Physical Digestion | Biology for GCSE/IGCSE - Year 11 PDF Download

Physical Digestion

• Physical digestion, also known as mechanical digestion, involves breaking down food into smaller pieces without altering the food molecules chemically.
• This process increases the surface area of food, making it more accessible for enzymes during chemical digestion.
• Physical digestion primarily occurs through the chewing action of the teeth, the churning action of the stomach, and the emulsification of fats by bile in the duodenum.

Chewing (Mastication)

• Teeth: The teeth play a vital role in physical digestion by cutting, grinding, and crushing food into smaller pieces. This process is known as mastication. Incisors cut the food, canines tear it, and molars grind it into a fine paste.
• Saliva: Saliva, produced by salivary glands, helps moisten the food, making it easier to chew and swallow. It also contains the enzyme amylase, which begins the chemical digestion of carbohydrates, although this is a secondary function of saliva.

Churning in the Stomach

• Stomach Muscles: The stomach's muscular walls contract and relax in a process known as peristalsis, which churns the food. This mechanical action further breaks down food particles and mixes them with gastric juices, creating a semi-liquid mixture called chyme.
• Gastric Juices: While primarily involved in chemical digestion, the gastric juices, which include hydrochloric acid and digestive enzymes, also help in breaking down the food physically by softening it and killing bacteria.

Emulsification of Fats by Bile

• Bile: Produced by the liver and stored in the gallbladder, bile is released into the duodenum (the first part of the small intestine) during digestion. Bile contains bile salts, which act as detergents that break down large fat globules into smaller droplets in a process called emulsification.
• Emulsification: This process increases the surface area of fats, making them more accessible to the enzyme lipase, which breaks down fats into fatty acids and glycerol.

Importance of Physical Digestion

• Surface Area: By breaking food into smaller pieces, physical digestion significantly increases the surface area for enzymes to act upon during chemical digestion. This ensures that nutrients can be more efficiently and thoroughly digested and absorbed.
• Efficiency: Physical digestion helps in mixing food with digestive enzymes and acids, which promotes more efficient chemical digestion.
• Preparation: It prepares food for its journey through the digestive tract, making it easier to handle and process in subsequent stages of digestion.