--- title: "The Journey of Food" book: "Primary & Middle School Biology" subject: biology language: en chapter: 25 exercises: 11 source: https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food --- # Chapter 25 — The Journey of Food One bite of bread: chewed, swallowed — gone. Gone where? By tonight, part of that bread will be helping your [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) run and your [bones](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton) grow, which means it must somehow get *out* of the tube it was swallowed into and *into* the rest of the body. Follow the bite: its journey is one of the body’s best stories. ## 25.1 The digestive tract **Definition 25.1 (The digestive tract).** Food travels through the *digestive tract*, one long tube running right through the body: 1. the *mouth* , where teeth grind ( [Chapter 12](https://one-course.com/books/biology/1/en/chapter/12-teeth-and-eating-well#ch-g2-teeth-and-eating-well) ) and *saliva* moistens; 2. the *esophagus* , the slide from throat to stomach; 3. the *stomach* , a muscular pouch where food is churned and mixed with [digestive juices](#def-g4-journey-of-food-digestion) for hours; 4. the *small intestine* , a folded tube several metres long, where [digestion](#def-g4-journey-of-food-digestion) finishes and the food’s goodness passes into the body; 5. the *large intestine* , where water is reclaimed from what remains, before the leftovers leave the body. ![The digestive tract: one tube from mouth to exit. The esophagus delivers to the stomach; the small intestine’s folded metres fill the middle, framed by the large intestine.](https://one-course.com/images/onecourse/chapters/biology-1/g4-journey-of-food/fig-59ff7d67153a.svg) *The [digestive tract](#def-g4-journey-of-food-tract): one tube from mouth to exit. The [esophagus](#def-g4-journey-of-food-tract) delivers to the [stomach](#def-g4-journey-of-food-tract); the [small intestine](#def-g4-journey-of-food-tract)’s folded metres fill the middle, framed by the [large intestine](#def-g4-journey-of-food-tract).* **Example 25.2 (Timing the journey).** The bite of bread spends seconds in the mouth, a couple of seconds sliding down the [esophagus](#def-g4-journey-of-food-tract), some hours being churned in the [stomach](#def-g4-journey-of-food-tract), several more working through the [small intestine](#def-g4-journey-of-food-tract), and up to a day or more finishing in the [large intestine](#def-g4-journey-of-food-tract). The whole journey commonly takes one to two days. ## 25.2 Digestion: from food to nutrients **Definition 25.3 (Digestion and nutrients).** *Digestion* is the transformation of food, along the tract, into components small and simple enough to enter the body: the *nutrients*. Teeth and the [stomach](#def-g4-journey-of-food-tract)’s churning break food up; *digestive juices* — [saliva](#def-g4-journey-of-food-tract) among them — dismantle it further, piece by piece, until what was bread is a liquid mix of nutrients. **Example 25.4 (The bread turns sweet).** Chew a piece of plain bread very long — a slow count of sixty — and a faint sweetness appears. Nothing was added: [saliva](#def-g4-journey-of-food-tract) is already dismantling the bread into simpler, sweeter pieces. You have tasted [digestion](#def-g4-journey-of-food-digestion) beginning. **Proposition 25.5 (Crossing into the body).** The tract’s inside is still, in a sense, outside the body — like the hole of a doughnut. The crossing happens in the *[small intestine](#def-g4-journey-of-food-tract)*: through its thin, deeply folded walls, [nutrients](#def-g4-journey-of-food-digestion) pass into the *blood*, which carries them to every part of the body (the delivery service that [Chapter 27](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#ch-g4-heart-and-blood) explores). What cannot cross — fibres, husks — travels on, loses its water in the [large intestine](#def-g4-journey-of-food-tract), and [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) as droppings. **Proof.** *Admitted at this level.* ∎ **Remark 25.6 (Why the folds and the metres).** Crossing takes wall surface, and the [small intestine](#def-g4-journey-of-food-tract) is built for it: metres of length, and walls folded and refolded inside. Stretched flat, the crossing surface would carpet a small room — packed into your middle. Big jobs, in the body, get big surfaces folded small. ## 25.3 Helping digestion do its work **Method 25.7 (Digestion-friendly habits).** 1. Chew properly: the [stomach](#def-g4-journey-of-food-tract) has no teeth — what the mouth skips, the [stomach](#def-g4-journey-of-food-tract) must struggle with; 2. eat calmly, at regular mealtimes; 3. drink water with meals; 4. eat fibre-rich foods — vegetables, [fruit](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) , whole bread: fibre keeps the leftovers moving; 5. leave hard play or swimming for after the meal has settled: a hard-working [stomach](#def-g4-journey-of-food-tract) and hard-working [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) compete. **Example 25.8 (The stomach-ache detective).** Wolfed-down lunch, then a sprint: the classic afternoon [stomach](#def-g4-journey-of-food-tract) ache. The meal was swallowed half-chewed (rule 1 broken) and the digesting [stomach](#def-g4-journey-of-food-tract) was hurried into a race (rule 5). The fix is not medicine but manners — toward one’s own [digestive tract](#def-g4-journey-of-food-tract). ## 25.4 Exercises **Exercise 25.1 ★.** Name the parts of the [digestive tract](#def-g4-journey-of-food-tract) in order, mouth to exit. **Solution of Exercise 25.1.** Mouth, [esophagus](#def-g4-journey-of-food-tract), [stomach](#def-g4-journey-of-food-tract), [small intestine](#def-g4-journey-of-food-tract), [large intestine](#def-g4-journey-of-food-tract) — then the leftovers leave the body. **Exercise 25.2 ★.** What happens to food in the [stomach](#def-g4-journey-of-food-tract), and roughly how long does it stay there? **Solution of Exercise 25.2.** It is churned by the [stomach](#def-g4-journey-of-food-tract)’s muscular walls and mixed with [digestive juices](#def-g4-journey-of-food-digestion) — for some hours. **Exercise 25.3 ★.** What is [digestion](#def-g4-journey-of-food-digestion)? What are [nutrients](#def-g4-journey-of-food-digestion)? **Solution of Exercise 25.3.** [Digestion](#def-g4-journey-of-food-digestion) is the transformation of food, along the tract, into components simple and small enough to enter the body. Those components are the [nutrients](#def-g4-journey-of-food-digestion). **Exercise 25.4 ★.** In which part do [nutrients](#def-g4-journey-of-food-digestion) cross into the [blood](#prop-g4-journey-of-food-absorb)? What happens to what cannot cross? **Solution of Exercise 25.4.** In the [small intestine](#def-g4-journey-of-food-tract), through its thin folded walls, into the [blood](#prop-g4-journey-of-food-absorb). What cannot cross travels on to the [large intestine](#def-g4-journey-of-food-tract), loses its water there, and [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) the body as droppings. **Exercise 25.5 ★.** Why does long-chewed bread turn faintly sweet? **Solution of Exercise 25.5.** [Saliva](#def-g4-journey-of-food-tract) is already dismantling the bread into simpler, sweeter components — [digestion](#def-g4-journey-of-food-digestion) begins in the mouth, and the sweetness is its taste. **Exercise 25.6 ★.** Why is the [small intestine](#def-g4-journey-of-food-tract) so long and so folded inside? **Solution of Exercise 25.6.** Because crossing into the [blood](#prop-g4-journey-of-food-absorb) takes wall surface: the metres of length and the deep inner folds pack an enormous crossing surface — flattened out, roughly a small room’s carpet — into the belly. **Exercise 25.7 ★.** Give three digestion-friendly habits and the reason for each. **Solution of Exercise 25.7.** For example: chew properly (the [stomach](#def-g4-journey-of-food-tract) has no teeth); eat calmly at regular times (the tract works best unhurried); drink water ([digestion](#def-g4-journey-of-food-digestion) runs wet); eat fibre (it keeps leftovers moving); rest after big meals ([stomach](#def-g4-journey-of-food-tract) and [legs](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-parts) compete for the body’s effort). **Exercise 25.8 ★.** Try it (at table): time your next meal. How long does your food spend in your mouth — and how does that compare with the days ahead of it? **Solution of Exercise 25.8.** Open answer. Typically well under half an hour in the mouth — against one to two days for the rest of the journey. The mouth’s share is tiny in time and decisive in effect: it is the only stop with teeth. **Exercise 25.9 ★★.** Diagnose [Example 25.8](#ex-g4-journey-of-food-ache)’s [stomach](#def-g4-journey-of-food-tract) ache: which two habits were broken, and what would you change tomorrow? **Solution of Exercise 25.9.** Broken: chew properly (lunch was wolfed half-chewed) and rest after meals (a sprint straight after). Tomorrow: eat slowly, and let the meal settle before racing. **Exercise 25.10 ★★.** “Food goes into the body the moment you swallow it.” Correct this with [Proposition 25.5](#prop-g4-journey-of-food-absorb): where does food really enter the body, and what must happen to it first? **Solution of Exercise 25.10.** Swallowed food is still inside the tube — like the hole of a doughnut, not yet truly in the body. It must first be digested into [nutrients](#def-g4-journey-of-food-digestion); only in the [small intestine](#def-g4-journey-of-food-tract) do those cross the wall into the [blood](#prop-g4-journey-of-food-absorb). Entering the mouth is not entering the body. **Exercise 25.11 ★★★.** The cow’s grass is far tougher food than your bread — and the cow has four [stomach](#def-g4-journey-of-food-tract) chambers, chews its food twice, and hosts helpful [living things](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living) in its gut to dismantle grass. Using [Definition 25.3](#def-g4-journey-of-food-digestion) and [Proposition 10.4](https://one-course.com/books/biology/1/en/chapter/10-what-animals-eat#prop-g2-what-animals-eat-match), explain why a grass-eater needs this heavier machinery. **Solution of Exercise 25.11.** [Digestion](#def-g4-journey-of-food-digestion) must dismantle food into [nutrients](#def-g4-journey-of-food-digestion), and grass is far harder to dismantle than bread — tough fibres, little goodness per mouthful ([Proposition 10.7](https://one-course.com/books/biology/1/en/chapter/10-what-animals-eat#prop-g2-what-animals-eat-daily)). So the grass-eater brings heavier machinery: grinding [molars](https://one-course.com/books/biology/1/en/chapter/12-teeth-and-eating-well#def-g2-teeth-and-eating-well-kinds) used twice over (chewing the cud), four [stomach](#def-g4-journey-of-food-tract) chambers for long processing, and gut helpers that can break down what its own juices cannot. Tougher input, bigger factory.