--- title: "What makes a sex cell special?" subtitle: "Year 9 Biology · Reproduction · Lesson 6" --- # What makes a sex cell special? **Name:** {{field-24}} **Date:** {{field-15}} **Class:** {{field-15}} > **Key question:** 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. :::teacher-note **In one sentence:** 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). **Students should be able to say:** 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. **Width:** the unmarked parts are the floor every student does (Engage, Explore, Explain, Evaluate). Typical and Wider sections add contexts, not difficulty. Students work in the online tutorial alongside this sheet. ::: ## Engage :::keep-together **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 My prediction, and why: {{lines-3}} ::: :::teacher-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. ::: ## Explore Try it on the board in the tutorial. Was your prediction right? What was different? {{lines-2}} ## Explain: built for the job :::keep-together ### Features **Match each feature to its job.** A. Enzymes digest a way into the egg B. Releases energy for swimming C. Changes after one sperm enters, so no others can D. Feeds the embryo in its first days E. Swims towards the egg {{columns 60% 40%}} {{rows 1}} | Part | Letter | |---|---| | `Sperm's tail` | | | `Midpiece full of mitochondria` | | | `Acrosome` | | | `Egg's food store` | | | `Jelly coat` | | ::: :::teacher-note **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 ::: :::keep-together ### 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 ::: :::teacher-note **Answer:** So that when two gametes join, the new cell has the full number again ::: :::keep-together ### 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 ::: :::teacher-note **Answer:** Most sperm never reach the egg; the egg is large and costly to make because it holds food ::: :::typical ## Elaborate: plants make gametes too :::keep-together ### 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 ::: :::teacher-note **Answer:** Wind or animals carry the pollen to a flower; a pollen tube then grows down to the ovule ::: :::keep-together ### 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 ::: :::teacher-note **Answer:** It keeps the egg protected, and after fertilisation the ovule grows into a seed with a food store ::: ::: :::wider ## Elaborate: counting chromosomes :::keep-together **A dog's body cells have 78 chromosomes. How many are in a dog's sperm cell?** Answer: {{field-15}} ::: :::teacher-note **Answer:** 39 Half of 78. The egg also has 39, so the puppy's cells have 78. ::: ::: ## 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. ## Exit ticket :::keep-together **Why do sperm and egg cells have 23 chromosomes, not 46?** {{lines-3}} ::: :::teacher-note **Answer:** So that when they fuse at fertilisation, the new cell has 46, the full number, again. :::