---
title: "Digestion and Nutrients"
book: "Primary & Middle School Biology"
subject: biology
language: en
chapter: 50
exercises: 12
source: https://one-course.com/books/biology/1/en/chapter/50-digestion-and-nutrients
---

# Chapter 50 — Digestion and Nutrients

[Chapter 25](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#ch-g4-journey-of-food) followed the bite of bread down the tube and promised that “[digestive juices](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-digestion) dismantle it”. This year we open the workshop doors: what exactly do the juices do, what are the [nutrients](#def-g7-digestion-and-nutrients-families) they produce, and how does the [small intestine](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract)’s wall manage the greatest border crossing in the body? The bite of bread is back — under better instruments.

## 50.1 The juices at work

**Proposition 50.1 (Digestion is chemical dismantling).**

Chewing and churning only break food *smaller*; the real dismantling is chemical. The *digestive juices* — [saliva](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract), the [stomach](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract)’s juice, the juices poured into the [small intestine](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract) — contain working substances that cut food’s large components into small ones: starch into sugars, meat and [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born)’ building matter into its small units, fats into theirs. Each juice has its specialties, and each works best at its own station’s [conditions](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions).

**Proof.** *Admitted at this level.* ∎

**Example 50.2 (The bench version).**

The classic experiment runs [digestion](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-digestion) in glass. Two tubes of cooked starch paste at body warmth: one gets fresh [saliva](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract), one gets boiled (spoiled) [saliva](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract). An hour later, the starch test finds tube 2 unchanged — still starch — while tube 1’s paste has turned to sugars, exactly the sweetness of [Example 25.4](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#ex-g4-journey-of-food-sweet), produced without any mouth. The juice alone did the dismantling — and the boiled control shows its working substance is destroyed by heat, as delicate machinery would be.

**Remark 50.3 (Why dismantle at all?).**

Food’s large components can no more cross the intestine’s wall than a wardrobe can pass a letterbox. And the body does not want them whole: it wants *parts* — small units it can rebuild into its own substance ([Proposition 41.1](https://one-course.com/books/biology/1/en/chapter/41-matter-from-food#prop-g6-matter-from-food-transform)). [Digestion](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-digestion) is dismantling for transport *and* for reassembly: furniture to flat-pack, at every meal.

## 50.2 The nutrients

**Definition 50.4 (The nutrient families).**

The dismantling yields the *nutrients* — the small, blood-ready components:

- *sugars* ( [glucose](https://one-course.com/books/biology/1/en/chapter/49-organs-at-work#prop-g7-organs-at-work-bill) above all, [Proposition 49.1](https://one-course.com/books/biology/1/en/chapter/49-organs-at-work#prop-g7-organs-at-work-bill) ’s fuel), from starchy and sweet foods;
- the small units of *building matter* , from meat, [fish](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) , [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) , dairy and [pulses](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse) — the bricks of [Proposition 18.1](https://one-course.com/books/biology/1/en/chapter/18-a-balanced-diet#prop-g3-balanced-diet-jobs) ’s construction;
- the units of *fats* — dense fuel and building material both;
- plus passengers needing no dismantling: *vitamins* , *[mineral salts](https://one-course.com/books/biology/1/en/chapter/22-what-plants-need#prop-g4-what-plants-need-needs)* , and water itself.

The [food families](https://one-course.com/books/biology/1/en/chapter/18-a-balanced-diet#def-g3-balanced-diet-families) of [Definition 18.3](https://one-course.com/books/biology/1/en/chapter/18-a-balanced-diet#def-g3-balanced-diet-families) are, at bottom, suppliers of these nutrient families — the shopping list behind the shopping list.

**Example 50.5 (One lunch, itemized).**

The balanced lunch of [Example 18.4](https://one-course.com/books/biology/1/en/chapter/18-a-balanced-diet#ex-g3-balanced-diet-lunch), re-read as [nutrients](#def-g7-digestion-and-nutrients-families): pasta — starch, dismantled to [glucose](https://one-course.com/books/biology/1/en/chapter/49-organs-at-work#prop-g7-organs-at-work-bill) by the litre; [fish](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups) — building matter, cut to its units; the oil on the beans — fat units; beans and apple — vitamins, minerals, water, plus fibre. Fibre, note, is the exception that proves the system: no juice of ours can cut it, so it crosses nothing and travels on — the leftovers’ bulk ([Proposition 25.5](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb)), doing its own useful sweeping.

## 50.3 The great crossing

**Proposition 50.6 (Absorption at the villi).**

The crossing promised in [Proposition 25.5](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) happens across the [small intestine](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract)’s inner wall, and the wall is built for it: folded, and carpeted with millions of *villi* — finger-shaped bumps a millimetre high, each wrapping its own fine [blood vessels](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-vessels). The [villi](#prop-g7-digestion-and-nutrients-absorption) are why the crossing surface reaches its carpet-a-room size ([Remark 25.6](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#rem-g4-journey-of-food-surface)): folds on folds on fingers. [Nutrients](#def-g7-digestion-and-nutrients-families) cross their thin walls into the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb), and the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) carries them first to a great sorting-and-storing organ — the *liver* — before general delivery.

**Proof.** *Admitted at this level.* ∎

![The intestine’s inner carpet: millions of villi, each a thin-walled finger full of blood vessels. Surface folded on surface — the body’s great border crossing.](https://one-course.com/images/onecourse/chapters/biology-1/g7-digestion-and-nutrients/fig-ae6b9713127d.svg)

*The intestine’s inner carpet: millions of [villi](#prop-g7-digestion-and-nutrients-absorption), each a thin-walled finger full of [blood vessels](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-vessels). Surface folded on surface — the body’s great border crossing.*

**Example 50.7 (The specification again).**

Large, thin, moist, blood-lined: the [villi](#prop-g7-digestion-and-nutrients-absorption) meet exactly [Proposition 47.1](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#prop-g7-how-animals-breathe-spec)’s specification — for [nutrients](#def-g7-digestion-and-nutrients-families) instead of [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration). [Lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four)’ pockets, [gills](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four)’ [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering), intestine’s [villi](#prop-g7-digestion-and-nutrients-absorption): wherever the body must move much across a border, it builds the same machine. Spot the specification once, and you recognize it everywhere.

**Method 50.8 (Tracing a nutrient end to end).**

For any mouthful, trace one [nutrient](#def-g7-digestion-and-nutrients-families):

1. name the food’s family and its main components;
2. station by station, note where its dismantling starts and finishes, and which juices worked on it;
3. cross at the [villi](#prop-g7-digestion-and-nutrients-absorption) with it, into the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) , via the [liver](#prop-g7-digestion-and-nutrients-absorption) ;
4. deliver it: to a working [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) as fuel ( [Proposition 49.1](https://one-course.com/books/biology/1/en/chapter/49-organs-at-work#prop-g7-organs-at-work-bill) ), or to a building site as material ( [Definition 41.4](https://one-course.com/books/biology/1/en/chapter/41-matter-from-food#def-g6-matter-from-food-production) ).

**Example 50.9 (The trace, run on bread).**

Bread: starchy family, mainly starch. Dismantling starts in the mouth ([saliva](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract) — the long-chew sweetness), pauses in the [stomach](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract)’s acid, finishes in the [small intestine](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract)’s juices: [glucose](https://one-course.com/books/biology/1/en/chapter/49-organs-at-work#prop-g7-organs-at-work-bill). Crossing: [villi](#prop-g7-digestion-and-nutrients-absorption), [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb), [liver](#prop-g7-digestion-and-nutrients-absorption) — which banks part of the [glucose](https://one-course.com/books/biology/1/en/chapter/49-organs-at-work#prop-g7-organs-at-work-bill) for later, one of its many services. Delivery: a thigh [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) mid-sprint burns it within the hour. The bite of grade four, fully accounted.

## 50.4 Exercises

**Exercise 50.1 ★.**

What do [digestive juices](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-digestion) do that teeth and churning cannot?

**Solution of Exercise 50.1.**

They dismantle food *chemically*: cutting its large components — starch, building matter, fats — into small, blood-ready units. Teeth and churning only make pieces smaller; the juices change what the pieces are.

**Exercise 50.2 ★.**

Describe the two-tube [saliva](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract) experiment: setup, result, and both conclusions.

**Solution of Exercise 50.2.**

Two tubes of starch paste at body warmth: one with fresh [saliva](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract), one with boiled [saliva](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract). After an hour the fresh tube’s starch has become sugars; the boiled tube is unchanged. Conclusions: the juice alone dismantles starch — no mouth needed — and boiling destroys its working substance.

**Exercise 50.3 ★.**

List the [nutrient](#def-g7-digestion-and-nutrients-families) families and one food source of each.

**Solution of Exercise 50.3.**

Sugars ([glucose](https://one-course.com/books/biology/1/en/chapter/49-organs-at-work#prop-g7-organs-at-work-bill)) — from starchy and sweet foods, like bread; building-matter units — from meat, [fish](https://one-course.com/books/biology/1/en/chapter/14-sorting-living-things#prop-g3-sorting-living-things-groups), [eggs](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born), dairy, [pulses](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#prop-g4-heart-and-blood-pulse); fat units — from oils and butter; plus vitamins, [mineral salts](https://one-course.com/books/biology/1/en/chapter/22-what-plants-need#prop-g4-what-plants-need-needs) and water — from [fruit](https://one-course.com/books/biology/1/en/chapter/16-flowers-fruits-and-seeds#prop-g3-flowers-fruits-seeds-transform) and vegetables among others.

**Exercise 50.4 ★.**

Which food components need no dismantling? And which one defeats our juices entirely — to what useful end?

**Solution of Exercise 50.4.**

Vitamins, [mineral salts](https://one-course.com/books/biology/1/en/chapter/22-what-plants-need#prop-g4-what-plants-need-needs) and water cross as they are. Fibre defeats our juices entirely: uncut, it crosses nothing and travels on as the leftovers’ useful sweeping bulk.

**Exercise 50.5 ★.**

What is a villus? What does each one wrap?

**Solution of Exercise 50.5.**

A finger-shaped bump of the [small intestine](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract)’s inner wall, about a millimetre high, one of millions. Each wraps its own fine [blood vessels](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-vessels).

**Exercise 50.6 ★.**

Where does [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) from the intestine go before general delivery, and name one service that organ renders.

**Solution of Exercise 50.6.**

To the [liver](#prop-g7-digestion-and-nutrients-absorption) — the sorting-and-storing organ. One service: banking part of the meal’s [glucose](https://one-course.com/books/biology/1/en/chapter/49-organs-at-work#prop-g7-organs-at-work-bill) for later release.

**Exercise 50.7 ★★.**

Explain the flat-pack image of [Remark 50.3](#rem-g7-digestion-and-nutrients-why): the two reasons food must be dismantled.

**Solution of Exercise 50.7.**

Transport: food’s large components cannot cross the intestine’s wall — the wardrobe and the letterbox. And reassembly: the body builds its own substance from small units, so it wants parts, not furniture — flat-pack for the journey and for the rebuild.

**Exercise 50.8 ★★.**

Why does the [boiled-saliva](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract) tube matter? Name the method, and what the boiling shows about the juice’s working substance.

**Solution of Exercise 50.8.**

It is the control of a fair test: identical in everything but the juice’s working state. Its unchanged starch pins the dismantling on fresh [saliva](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract) specifically — and shows the working substance is heat-fragile, destroyed by boiling like delicate machinery.

**Exercise 50.9 ★★.**

Set the [villi](#prop-g7-digestion-and-nutrients-absorption) beside the [lungs](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four)’ pockets and the [gills](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four)’ [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering): state the shared specification, and each machine’s cargo.

**Solution of Exercise 50.9.**

All three are large, thin, moist, blood-lined crossing surfaces. Cargo: [oxygen](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) and [carbon dioxide](https://one-course.com/books/biology/1/en/chapter/46-what-is-respiration#def-g7-what-is-respiration-respiration) for [lung](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) pockets and [gill](https://one-course.com/books/biology/1/en/chapter/47-how-animals-breathe#def-g7-how-animals-breathe-four) [feathers](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-covering); [nutrients](#def-g7-digestion-and-nutrients-families) for the [villi](#prop-g7-digestion-and-nutrients-absorption). One specification, three borders.

**Exercise 50.10 ★★.**

Run [Method 50.8](#met-g7-digestion-and-nutrients-trace) on a boiled [egg](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) (building matter), to a repair site in a healing scrape ([Exercise 45.10](https://one-course.com/books/biology/1/en/chapter/45-the-cell-unit-of-life#exo-g6-cell-unit-of-life-10)).

**Solution of Exercise 50.10.**

[Egg](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born): building-matter family. Dismantling starts in the [stomach](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract)’s acid juice (the cubes’ fate in the glass gut), finishes in the [small intestine](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract): small building units. Crossing: [villi](#prop-g7-digestion-and-nutrients-absorption), [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb), [liver](#prop-g7-digestion-and-nutrients-absorption). Delivery: to the healing scrape, where dividing [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) take the units as material for the rebuild.

**Exercise 50.11 ★★.**

A crash diet skips all building-matter foods for weeks. Using [Definition 50.4](#def-g7-digestion-and-nutrients-families) and [Definition 41.4](https://one-course.com/books/biology/1/en/chapter/41-matter-from-food#def-g6-matter-from-food-production), predict what runs short and what suffers first in a still-growing body.

**Solution of Exercise 50.11.**

The building-matter units run short. First to suffer is everything under construction and renewal: growth itself, [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) upkeep, repairs, and the constantly renewed linings and [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) components — [fuel-nutrients](#def-g7-digestion-and-nutrients-families) cannot substitute, because bricks and firewood are not interchangeable.

**Exercise 50.12 ★★★.**

“[Digestion](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-digestion) turns a meal into a delivery list.” Defend the sentence end to end — juices, [nutrients](#def-g7-digestion-and-nutrients-families), [villi](#prop-g7-digestion-and-nutrients-absorption), [liver](#prop-g7-digestion-and-nutrients-absorption), deliveries — in one paragraph.

**Solution of Exercise 50.12.**

For example: a meal enters as furniture — starch, building matter, fat. Station by station its juices cut each into its units: sugars, building units, fat units — the delivery list. At the [villi](#prop-g7-digestion-and-nutrients-absorption) the list crosses into the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb), the [liver](#prop-g7-digestion-and-nutrients-absorption) sorts, banks and releases, and the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb) distributes: fuel to working [muscles](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle), bricks to building sites, the rest to stores. What sat on the plate as food circulates within hours as deliveries with addresses.

## 50.5 Problem: The Glass Gut

**Problem 50.1.**

Weekend problem — building digestion on the bench, tube by tube

The class builds a “glass gut”: a rack of tubes at body warmth, each simulating one station of the tract. Tube M (mouth): starch paste plus fresh [saliva](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract). Tube M$'$: starch paste plus boiled [saliva](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract). Tube S ([stomach](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract)): [egg](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) white cubes plus [stomach](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract) juice from the pharmacist. Tube S$'$: the same cubes in plain water. All run for two hours; then the tests — for starch, for sugars, and a look at the cubes.

**Part I — Predictions and results.**

1. Predict the two-hour results of all four tubes.
2. M turns sugary, M $'$ does not. State the conclusion, and the exact role of M $'$ .
3. S’s cubes shrink, soften and clear; S $'$ ’s sit unchanged. What has the [stomach](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract) juice done — and what kind of [nutrient](#def-g7-digestion-and-nutrients-families) family is being produced?
4. No tube was chewed or churned. What does the rack prove about where [digestion](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-digestion) ’s real work lies?

**Part II — Improving the model.**

5. A student proposes running tube M at fridge [temperature](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions) “for tidiness”. Object, with [Proposition 42.8](https://one-course.com/books/biology/1/en/chapter/42-decomposers-and-the-soil#prop-g6-decomposers-and-soil-speed) ’s pattern applied to juices.
6. Another proposes one big tube with both juices mixed. What station-by-station fact of [Proposition 50.1](#prop-g7-digestion-and-nutrients-chemical) would the mixture blur?
7. The glass gut has no wall: nothing is absorbed. Which structure would a full model need next, and what two properties must its material have?
8. Add fibre — bran flakes — to a fifth tube with every juice available. Predict its two-hour state, and its meaning for the model.

**Part III — From glass to gut.**

9. In the body, M’s sugars would next meet the [villi](#prop-g7-digestion-and-nutrients-absorption) . Continue their journey in four stops.
10. The [stomach](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract) ’s juice works in strong acid; the intestine’s juices need that acid neutralized. What does this say about “each juice at its own station’s [conditions](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions) ” — and about swallowing the stations in order?
11. A pharmaceutical company sells juice capsules for people whose own stations underperform. Explain, with the rack, why such a therapy is even possible.
12. Summarize the practical in one sentence containing *juices* , *[nutrients](#def-g7-digestion-and-nutrients-families)* and *control tubes* .

**Solution of Problem 50.1.**

**1.** M: starch gone, sugars present. M$'$: starch unchanged. S: cubes attacked — shrunken, softened. S$'$: cubes unchanged.

**2.** [Saliva](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract)’s working substance dismantles starch into sugars. M$'$ is the control: identical but for the boiled juice, it proves the dismantler was the fresh [saliva](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-tract) and nothing else in the tube.

**3.** It has begun dismantling the [egg](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born)’s building matter — cutting it toward its small units, the building-matter [nutrients](#def-g7-digestion-and-nutrients-families). (The clearing, shrinking cubes are that cutting made visible.)

**4.** That the real work is chemical: with no tooth and no churn in any tube, dismantling ran wherever the right juice met the right food. Mechanical breaking only serves the chemistry.

**5.** Juices are working substances with working [conditions](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions): chilled, they idle — like every worker from [decomposer](https://one-course.com/books/biology/1/en/chapter/42-decomposers-and-the-soil#def-g6-decomposers-and-soil-decomposers) to [yeast](https://one-course.com/books/biology/1/en/chapter/43-producing-our-food#ex-g6-producing-our-food-bread). The tube would sit undigested, and prove only that fridges pause chemistry.

**6.** That each juice has its specialties and its own station’s [conditions](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions): mixed in one tube, neither works at its best, and the model loses the tract’s station-by-station order — the very thing it is meant to teach.

**7.** The absorbing wall: a villus-carpeted surface. Its material must be thin (an easy crossing) and selective-but-permeable to the small units — with, in the body, [blood vessels](https://one-course.com/books/biology/1/en/chapter/27-the-heart-and-the-blood#def-g4-heart-and-blood-vessels) behind it to carry the crossers away.

**8.** Unchanged flakes: no juice of ours cuts fibre. Meaning: the model correctly predicts fibre’s real fate — crossing nothing, sweeping on — the exception that maps the system’s limits.

**9.** Across the [villi](#prop-g7-digestion-and-nutrients-absorption)’s thin walls; into the [blood](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#prop-g4-journey-of-food-absorb); through the [liver](#prop-g7-digestion-and-nutrients-absorption), which banks part; out on general delivery to the body’s [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) — a working [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) burning them within the hour.

**10.** [Conditions](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions) are part of the machinery: each station is tuned — acid here, neutralized there — so the juices must meet food in the right order, which is exactly what swallowing through the stations provides. The tract is a production line, not a bucket.

**11.** Because [digestion](https://one-course.com/books/biology/1/en/chapter/25-the-journey-of-food#def-g4-journey-of-food-digestion)’s workers are substances, not organs: as the rack shows, a juice dismantles its food wherever the two meet in the right [conditions](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions). A capsule delivering the missing juice to the right station can therefore do the station’s chemistry.

**12.** “The juices, each at its station, cut food into [nutrients](#def-g7-digestion-and-nutrients-families) — and the control tubes proved it was the juices, and nothing else, doing the cutting.”