---
title: "A First Look at Cells"
book: "Primary & Middle School Biology"
subject: biology
language: en
chapter: 29
exercises: 11
source: https://one-course.com/books/biology/1/en/chapter/29-a-first-look-at-cells
---

# Chapter 29 — A First Look at Cells

Look at the back of your hand: smooth [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense), one seamless surface. Now imagine a magnifying glass strong enough to zoom in a thousand times. The seamless surface would dissolve into a pavement of tiny compartments, fitted together like flagstones — and every [living thing](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living) you have met in this book, moss to whale, is paved the same way. [Biology](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living)’s biggest secret is small.

## 29.1 Living things are made of cells

**Definition 29.1 (Cell).**

A *cell* is the smallest living building block of a [living thing](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living). Almost all cells are far too small to see with the naked [eye](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense); a body is built of them as a wall is built of bricks. Your own body is made of tens of thousands of billions of cells.

**Proposition 29.2 (The cell principle).**

Every [living thing](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living) is made of [cells](#def-g5-first-look-at-cells-cell) — every [animal](https://one-course.com/books/biology/1/en/chapter/2-animals-around-us#def-g1-animals-around-us-animal), every [plant](https://one-course.com/books/biology/1/en/chapter/3-plants-around-us#def-g1-plants-around-us-plant), every mushroom, down to [living things](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living) that are just *one* [cell](#def-g5-first-look-at-cells-cell). This shared construction is one of the great signs of the *unity of life*: however different the daisy and the whale, up close they are built of the same kind of small living units.

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

**Example 29.3 (How small is small?).**

A typical [cell](#def-g5-first-look-at-cells-cell) of your body measures about a fiftieth of a millimetre. Line up $50$ of them and they span one millimetre — a pencil dot could cover the parade. That is why nobody suspected [cells](#def-g5-first-look-at-cells-cell) for most of human history: the building blocks of life are smaller than the [eye](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense)’s smallest visible speck.

## 29.2 The instrument that revealed them

**Definition 29.4 (Microscope).**

A *microscope* is an instrument whose lenses magnify very small things hundreds of times. Pointed at thin slices of living material, it revealed what no [eye](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) had ever seen: the [cells](#def-g5-first-look-at-cells-cell). The name itself was coined by an early observer who thought the little compartments in a slice of cork looked like monks’ small rooms — *[cells](#def-g5-first-look-at-cells-cell)*.

![The instrument that revealed the cells: lenses, light, and a thin slice on a glass slide.](https://one-course.com/images/onecourse/chapters/biology-1/g5-first-look-at-cells/img-37d92f3501dd.jpg)

*The instrument that revealed the [cells](#def-g5-first-look-at-cells-cell): lenses, light, and a thin slice on a glass slide.*

**Example 29.5 (Two classics under the lens).**

Two observations any school [microscope](#def-g5-first-look-at-cells-microscope) can repeat: a shred of the thin [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) inside an onion shows neat rows of brick-like [cells](#def-g5-first-look-at-cells-cell), wall against wall; a gentle scrape from the inside of a cheek shows scattered, rounder [cells](#def-g5-first-look-at-cells-cell) — yours. [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), side by side, both cellular.

![Under the microscope: onion cells and cheek cells. Each compartment — with its small dark spot — is one cell.](https://one-course.com/images/onecourse/chapters/biology-1/g5-first-look-at-cells/img-4e6b6957cf80.jpg)

![Under the microscope: onion cells and cheek cells. Each compartment — with its small dark spot — is one cell.](https://one-course.com/images/onecourse/chapters/biology-1/g5-first-look-at-cells/img-11593986038d.jpg)

*onion [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense): brick-like [cells](#def-g5-first-look-at-cells-cell) in neat rows*

*cheek scrape: rounder [cells](#def-g5-first-look-at-cells-cell), scattered*

*Under the [microscope](#def-g5-first-look-at-cells-microscope): onion [cells](#def-g5-first-look-at-cells-cell) and cheek [cells](#def-g5-first-look-at-cells-cell). Each compartment — with its small dark spot — is one [cell](#def-g5-first-look-at-cells-cell).*

**Remark 29.6 (The dark spot).**

In both views, most [cells](#def-g5-first-look-at-cells-cell) show a small darker spot inside: the [cell](#def-g5-first-look-at-cells-cell)’s command center, called its *nucleus*. What the nucleus commands with — the famous thread of instructions coiled inside it — is a story for the later years of this book; for now, note only that the little rooms are not empty.

## 29.3 From one cell to a body

**Proposition 29.7 (Bodies grow by their cells).**

A body grows, and repairs itself, because its [cells](#def-g5-first-look-at-cells-cell) can *divide*: one [cell](#def-g5-first-look-at-cells-cell) splits into two living [cells](#def-g5-first-look-at-cells-cell), two into four, and so on. Growing taller ([Proposition 15.7](https://one-course.com/books/biology/1/en/chapter/15-the-stages-of-life#prop-g3-stages-of-life-animals)) is, underneath, [cells](#def-g5-first-look-at-cells-cell) multiplying; a healing scrape ([Remark 17.3](https://one-course.com/books/biology/1/en/chapter/17-bones-and-muscles#rem-g3-bones-and-muscles-alive)) is [cells](#def-g5-first-look-at-cells-cell) rebuilding. And every body, even the largest, began as a *single [cell](#def-g5-first-look-at-cells-cell)* — the fertilized [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) of [Definition 23.1](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual).

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

**Example 29.8 (From one to trillions).**

The fertilized [egg](https://one-course.com/books/biology/1/en/chapter/8-how-animals-grow-up#def-g2-animals-grow-up-born) divides into $2$ [cells](#def-g5-first-look-at-cells-cell), then $4$, $8$, $16$ … Repeated division, over months and years, built the tens of thousands of billions of [cells](#def-g5-first-look-at-cells-cell) reading this sentence. Every [cell](#def-g5-first-look-at-cells-cell) of you descends from that first one.

**Remark 29.9 (Whole lives in one cell).**

Some [living things](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living) never go beyond one [cell](#def-g5-first-look-at-cells-cell) — and need nothing more. A drop of pond water teems with them: single [cells](#def-g5-first-look-at-cells-cell) that swim, feed, and divide, whole lives at [microscope](#def-g5-first-look-at-cells-microscope) scale. The [germs](https://one-course.com/books/biology/1/en/chapter/6-healthy-habits#def-g1-healthy-habits-germs) of [Definition 6.1](https://one-course.com/books/biology/1/en/chapter/6-healthy-habits#def-g1-healthy-habits-germs) are mostly such single-celled [living things](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living) — old acquaintances, now with an explanation.

## 29.4 Exercises

**Exercise 29.1 ★.**

What is a [cell](#def-g5-first-look-at-cells-cell)? Roughly how many make up your body?

**Solution of Exercise 29.1.**

The smallest living building block of a [living thing](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living). Your body is made of tens of thousands of billions of them.

**Exercise 29.2 ★.**

Why did nobody know about [cells](#def-g5-first-look-at-cells-cell) for most of history? Which instrument changed that?

**Solution of Exercise 29.2.**

Because [cells](#def-g5-first-look-at-cells-cell) are smaller than the smallest speck the [eye](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense) can see. The [microscope](#def-g5-first-look-at-cells-microscope) — magnifying thin slices hundreds of times — revealed them.

**Exercise 29.3 ★.**

Describe what the [microscope](#def-g5-first-look-at-cells-microscope) shows in onion [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense), and in a cheek scrape. What do the two views have in common?

**Solution of Exercise 29.3.**

Onion [skin](https://one-course.com/books/biology/1/en/chapter/5-the-five-senses#def-g1-the-five-senses-sense): brick-like [cells](#def-g5-first-look-at-cells-cell) in neat rows, wall against wall. Cheek scrape: rounder [cells](#def-g5-first-look-at-cells-cell), scattered. In common: both materials are made entirely of [cells](#def-g5-first-look-at-cells-cell), most showing a small dark spot inside.

**Exercise 29.4 ★.**

What is the small dark spot visible in most [cells](#def-g5-first-look-at-cells-cell) called?

**Solution of Exercise 29.4.**

The nucleus — the [cell](#def-g5-first-look-at-cells-cell)’s command center.

**Exercise 29.5 ★.**

About how many body [cells](#def-g5-first-look-at-cells-cell), lined up, span one millimetre — and how long is one [cell](#def-g5-first-look-at-cells-cell), in millimetres?

**Solution of Exercise 29.5.**

About $50$ [cells](#def-g5-first-look-at-cells-cell) span one millimetre; one [cell](#def-g5-first-look-at-cells-cell) is about a fiftieth of a millimetre — $0.02\,\mathrm{mm}$.

**Exercise 29.6 ★.**

What are [cells](#def-g5-first-look-at-cells-cell) doing, underneath, when a child grows taller? And when a scraped [knee](https://one-course.com/books/biology/1/en/chapter/4-my-body#def-g1-my-body-joint) heals?

**Solution of Exercise 29.6.**

Growing taller: the body’s [cells](#def-g5-first-look-at-cells-cell) are dividing and multiplying. Healing: [cells](#def-g5-first-look-at-cells-cell) are dividing to rebuild the damaged patch.

**Exercise 29.7 ★.**

Every human body began as how many [cells](#def-g5-first-look-at-cells-cell)? What was that first [cell](#def-g5-first-look-at-cells-cell), by [Definition 23.1](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual)?

**Solution of Exercise 29.7.**

One. It was the fertilized [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) — the joining of one [sperm cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual) and one [egg cell](https://one-course.com/books/biology/1/en/chapter/23-how-animals-reproduce#def-g4-animal-reproduction-sexual).

**Exercise 29.8 ★.**

A [cell](#def-g5-first-look-at-cells-cell) divides into $2$; each of those divides again, and so on. Continue the doubling from $1$: how many [cells](#def-g5-first-look-at-cells-cell) after $5$ divisions? After $10$?

**Solution of Exercise 29.8.**

Doubling from $1$ gives $2$, $4$, $8$, $16$, $32$: there are $32$ [cells](#def-g5-first-look-at-cells-cell) after $5$ divisions. Five more doublings give $64$, $128$, $256$, $512$, $1024$: so $1024$ [cells](#def-g5-first-look-at-cells-cell) after $10$.

**Exercise 29.9 ★★.**

Explain the sentence: “the daisy and the whale show the [unity of life](#prop-g5-first-look-at-cells-principle).” What do they share, by [Proposition 29.2](#prop-g5-first-look-at-cells-principle)?

**Solution of Exercise 29.9.**

However different in size, shape and life, both are built of the same kind of small living units — [cells](#def-g5-first-look-at-cells-cell). The shared construction plan, common to all [living things](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living), is what “[unity of life](#prop-g5-first-look-at-cells-principle)” means.

**Exercise 29.10 ★★.**

A drop of pond water holds [living things](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living) of one single [cell](#def-g5-first-look-at-cells-cell) each. Check them against the signs of life of [Proposition 1.3](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#prop-g1-living-or-not-signs): which signs can a single [cell](#def-g5-first-look-at-cells-cell) show?

**Solution of Exercise 29.10.**

All of them: a single [cell](#def-g5-first-look-at-cells-cell) is born (of a dividing parent [cell](#def-g5-first-look-at-cells-cell)), feeds, grows, divides — making new [living things](https://one-course.com/books/biology/1/en/chapter/1-living-or-not-living#def-g1-living-or-not-living) like itself — and dies. One [cell](#def-g5-first-look-at-cells-cell) is enough for a complete life.

**Exercise 29.11 ★★★.**

An elephant’s [cells](#def-g5-first-look-at-cells-cell) are about the same size as a mouse’s. What, then, makes the elephant bigger — larger [cells](#def-g5-first-look-at-cells-cell), or more [cells](#def-g5-first-look-at-cells-cell)? What does your answer say about how growing works, by [Proposition 29.7](#prop-g5-first-look-at-cells-growth)?

**Solution of Exercise 29.11.**

More [cells](#def-g5-first-look-at-cells-cell) — [cell](#def-g5-first-look-at-cells-cell) size barely changes between mouse and elephant. Growing, then, is not inflating one’s [cells](#def-g5-first-look-at-cells-cell) but multiplying them: bodies get big by division after division, exactly as the proposition says.