Human Digestion

• Digestion is the process of breaking down complex organic food substances into simpler inorganic ones.

Types of Digestion

• Mechanical digestion involves the physical breakdown of food to increase surface area.
• Examples of mechanical digestion include using teeth, tongue, and stomach muscles.
• Chemical digestion uses enzymes, water, and acids to chemically change and break down food.

Mouth

• Teeth mechanically digest food.
• The tongue rolls food into a bolus and pushes it to the pharynx.
• Salivary amylase chemically digests food, breaking down starch into maltose.
• Salivary amylase functions at an alkaline to neutral pH (7.1-7.2).
• Saliva consists of water, salivary amylase, and mucus.
• The epiglottis prevents food from entering the trachea.

Oesophagus

• Peristalsis, wave-like muscle contractions, moves food from the mouth to the stomach.
• No enzymes are present, so no chemical digestion occurs here.

Stomach

• Mechanical digestion occurs via churning of the stomach's muscular walls.
• Gastric juice chemically digests food.
• Gastric juice includes hydrochloric acid (HCl), mucus, pepsin (pH 2-3), and rennin.
• Pepsin breaks down large proteins into polypeptides.
• Rennin clots milk in babies by converting casein into caseinogen.

Hydrochloric Acid (HCl) Functions

• Stops salivary amylase from working by denaturing it.
• Destroys harmful microorganisms.
• Creates an acidic environment for pepsin and rennin to work efficiently.
• Mucus traps microorganisms.
• The pyloric sphincter regulates the movement of food out of the stomach.

Pancreas

• Produces pancreatic juice containing trypsin, amylase, and lipase.
• Pancreatic juice is secreted into the duodenum and has an alkaline pH.

Liver

• Produces bile for the emulsification of fats.
• Bile breaks fats into tiny droplets by lowering surface tension.

Gall Bladder

• Stores and concentrates bile produced by the liver.

Small Intestine

Duodenum

• Secretes intestinal juice with maltase, peptidase, and lipase.

Intestinal Enzymes

• Maltase breaks down maltose into glucose.
• Peptidase breaks down peptides into amino acids.
• Lipase breaks down fats into fatty acids and glycerol.
• Receives pancreatic juice and bile.

Pancreatic Enzymes

• Trypsin breaks down peptones into peptides.
• Amylase continues starch digestion into maltose.

Bile From Gall Bladder

• Neutralizes stomach acid and emulsifies fats to increase surface area for lipase.
• Alkaline conditions are present.

Ileum

• Some intestinal juice is secreted here.
• Absorbs most nutrients, including glucose, amino acids, fatty acids, glycerol, vitamins, and minerals.
• Villi increase the surface area for nutrient absorption.

Large Intestine

Colon

• No enzymes are produced.
• Absorbs large amounts of water from undigested food.

Rectum

• Faeces are formed and stored until defecation.

Villi

• Finger-like projections in the ileum increase the surface area for nutrient absorption.
• More villi enable maximum absorption.

Digestion Summary

• Starch is broken down by salivary amylase into maltose, then by maltase into glucose.
• Proteins are broken down by pepsin into polypeptides, then by proteases (trypsin) into amino acids.
• Fats are broken down by lipase into fatty acids and glycerol.

End Products of Digestion

• Glucose results from starch breakdown.
• Amino acids result from protein breakdown.
• Fatty acids and glycerol result from fat breakdown.
• Glucose, amino acids, and water-soluble vitamins are absorbed into blood capillaries in the villi.
• Fatty acids, glycerol, and fat-soluble vitamins are absorbed into the lacteal in the villi.

Adaptations of Villi

• Abundant villi increase the surface area for absorption.
• Thin epithelial walls allow easy diffusion of substances.
• A large network of blood capillaries allows maximum absorption and transport.
• Microvilli further increase the surface area.
• The long ileum allows maximum nutrient absorption.

Assimilation and Deamination

• Assimilation is the movement of nutrients into cells for use.
• Deamination is removing the amino group (N) from proteins.

Chemical Structures

• Proteins contain carbon, hydrogen, oxygen, and nitrogen (CHON).
• Carbohydrates contain carbon, hydrogen, and oxygen (CHO).

After Absorption

• Fats are carried by lymph through lymph vessels to the blood.
• Fats are used for energy or stored for insulation.
• Glucose is oxidized to release energy.
• Excess glucose is converted into glycogen and stored in the liver and muscles. Amino acids are used for growth, repair, and maintenance.
• Excess amino acids are deaminated in the liver, converting them into carbohydrates.
• Nitrogen is converted to urea and excreted as urine by the kidneys.