What you need to know
- 3.11Specialisations of mammalian gameteA haploid sex cell, such as a sperm or an egg cell., including acrosomeA vesicle at the tip of the sperm head containing enzymes that digest the zona pellucida. and zona pellucidaThe glycoprotein layer around a mammalian egg cell. Sperm must digest a path through it..
By the end, you should be able to…
- Explain sperm structure–function relationships.
- Explain egg-cell specialisations.
- Distinguish the acrosome, zona pellucida and cortical granules.
Build your understanding
Sperm: deliver a haploid nucleus
The sperm head contains a haploidHaving one set of chromosomes (n), as in gametes. nucleusThe organelle that contains the cell’s chromosomes, surrounded by a nuclear envelope with pores. and an acrosomeA vesicle at the tip of the sperm head containing enzymes that digest the zona pellucida. containing enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. involved in penetration of the egg’s surrounding layers. The membrane carries molecules involved in recognition and fusion. The acrosome is not the nucleus or an energy store.
The flagellum provides propulsion. Many mitochondrionThe organelle where the link reaction, Krebs cycle and oxidative phosphorylation of aerobic respiration take place. in the midpiece supply ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi. for movement; ATP transfers energy to cellular processes; mitochondria do not create energy. A relatively small amount of cytoplasmEverything inside the cell surface membrane except the nucleus: the fluid cytosol and the organelles. and streamlined form help sperm movement. Features operate together: motility alone does not establish fertilising ability.
Egg: cytoplasm and protection
A mammalian egg is much larger than a sperm and has abundant cytoplasm containing organelleA structure inside a cell with a particular function, such as a mitochondrion or ribosome., molecules and resources needed for early development. It does not have a propulsive flagellum. Its haploid genetic contribution combines with the sperm’s at fertilisationThe fusion of the nuclei of a male and a female gamete to form a diploid zygote..
The zona pellucidaThe glycoprotein layer around a mammalian egg cell. Sperm must digest a path through it. is a glycoproteinA protein with carbohydrate chains attached. In membranes, glycoproteins act as receptors and as antigens for cell recognition. layer outside the egg cell surface membraneThe partially permeable membrane around a cell that controls which substances enter and leave.. It participates in sperm interaction and changes after fertilisation to help prevent further sperm entry. Cortical granules lie beneath the cell membrane; their contents are released during the cortical reactionAfter one sperm enters, cortical granules release their contents, which harden the zona pellucida and block entry of further sperm.. The zona pellucida is not a cell wallA rigid layer outside the cell membrane. In plants it is made mainly of cellulose; it supports the cell and resists bursting..
Read a structure as evidence
A long flagellum suggests a role in movement; a concentrated mitochondrial region supports ATP demand; an enzyme-containing cap supports penetration. In an explanation state both the feature and the mechanism it enables.
In the human detail, the ovulated cell is a secondary oocyte arrested in meiosisTwo nuclear divisions that produce four genetically different haploid nuclei, used to make gametes. II; sperm entry triggers completion. The specification’s “egg cell” wording is used here for accessibility. Do not infer that every mammalian egg is a fully completed meiotic product before sperm contact.
Follow the process
Sperm reaches the egg → the flagellum has used ATP from midpiece mitochondria for propulsion → recognition molecules on the sperm surface meet the zona pellucida → the acrosome releases enzymes → those enzymes help digest a path through the zona → membranes can fuse and the haploid nuclei can later combine. Entry is not the same event as swimming.
After entry, the egg’s cortical granules release their contents and the zona changes, which helps block further sperm. You may start an answer at fertilisation and work back, if each step still names the structure and what it does.
NOT: the zona pellucida is not the plasma membrane and not a cell wall.
- Both gametes are haploid
- Mitochondria supply ATP for the flagellum
- Acrosome enzymes help entry; they are not the energy store
- The egg has no flagellum
Two traps this diagram blocks
- Do not say mitochondria digest the zona.
- Do not call the zona the plasma membrane.
From the diagram alone
Question
Explain how the acrosome and the midpiece mitochondria each help a sperm fertilise an egg. [3]
Show a model answer
Mitochondria in the midpiece supply ATP so the flagellum can propel the sperm to the egg. Acrosome enzymes then help break down part of the zona pellucida so the sperm membrane can meet the egg membrane.
Make your reasoning visible
Build the explanation
X happens because Y, leading to Z. X: the sperm can pass the zona pellucidaThe glycoprotein layer around a mammalian egg cell. Sperm must digest a path through it.. Y: acrosomeA vesicle at the tip of the sperm head containing enzymes that digest the zona pellucida. enzymeA globular protein that acts as a biological catalyst, speeding up a specific reaction. digest part of that glycoproteinA protein with carbohydrate chains attached. In membranes, glycoproteins act as receptors and as antigens for cell recognition. layer. Z: the sperm plasma membrane can meet the egg plasma membrane. Incomplete: “The acrosome helps the sperm.” Improved: “Acrosome enzymes break down part of the zona pellucida, so the sperm can reach the egg plasma membrane.” The improvement names the layer and the consequence. mitochondrionThe organelle where the link reaction, Krebs cycle and oxidative phosphorylation of aerobic respiration take place. supply ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi. for swimming; they do not digest the zona.
Keep the two reactions distinct
Sperm acrosome enzymes assist entry; egg cortical granule release helps prevent additional sperm entry.
Avoid this misconception
The claim: The egg’s zona pellucida is the same as its plasma membrane.
The correction: It is a surrounding glycoprotein layer outside the plasma membrane.
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
Explain two sperm adaptations for reaching and entering an egg.
Show suggested answer
A flagellum propels the sperm, with ATP supplied by mitochondria; acrosomal enzymes help penetration of the egg’s surrounding layers.
Practice question
Retrieval and explanation · suggested answer
Why is a large cytoplasm useful in an egg?
Show suggested answer
It supplies organelles and molecules supporting the early stages of development before the embryo can obtain all its own resources.
Practice question
Retrieval and explanation · suggested answer
Distinguish acrosome and cortical granules.
Show suggested answer
The acrosome is in the sperm head and helps penetration. Cortical granules are in the egg and release contents that help prevent additional sperm entry.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A fictional sperm has an active flagellum but lacks functional acrosomal enzymes. Predict an effect and a limitation of the prediction.
Show a hint
Separate reaching the egg from entering its surrounding layers.
Explore the reasoning
It may reach the egg but have impaired penetration. Other molecular interactions also matter, so a structural observation alone cannot quantify fertilisation probability.
Recall this lesson
Lesson recall questions
Question 1 of 5: State the function of the acrosome in a sperm cell.
Mammalian gamete specialisation
Practice question
Retrieval and explanation · suggested answer
State the function of the acrosome in a sperm cell.
Lesson: 3B 4: Gametes: Structure and Function
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
The work of an embryologist
3B · Gametes: Structure and FunctionIn ICSI, an embryologist injects one sperm into an egg. This illustrates how laboratory techniques can bypass some steps required for sperm entry. An embryologist assesses gametes and…Specification 3.11Read & use the scienceClose article
Why this matters
In ICSI, an embryologist injects one sperm into an egg. This illustrates how laboratory techniques can bypass some steps required for sperm entry. An embryologist assesses gametes and records each laboratory step. ICSI places a sperm inside an egg, so sperm motility and membrane entry can be bypassed, but the egg must still support subsequent events. This makes a clear comparison with the roles of the sperm tail, acrosome and egg zona pellucida in natural fertilisation. It would be misleading to describe ICSI as repairing all genetic or developmental problems. The technique is one intervention at one stage, and its success has to be measured by later outcomes too.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.11 understand how mammalian gametes are specialised for their functions (including the acrosome in sperm and the zona pellucida in the egg cell)
Use the science
Does bypassing entry establish that the sperm and egg have no other problems?
Explore a possible response
No. Entry is only one step; later fertilisation and development also depend on the cells’ other properties.
Explore the source
- HFEA: intracytoplasmic sperm injectionHuman Fertilisation and Embryology Authority
What a semen analysis can and cannot show
3B · Gametes: Structure and FunctionAn NHS laboratory examines sperm number, movement and appearance when assessing fertility. Movement links directly to the specialised flagellum and the energy supply needed for a sperm to…Specification 3.11Read & use the scienceClose article
Why this matters
An NHS laboratory examines sperm number, movement and appearance when assessing fertility. Movement links directly to the specialised flagellum and the energy supply needed for a sperm to reach an egg. Yet a normal motility result does not prove every later fertilisation step will succeed, and one abnormal test does not settle every possible cause. An andrology scientist interprets measurements in context and may request further investigation. This application encourages students to explain a structural feature by its function rather than memorising a labelled gamete. It also reminds them that a clinical measure answers a limited biological question. Online fertility advice can turn a single laboratory number into a verdict; the biology supports a narrower conclusion.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 2, pp. 21–24. The application extends beyond required recall.
- 3.11 understand how mammalian gametes are specialised for their functions (including the acrosome in sperm and the zona pellucida in the egg cell)
Use the science
If sperm motility is normal, has an acrosome reaction been demonstrated?
Explore a possible response
No. Motility shows movement; the acrosome reaction is a separate event at the egg surface and requires different evidence.
Explore the source
- Diagnostic semen analysis: information for patientsCambridge University Hospitals NHS Foundation Trust