Distinguish egg coat change from sperm acrosome action
Sperm enzymes assist entry; egg coat change helps restrict further entry at fertilisation. Both functions differ from the nuclei that fuse.
Section B14.2 · Sexual reproduction in humans
Cross-referenced to the 2028–2029 0653 section using original teaching from the related single-science courses. A 0653-specific objective-by-objective and Core/Supplement review is in progress. Written practice is self-checked and does not demonstrate supervised laboratory competence or award an official Cambridge grade.
Before you begin
- Recall cell nuclei, gametes, diffusion, respiration and the difference between a cell and an organ.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Which belongs to the sperm?
What you will learn
- Order entry assistance and further-entry restriction
- Keep gamete adaptation roles separate
Original teaching model.
| Feature | Meaning |
|---|---|
| Acrosome | Sperm entry enzymes |
| Nuclei | Fuse during fertilisation |
| Jelly coat | Restricts further entry after change |
| Egg reserves | Support early development |
Order entry assistance and further-entry restriction
Before successful fertilisation, enzymes in a sperm’s acrosome help it penetrate the egg’s surrounding coat. This is an entry adaptation of the sperm. At fertilisation, the egg coat changes in a way that helps prevent additional sperm entry. The two descriptions concern different gametes and different roles; they are not contradictory statements about one unchanged barrier.
The sperm nucleus and egg nucleus provide the nuclear contributions that fuse. Neither the acrosome nor the jelly coat is itself a nucleus. Entry support must not replace nuclear fusion in the definition of fertilisation. A learner who writes “fertilisation is when the acrosome becomes the coat” has confused cell structures, their functions and the biological event.
Keep gamete adaptation roles separate
Egg energy stores have a third role: supporting early development. They do not swim toward sperm or chemically supply the sperm’s mitochondrial energy from a distance. Each adaptation can be placed in a clear explanation: sperm movement toward egg, enzyme-supported coat penetration, gamete nuclear fusion, coat change restricting further entry, and early development using reserves.
This ordered model is deliberately limited. It explains why several features are useful without guaranteeing every encounter succeeds or every barrier is absolute. If a question provides only enzyme action, later fusion remains unproven. If it provides fusion and a stated coat change, explain those events directly, retaining the distinction between entry, fertilisation and subsequent development.
Sources for this lesson
Pause and explain
What is missing if only coat penetration is recorded?
Worked example
Compare acrosome enzymes with the fertilisation-related coat change.
Show the worked solution
- Assign enzymes to the sperm.
- Assign later further-entry restriction to the egg coat.
Answer Acrosome enzymes help sperm penetrate the egg coat; the coat changes at fertilisation to help prevent additional sperm entry.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
State the different roles of acrosome enzymes and changing jelly coat.
Give me a hint
Use entry versus additional-entry restriction.
Compare my reasoning
- Locate the enzymes in sperm.
- Locate the changing coat around the egg.
Sperm acrosome enzymes support entry through the coat. Egg jelly coat changes at fertilisation to help prevent additional sperm entry.
Look for these in your work
- Assign each to its gamete
- Distinguish the roles
Order enzyme-assisted penetration, nuclear fusion and coat change at fertilisation.
Give me a hint
Separate the preparatory entry step from the fertilisation events.
Compare my reasoning
- Place penetration before fusion.
- Associate coat change with fertilisation.
Enzyme-assisted penetration precedes sperm–egg nuclear fusion; coat change at fertilisation helps restrict later additional sperm entry.
Look for these in your work
- Preserve the event order
- State the change function
An invented observation records one sperm penetrating the coat but no nuclei are resolved. Evaluate “a zygote has certainly formed”.
Give me a hint
The defining fusion has not been observed.
Compare my reasoning
- Identify the demonstrated entry step.
- Name the missing nuclear evidence.
Entry is recorded, but zygote formation is not established without evidence of sperm and egg nuclear fusion.
Look for these in your work
- Use the actual observation
- Avoid claiming completed fusion
Common mistakes
- Neither coat nor acrosome is a nucleus.
- Entry assistance does not prove fusion.
What can you explain now?
What is missing if only coat penetration is recorded?
Compare with the explanation
Evidence of sperm and egg nuclear fusion
Penetration and fusion are distinct events.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
Explain order entry assistance and further-entry restriction and keep gamete adaptation roles separate. Use the diagram key and check your reasoning before opening the solutions.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.