Role of Digestive Enzymes

Most of the food you eat is made of large molecules — starch, proteins and lipids. These molecules are:

• Too big to pass through the gut wall into the blood.
• Insoluble — they will not dissolve, so they cannot be carried in the blood plasma.

Digestion solves this. It breaks the large, insoluble molecules into small, soluble ones:

Large molecule	Small soluble product(s)
Starch (a carbohydrate)	Glucose (a sugar)
Protein	Amino acids
Lipid (fat/oil)	Fatty acids + glycerol

These small products can then be absorbed through the wall of the small intestine into the blood.

Enzymes do the cutting. Digestive enzymes are biological catalysts — they speed up the breakdown reactions, are specific to one type of substrate, and are not used up, so a small amount digests a lot of food.

Exam tip. The whole point of digestion in one sentence: "large insoluble molecules are broken down into small soluble molecules so they can be absorbed." This sentence scores marks again and again.

Starch is digested in two steps, by two enzymes:

Step 1 — Amylase (a carbohydrase)

• Substrate → product: starch → maltose.
• Maltose is a smaller sugar made of two glucose units joined together.
• Made in: the salivary glands (saliva) and the pancreas.
• Acts in: the mouth and the small intestine.

Step 2 — Maltase

• Substrate → product: maltose → glucose.
• Made in / acts in: the wall of the small intestine.
• Glucose is small and soluble, so it is now ready to be absorbed.

So the full pathway is:

$\text{starch} \xrightarrow{\text{amylase}} \text{maltose} \xrightarrow{\text{maltase}} \text{glucose}$

Exam tip. Many students stop at maltose. Maltose is still a sugar that cannot be absorbed efficiently — you must mention maltase converting it to glucose to score full marks.

Proteases — digest proteins

• Substrate → product: protein → amino acids.
• Made in: the stomach (e.g. an acid-tolerant protease), the pancreas and the small intestine.
• Acts in: the stomach and the small intestine.
• Amino acids are small and soluble, so they can be absorbed.

Lipases — digest lipids (fats and oils)

• Substrate → products: lipid → fatty acids + glycerol (two products).
• Made in: the pancreas and the small intestine.
• Acts in: the small intestine.

$\text{protein} \xrightarrow{\text{protease}} \text{amino acids}$

$\text{lipid} \xrightarrow{\text{lipase}} \text{fatty acids} + \text{glycerol}$

Watch the products. Lipids give two named products — fatty acids AND glycerol. Forgetting glycerol is the single most common slip in this topic.

Exam tip. A neat way to remember the matches:

• carbohydrase → carbohydrate (starch) → glucose
• protease → protein → amino acids
• lipase → lipid → fatty acids + glycerol

The enzyme is usually named after the food it digests, with the ending -ase.

Questions often ask where an enzyme is made or where it works. Learn this summary table — it is worth easy marks.

Enzyme	Digests	Products	Made in	Acts in
Amylase	starch	maltose	salivary glands, pancreas	mouth, small intestine
Maltase	maltose	glucose	small intestine wall	small intestine
Protease	protein	amino acids	stomach, pancreas, small intestine	stomach, small intestine
Lipase	lipid	fatty acids + glycerol	pancreas, small intestine	small intestine

Exam tip. If unsure where an enzyme acts, the small intestine is the safest answer — amylase, maltase, protease and lipase all complete their work there.

Digestion is only useful because of what happens next — absorption.

• Once large molecules become small and soluble (glucose, amino acids, fatty acids, glycerol), they are small enough to pass through the wall of the small intestine.
• They are absorbed into the blood (glucose and amino acids) or the lacteals (fatty acids and glycerol), and carried around the body to be used by cells.

So the chain is:

large insoluble food → (enzymes) → small soluble molecules → absorbed into blood → used by cells

Why "insoluble" matters. A molecule that will not dissolve cannot be carried in blood plasma and cannot diffuse through a membrane. Digestion makes molecules both small and soluble — both properties are needed for absorption.

Exam tip. When a question asks "why must food be digested?", always link back to absorption: "so that small, soluble molecules can be absorbed into the blood." The mark is for the absorption link, not just for saying "to break it down".