Digestion in Mammals

HSC Biology | Free Study Notes

In this lesson

• what physical digestion is

• what chemical digestion is

• how absorption happens

• how elimination happens

• why digestion is important in mammals

Why mammals need digestion

Mammals are heterotrophs, which means they cannot make their own food and must obtain organic nutrients from other organisms.

Food often contains large, complex molecules that cannot pass straight through the wall of the digestive system. Digestion breaks food down into smaller molecules so they can be absorbed and used by the body.

Physical digestion

Physical digestion is the mechanical breakdown of food into smaller pieces.

What physical digestion does

Physical digestion:

• increases the surface area of food

• makes it easier for enzymes to act

• helps move food through the digestive system

Where it happens

Physical digestion begins in the mouth, where teeth cut, tear and grind food.

It also occurs in the stomach, where muscular churning mixes food with digestive juices.

Why it matters

Physical digestion does not change the chemical composition of food, but it makes chemical digestion more efficient.

Chemical digestion

Chemical digestion is the breakdown of large food molecules into smaller soluble molecules by enzymes.

What chemical digestion does

Chemical digestion changes the chemical structure of food molecules so they can be absorbed into the blood or lymph.

Examples of chemical digestion

• carbohydrates are broken down into simple sugars

• proteins are broken down into amino acids

• lipids are broken down into fatty acids and glycerol

Where it happens

Chemical digestion begins in the mouth with enzymes in saliva.

It continues in the stomach and especially in the small intestine, where many digestive enzymes act.

Sequence of digestion in mammals

Mouth

In the mouth:

• food is physically broken down by chewing

• saliva begins chemical digestion

• food is formed into a bolus for swallowing

Oesophagus

The oesophagus moves food to the stomach by muscular contractions called peristalsis.

Stomach

In the stomach:

• food is churned physically

• chemical digestion continues, especially of proteins

• food is mixed into a semi-liquid form

Small intestine

The small intestine is the main site of:

• chemical digestion

• absorption of nutrients

Enzymes continue breaking down food into small soluble molecules.

Large intestine

The large intestine mainly absorbs:

• water

• some mineral ions

This helps form solid waste.

Absorption

Absorption is the movement of digested nutrients, minerals and water from the digestive system into the body.

Absorption in the small intestine

The small intestine is adapted for absorption because it has:

• a large surface area

• a thin lining

• a rich blood supply

Digested nutrients such as glucose and amino acids are absorbed here.

Absorption in the large intestine

The large intestine absorbs:

• water

• some mineral ions

This is important for maintaining water balance and reducing water loss.

Why absorption matters

Without absorption, even fully digested food would be useless because the body could not take the substances into its transport system.

Elimination

Elimination is the removal of undigested and unabsorbed material from the body as solid waste.

What is eliminated

After digestion and absorption:

• some food remains undigested

• some substances are not absorbed

• these materials move into the rectum and are removed through the anus

Why elimination matters

Elimination removes material the body cannot use and prevents waste from building up in the digestive tract.

Digestion compared

Process	Main idea	Example
Physical digestion	Breaks food into smaller pieces	Chewing, stomach churning
Chemical digestion	Breaks large molecules into small soluble molecules	Enzyme action in the mouth, stomach and small intestine
Absorption	Moves useful substances into the body	Nutrients absorbed in the small intestine
Elimination	Removes undigested waste	Solid waste removed from the body

Why digestion is efficient in mammals

Digestion in mammals is efficient because different regions of the digestive system are specialised for different functions.

For example:

• the mouth starts physical and chemical digestion

• the stomach churns food and continues digestion

• the small intestine completes digestion and absorbs nutrients

• the large intestine absorbs water

• the rectum and anus eliminate waste

This shows how organs in a system work together, which links directly to Module 2 ideas about organisation in multicellular organisms. 

Worked example

Exam-style question

Explain the difference between physical digestion and chemical digestion in mammals.

Worked answer

Physical digestion is the mechanical breakdown of food into smaller pieces, for example by chewing and stomach churning. Chemical digestion is the breakdown of large food molecules into smaller soluble molecules by enzymes, so they can be absorbed.

Why this works

This answer:

• defines both processes clearly

• includes an example of each

• links chemical digestion to absorption

Common mistakes

• Saying physical digestion breaks food into nutrients. It only breaks food into smaller pieces.

• Forgetting that enzymes are involved in chemical digestion.

• Confusing absorption with digestion.

• Saying water is mainly absorbed in the stomach.

• Mixing up elimination with excretion. In this topic, elimination refers to solid digestive waste leaving the body.

Quick quiz

1. What is physical digestion?

2. What is chemical digestion?

3. Where does most absorption of nutrients occur?

4. What is absorbed in the large intestine?

5. What is elimination?

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