Digestion Lecture Notes

Digestion

Types of Digestion

• Intracellular Digestion:
  • Digestion occurs inside the cell.
  • Involves phagocytosis, where the cell engulfs food.
  • Example: Single-celled organisms like paramecium and amoeba.
• Extracellular Digestion:
  • Digestion occurs outside the cells.
  • Food enters a tube and exits from the other end.
  • Example: Most animals, including humans (digestive tract).

Animal Feeding Methods

• Filter Feeders:
  • Aquatic animals that use a filter basket-like structure to gather small organisms suspended in the water.
  • They siphon water into their mouth and then filter it to obtain small organisms for digestion.
  • Examples: Flamingoes, tube worms, clams, barnacles, and baleen whales.
• Fluid Feeders:
  • Obtain food by sucking or licking nutrient-rich fluids from live plants or animals.
  • Their mouthparts are adapted to pierce or rip skin or leaf tissue to suck or lick blood or sap.
  • Examples: Mosquitoes, ticks, aphids, spiders, bees, butterflies, vampire bats, and hummingbirds.
• Substrate Feeders:
  • Live in or on their food source and eat their way through it.
  • Examples: Caterpillars and earthworms; caterpillars eat through the green tissues of leaves.
• Bulk Feeders:
  • Ingest fairly large pieces of food; some swallow their food whole.
  • Use tentacles, pincers, claws, fangs, jaws, or teeth to kill prey or tear off pieces of meat or vegetation.

Mechanical Digestion

• Physical breakdown of large food particles into smaller ones.
• Achieved through chewing, mashing, chopping, breaking food into smaller pieces.
• Increases the surface area of the food, allowing more enzymes to come into contact with it.

Chemical Digestion

• Chemical breakdown of large molecules into smaller ones.
  • Carbohydrates: Polysaccharides (starch) are broken down into monosaccharides (simple sugars).
  • Proteins: Polypeptides are broken down into amino acids.
  • Lipids: Triglycerides are broken down into fatty acids and glycerol.
  • Nucleic acids: DNA is broken down into nucleotides.
• Food particles are broken down by enzymes, each with an optimal pH.

Human Digestive System

• Consists of the mouth, esophagus, stomach, small intestine, and large intestine.
• The liver, pancreas, and gallbladder also contribute to the digestive process.
• Digestion is the process by which food and drink are broken down into small molecules of nutrients that can be absorbed into the blood and distributed to the body for energy, cell growth, and repair.
• Accomplished through both mechanical and chemical processes.

Organs and Their Functions

• Stomach:
  • Secretes enzymes and hydrochloric acid (HCl) to break down protein.
  • Empties all food into the small intestine within 2-6 hours.
• Small Intestine:
  • Over 90% of digestion and absorption take place here.
  • Neutralizes stomach acid and mixes food with enzymes, bile, and pancreatic juices.
• Liver:
  • Aids in digestion and detoxification of food impurities.
  • Inspects nutrients before allowing them into the bloodstream.
• Gall Bladder:
  • Stores bile used to break down dietary fat.
• Pancreas:
  • Produces digestive juices and helps control blood sugar.

Additional Facts

• Digestive problems cost Americans 50 billion each year.
• 70-year-olds may produce as little as half the enzymes they produced when they were 20.
• By age 50, many people will produce only 15% of the HCL they released at age 25.
• About a third of all people over the age of 65 secrete almost no stomach acid.

Introduction to the Digestive System

• The digestive system breaks food into nutrients, which pass into the circulatory system for distribution throughout the body.
• Four stages of food processing:
  • Ingestion: Taking in food.
  • Digestion: Breaking down food into nutrients.
  • Absorption: Taking in nutrients by cells.
  • Egestion: Removing any leftover wastes.

Physical Digestion - Oral Cavity

• Begins when food enters the mouth (oral cavity).
• Physically broken down by the teeth (mechanical digestion).
• Chemically broken down by enzymes released from the salivary glands.

Human Digestive System - Bolus Formation and Movement

• The tongue moves the food around until it forms a ball called a bolus.
• The bolus passes to the pharynx (throat).
• The epiglottis ensures the bolus enters the esophagus and not the trachea.

Peristalsis

• The bolus passes down the esophagus via peristalsis.

• Peristalsis: a wave of muscular contractions that push the bolus down towards the stomach.

• To enter the stomach, the bolus must pass through the lower esophageal sphincter, a bundle of muscles that keeps stomach acid out of the esophagus.

The Stomach

• The stomach has folds called rugae and is a muscular pouch that churns the bolus (physical digestion).
• The bolus mixes with gastric juice, a mixture of stomach acid and enzymes (chemical digestion).
• Some absorption occurs in the stomach:
  • Certain medicines (e.g., aspirin), water, and alcohol are absorbed.
• The digested bolus is now called chyme and leaves the stomach through the pyloric sphincter.

Small Intestine

• Food enters the small intestine.
• The majority of absorption occurs here.
• The liver and pancreas aid the small intestine to maximize absorption.
• The small intestine has three parts:
  • Duodenum:
    • Bile enters through the bile duct to break down fats.
    • The pancreas secretes pancreatic juice to reduce the acidity of the chyme.
  • Jejunum:
    • The majority of absorption takes place here.
    • Lined with villi (finger-like projections) to increase the surface area for absorption.
    • Each villus has microvilli (tiny projections) to further increase the surface area for absorption.
  • Ileum:
    • Compacts the leftovers to pass through the cecum into the large intestine.

Large Intestine (Colon)

• Absorbs water from the waste material.
• Produces vitamin K and some B vitamins using helpful bacteria.

Waste Elimination

• Leftover waste is compacted and stored in the rectum.
• When full, the anal sphincter loosens, and waste (feces) passes out of the body through the anus.

Chemical Digestion

• Food cannot be sufficiently broken down by physical digestion alone.
• Special enzymes chemically break down food into smaller nutrients.

Enzymes

• Special proteins made at ribosomes.
• Classes of enzymes:
  • Carbohydrases: Break down carbohydrates.
  • Proteinases: Break down proteins.
  • Lipases: Break down lipids.
  • Nucleases: Break down nucleic acids.
• Secreted into the digestive system by secretory cells called glands, connected through ducts.
• The substrate is the item that an enzyme breaks down; the enzyme remains unchanged.
• Optimal conditions for enzyme function depend on temperature, pH, and the presence/absence of certain ions, vitamins, and minerals.

Enzyme Activity in Different Locations

• In the Mouth:
  • Three salivary glands produce saliva containing:
    • Water and mucus: lubricates the food.
    • Sodium bicarbonate: reduces the acidity of the bolus.
    • Salivary amylase: enzyme that begins to break down carbohydrates.
• In the Stomach:
  • Hydrochloric acid (HCl) kills invading bacteria and viruses.
  • Pepsin breaks down proteins into shorter amino acid chains.
  • Lipase breaks down lipids.
  • Mucus protects the stomach lining from acid.
• In the Pancreas:
  • Releases enzymes into the small intestine that act on proteins, carbohydrates, and fats.
  • Releases bicarbonate solution to raise the pH of the chyme.
  • Releases insulin into the bloodstream, converting glucose to glycogen for storage.
  • Releases glucagon to raise blood sugar levels.
  • Enzymes include:
    • Pancreatic amylase: breaks down carbohydrates.
    • Trypsin: breaks down proteins.
    • Lipase: breaks down fats.
    • Nucleases: breaks down nucleic acids.
• The Liver:
  • Makes bile, which aids in fat digestion.
  • Stores most of the glycogen.
  • Stores vitamins A, D, E, and K.
  • Detoxifies poisons, including ethanol.
  • Cirrhosis results from the breakdown of liver cells due to high levels of poisons.
• The Gall Bladder:
  • Stores bile produced in the liver.
  • Releases bile into the small intestine.
  • Illnesses:
    • Gallstones: crystals of bile salts around cholesterol.
    • Jaundice: collection of bile pigment in the blood.

Digestion and Homeostasis

• A large meal activates receptors to churn the stomach and empty it faster.
• High-fat meals slow digestion, allowing time for fat breakdown, leading to a feeling of fullness.
• The endocrine, nervous, digestive, and circulatory systems work together to control digestion.
• Smelling food releases saliva and gastrin to prepare the body for digestion.

Hormones

• Chemical regulators that control digestive enzymes.
  • Gastrin: stimulates digestive glands around the stomach.
  • Secretin: stimulates the pancreas to release enzymes that neutralize stomach contents in the small intestine.
  • Motilin: in the duodenum, stimulates pepsin production.
  • Cholecystokinin (CCK): stimulates the release of gastric juice.

Some Hormones That Regulate Digestion

Major Digestive Enzymes