🍫 Imagine a huge chocolate bar that won't fit through a letterbox. You need to snap it into tiny pieces first. That's exactly what your body does with food.
The molecules in your sandwich — starch, protein, fat — are too large and insoluble (they don't dissolve) to pass through the wall of your intestine into your blood. Chemical digestion uses enzymes as molecular scissors to cut them into pieces small enough to be absorbed.
Chemical digestion is the breakdown of large, insoluble molecules into small, soluble molecules using enzymes.
Why? To produce small soluble molecules that can be absorbed through the intestine wall into the blood.
Your body has three types of molecular scissors. Each one cuts a specific type of food molecule. Remember: enzymes are specific — amylase won't cut protein, just like scissors won't work on a rock.
Amylase → A for stArch (both start with A-sounds)
Protease → P for Protein (same letter!)
Lipase → L for Lipids (fats) (same letter!)
Take a piece of plain bread or cracker. Chew it slowly for 60 seconds without swallowing. Notice it starts tasting sweet?
That's amylase in your saliva breaking starch into sugar — you're experiencing chemical digestion in real time! 🤯
Follow the food from mouth to intestine. At each stop, different enzymes are waiting to do their job:
Salivary glands release saliva containing amylase. Starch digestion begins here.
Gastric juice contains protease (pepsin) and hydrochloric acid (HCl).
HCl does two jobs:
1. Kills harmful microorganisms in food
2. Provides acidic pH (pH 2) for pepsin to work at its best
Bile arrives from the liver (stored in gall bladder). Bile is NOT an enzyme.
Bile does two jobs:
1. Emulsifies fats — breaks large fat droplets into tiny droplets (like dish soap on grease), increasing surface area for lipase
2. Neutralises the acidic food from the stomach — bile is alkaline, creating the right pH for intestinal enzymes
All three enzyme types work here. This is where most chemical digestion happens.
SUPPLEMENT detail:
Starch: amylase → maltose, then maltase (on epithelium membranes) → glucose
Protein: trypsin works in alkaline conditions (vs pepsin in acid)
The supplement content adds specific names and conditions. Here's the key extra detail:
| Enzyme | What it does | Where | Conditions |
|---|---|---|---|
| Pepsin | Breaks down protein | Stomach | Acidic (HCl provides pH 2) |
| Trypsin | Breaks down protein | Small intestine | Alkaline (bile neutralises acid) |
| Maltase | Breaks maltose → glucose | Small intestine epithelium membranes | Alkaline |
Stage 1: Amylase chops starch into maltose (a medium-sized sugar — like snapping a chocolate bar in half)
Stage 2: Maltase chops maltose into glucose (the final tiny piece your body can actually use)
Starch → amylase → Maltose → maltase → Glucose
Bile is NOT an enzyme. Think of it like washing-up liquid. When you squirt dish soap on a greasy plate, the big blob of grease breaks into hundreds of tiny droplets. That's emulsification.
This gives lipase a much bigger surface area to work on — so fat digestion happens faster.
Bile is also alkaline, which neutralises the acid from the stomach. Intestinal enzymes (trypsin, lipase, maltase) need alkaline conditions.
Q1: What breaks down maltose into glucose?
Q2: Pepsin works in acidic conditions. Where does it work?
Q3: Why is bile important even though it's NOT an enzyme?
Q4: Trypsin works in _____ conditions in the _____.
Digestion = chopping food into tiny pieces (what we just learnt)
Absorption = those tiny pieces passing through the intestine wall into the blood
Think of digestion as unwrapping a sweet, and absorption as putting it in your mouth. Two different steps!
• The small intestine is where most nutrients are absorbed
• Most water is absorbed in the small intestine, but some water is also absorbed from the colon (large intestine)
Imagine a flat towel vs a fluffy towel. The fluffy one has a much bigger surface area because of all the tiny loops of fabric. That's what villi do — they're tiny finger-like projections that massively increase the surface area inside the small intestine.
And on each villus, there are even tinier projections called microvilli — like putting fuzz on the fluffy towel loops. This means maximum surface area for absorption.
Each villus contains:
Capillaries (tiny blood vessels) — absorb amino acids, glucose, and other nutrients into the blood
Lacteals (tiny lymph vessels) — absorb fatty acids and glycerol (the products of fat digestion) into the lymph
Q5: Why do villi increase absorption?
Q6: Fatty acids and glycerol are absorbed into lacteals. What are lacteals?
These are the types of questions Cambridge will ask. If you can answer these, you own this topic.
Q7: A student eats a meal containing starch, protein and fat. Describe the chemical digestion of the starch. [3 marks]
Q8: Explain the role of bile in digestion. [3 marks]
Q9: Compare the digestion of protein in the stomach and small intestine. [2 marks]
Q10: Explain how the structure of a villus is adapted for absorption. [3 marks]
| Enzyme | Substrate | Products | Location |
|---|---|---|---|
| Amylase | Starch | Maltose / simple sugars | Mouth + small intestine |
| Maltase ★ | Maltose | Glucose | Small intestine epithelium |
| Pepsin ★ | Protein | Amino acids | Stomach (acidic) |
| Trypsin ★ | Protein | Amino acids | Small intestine (alkaline) |
| Lipase | Fats & oils | Fatty acids + glycerol | Small intestine |
★ = Supplement content
Non-enzyme helpers:
HCl — kills bacteria + provides acidic pH for pepsin
Bile — emulsifies fats (more surface area for lipase) + neutralises acid (alkaline pH for intestinal enzymes)