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Digestion

Subject: Biology
Topic: 7
Cambridge Code: 0610 / 0970 / 5090

Digestive System​

Digestion - Breakdown of food into particles small enough to absorb

Two Types​

Mechanical digestion: Physical breakdown

  • Chewing, churning
  • Increases surface area

Chemical digestion: Enzyme breakdown

  • Breaks chemical bonds
  • Produces smaller molecules

Enzymes in Digestion​

Carbohydrase​

Breaks down carbohydrates

Amylase: Starch → Maltose

  • Location: Saliva, pancreas
  • Optimum pH: Neutral (6.5)

Maltase: Maltose → Glucose

  • Location: Small intestine
  • Optimum: Neutral

Protease​

Breaks down proteins

Pepsin: Protein → Peptides (in stomach)

  • Location: Stomach
  • Optimum pH: Acidic (1.5-2)

Trypsin: Peptides → Amino acids

  • Location: Pancreas into small intestine
  • Optimum pH: Alkaline (8)

Lipase​

Breaks down fats

Lipase: Lipids → Fatty acids + Glycerol

  • Location: Pancreas
  • Optimum pH: Neutral

Path Through Digestive System​

Mouth​

  • Mechanical: Teeth chew food
  • Chemical: Salivary amylase
  • Result: Food bolus formed

Oesophagus​

  • Muscular tube
  • Peristalsis (muscle contractions) pushes food
  • ~25 seconds to stomach

Stomach​

  • Mechanical: Churning breaks food
  • Chemical: Pepsin breaks proteins
  • Acid: HCl environment (pH 1.5-2)
  • Time: 2-4 hours
  • Result: Chyme (semi-liquid mixture)

Small Intestine​

Location of maximum digestion and absorption

  • Duodenum: First part

    • Receives pancreatic juices (trypsin, lipase)
    • Receives bile (emulsifies fats)
    • pH raised by bicarbonate

  • Jejunum/Ileum: Remaining small intestine

    • Absorption of nutrients
    • Villi increase surface area

Large Intestine​

  • Absorption of water
  • Formation of feces
  • Bacterial action (vitamin K production)

Rectum and Anus​

  • Temporary storage
  • Defecation

Absorption in Small Intestine​

Intestinal Villi​

Villi features:

  • Single cell lining
  • Microvilli on epithelial cells
  • Capillary networks inside
  • Lacteal (lymphatic vessel) inside

Function: Maximize absorption surface area

Glucose and Amino Acids​

  • Method: Active transport (requires energy)
  • Direction: Into epithelial cell then blood
  • Carried by: Blood to liver

Fatty Acids and Glycerol​

  • Method: Diffusion then resynthesis
  • In cell: Recombined to triglycerides
  • Transport: Via lacteals in lymphatic system

Water​

  • Method: Osmosis
  • Direction: Into blood (absorbed in small and large intestine)

Mineral Ions​

  • Method: Active transport
  • Carried by: Blood

Role of Bile​

Bile - Produced by liver, stored in gallbladder

Functions​

  • Emulsifies fats - Breaks into small droplets
  • Increases surface area - More accessible to lipase
  • Aids absorption - Fat solubility

Salts: Reduce surface tension Cholesterol: Component

Role of Pancreas​

Pancreatic enzymes secreted into duodenum:

  • Trypsin - Protein digestion
  • Amylase - Starch digestion
  • Lipase - Fat digestion

Bicarbonate - Neutralizes stomach acid

Control of Digestion​

Nervous Control​

Reflex actions:

  • Swallowing
  • Peristalsis
  • Secretions triggered by food

Hormonal Control​

Gastrin:

  • Released in stomach
  • Stimulates: Stomach acid, pepsin

Secretin:

  • Released in duodenum (by acid)
  • Stimulates: Pancreatic bicarbonate

Cholecystokinin (CCK):

  • Released by duodenum (by lipids, peptides)
  • Stimulates: Gallbladder, pancreatic enzymes

Key Points​

  1. Mechanical digestion increases surface area
  2. Chemical digestion by enzymes
  3. Different enzymes in different locations
  4. Optimum pH varies by enzyme
  5. Maximum absorption in small intestine
  6. Villi adapt structure to function
  7. Active transport requires energy

Practice Questions​

  1. Describe path of carbohydrate through digestive system
  2. Explain action of amylase
  3. Compare digestion in stomach vs small intestine
  4. Describe villi structure and function
  5. Explain role of bile
  6. Identify enzyme optimum conditions

Revision Tips​

  • Learn enzyme locations and substrates
  • Know optimum pH for each enzyme
  • Understand peristalsis
  • Know absorption methods
  • Relate structure to function of villi
  • Learn hormonal controls