🧱 Building new molecules
The products of digestion are used to build new carbohydrates, lipids and proteins. For example, amino acids can be joined together to make the proteins your body needs.
Animal Tissues, Organs and Systems » Digestive Enzymes and Bile
What you'll learn this session
Study time: 30 minutes
AQA spec: 4.2.2.1
- Why food must be broken down into small soluble molecules
- What carbohydrases (including amylase), proteases and lipases break down, and where they are made
- How the products of digestion are used in the body
- How bile is made and why it speeds up fat digestion
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Unlock This CourseWhy food must be digested
The food you eat is made of large molecules such as starch, proteins and fats. These molecules are too big to pass through the wall of the gut and into the blood. They also do not dissolve well.
So the body uses digestive enzymes to convert food into small soluble molecules. These are small enough to be absorbed into the bloodstream and carried around the body. Digestive enzymes are made by glands and by the lining of the gut, and they work in the gut itself.
You will see that each digestive enzyme works on one type of food. This fits with what you already know: every enzyme is specific to its own substrate.
Key terms:
- Digestive enzyme: an enzyme that breaks down large food molecules into small soluble molecules.
- Carbohydrase: an enzyme that breaks down carbohydrates into simple sugars.
- Amylase: a carbohydrase that breaks down starch.
- Protease: an enzyme that breaks down proteins into amino acids.
- Lipase: an enzyme that breaks down lipids (fats) into glycerol and fatty acids.
- Bile: an alkaline liquid made in the liver that helps the digestion of fat.
- Emulsify: to break a large drop of fat into many small droplets.
The three groups of digestive enzymes
🍞 Carbohydrases
Break down carbohydrates into simple sugars. Amylase is a carbohydrase that breaks down starch.
🍳 Proteases
Break down proteins into amino acids.
🧀 Lipases
Break down lipids (fats) into glycerol and fatty acids.
Because each enzyme only fits one type of substrate, a protease cannot break down starch and amylase cannot break down fat. The body needs all three groups to digest a meal.
Word equations
You only need to understand simple word equations for digestion. You do not need chemical symbol equations. The substrate goes on the left, an arrow shows the reaction, and the products go on the right. The enzyme is written above the arrow.
The three word equations
starch → (amylase) → sugars
proteins → (proteases) → amino acids
lipids (fats) → (lipases) → glycerol and fatty acids
Notice that the large molecule is always broken down into smaller ones. Starch becomes simple sugars, proteins become amino acids, and fats become glycerol and fatty acids.
Where the enzymes are made
You need to recall the sites of production of amylase, proteases and lipases.
| Enzyme | Made in | Acts in |
|---|---|---|
| Amylase | Salivary glands, pancreas, small intestine | Mouth and small intestine |
| Proteases | Stomach, pancreas, small intestine | Stomach and small intestine |
| Lipases | Pancreas, small intestine | Small intestine |
The pancreas releases its enzymes into the small intestine. So most digestion is finished in the small intestine, where the small molecules can then be absorbed.
What happens to the products of digestion?
Once the small molecules have been absorbed into the blood, the body uses them in two main ways.
⚡ Respiration
Some glucose is used in respiration, which releases energy for the cell.
Bile
Bile is not an enzyme, but it helps lipase to work. Bile is made in the liver and stored in the gall bladder. It is then released into the small intestine, where it does two jobs.
🧪 It is alkaline
Bile neutralises the hydrochloric acid that comes from the stomach. This gives alkaline conditions in the small intestine, which suit the enzymes there.
💧 It emulsifies fat
Bile breaks large drops of fat into small droplets. This increases the surface area of the fat.
Alkaline conditions and a large surface area both increase the rate of fat breakdown by lipase. With more surface area, more lipase molecules can reach the fat at the same time.
Worked example
A person eats a meal of buttered toast and a boiled egg. Which enzymes digest each food, and what are the products?
Toast: the starch is broken down by amylase into sugars. Butter: the fat is emulsified by bile, then broken down by lipase into glycerol and fatty acids. Egg: the protein is broken down by proteases into amino acids.
Common mistakes
Saying that bile is an enzyme. It is not. It does not break down fat chemically. It only breaks the fat into droplets so lipase can work faster.
Saying that bile is made in the gall bladder. It is made in the liver and only stored in the gall bladder.
Saying that emulsifying makes the fat smaller. The amount of fat stays the same. It is split into many droplets, which gives a larger surface area.
Exam-style question
(a) Name the enzyme that breaks down starch, and name its product. [2 marks]
(b) Name two places where proteases are made. [2 marks]
(c) Explain how bile helps the digestion of fat. [3 marks]
Model answer
(a) Amylase (1). It makes sugars (1).
(b) Any two from: stomach (1), pancreas (1), small intestine (1).
(c) Bile emulsifies the fat into small droplets (1). This increases the surface area (1) so lipase can break the fat down faster (1). Also accept: bile is alkaline and neutralises stomach acid, giving better conditions for lipase (1).
Exam tip
In "explain" questions, link each fact to its effect: droplets, so more surface area, so a faster rate of breakdown by lipase.