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.
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.
In Year 8 you met specialised cells. This lesson: the cells made for reproduction.
Engage: Commit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Explore: Try it on the board and compare what happens with your prediction.
Explain: Use the particle model to explain what you saw.
Elaborate: Use the same idea in a new context.
Evaluate: Look back at your first ideas and what changed.
WidthMore contexts, same idea.
Make your prediction first. Help opens once you've locked it in.
EngageCommit to a prediction before you test anything. A wrong guess is fine: it shows you what to look for.
Engage: how many chromosomes?
ExploreTry it on the board and compare what happens with your prediction.
Explore: a sperm and an egg
Click each part to see what it is and what it does.
Try it yourselfYour edits are saved in this browser.
Find the nucleus and the special features of the sperm and the egg.
Use the board. This ticks when you get there.
ExplainUse the particle model to explain what you saw.
Explain: built for the job
In your book: draw a sperm and an egg, label three features, and write what each one is for.
Match each feature to its job.
Sperm's tail
Midpiece full of mitochondria
Acrosome
Egg's food store
Jelly coat
ElaborateUse the same idea in a new context.
Elaborate: plants make gametes too
Try it yourselfYour edits are saved in this browser.
Find the nucleus in the pollen grain and the egg, and the pollen's wall.
Use the board. This ticks when you get there.
Elaborate WiderUse the same idea in a new context.
Elaborate: counting chromosomes
A dog's body cells have 78 chromosomes. How many are in a dog's sperm cell?
EvaluateLook back at your first ideas and what changed.
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.
For teachers
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.
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.
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.
Running the lesson
Engage: count how many chose 46.
Explore: ask which feature they think is most important for getting in.
Explain: the 'why half' question is the checkpoint.
The hard part (on purpose)
Why a gamete needs half the chromosomes. When a student is stuck, ask how many chromosomes a fertilised egg would have if each gamete carried the full set.
When students get stuck
A student says the baby gets half the genes. Ask: How many chromosomes does the fertilised egg have? Fix: 46: a full set, half from each parent.
A student thinks pollen is the plant's sperm and swims. Ask: What carries pollen? Fix: Wind or animals; then a pollen tube grows to the ovule.
Stretch and support
Stretch: Why do sperm need so many mitochondria, but egg cells don't need a tail?
Support: Give a labelled sperm and egg with the jobs listed in a box to match.
Where it leads
Next: sexual and asexual reproduction and variation (node 3). Year 10: meiosis, the cell division that halves the number.
What each step is for
Engage: how many chromosomes?: Students who think gametes have the full set won't see why fertilisation doesn't double the number; the answer sets up the 'half' idea.
Explore: a sperm and an egg: Clicking each feature and reading what it does links structure to the gamete's job.
Explain: built for the job: Matching each feature to the job it makes possible is the hinge; the chromosome number question checks 'half'.
Elaborate: plants make gametes too: Seeing pollen's tough wall (no tail) shows plant gametes solve the same problem, getting the male nucleus to the egg, another way.
Elaborate: counting chromosomes: Applying 'half' to an unfamiliar organism checks the rule, not the number 46.
Now you can
Explain why gametes have half the usual number of chromosomes.
Link three features of sperm and egg cells to their jobs.
Describe how a plant's male nucleus reaches the egg.