The Human Body and Its Systems · Grade 8
What happens to a sandwich between the mouth and the bloodstream?
It goes through two kinds of breakdown. Mechanical breakdown cuts it into smaller pieces and increases the contact area; chemical breakdown uses enzymes to split large molecules into small ones. Only at the end of that, in the small intestine, are the molecules small enough to cross the wall into the blood. Everything else carries on to the large intestine.
Learning objectives
- Follow a mouthful of food through the mouth, the oesophagus, the stomach, the small intestine and the large intestine
- Distinguish mechanical breakdown from chemical breakdown and give an example of each
- Explain what an enzyme does and why each enzyme works on only one kind of food
- Explain how the folds and the villi of the small intestine speed up absorption
The route, station by station
In the mouth the teeth cut and grind, and saliva adds an enzyme that begins breaking down starch. The tongue pushes the mouthful into the oesophagus, where waves of muscle push it down — not gravity, which is why you can swallow lying down.
In the stomach powerful muscles churn the food in a strongly acidic liquid. The acid kills many bacteria and lets the protein-splitting enzyme work; many molecules are opened up there for the first time.
In the small intestine further enzymes arrive from the pancreas and bile from the liver, and the breakdown is completed. Almost all absorption happens there too. The large intestine absorbs mainly water and minerals, and what is left leaves.
Two completely different breakdowns
Mechanical breakdown changes size, not identity. A piece of bread that has been ground up is still bread, only in smaller pieces — and its contact area with the enzymes is now far larger.
Chemical breakdown changes identity. Starch that has been broken down is not smaller starch but simple sugars, entirely different molecules. The two work together: the mechanical prepares the surface on which the chemical acts.
An enzyme is a protein that speeds up one particular reaction and is not used up by it. Each enzyme has an active site of a particular shape, so it binds only a molecule that fits that shape — a protein-splitting enzyme will not touch starch.
Why the small intestine specifically
The small intestine is several metres long and its wall is folded. On the folds sit villi, and on each villus cell sit tiny microvilli. Three levels of folding multiply the absorbing surface hundreds of times over.
Every villus contains a blood capillary and a lymph vessel. Sugars and amino acids pass into the blood; fats enter the lymph. The wall is very thin, because every extra layer would slow the crossing.
Worked examples
Why is it worth chewing thoroughly before swallowing?
- Chewing is mechanical breakdown
- Smaller pieces give a larger contact area
- Enzymes act on the surface only
Answer: Because chemical breakdown is faster the larger the contact area
Someone swallows a slice of bread. Where does its breakdown begin and where does it end?
- Bread is mostly starch
- An enzyme in saliva begins breaking down starch in the mouth
- In the stomach the acid stops that enzyme working
- In the small intestine an enzyme from the pancreas finishes the job
Answer: It starts in the mouth, pauses in the stomach and finishes in the small intestine
Why might someone who has had part of the small intestine removed become deficient?
- Almost all absorption happens in the small intestine
- A shorter length means less absorbing surface
- Food moves on to the large intestine before it has been fully absorbed
Answer: Because some of the food passes through without being absorbed
Common mistakes
- Saying food is absorbed in the stomach
- The stomach breaks down and stores; almost all absorption is in the small intestine. Very few substances cross the stomach wall, and food is not among them.
- Treating mechanical breakdown as finished digestion
- A small piece of bread is still far too large to cross the intestinal wall. Only chemical breakdown produces molecules of a size that can get through.
- Assuming one enzyme handles every kind of food
- Each enzyme has an active site of a particular shape and binds only a molecule that fits it. That is why the body makes different enzymes for starch, protein and fat.
What to remember
- Mechanical changes size; chemical changes identity.
- One enzyme, one shape of molecule.
- Almost all absorption happens in the small intestine.
- Folds, villi and microvilli — three levels of surface area.
More in The Human Body and Its Systems
- Why does a large body need organ systems at all?
- What is the difference between breathing, gas exchange and cellular respiration?
- Why does blood pass through the heart twice on every complete circuit?
- How does a kidney decide what to remove from the blood and what to keep?
- When does the body send an instruction by nerve and when by hormone?