Chapter 4: Food and Digestion (2) Notes

Human Alimentary Canal: Structure and Function

  • The human alimentary canal consists of the mouth, oesophagus, stomach, small intestine (duodenum and ileum), large intestine (colon and rectum), pancreas, liver, and gall bladder.

Food Movement Through the Gut

  • Peristalsis: Wave-like muscle contractions that move food through the alimentary canal.

Digestive Enzymes

  • Amylase and maltase: Digest starch to glucose.
  • Proteases: Digest proteins to amino acids.
  • Lipases: Digest lipids to fatty acids and glycerol.

Bile

  • Produced by the liver and stored in the gall bladder.
  • Role: Emulsifies fats for digestion.

Small Intestine Adaptations for Absorption

  • Villi and microvilli: Increase surface area for absorption.

Human Nutrition: Five Processes

  • Ingestion: Intake of food (Mouth).
  • Digestion: Breaking down of large molecules into small molecules (Mouth, stomach, small intestine).
  • Absorption: Entrance of food into bloodstream (Mainly small intestine, some in stomach, large intestine).
  • Assimilation: Absorbed nutrients used in building new tissues (All over the body).
  • Egestion: Removal of undigested food (Anus).

Human Digestive System

  • Alimentary canal facilitates movement of food.
  • Mouth cavity: Ingestion and digestion.
  • Oesophagus: Transports food (bolus) to stomach via peristalsis.
  • Stomach: Secretes gastric juice; digestion.
  • Small intestine: Duodenum (digestion), ileum (absorption).
  • Large intestine: Stores feces temporarily; colon removes water and salts.
  • Liver: Produces bile.
  • Gall bladder: Stores bile temporarily.
  • Pancreas: Secretes pancreatic juice.

Alimentary Canal and Food Movement

  • Epithelium: Inner surface of the alimentary canal, produces mucus.
  • Functions of mucus:
    • Lubricates food passage and prevents wear.
    • Protects inner surface from digestive enzymes.
  • Circular and longitudinal muscles: Contract and relax alternately, causing peristalsis.
  • Functions of peristalsis:
    • Pushes food forward.
    • Mixes food with digestive juices.

Digestion: Breaking Down Food

  • Large, complex substances broken into smaller, simpler molecules for absorption.
    • Carbohydrates → simple sugars
    • Proteins → amino acids
    • Fats → fatty acids and glycerol
  • Vitamins and minerals: Absorbed without digestion.
  • Water: Absorbed directly.

Types of Digestion

  • Mechanical Digestion: Physical breakdown of food into smaller pieces without chemical change. Increases surface area for enzyme action.
    • Examples: Chewing, churning, emulsification by bile.
  • Chemical Digestion: Breakdown of large molecules into smaller molecules via digestive enzymes.
    • Carbohydrases: Carbohydrates into simple sugars.
    • Lipases: Fats into fatty acids and glycerol.
    • Proteases: Proteins into amino acids.

Digestion in Mouth

  • Site: Mouth cavity.
  • Mechanical: Chewing.
  • Chemical: Saliva (salivary amylase).
    • pH: 6.5-7.5 (neutral to slightly alkaline).
  • Action: Starch broken down into maltose by salivary amylase.
  • Functions of saliva:
    • Water: Moistens and softens food.
    • Mucus: Lubricates food for swallowing.
    • Salivary amylase: Starch → maltose.
  • Food mixed with saliva → bolus.

Digestion in Stomach

  • Food enters via cardiac sphincter, leaves via pyloric sphincter.
  • Food leaving the stomach: acidic chyme.
  • Mechanical: Churning.
  • Chemical: Gastric juice (protease).
    • pH: 2.
  • Action: Protease (pepsin) breaks down protein into peptides.
  • Function of Hydrochloric Acid:
    • Kills bacteria.
    • Provides optimum pH for pepsin.
    • Activates pepsin.
  • Stomach wall secretes mucus: Protects against protease and acid, lubricates surface.

Additional Information

  • Rennin: Protease in young children, coagulates milk proteins for digestion.
  • Peptic Ulcer: Erosion in GI tract due to weakened mucus coating, often caused by bacterial infection, treated with antibiotics.

Small Intestine: Digestion

  • Site for both digestion and absorption.
  • Duodenum:
    • Mechanical Digestion: Emulsification of fats into oil droplets by bile.
    • Bile:
      • Produced by liver, stored in gall bladder.
      • Contains no digestive enzymes.
      • Components:
        • Bile salts: Emulsify fats.
        • Sodium hydrogencarbonate (NaHCO3): Neutralizes acidic chyme, provides alkaline medium.
        • Bile pigment: Waste from haemoglobin breakdown, excreted in feces (color).
    • Chemical Digestion:
      • Pancreatic juice (pancreas):
        • Amylase (pancreatic amylase): Starch → maltose.
        • Protease (Trypsin): Proteins → peptides.
        • Lipase: Lipids → fatty acids and glycerol.

Additional Information:

  • Pancreas produces insulin and glucagon for blood glucose regulation.

Digestion in Small Intestine (cont.)

  • Intestinal juice (intestinal glands in duodenum wall):
    • Alkaline solution with digestive enzymes.
  • Mechanical: Emulsification (bile) - Fat → tiny oil droplets.
  • Chemical (Pancreatic juice):
    • Pancreatic amylase: starch → maltose.
    • Protease (Trypsin): proteins → peptides.
    • Lipase: lipids → glycerol and fatty acids.
  • Chemical (Intestinal juice):
    • Peptidases: peptides → amino acids.
    • Carbohydrases:
      • Maltase: maltose → glucose + glucose.
      • Sucrase: sucrose → glucose + fructose.
      • Lactase: lactose → glucose + galactose.
  • Ileum:
    • Intestinal glands secrete intestinal juice.
    • Digestion complete: All food substances broken down into simplest forms.
    • Food becomes watery fluid.
  • Lactose Intolerance: Shortage of lactase, inability to break down lactose, causing nausea, cramps, bloating, gas, diarrhea.

Summary of Digestion

  • Mouth (Salivary gland): Saliva (salivary amylase) - Starch → Maltose (pH 6.5-7.5).
  • Stomach (Gastric gland): Gastric juice (HCl, Pepsin) - Proteins → Peptides (pH 1-2).
  • Small Intestine (Liver): Bile (Bile salts, Alkaline salt) - Fat → Tiny oil droplets (pH 8).
  • Small Intestine (Pancreas): Pancreatic juice (Pancreatic Amylase, Lipase, Trypsin, Alkaline salt) - Starch → Maltose, Fats, Proteins → Peptides (pH 8).
  • Small Intestine (Intestinal glands): Intestinal juice (Maltase, Sucrase, Lactase, Peptidase) - Maltose → Glucose, Sucrose → Glucose + Fructose, Lactose → Glucose + Galactose, Peptides → Amino acid (pH 8).

Absorption

  • Occurs in stomach, small intestine, and colon.
  • Stomach:
    • Water
    • Alcohol
    • Little simple sugar
    • Minerals
    • Water-soluble vitamins
  • Small Intestine: Main site of absorption (water and nutrients) via diffusion, osmosis, active transport.
    • Inner lining folded to increase surface area.
    • Villi (finger-like projections) increase surface area.
    • Peristalsis brings food into contact with villi.
  • Colon: Absorbs remaining water and minerals.

Villi

  • Finger-like projections increase surface area.
  • Microvilli on surface cells further increase surface area.
  • One-cell thick epithelium shortens diffusion distance.
  • Each villus contains:
    • Blood capillaries: Transport simple sugars, minerals, and amino acids.
    • Lacteal: Glycerol and fatty acids recombine to form lipid, transported via lymph.

Structural Adaptations of Small Intestine

  • Very long - More time for absorption.
  • Numerous villi - Increase surface area for absorption.
  • Thin villi walls - Shorten diffusion distance.
  • Dense network of blood capillaries - Maintain steep diffusion gradient.
  • Epithelial cells with many mitochondria - Energy for active transport.

Absorption in the Small Intestine

  • Glucose, amino acids, salts, and water absorbed into epithelial cells.
  • Water-soluble nutrients transported via hepatic portal vein to liver.
  • Lipids absorbed into lacteals.

Assimilation

  • Liver plays a key role.

Liver Functions

  • Regulates blood glucose level (glucose → glycogen, glycogen → glucose).
  • Deaminates excess amino acids.
  • Stores iron and vitamins A & D.
  • Detoxifies toxins (e.g., alcohol).

Egestion

  • Removal of faeces (undigested food, bile pigment, cells, bacteria).
  • Faeces temporarily stored in rectum.
  • Defaecation when anal sphincter relaxes.

Blood Transport

  • Blood from lower body → hepatic vein → liver → vena cava → heart.
  • Blood from heart → aorta → hepatic artery → rest of body.
  • Hepatic portal vein transports blood from small intestine, colon, caecum, and appendix to the liver.