Physical DigestionCambridge IGCSE Biology: Revision notes

Section 1

What Is Physical Digestion?

Physical digestion is the breakdown of food into smaller pieces without any chemical change to the food molecules themselves.

No new substances are formed - the food is simply broken into smaller physical pieces
This is different from chemical digestion, where enzymes break large molecules into smaller, different molecules

Key termsphysical digestion

Common mistake

Physical digestion does not produce new molecules - chewing a starch-rich food into smaller pieces is physical digestion; amylase turning starch into maltose is chemical digestion.

Section 2

Why Does Physical Digestion Matter? Surface Area

Physical digestion increases the surface area of food.

Breaking food into smaller pieces increases its total surface area relative to volume
A larger surface area allows digestive enzymes to act on the food faster during chemical digestion
This makes chemical digestion more efficient and complete

Key termssurface area

Think of it like this

Think of it like crushing ice - one big block melts slowly, but the same ice crushed into small pieces melts much faster because more surface is exposed.

Section 3

Teeth and Physical Digestion

Different types of teeth carry out physical digestion (chewing) in different ways:

Incisors - sharp, chisel-shaped, used for biting and cutting food
Canines - pointed, used for tearing food
Premolars and molars - broad, ridged surfaces, used for crushing and grinding food

Together, chewing breaks food into small pieces and mixes it with saliva, increasing surface area for enzymes.

Section 4

The Stomach's Role in Physical Digestion

The stomach carries out physical digestion by churning food.

Muscular walls of the stomach contract and relax, mixing food with gastric juice
This physically breaks food into smaller pieces and forms a semi-liquid mixture (chyme)
Mixing increases contact between food and digestive enzymes

Section 5

Bile and Emulsification

Bile plays a physical (not chemical) role in fat digestion.

Bile emulsifies fats and oils, breaking large fat droplets into many smaller droplets
This greatly increases the surface area of fat exposed to lipase
Emulsification is a physical process - bile does not chemically break down the fat molecules themselves; that is done later by lipase

Key termsemulsificationbile

Exam tip

Bile emulsifying fat is a classic 'increases surface area' exam answer - remember it is physical digestion, not chemical.

Must Know

Physical digestion breaks food into smaller pieces with no chemical change
It increases surface area so enzymes can act faster in chemical digestion
Teeth (incisors, canines, premolars, molars) physically break down food by cutting, tearing, crushing and grinding
The stomach churns food, mixing it with gastric juice
Bile emulsifies fats and oils into smaller droplets, increasing surface area for lipase

That's the notes covered.

Carry on to the next subtopic.