Physical Digestion

• Physical digestion is the breakdown of food into smaller pieces without chemical change to the food molecules.
• Physical digestion increases the surface area of food for the action of enzymes in chemical digestion.
  • When large lumps of food are broken into many smaller pieces, the total surface area exposed to the outside increases significantly.
  • This provides a much larger area for digestive enzymes to collide with the food molecules, speeding up the rate of chemical digestion.

Teeth

Diagram of types of human teeth

Diagram of tooth structure

Types of human teeth

• Incisors
  • Shape: Chisel-shaped with a sharp, flat edge. Located at the front of the mouth.
  • Function: Used for biting, cutting, and slicing pieces off chunks of food.
• Canines
  • Shape: Pointed, dagger-like teeth situated next to the incisors.
  • Function: Used for tearing and gripping tough food.
• Premolars
  • Shape: Broad, flat top surface with two pointed ridges (cusps)
  • Function: Used for crushing and chewing food down into manageable pieces.
• Molars
  • Shape: Larger, broader flat top surface with four or five ridges (cusps). Located at the back of the mouth.
  • Function: Used for grinding and chewing food into a smooth paste before swallowing.

Tooth structure

• The enamel is the extremely hard, outer layer covering the crown (visible part) of the tooth. It contains calcium salts and is the hardest substance made by the human body. It protects the tooth from the mechanical wear and tear of chewing.
• The dentine (labeled as dentin) is the layer directly beneath the enamel. It forms the bulk of the tooth structure and is similar in texture to bone but slightly softer than enamel.
• The pulp cavity is the central living region of the tooth. It contains blood vessels (to deliver nutrients and oxygen to the tooth cells) and nerves (to detect pressure and temp. changes).
• The cement is a specialized bone-like substance covering the root of the tooth. Itt helps anchor the root firmly to the surrounding fibers within the jawbone.

Function of stomach in physical digestion

• When the mouth starts physical digestion, the stomach continues it mechanically through churning.
• The stomach wall contains three strong layers of smooth muscle oriented in different directions. These muscles periodically contract and relax, rhythmically squeezing, pounding, and mixing the food with gastric juices.
  • This mechanical action breaks the solid bolus down into a thick, semi-liquid mixture called chyme, maximizing its exposure to stomach acid and pepsin.

Role of bile in emulsification

• Fats and oils are hydrophobic and naturally clump together into large, dense globules when mixed with water in the watery environment of the duodenum.
  • This presents a major obstacle for the water-soluble enzyme lipase, only attacking the outer edges of these giant droplets.

The mechanical action of bile

• Bile is produced by the liver and stored in the gall bladder. It does not contain enzymes, so its action is entirely physical.
• When bile enters the duodenum, it breaks down these big, greasy fat globules into millions of tiny microscopic droplets.
• This physical process is called emulsification.
  • Emulsification does not break any chemical bonds within the lipids. Instead, it drastically increases the total surface area of the fats and oils, giving the enzyme lipase vastly more physical space to bind and rapidly digest lipids into fatty acid and glycerol.