GCSE · Biology · Edexcel · Spec 1BI0

Egg cell adaptations

One sperm gets in. Now a single cell must keep the others out, hand over the right number of chromosomes and feed a brand-new embryo. The egg is ready.

Biology · Cell structure

Inside the egg cell

View: Before fertilisation. Showing 3 layers: Cytoplasm, Nucleus, Membrane before fertilisation

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switch the view, then tap each part

Sperm cells have reached the egg. The membrane is the way in, and right now a sperm can still get through it.

An egg cell drawn with its three adapted parts: the cell membrane, the cytoplasm and the nucleus.

Watch out: The membrane change usually stops more sperm getting in. Keep the word “usually” in your own answers.

Predict, then check

Commit to an answer before you look.

Suppose the egg nucleus carried a FULL set of chromosomes, and so did the sperm nucleus. At fertilisation the two nuclei fuse. What would the new cell end up with?

From fertilisation to early embryo

Follow the story and spot the moment each adaptation does its job.

One sperm gets inA sperm cell reaches the egg and fertilises it. This is the egg cell's function: it is the female sex cell, and it exists to be fertilised by a sperm.
A zygote formsThe result is a zygote: the first cell of the new individual. It is one single cell with the full set of genetic information.

What about all the other sperm?

Picture several sperm cells arriving at the same egg. One of them gets in first and fertilises it.

Which is closest to what you think happens next?

The more sperm that get into the egg, the better the chance of a healthy embryo.The egg just lets in whatever sperm reach it. It has no say in what gets through.The membrane changes before any sperm arrive, so that only the fastest one can get through.After one sperm fertilises the egg, the egg membrane changes, so usually no more sperm can get in.

How sure are you?

I'm sureI think soI'm guessing

The more sperm that get into the egg, the better the chance of a healthy embryo.

This is a common misconception

It feels as if more should help, but the egg's function is to be fertilised by a sperm: one. That single haploid sperm nucleus, fused with the haploid egg nucleus, already gives the zygote its full set of chromosomes. And once that one sperm is in, the membrane changes, so usually no more sperm can enter.

The egg just lets in whatever sperm reach it. It has no say in what gets through.

This is a common misconception

The egg isn't a passive bystander here. Its membrane is adapted: it changes after fertilisation by one sperm. That change is what usually stops the others getting in.

The membrane changes before any sperm arrive, so that only the fastest one can get through.

This is a common misconception

Check the timing. The membrane changes AFTER one sperm has fertilised the egg, not before. Before that, a sperm has to be able to get in, or fertilisation could never happen at all.

After one sperm fertilises the egg, the egg membrane changes, so usually no more sperm can get in.

This matches the evidence

That's it. The trigger is fertilisation by one sperm, and the result is a changed membrane that usually keeps the rest out. “Usually” is the honest word here, so keep it.

One sperm in, then the membrane changes. The egg is adapted so that usually no more sperm can enter after the first.

Put it into words

Explain how the egg cell is adapted to carry out its function. [4 marks]

Your answer

0 words · your answer stays on this page and is not sent anywhere.

The mark scheme

States the function: to be fertilised by a sperm, forming a zygote, and to support the developing embryo. (1 mark)
Cytoplasm contains lots of nutrients, which sustain the early growing embryo. (1 mark)
Nucleus is haploid (half the number of chromosomes), so after fusing with the haploid sperm nucleus the new cell has a full set. (1 mark)
Membrane changes after fertilisation by one sperm, so usually no more sperm can enter. (1 mark)

Model answer

The egg cell's job is to be fertilised by a sperm to form a zygote, and then to support the developing embryo. Its cytoplasm contains lots of nutrients, which sustain the embryo while it is growing early on. Its nucleus is haploid, holding half the number of chromosomes, so when it fuses with the haploid sperm nucleus the zygote has a full set. After one sperm has fertilised it, the egg's membrane changes so that usually no more sperm can enter.

An adaptation only earns credit with its reason attached: say what the part is like, then what that does for fertilisation or the embryo.

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

How the egg's cytoplasm, nucleus and membrane each prepare it to be fertilised by one sperm and to support the embryo that follows.

What you need to know

The egg cell is the female sex cell. Its function is to be fertilised by a sperm, forming a zygote, and to support the developing embryo.
Cytoplasm: contains lots of nutrients, which sustain the early growing embryo.
Nucleus: haploid, meaning it has half the number of chromosomes, so after fusing with the haploid sperm nucleus the new cell has a full set.
Membrane: changes after fertilisation by one sperm, so usually no more sperm can enter.

The big picture

The egg cell is the female sex cell. Its function is to be fertilised by a sperm, forming a zygote, and to support the developing embryo. Three adaptations fit it for that job. Its cytoplasm contains lots of nutrients to sustain the early growing embryo. Its nucleus is haploid, with half the number of chromosomes, so that fusing with the haploid sperm nucleus gives the new cell a full set. And its membrane changes after fertilisation by one sperm, so that usually no more sperm can enter.

Key points

1A specialised cell has a structure (its shape and the sub-cellular structures it contains) that adapts it to one specific function.

2A zygote is the first cell formed when a sperm and an egg fertilise: one cell with the full set of genetic information.

3The zygote divides to form two cells, then four, and so on, as the embryo develops.

4Half plus half makes one full set. That is why the egg nucleus needs to be haploid.

5The membrane changes after the first sperm, not before, and it usually stops more getting in, so say “usually”, not “always”.

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Worked example

Problem

A made-up animal has a full set of 12 chromosomes. How many chromosomes are in the nucleus of its egg cell, and how many are in the zygote after fertilisation? Explain your answer.

⚠ Watch out

Mixing up which part carries which adaptation, for example saying the membrane stores the nutrients, or the nucleus changes to keep other sperm out. Match each one to its part: nutrients go with the cytoplasm, half the chromosomes with the nucleus, and changing after fertilisation with the membrane.

🧠

Memory hook

FEED, HALVE, LOCK. The cytoplasm FEEDS the early embryo with nutrients, the nucleus is HALVED (haploid) so the zygote ends up with a full set, and the changed membrane usually LOCKS the other sperm out.

✓

Check yourself

Cover the page. Name the egg's three adapted parts and, for each, the problem it solves. Then say what would happen to the chromosome number if the egg nucleus held a full set.

Flashcards

(9)

What is the function of an egg cell?

To be fertilised by a sperm, forming a zygote, and to support the developing embryo.

Is the egg the male or the female sex cell?

The female sex cell.

What makes a cell 'specialised'?

Its structure (its shape and the sub-cellular structures it contains) adapts it to carry out a specific function.

How is the egg cell's cytoplasm adapted?

It contains lots of nutrients, which sustain the early growing embryo.

What does 'haploid' mean?

Having half the number of chromosomes.

Why does the egg cell need a haploid nucleus?

So that when it fuses with the haploid sperm nucleus at fertilisation, the new cell has a full set of chromosomes.

What happens to the egg cell membrane after fertilisation by one sperm?

It changes, so that usually no more sperm can enter and fertilise the egg.

What is a zygote?

The first cell formed after a sperm and an egg fertilise: one cell with the full set of genetic information.

What does the zygote do next?

It divides to form two cells, then four, and so on, as the embryo develops.

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What about all the other sperm?

Put it into words

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