---
title: "The Cell, Unit of Life"
book: "Primary & Middle School Biology"
subject: biology
language: en
chapter: 45
exercises: 12
source: https://one-course.com/books/biology/1/en/chapter/45-the-cell-unit-of-life
---

# Chapter 45 — The Cell, Unit of Life

[Chapter 29](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#ch-g5-first-look-at-cells) ended with a promise: the little rooms are not empty. This year the school [microscope](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-microscope) comes back out — with better slides and sharper questions. What exactly is in a [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell)? Do [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) and [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) differ? And is a [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) a *part* of life, or can one [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) be a whole [living thing](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-components)? The smallest unit of [biology](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living) deserves one thorough chapter.

## 45.1 Inside the cell

**Definition 45.1 (The cell’s parts).**

Seen properly, every [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) shows the same basic equipment:

1. the *membrane* , the [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) ’s thin outer envelope, which holds it together and controls what enters and [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts) ;
2. the *cytoplasm* , the jelly filling where the [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) ’s work goes on;
3. the *nucleus* ( [Remark 29.6](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#rem-g5-first-look-at-cells-nucleus) ’s dark spot), the command center that directs the [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) ’s life and its division.

Membrane, cytoplasm, nucleus: the three-part kit of nearly every [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) in every [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant), [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) and [fungus](https://one-course.com/books/biology/1/en/chapter/42-decomposers-and-the-soil#def-g6-decomposers-and-soil-decomposers).

**Proposition 45.2 (Plant cells carry extras).**

[Plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) add equipment of their own:

- a stiff *cell wall* outside the [membrane](#def-g6-cell-unit-of-life-parts) — the walls that make onion [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) ’s brickwork ( [Example 29.5](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#ex-g5-first-look-at-cells-classics) ) and give [plants](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) their standing stiffness without [bones](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton) ;
- green grains of the light-catching substance in the [cytoplasm](#def-g6-cell-unit-of-life-parts) — the [producers](https://one-course.com/books/biology/1/en/chapter/34-food-webs#def-g5-food-webs-roles) ’ workshops ( [Proposition 40.3](https://one-course.com/books/biology/1/en/chapter/40-plants-are-producers#prop-g6-plants-are-producers-produce) ), absent from every [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) ;
- often a large water-filled pocket whose pressure keeps the [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) plump — wilting ( [Example 3.9](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#ex-g1-plants-around-us-wilt) ) is these pockets going slack.

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

![The two classic cells side by side. Same three-part kit; the plant cell adds its wall, its green grains and its plump water pocket.](https://one-course.com/images/onecourse/chapters/biology-1/g6-cell-unit-of-life/fig-8a6b2d102726.svg)

![The two classic cells side by side. Same three-part kit; the plant cell adds its wall, its green grains and its plump water pocket.](https://one-course.com/images/onecourse/chapters/biology-1/g6-cell-unit-of-life/fig-364244e2da8e.svg)

*[animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell): [membrane](#def-g6-cell-unit-of-life-parts), [cytoplasm](#def-g6-cell-unit-of-life-parts), [nucleus](#def-g6-cell-unit-of-life-parts)*

*[plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell): the same kit, plus wall, green grains and water pocket*

*The two classic [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) side by side. Same three-part kit; the [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) adds its wall, its green grains and its plump water pocket.*

**Example 45.3 (Telling the slides apart).**

Back at the [microscope](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-microscope) with [Example 29.5](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#ex-g5-first-look-at-cells-classics)’s two slides, the differences now have names. Onion [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense): straight walls meeting at corners — [cell walls](#prop-g6-cell-unit-of-life-plantextras); a [nucleus](#def-g6-cell-unit-of-life-parts) pushed to the side — the water pocket’s doing. Cheek [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell): no walls, soft rounded outlines — [membrane](#def-g6-cell-unit-of-life-parts) only — [nucleus](#def-g6-cell-unit-of-life-parts) near the middle. One glance now reads [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) or [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal).

## 45.2 Whole lives in one cell

**Definition 45.4 (Unicellular living things).**

A *unicellular* [living thing](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-components) is a complete organism of exactly one [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) ([Remark 29.9](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#rem-g5-first-look-at-cells-onecelled)). In one [membrane](#def-g6-cell-unit-of-life-parts) it feeds, moves, senses and reproduces — by dividing in two ([Proposition 29.7](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#prop-g5-first-look-at-cells-growth)’s division, as a whole life story). The [yeast](https://one-course.com/books/biology/1/en/chapter/43-producing-our-food#ex-g6-producing-our-food-bread) of [Example 43.5](https://one-course.com/books/biology/1/en/chapter/43-producing-our-food#ex-g6-producing-our-food-bread) is one; so are the swimmers of any pond drop, and most [microbes](https://one-course.com/books/biology/1/en/chapter/43-producing-our-food#def-g6-producing-our-food-fermentation) of [soil](https://one-course.com/books/biology/1/en/chapter/42-decomposers-and-the-soil#def-g6-decomposers-and-soil-soil), yogurt and hands.

**Example 45.5 (A drop of pond water, toured).**

One drop from the aquarium’s green wall, under the lens: a slipper-shaped [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) rowing past with beating hairs, faster than anything else in view; green single [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) — swimming [producers](https://one-course.com/books/biology/1/en/chapter/34-food-webs#def-g5-food-webs-roles), catching [light](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions) like any [leaf](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts); a blob that flows around its food and swallows it whole. Each is feeding, moving, avoiding — a complete animal-or-plant’s life, budgeted into one [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell).

**Remark 45.6 (One cell or many: two ways to be alive).**

A [unicellular](#def-g6-cell-unit-of-life-unicellular) organism does everything with one [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell); your body does everything with tens of thousands of billions, *specialized* — cheek [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) for lining, [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) that contract ([Definition 17.6](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle)), [nerve](https://one-course.com/books/biology/1/en/chapter/33-the-brain-in-command#def-g5-brain-in-command-system) [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) that carry [Proposition 33.2](https://one-course.com/books/biology/1/en/chapter/33-the-brain-in-command#prop-g5-brain-in-command-loop)’s messages. Many [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) allow division of labor; one [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) means every trade in one room. Both plans work: the [unicellulars](#def-g6-cell-unit-of-life-unicellular) are the most numerous [living things](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-components) on Earth.

## 45.3 The cell theory

**Proposition 45.7 (The cell theory).**

Two sentences, centuries in the making, sum up this chapter and [Chapter 29](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#ch-g5-first-look-at-cells):

1. every [living thing](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-components) is made of one or more [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) , the smallest units of life;
2. every [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) comes from the division of an existing [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) .

No exception has ever been found — not in the deepest sea, not in the oldest rock pool. The second sentence has a sweeping consequence: every [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) alive today continues an unbroken chain of divisions reaching back to the earliest life — [kinship](https://one-course.com/books/biology/1/en/chapter/44-classification-and-kinship#prop-g6-classification-kinship-kinship) ([Proposition 44.4](https://one-course.com/books/biology/1/en/chapter/44-classification-and-kinship#prop-g6-classification-kinship-kinship)) written at the [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell)’s scale.

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

**Method 45.8 (Reading any organism at cell scale).**

Faced with any [living thing](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-components):

1. ask first: one [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) , or many?
2. if many: expect specialized kinds — lining, carrying, contracting, commanding;
3. [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) or [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) ? check for walls and green grains at the [microscope](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-microscope) ;
4. every growth, repair or reproduction you observe: translate it into divisions of [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) — sentence 2 guarantees the translation exists.

**Example 45.9 (The theory at work in this book).**

Sentence 1 underwrote the [unity of life](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#prop-g5-first-look-at-cells-principle) ([Proposition 29.2](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#prop-g5-first-look-at-cells-principle)); sentence 2 underwrote growth-by-division ([Example 29.8](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#ex-g5-first-look-at-cells-one)), the [yeast](https://one-course.com/books/biology/1/en/chapter/43-producing-our-food#ex-g6-producing-our-food-bread) doubling in the dough, [decomposers](https://one-course.com/books/biology/1/en/chapter/42-decomposers-and-the-soil#def-g6-decomposers-and-soil-decomposers) multiplying in the warm heap, and the microbe workforces of [Chapter 43](https://one-course.com/books/biology/1/en/chapter/43-producing-our-food#ch-g6-producing-our-food). One small theory, carrying a large share of this year’s [biology](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living).

## 45.4 Exercises

**Exercise 45.1 ★.**

Name the three-part kit of every [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell), with each part’s job.

**Solution of Exercise 45.1.**

The [membrane](#def-g6-cell-unit-of-life-parts) — the thin envelope holding the [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) together and controlling what enters and [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts); the [cytoplasm](#def-g6-cell-unit-of-life-parts) — the jelly filling where the work goes on; the [nucleus](#def-g6-cell-unit-of-life-parts) — the command center directing the [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell)’s life and division.

**Exercise 45.2 ★.**

Name the [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell)’s three extras and what each does.

**Solution of Exercise 45.2.**

The stiff [cell wall](#prop-g6-cell-unit-of-life-plantextras) — [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) stiffness without [bones](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton); the green grains — the light-catching workshops of the [producers](https://one-course.com/books/biology/1/en/chapter/34-food-webs#def-g5-food-webs-roles)’ trade; the large water pocket — its pressure keeps the [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) plump.

**Exercise 45.3 ★.**

At the [microscope](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-microscope), how do you tell an [onion-skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) from a cheek [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell)? Two visible differences.

**Solution of Exercise 45.3.**

Onion: straight walls meeting at corners, [nucleus](#def-g6-cell-unit-of-life-parts) pushed to the side. Cheek: soft rounded outline with [membrane](#def-g6-cell-unit-of-life-parts) only, [nucleus](#def-g6-cell-unit-of-life-parts) near the middle.

**Exercise 45.4 ★.**

What is a [unicellular](#def-g6-cell-unit-of-life-unicellular) [living thing](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-components)? Name two from this chapter.

**Solution of Exercise 45.4.**

A complete organism of exactly one [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell). The [yeast](https://one-course.com/books/biology/1/en/chapter/43-producing-our-food#ex-g6-producing-our-food-bread); the pond drop’s slipper-shaped rower (or its green swimmers, the flowing blob, yogurt’s [microbes](https://one-course.com/books/biology/1/en/chapter/43-producing-our-food#def-g6-producing-our-food-fermentation)).

**Exercise 45.5 ★.**

State the [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) theory’s two sentences.

**Solution of Exercise 45.5.**

Every [living thing](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-components) is made of one or more [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell), the smallest units of life; and every [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) comes from the division of an existing [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell).

**Exercise 45.6 ★.**

How does a [unicellular](#def-g6-cell-unit-of-life-unicellular) organism reproduce?

**Solution of Exercise 45.6.**

By dividing in two: one [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) becomes two complete organisms.

**Exercise 45.7 ★★.**

Why do [plants](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) stand up without [bones](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton)? Which two pieces of [plant-cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) equipment share the job — and what is wilting, at [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) scale?

**Solution of Exercise 45.7.**

The stiff [cell walls](#prop-g6-cell-unit-of-life-plantextras) and the plump water pockets: millions of walled, pressurized [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) brace one another like inflated brickwork. Wilting is the pockets going slack — pressure lost for lack of water, the whole structure sagging.

**Exercise 45.8 ★★.**

Which [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) part is the [producers](https://one-course.com/books/biology/1/en/chapter/34-food-webs#def-g5-food-webs-roles)’ workshop of [Proposition 40.3](https://one-course.com/books/biology/1/en/chapter/40-plants-are-producers#prop-g6-plants-are-producers-produce)? Why must every [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) lack it, by [Proposition 41.1](https://one-course.com/books/biology/1/en/chapter/41-matter-from-food#prop-g6-matter-from-food-transform)?

**Solution of Exercise 45.8.**

The green grains. [Animals](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) are [consumers](https://one-course.com/books/biology/1/en/chapter/34-food-webs#def-g5-food-webs-roles) — transformers of eaten [organic matter](https://one-course.com/books/biology/1/en/chapter/40-plants-are-producers#def-g6-plants-are-producers-matter), never makers-from-mineral — so their [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) carry no manufacturing grains: the equipment matches the trade, [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) by [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell).

**Exercise 45.9 ★★.**

Give three specialized [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) kinds of your body and each one’s trade, from [Remark 45.6](#rem-g6-cell-unit-of-life-twoways).

**Solution of Exercise 45.9.**

For example: lining [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) (the cheek’s), covering surfaces; [muscle](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-muscle) [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell), contracting to pull [bones](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#def-g3-bones-and-muscles-skeleton); [nerve](https://one-course.com/books/biology/1/en/chapter/33-the-brain-in-command#def-g5-brain-in-command-system) [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell), carrying the reports and orders of the [brain](https://one-course.com/books/biology/1/en/chapter/33-the-brain-in-command#def-g5-brain-in-command-system)’s loop.

**Exercise 45.10 ★★.**

Translate into [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) divisions, per [Method 45.8](#met-g6-cell-unit-of-life-read): a healing scrape; dough doubling overnight; a growing tadpole.

**Solution of Exercise 45.10.**

Healing scrape: [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) dividing to rebuild the patch. Dough doubling: [yeast](https://one-course.com/books/biology/1/en/chapter/43-producing-our-food#ex-g6-producing-our-food-bread) [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) dividing again and again in the warm sugar-fed dough. Growing tadpole: its [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) dividing and specializing as the body builds.

**Exercise 45.11 ★★.**

The pond drop’s green swimmers are single [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) that catch [light](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions). Which trade of [Definition 34.3](https://one-course.com/books/biology/1/en/chapter/34-food-webs#def-g5-food-webs-roles) do they run, and what does that make them in the pond’s [food web](https://one-course.com/books/biology/1/en/chapter/34-food-webs#def-g5-food-webs-web)?

**Solution of Exercise 45.11.**

The [producers](https://one-course.com/books/biology/1/en/chapter/34-food-webs#def-g5-food-webs-roles)’ trade: single-celled manufacture of [organic matter](https://one-course.com/books/biology/1/en/chapter/40-plants-are-producers#def-g6-plants-are-producers-matter) from mineral supplies and [light](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-conditions). In the pond web they are first links — the green floor the [tadpoles](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#ex-g2-animals-grow-up-frog) and water fleas graze.

**Exercise 45.12 ★★★.**

Take the [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) theory’s second sentence seriously, as [Remark 44.8](https://one-course.com/books/biology/1/en/chapter/44-classification-and-kinship#rem-g6-classification-kinship-implies) took the tree: what unbroken chain connects one of your cheek [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) to the distant past? State the consequence in one sentence.

**Solution of Exercise 45.12.**

Each of your [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) came from a division, whose parent [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) came from a division, without one break — back through the fertilized [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual), through your parents’ [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell), and beyond into the deep past of life. Consequence in one sentence: every living [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell), yours included, is the latest link of one unbroken chain of divisions as old as life itself.

## 45.5 Problem: The Microscopy Afternoon

**Problem 45.1.**

Weekend problem — four slides, one theory

The class’s microscopy afternoon: four stations, four slides — onion [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) (A), cheek scrape (B), a drop of pond water (C), a smear of yogurt (D). Each team tours all four and keeps drawings.

**Part I — Stations A and B.**

1. At station A the team draws straight-walled “bricks”, each with a dark spot squeezed to one side. Name the wall, the spot, and the crowding culprit.
2. At station B: rounded [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) , no walls, spots central. List the equipment A’s [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) have that B’s lack — and the kit both share.
3. A teammate asks why the onion [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) , from a [bulb](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#ex-g4-plants-without-seeds-bulbtuber) grown underground, show no green grains. Answer with the grains’ job and the [bulb](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#ex-g4-plants-without-seeds-bulbtuber) ’s address.
4. Whose [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) are on slide B? State what that implies by [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) theory sentence 2: where did each of those [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) come from?

**Part II — Stations C and D.**

5. Station C: the slipper-shaped rower, green swimmers, the flowing blob. For each, name the life activities visible in one minute’s watching — and conclude what each one is.
6. The green swimmers are single [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) yet [producers](https://one-course.com/books/biology/1/en/chapter/34-food-webs#def-g5-food-webs-roles) . Which piece of equipment must the drawing show to prove it?
7. Station D’s yogurt smear shows tiny paired and chained [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) in vast numbers. Connect them to [Example 43.6](https://one-course.com/books/biology/1/en/chapter/43-producing-our-food#ex-g6-producing-our-food-yogurt) : what did these [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) do to the [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) , and what are they, in this chapter’s vocabulary?
8. One team claims slide D shows “no [living things](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-components) , just specks”. What single observation — given patience or a warm slide — would settle whether the specks live, by [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) theory sentence 2?

**Part III — Writing up.**

9. The write-up must sort all four slides by: one [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) or many? [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) , [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) or neither? Do it.
10. Every drawing of the afternoon shows the same three-part kit. State the kit, and what its universality is evidence of ( [Proposition 29.2](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#prop-g5-first-look-at-cells-principle) ).
11. The teacher closes: “everything you drew today was once one [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) .” Defend the sentence for the onion, the cheek [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) ’ owner, and the yogurt chains.
12. Formulate the afternoon’s finding as the [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) theory — two sentences — and mark, for each slide, which sentence it illustrated best.

**Solution of Problem 45.1.**

**1.** The wall: the [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) [cell wall](#prop-g6-cell-unit-of-life-plantextras). The spot: the [nucleus](#def-g6-cell-unit-of-life-parts). The crowding culprit: the large water pocket, pressing the [nucleus](#def-g6-cell-unit-of-life-parts) to the side.

**2.** A has the wall, the water pocket (and, in green parts, green grains) that B lacks. Both share the kit: [membrane](#def-g6-cell-unit-of-life-parts), [cytoplasm](#def-g6-cell-unit-of-life-parts), [nucleus](#def-g6-cell-unit-of-life-parts).

**3.** The grains are the light-catching workshops — and an underground [bulb](https://one-course.com/books/biology/1/en/chapter/24-plants-without-seeds#ex-g4-plants-without-seeds-bulbtuber) lives in the dark, where workshops would stand idle: the [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) builds them in its lit [leaves](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-parts), not its buried storeroom.

**4.** The students’ own [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell). By sentence 2, each came from the division of an existing [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) — a chain of divisions reaching back to each student’s own fertilized [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual).

**5.** The rower: moving, steering, feeding — a complete [unicellular](#def-g6-cell-unit-of-life-unicellular) organism (an animal-like one). The green swimmers: moving and light-catching — [unicellular](#def-g6-cell-unit-of-life-unicellular) [producers](https://one-course.com/books/biology/1/en/chapter/34-food-webs#def-g5-food-webs-roles). The blob: flowing, engulfing food — a [unicellular](#def-g6-cell-unit-of-life-unicellular) hunter. Each shows a whole life’s activities in one [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell).

**6.** The green grains — the light-catching equipment that runs the [producers](https://one-course.com/books/biology/1/en/chapter/34-food-webs#def-g5-food-webs-roles)’ manufacture.

**7.** They fed on the [milk](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born)’s sugar and released the acid that set it into yogurt: they are [unicellular](#def-g6-cell-unit-of-life-unicellular) [living things](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-components) — the microbe workforce, seen in person at last.

**8.** Watch a speck divide: one becoming two, two becoming four — division is the signature sentence 2 demands of the living. (On a warm slide, yogurt [microbes](https://one-course.com/books/biology/1/en/chapter/43-producing-our-food#def-g6-producing-our-food-fermentation) oblige within hours.)

**9.** A: many [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell), [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant). B: many [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) (a sample of a many-celled [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal)), [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal). C: single [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) — animal-like (rower, blob) and plant-like (green swimmers) side by side. D: single [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell), neither [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant) nor [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal) — [microbes](https://one-course.com/books/biology/1/en/chapter/43-producing-our-food#def-g6-producing-our-food-fermentation).

**10.** [Membrane](#def-g6-cell-unit-of-life-parts), [cytoplasm](#def-g6-cell-unit-of-life-parts), [nucleus](#def-g6-cell-unit-of-life-parts). Its universality across onion, cheek, pond and yogurt is evidence of the [unity of life](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#prop-g5-first-look-at-cells-principle): one construction plan under all of it.

**11.** The onion grew from one [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) of its parent [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant)’s making, by divisions; the student began as one fertilized [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual); each yogurt chain began as one microbe dividing and dividing. Everything drawn traces to a single starting [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell).

**12.** Sentence 1 — every [living thing](https://one-course.com/books/biology/1/en/chapter/37-exploring-our-environment#def-g6-exploring-our-environment-components) is made of [cells](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell): best shown by A and B (and the tour as a whole). Sentence 2 — every [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell) from a dividing [cell](https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells#def-g5-first-look-at-cells-cell): best shown by C and D, where division is a visible way of life.