Physical digestion

Physical digestion breaks food into smaller pieces without chemically changing the molecules it contains. The smaller pieces expose more of the food’s surface area, giving enzymes more surface to act on during chemical digestion.

In the mouth, chewing breaks food into smaller pieces. In the stomach, contractions of the muscular wall churn the food, mixing it and physically breaking it up further.

Types of human teeth

Human teeth break food into smaller pieces during physical digestion. The visible part of a tooth, called the crown, has a shape suited to the way the tooth handles food.

• Incisors have relatively thin, sharp edges. They bite into and cut food.
• Canines have pointed crowns. They grip and tear food.
• Premolars have broader chewing surfaces. They crush and grind food.
• Molars are larger than premolars and have broad surfaces that grind food into smaller pieces.

Identifying teeth in diagrams and images

Look at the shape of the crown: a thin cutting edge indicates an incisor, while a pointed crown indicates a canine. Premolars and molars have broader chewing surfaces; molars are generally the larger of the two.

Emulsification of fats by bile

Bile disperses large droplets of fats and oils into many smaller droplets. This process is called emulsification.

Emulsification is a physical change: bile changes the size of the droplets but does not chemically break down the fat molecules. The same amount of fat is spread across more droplets, giving it a greater total surface area.

The greater surface area exposes more fat to digestive enzymes. These enzymes can then act on the fat during chemical digestion.