The digestive system: from peristalsis to absorption

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The **breakdown of large food molecules** into small, soluble ones that can be absorbed.

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The movement of the **small, soluble products of digestion** out of the gut and into the **blood** (or lymph).

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**Waves of muscle contraction** in the gut wall that **push food along** the digestive tract.

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**Involuntary smooth muscle** — it is **not** under conscious control.

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The **autonomic nervous system** — the nerves in the gut wall (the **enteric nervous system**).

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**Hydrochloric acid (HCl)** — giving a very **low pH** (about 1.5–2).

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(1) It **kills most ingested bacteria**; (2) it gives the enzyme **pepsin** its **optimum (acidic) pH**.

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It **denatures** (unfolds) proteins and provides the acidic pH that lets **pepsin** (a protease) break them into shorter chains.

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**Proton-pump inhibitors** (antacids neutralise acid that is already there).

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**Amylase** (starch), **protease** (protein) and **lipase** (fat).

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Starch → **glucose**; protein → **amino acids**; triglyceride (fat) → **fatty acids and glycerol**.

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**Villi** (large surface area), a **thin (one-cell) wall** (short diffusion distance), and a **rich blood supply** (steep concentration gradient).

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To **reabsorb water** (and mineral ions) and form faeces — it does no enzyme digestion.

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**Glucose is small** enough to cross the partially permeable membrane; **starch is too large** — it must be digested first. This models absorption in the gut.