Chemical Digestion

Chemical digestion is the breakdown of large, insoluble molecules into small, soluble molecules. That solubility is not a side detail — it is the entire reason chemical digestion has to happen at all. Large insoluble molecules cannot cross the intestinal wall, so however much physical digestion has broken food into smaller pieces, none of it can be absorbed until enzymes have converted it into small soluble products. Large molecules are never absorbed first and broken down afterwards — digestion has to come before absorption, not the other way round.

Following one meal through the system

The clearest way to hold this chapter together is to trace a single meal from mouth to small intestine and notice that each stage causes the next one, rather than treating the organs as an unconnected list.

1. Mouth. Teeth cut and grind the food (see human teeth), increasing its surface area. Saliva, secreted by the salivary glands, contains amylase, which begins starch digestion here.
2. Oesophagus. No digestion happens at this stage — the swallowed food is simply carried down to the stomach.
3. Stomach. The muscular wall churns the food (see physical digestion), mixing it thoroughly with gastric juice. That juice contains hydrochloric acid, which kills harmful microorganisms and creates the acidic conditions the stomach’s protease needs. Protein digestion begins here — at Supplement level, that protease is named pepsin.
4. Duodenum. The acidic mixture leaving the stomach meets two secretions at once: the pancreas delivers amylase, protease and lipase, acting on starch, protein and fat respectively, and bile arrives from the liver by way of the gall bladder. At Supplement level, bile does two separate jobs here — see bile.
5. Small intestine (ileum). Digestion is completed and the small, soluble products are absorbed through the wall — see absorption. At Supplement level, an enzyme called maltase, held on the epithelium itself, completes the final step of starch digestion — see digestive enzymes.
6. Colon, rectum, anus. Some remaining water is absorbed in the colon; undigested material is stored as faeces in the rectum and egested through the anus.

Notice that the site of action also fixes the pH an enzyme has to work in: the stomach is acidic, because of hydrochloric acid, while the small intestine is alkaline — at Supplement level, because bile neutralises the acid arriving from the stomach. That is exactly why the same food group needs different named enzymes in different regions of the gut, each suited to the conditions it actually meets there.

Hydrochloric acid’s two jobs

Hydrochloric acid in the stomach has two required functions, and a full-mark answer states both separately: it kills harmful microorganisms that were swallowed with food, and it produces the acidic pHthat gives the stomach’s own protease its optimum conditions for activity. What it does not do is digest anything itself — the enzyme does the digesting, not the acid. “The acid breaks down the food” is a specific and common way to lose this mark, because it hands the acid a job that actually belongs to protease.

It is also worth noticing that these acidic conditions suit only the stomach’s own enzyme. They are not suitable for the enzymes working further along in the small intestine, which is exactly why that acid has to be neutralised before food moves on — the mechanism behind that neutralisation, and bile’s second role alongside it, is covered on the bile page.

For the exact substrate-and-product relationships behind amylase, protease and lipase — and the Supplement detail behind starch digestion and the pepsin/trypsin comparison — see digestive enzymes.