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# What makes a sex cell special?

Year 9 Biology · Reproduction

<!-- Speaker note: Students explain that gametes have half the chromosome number and relate the features of sperm and egg cells to their jobs (AC9S9U02, node 1, a hinge node). About 20 minutes (Engage and Explore 6 min · Explain 9 min · Plant gametes 3 min · Evaluate 3 min · Wider adds about 1 min). -->

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## Today

**We are learning to** gametes (sperm and egg, or the nuclei in pollen and ovules) carry half the usual number of chromosomes, so fertilisation restores the full set. Each is specialised: sperm to swim and get in, the egg to feed the embryo and let only one sperm in.

- I can explain why gametes have half the usual number of chromosomes.
- I can link three features of sperm and egg cells to their jobs.
- I can describe how a plant's male nucleus reaches the egg.

<!-- Speaker note: Before class: A concept tutorial for introducing or revising the idea. Set the width with `?width=minimum`, `typical` (default) or `wider`. The hinge misconception is that gametes have the full set of chromosomes. The Engage question asks it directly. Board notes: diagrams are not to scale (a human egg is about 0.1 mm across, roughly 20 times the width of a sperm's head). The pollen grain carries the male nucleus; strictly, the male gametes form inside it. -->

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## Why it matters

> A human egg is just big enough to see without a microscope. A sperm is one of the smallest cells in the body. Together they make one cell that grows into a person.

<!-- Speaker note: In Year 8 you met specialised cells. This lesson: the cells made for reproduction. -->

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<span style="background:#dbeafe; color:#1a1a1a; border:1.5px solid #3b82f6; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">💬 Discussion</span>

## Engage: predict

**Your body cells have 46 chromosomes. How many does a human sperm or egg cell have?**

- 92: double
- 46, like every other cell
- 23: half
- None: gametes have no nucleus

<!-- Speaker note: What happens: 23: half. When a sperm (23) fertilises an egg (23), the new cell has 46 again. If gametes had 46, the number would double every generation. Take two or three predictions before anyone runs it. -->

<!-- Speaker note: Teacher: count how many chose 46. -->

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## Explore it

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fbiology-9%2Fgametes%2Fstep-2&code=widget%3A+cell%0Ashow%3A+sperm%2C+egg%0Afind%3A+nucleus%2C+acrosome%2C+midpiece%2C+tail%2C+jelly%2C+yolk%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/biology-9/gametes/?full=step-2#step-2)

<!-- Speaker note: Click each part to see what it is and what it does. -->

<!-- Speaker note: Teacher: ask which feature they think is most important for getting in. -->

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## What happened?

* 23: half. When a sperm (23) fertilises an egg (23), the new cell has 46 again. If gametes had 46, the number would double every generation.

<!-- Speaker note: Reveal after running. Ask who predicted it, and what made them think so. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Features

**Match each feature to its job.**

| | |
|---|---|
| `Sperm's tail` | A. Enzymes digest a way into the egg |
| `Midpiece full of mitochondria` | B. Releases energy for swimming |
| `Acrosome` | C. Changes after one sperm enters, so no others can |
| `Egg's food store` | D. Feeds the embryo in its first days |
| `Jelly coat` | E. Swims towards the egg |

* Answer: Sperm's tail → E. Swims towards the egg; Midpiece full of mitochondria → B. Releases energy for swimming; Acrosome → A. Enzymes digest a way into the egg; Egg's food store → D. Feeds the embryo in its first days; Jelly coat → C. Changes after one sperm enters, so no others can

<!-- Speaker note:  -->

<!-- Speaker note: Teacher: the 'why half' question is the checkpoint. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Half

**Why must a gamete have half the number of chromosomes?**

- So that when two gametes join, the new cell has the full number again
- Because gametes are small
- So the offspring has half as many genes
- To save energy

* Answer: So that when two gametes join, the new cell has the full number again

<!-- Speaker note:  -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Numbers

**Why are millions of sperm released, but only one egg?**

- Most sperm never reach the egg; the egg is large and costly to make because it holds food
- Many sperm join the egg, each adding some chromosomes
- Sperm have no nucleus
- Eggs are smaller

* Answer: Most sperm never reach the egg; the egg is large and costly to make because it holds food

<!-- Speaker note:  -->

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<span style="background:#fef3c7; color:#1a1a1a; border:1.5px solid #f59e0b; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✍️ Activity</span>

## Elaborate: plants make gametes too

{{embed: https://studio.learningwaypoints.com/tutorials/_playground-board/index.html?id=deck%2Fscience%2Fbiology-9%2Fgametes%2Fstep-4&code=widget%3A+cell%0Ashow%3A+pollen%2C+egg%0Afind%3A+nucleus%2C+pwall%0A&fresh=1}}

[Open this step in the tutorial](https://studio.learningwaypoints.com/tutorials/science/biology-9/gametes/?full=step-4#step-4)

<!-- Speaker note: What this step is for: Seeing pollen's tough wall (no tail) shows plant gametes solve the same problem, getting the male nucleus to the egg, another way. -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Getting there

**A pollen grain can't swim. How does its male nucleus reach the egg?**

- The egg moves to the pollen
- It doesn't: plants don't use gametes
- Wind or animals carry the pollen to a flower; a pollen tube then grows down to the ovule
- It swims through the air

* Answer: Wind or animals carry the pollen to a flower; a pollen tube then grows down to the ovule

<!-- Speaker note:  -->

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## The ovule

**A flowering plant's egg sits inside an ovule, deep in the flower's ovary. How does that suit its job?**

- It keeps the egg protected, and after fertilisation the ovule grows into a seed with a food store
- It stops pollen ever reaching it
- It lets the egg swim out to meet the pollen
- It is where the pollen is made

* Answer: It keeps the egg protected, and after fertilisation the ovule grows into a seed with a food store

<!-- Speaker note:  -->

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Elaborate: counting chromosomes

**A dog's body cells have 78 chromosomes. How many are in a dog's sperm cell?**

* Answer: 39

<!-- Speaker note: Half of 78. The egg also has 39, so the puppy's cells have 78. What this step is for: Applying 'half' to an unfamiliar organism checks the rule, not the number 46. -->

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## Evaluate: look back

Write one sentence: "At first I thought… Now I think… because…"

**Revision summary**

- **Gametes** are sex cells: sperm and egg in animals; the male nucleus in pollen and the egg in the ovule in flowering plants.
- Gametes have **half** the chromosomes (23 in humans), so **fertilisation** restores the full number (46).
- **Sperm:** tail, midpiece of mitochondria, acrosome. **Egg:** large, food store, a coat that lets in only one sperm.

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<span style="background:#dcfce7; color:#1a1a1a; border:1.5px solid #22c55e; padding:6px 18px; border-radius:8px; font-size:1em; font-weight:500;">✅ Check</span>

## Exit ticket

**Why do sperm and egg cells have 23 chromosomes, not 46?**

* So that when they fuse at fertilisation, the new cell has 46, the full number, again.

<!-- Speaker note: Students answer on paper or a mini whiteboard before the reveal. Anyone who cannot answer needs the main idea again next lesson. Where this leads: Next: sexual and asexual reproduction and variation (node 3). Year 10: meiosis, the cell division that halves the number. -->