9.2 - Digestion

Types of Digestion

• Digestion breaks down large and complex pieces of food into smaller and simple pieces that can be dissolved for easy absorption.

Physical Digestion

• Mechanical breakdown of food into small particles.
• Involves chewing and peristalsis.

Chemical Digestion

• Decomposition of complex molecules into simple molecules.
• Involves enzyme reactions.

Digestion of Carbohydrates in the Mouth

1. Food stimulates salivary glands to secrete saliva.
2. Salivary amylase hydrolyses starch to maltose.

$\text{Starch}+\text{Water}\stackrel{\text{Salivary amylase}}{\to }\text{Maltose}$

3. Saliva at pH $6.5-7.5$ allows salivary amylase to act at its optimum.
4. Saliva helps food form a bolus for easier swallowing.
5. During swallowing, the epiglottis closes the trachea opening.
6. Peristalsis—the rhythmic contraction and relaxation of muscles along the alimentary canal—pushes the bolus through the oesophagus into the stomach.

Digestion of Proteins in the Stomach

• Gastric glands contain:
  • Chief cells—secrete pepsinogen.
  • Parietal cells—secrete hydrochloric acid.
  • Mucous cells—secrete mucus.
• Hydrochloric acid activates inactive pepsinogen into pepsin.
• Pepsin hydrolyses proteins into polypeptides.

$\text{Protein}+\text{Water}\stackrel{\text{Pepsin}}{\to }\text{Polypeptides}$

• Hydrochloric acid:
  • Provides pH $1.5-2.0$ for pepsin action.
  • Stops salivary amylase activity.
  • Kills bacteria in food.
• Mucus protects the stomach wall from hydrochloric acid and digestive enzymes.
• Peristalsis churns food with gastric juice for a few hours to form semi-fluid chyme.
• Chyme enters the duodenum slowly when the sphincter muscle relaxes.

Digestion in the Small Intestine

• The small intestine consists of the duodenum, jejunum and ileum.
• The duodenum receives chyme, bile and pancreatic juice.

Bile

• The liver produces bile; the gallbladder stores it.
• Bile enters the duodenum through the bile duct.
• Bile neutralises acidic chyme, provides an alkaline condition at pH $7.6-8.6$, and emulsifies lipids into tiny droplets to increase surface area for lipase activity.

Pancreatic Juice

• The pancreas secretes pancreatic amylase, trypsin and lipase into the duodenum through the pancreatic duct.

$\text{Starch}+\text{Water}\stackrel{\text{Pancreatic amylase}}{\to }\text{Maltose}$

$\text{Polypeptides}+\text{Water}\stackrel{\text{Trypsin}}{\to }\text{Shorter peptides}$

$\text{Lipids}+\text{Water}\stackrel{\text{Lipase}}{\to }\text{Fatty acids}+\text{Glycerol}$

Intestinal Juice

• Ileum glands secrete mucus and intestinal juice containing maltase, sucrase, lactase, lipase and erepsin.
• The alkaline medium allows the enzymes to act at their optimum.

$\text{Maltose}+\text{Water}\stackrel{\text{Maltase}}{\to }\text{Glucose}$

$\text{Sucrose}+\text{Water}\stackrel{\text{Sucrase}}{\to }\text{Glucose}+\text{Fructose}$

$\text{Lactose}+\text{Water}\stackrel{\text{Lactase}}{\to }\text{Glucose}+\text{Galactose}$

$\text{Peptides}+\text{Water}\stackrel{\text{Erepsin}}{\to }\text{Amino acids}$

$\text{Lipids}+\text{Water}\stackrel{\text{Lipase}}{\to }\text{Fatty acids}+\text{Glycerol}$

Experiments on Food Digestion

Starch

• Hypothesis: Amylase hydrolyses starch to a reducing sugar.
• Set-up: Compare starch + amylase with starch + distilled water; incubate at $37{}^{\circ }\text{C}$ for 10 minutes.
• Tests: Iodine solution detects starch; Benedict’s solution with heat detects reducing sugar.
• Conclusion: Amylase hydrolyses starch to a reducing sugar.

Protein

• Hypothesis: Pepsin hydrolyses protein in albumen suspension into polypeptides.
• Set-up: Compare albumen + dilute hydrochloric acid + pepsin with albumen + dilute hydrochloric acid + distilled water; incubate at $37{}^{\circ }\text{C}$ for 20 minutes.
• Observation: The mixture containing pepsin becomes clearer.
• Conclusion: Pepsin hydrolyses protein into polypeptides in an acidic medium.

Lipid

• Hypothesis: Lipase hydrolyses lipids into fatty acids and glycerol.
• Set-up: Add cooking oil, sodium carbonate, dishwashing liquid and phenolphthalein to two test tubes; add lipase to one and distilled water to the control; incubate at $37{}^{\circ }\text{C}$.
• Observation: The lipase mixture changes from pink to colourless as fatty acids lower its pH.
• Conclusion: Lipase hydrolyses lipids into fatty acids and glycerol.