Where Does Your Lunch Go?
Extension topic – this chapter is Extension Part content in the 2025 S1-3 Science curriculum.
You eat char siu rice at noon, and hours later it gives you energy to run in PE. How? Through a long journey in your digestive system!
In this chapter you will learn the main parts of the digestive system, why food must be digested, the difference between mechanical and chemical digestion, how enzymes work, and how digested food is absorbed. You will also model the gut with dialysis tubing.
1. Why Must Food Be Digested?
Most of our food is made of large molecules, such as starch, proteins and fats. They are too big to pass through the gut wall into the blood, and most do not dissolve in water (they are insoluble).
Digestion breaks large, insoluble food molecules into small, simple, soluble molecules. Only these can be absorbed into the blood and used by body cells.
Analogy: A big sofa will not fit through a narrow door, but its parts will. Digestion takes food "apart" so it can pass through the gut wall.
You met these building blocks in the chapter Nutrition and health:
\(\text{starch} \rightarrow \text{simple sugars (e.g. glucose)}\)
\(\text{proteins} \rightarrow \text{amino acids}\)
\(\text{fats} \rightarrow \text{fatty acids} + \text{glycerol}\)
Vitamins, minerals and water are already small, so they need no digesting.
Key takeaway: Food must be digested into small, soluble substances before it can be absorbed and used by body cells.
2. A Tour of the Digestive System
The digestive system is a long tube called the gut (alimentary canal), plus some associated organs that help but which food does not pass through.
The path of food through the gut
Mouth – Teeth chew food into small pieces. Saliva wets food so it is easy to swallow. It contains amylase, so starch digestion starts here.
Oesophagus (gullet) – A muscular tube from mouth to stomach. Its muscles squeeze in waves to push food down.
Stomach – A muscular bag that churns food with gastric juice. This juice contains protease to start protein digestion, and hydrochloric acid to kill most germs in food.
Small intestine – A long, coiled tube. Digestion is completed here, and most digested food is absorbed here.
Large intestine – Absorbs much of the remaining water. Undigested material, such as dietary fibre, becomes faeces.
Rectum – Stores faeces for a short time.
Anus – Faeces leave the body through this opening (egestion).
Memory aid: "Many Old Sharks Swim Like Rockets Always" = Mouth, Oesophagus, Stomach, Small intestine, Large intestine, Rectum, Anus.
The helpers: associated organs
Liver – Makes bile. Bile has no enzymes, but it breaks fats into tiny droplets so lipase can work faster.
Gall bladder – Stores bile and releases it into the small intestine.
Pancreas – Makes pancreatic juice (with amylase, protease and lipase), which flows into the small intestine.
Did you know? Swallowing does not need gravity, because oesophagus muscles push the food along. That is why astronauts on China's Tiangong space station can still swallow while floating!
Key takeaway: Food travels mouth → oesophagus → stomach → small intestine → large intestine → rectum → anus. The liver, gall bladder and pancreas help, but food never passes through them.
3. Two Ways to Break Down Food
Mechanical digestion
Mechanical digestion physically breaks food into smaller pieces. It is still the same substance, just in smaller bits. Examples: teeth chewing (see the chapter Dental care) and the stomach churning.
Smaller pieces have a larger surface area, so enzymes can work faster, just as granulated sugar dissolves faster than a sugar cube.
Chemical digestion
Chemical digestion uses enzymes to break large food molecules into small, soluble ones. New substances form, e.g. starch becomes sugars.
Try it: Chew plain white bread for a minute. It starts to taste slightly sweet, because amylase in saliva is turning starch into sugar.
Key takeaway: Mechanical digestion makes pieces smaller; chemical digestion uses enzymes to change big molecules into small, soluble ones.
4. Enzymes: The Body's Chemical Scissors
What is an enzyme?
An enzyme is a substance made by living cells that speeds up a chemical reaction, such as breaking down food, without being used up.
1. Each enzyme does one job – amylase acts on starch only, not on protein.
2. Our digestive enzymes work best at body temperature (about 37 °C). Boiling destroys them.
Memory aid: Enzyme names often end in "-ase". Amylase → starch, Protease → protein, Lipase → lipid (fat).
Digestive juices and their enzymes
Saliva (mouth) – amylase.
Gastric juice (stomach) – protease, plus hydrochloric acid.
Pancreatic juice (into small intestine) – amylase, protease and lipase.
Small intestine wall – enzymes that finish digesting sugars and protein pieces.
Bile (liver, stored in gall bladder) – no enzymes.
The three key enzymes
Amylase: \(\text{starch} \rightarrow \text{sugars}\)
Protease: \(\text{proteins} \rightarrow \text{amino acids}\)
Lipase: \(\text{fats} \rightarrow \text{fatty acids} + \text{glycerol}\)
Investigating enzymes in the lab
Keep the tubes in a water bath at about 37 °C. A control uses boiled enzyme (or water) instead of active enzyme. The food tests come from the chapter Nutrition and health.
Amylase and starch
Mix starch solution with amylase. Every minute, add a drop to iodine solution on a spotting tile. At first it turns blue-black (starch present); later it stays yellow-brown (starch gone). A Benedict's test (with heating) then gives a brick-red precipitate, showing sugar has formed. The control stays blue-black.
Protease and egg white
Egg white in water looks cloudy. With protease, it slowly turns clear as the protein is broken into small, soluble substances. The control stays cloudy.
Lipase and milk
Add a little alkali and phenolphthalein to milk so it is pink. As lipase digests the fat, acidic fatty acids form and the pink colour fades. The control stays pink.
Fair test: independent variable = active or boiled enzyme; dependent variable = colour change (or time taken); control variables = temperature, volumes and concentrations.
Key takeaway: Some digestive juices contain enzymes. Amylase digests starch, protease digests proteins and lipase digests fats.
5. Absorption: From Gut to Blood
Absorption is when small, soluble digested food passes through the gut wall into the blood.
Why the small intestine is so good at absorption
1. It is very long – about 6 m in an adult.
2. It has millions of villi – tiny finger-like bumps on the inner wall (one is a villus) that give a huge surface area.
3. Villi have very thin walls – food only travels a short distance.
4. Villi have many blood capillaries – blood quickly carries absorbed food away.
Analogy: A fluffy towel with many tiny loops soaks up far more water than a smooth plastic sheet. Villi work like those loops.
Delivered to every cell
The blood, part of the body's transport system, carries digested food to all parts of the body. Cells use glucose for energy, amino acids to grow and repair, and fatty acids and glycerol for energy and to build cell parts.
The large intestine: saving water
The small intestine absorbs a lot of water too, and the large intestine absorbs much of what is left, so we do not lose too much water. Undigested food, mainly dietary fibre, is egested as faeces.
Key takeaway: Most digested food is absorbed in the small intestine, whose villi give a huge surface area, and blood carries it to all body cells. The large intestine mainly absorbs water.
6. Model It: The Dialysis Tubing Gut (S)
Dialysis tubing (Visking tubing) is a thin, partially permeable membrane with tiny holes. Small molecules (water, glucose) pass through; large molecules (starch) cannot.
What each part represents
Dialysis tubing = wall of the small intestine
Liquid inside = food in the gut
Water outside = blood
Steps
1. Tie one end of wet dialysis tubing, fill it with starch solution and glucose solution, and tie the other end.
2. Rinse the outside, then put the tubing in a boiling tube of distilled water.
3. After about 30 minutes, test the water outside with iodine solution (starch) and Benedict's solution (glucose).
Results and conclusion
Iodine test on water outside: stays yellow-brown → no starch.
Benedict's test on water outside (heated): brick-red precipitate → glucose present.
Conclusion: Glucose molecules are small enough to pass through, but starch molecules are not. Likewise, only small, digested food molecules can be absorbed through the gut wall, so starch must be digested first.
Extra challenge: Put starch solution and amylase inside the tubing at 37 °C. Later, sugar appears in the water outside, because amylase has digested starch into small sugar molecules.
Limits of the model
The real gut wall is living tissue with villi and flowing blood; the tubing is a flat, non-living membrane. A model helps, but it is not the real thing.
Key takeaway: In the dialysis tubing model, glucose passes out but starch stays inside, so food must be small and soluble to be absorbed.
Common Mistakes to Avoid
Common mistake 1: "Chewing turns starch into sugar." Chewing is mechanical digestion. It is amylase in saliva that turns starch into sugar.
Common mistake 2: "Food passes through the liver and pancreas." No! They only send juices into the gut. Also, bile is not an enzyme.
Common mistake 3: "The large intestine absorbs most digested food." Most digested food is absorbed in the small intestine; the large intestine mainly absorbs water.
Quick Review
1. Digestion turns large, insoluble molecules into small, soluble ones that cells can use.
2. Gut: mouth → oesophagus → stomach → small intestine → large intestine → rectum → anus. Helpers: liver, gall bladder, pancreas.
3. Mechanical digestion: chewing, churning. Chemical digestion: enzymes in digestive juices.
4. Amylase digests starch, protease digests proteins, lipase digests fats.
5. Most digested food is absorbed in the small intestine (villi) and carried by blood to all parts of the body; the large intestine absorbs water.
6. Dialysis tubing model: glucose passes through, starch does not.
Well done! Next time you eat, thank your hard-working digestive system!