    • Submucosal layer: Contains lymph and blood vessels for nutrient transport, and nerves.
    • Smooth muscle layers:
      • Circular layer: Innermost cells constrict the gut.
      • Longitudinal layer: Outermost cells shorten the gut.
    • Nerves coordinate gut movement between smooth muscle layers.

Compartmentalization of the GI Tract

  • Different compartments in the GI are specialized in:
    • Mechanical and chemical digestion.
    • Absorption of nutrients.
    • Storing and eliminating wastes.
  • Includes the mouth, pharynx, esophagus, stomach, small and large intestines, and accessory digestive organs like teeth, tongue, gallbladder, salivary glands, liver, and pancreas.

Six Main Steps in Digestion

  1. Ingestion
  2. Propulsion
  3. Mechanical breakdown
  4. Chemical digestion
  5. Absorption
  6. Defecation

Swallowing and Peristalsis

  • Mechanical and chemical digestion begins in the mouth.
  • The tongue positions food for the teeth and pushes a bolus to the soft palate.
  • Salivary glands produce saliva that:
    • Holds food together.
    • Starts breaking down carbohydrates into sugars (amylase).
    • Kills bacteria (lysozyme).
    • Lubricates for swallowing.

Swallowing Mechanism

  • The tongue initiates swallowing (propulsion); food passes from the mouth into the esophagus.
  • Food is kept out of the trachea by the closed larynx and the epiglottis.
  • As food reaches the smooth muscle in the esophagus, the muscle contracts and pushes the food toward the stomach (peristalsis).
  • The lower esophageal sphincter prevents food from moving backward into the esophagus.
  • The pyloric sphincter controls the passage of food into the small intestine.

The Stomach

  • The stomach stores and breaks down ingested food (digestion).
  • Gastric glands in gastric pits have 3 types of secretory cells:
    • Chief cells: Secrete inactive pepsinogen, which is activated into pepsin in the acidity of the stomach to break down proteins.
    • Parietal cells: Secrete hydrochloric acid, maintaining the stomach pH below 1 (kills most bacteria in food).
    • Mucus-secreting cells: Located in the gastric glands and the gastric mucosa, protect the stomach.

Chyme and Small Intestine

  • Chyme, a mixture of gastric juice and partly digested food, passes to the bottom of the stomach when stomach walls contract.
  • The pyloric sphincter allows small chyme amounts to enter the small intestine.
  • The initial section of the small intestine is the duodenum, the site of most digestion.
  • The jejunum and ileum carry out most absorption.
  • Epithelial cells secrete enzymes to cleave peptides, disaccharides, and lipids.
  • The microvilli of the small intestine absorb nutrients and inorganic ions.

The Pancreas

  • The pancreas is an endocrine and exocrine gland located behind the stomach (e.g., pancreatic hormones regulate blood sugar levels).
  • The pancreas makes pancreatic juices (lipase, protease, amylase) to break down fats, proteins, and starches in the duodenum.
  • It also secretes bicarbonate to neutralize chyme in the intestine.

The Liver and Gallbladder

  • The liver synthesizes bile, which is essential for the digestion of lipids in the small intestine.
  • Bile produced in the liver flows through the hepatic duct to the duodenum and to the gallbladder (where bile is stored).
  • Fat in the duodenum causes the gallbladder to contract and release bile.
  • Bile salts emulsify fats and form micelles (small fat particles) that are digested by lipases into fatty acids and monoglycerides.

Absorption of Fats

  • Fatty acids and monoglycerides pass through the intestinal microvilli membranes.
  • In the cells, fats are re-formed into triglycerides, exported to lymphatic vessels, and then to the bloodstream.
  • Lipoproteins transport fats in the blood, from sites of synthesis to sites of storage and vice versa (chylomicrons are lipoproteins produced in the small intestine; other lipoproteins are produced in the liver).

Defecation

  • Contents of the small intestine pass into the large intestine, or colon.
  • The colon absorbs water and ions, and produces feces (indigestible material).
  • Too much water absorption in the colon leads to constipation; too little leads to diarrhea.