Biology · 16 Reproduction

16.4 Sexual reproduction in humans

Before you start

Sexual reproduction is the fusion of the nuclei of two gametes to form a zygote (16.1).
Diffusion is the net movement of particles from a higher to a lower concentration, and it is faster across a surface that is large in area and thin (Topic 3).
Aerobic respiration takes place in mitochondria and uses oxygen to release energy from glucose (Topic 12).
Mitosis produces cells that are genetically identical to the parent cell (17.2).
An exchange surface works well when it has a large surface area, a short diffusion distance and a good blood supply (Topics 7 and 11).

Syllabus objectives covered (10)

Identify on diagrams and state the functions of the following parts of the male reproductive system: testes, scrotum, sperm ducts, prostate gland, urethra and penis
Identify on diagrams and state the functions of the following parts of the female reproductive system: ovaries, oviducts, uterus, cervix and vagina
Describe fertilisation as the fusion of the nuclei from a male gamete (sperm) and a female gamete (egg cell)
Explain the adaptive features of sperm, limited to: flagellum, mitochondria and enzymes in the acrosome
Explain the adaptive features of egg cells, limited to: energy stores and the jelly coat that changes at fertilisation
Compare male and female gametes in terms of: size, structure, motility and numbers
State that in early development, the zygote forms an embryo which is a ball of cells that implants into the lining of the uterus
Identify on diagrams and state the functions of the following in the development of the fetus: umbilical cord, placenta, amniotic sac and amniotic fluid
Describe the function of the placenta and umbilical cord in relation to the exchange of dissolved nutrients, gases and excretory products between the blood of the mother and the blood of the fetus
State that some pathogens and toxins can pass across the placenta and affect the fetus

Part 1 of 9

The male reproductive system, following the route of a sperm

Six parts to know. Learning them as a scattered list is hard; learning them as a route is easy, because a sperm passes through them in a fixed order and the order is also roughly the order down the body.
Testes. Two oval organs, held below the body in a bag of skin. This is where sperm are made. Their function: to produce sperm, which are the male gametes. (They also produce the hormone testosterone, which is the business of 16.5.)
Scrotum. The bag of skin that holds the testes, hanging outside the main body cavity. Its function is to keep the testes at a temperature slightly below body temperature, which is the temperature sperm production requires.
That is the whole reason the testes are outside rather than tucked away safely inside — an odd design that only makes sense once you know the temperature point, and a question asking why the testes hang outside the body is asking for exactly that.
Sperm ducts. One tube leading from each testis. Their function: to carry sperm from the testis towards the urethra.
Prostate gland. A gland sitting where the sperm ducts join the urethra. Its function: to add fluid to the sperm. Sperm plus that fluid is called semen, and the fluid contains nutrients that keep the sperm alive and a liquid medium they can swim in.
Urethra. The tube running from that junction, down the centre of the penis, to the outside. Its function: to carry semen out of the body. It also carries urine, at other times — one tube with two jobs, never both at once.
Penis. Its function is to become firm so that it can be inserted into the vagina, and to transfer semen into the vagina.
Say the route once and it sticks: made in the testis, along the sperm duct, fluid added at the prostate gland, along the urethra, out through the penis.
The six parts laid out along the route a sperm actually takes.
One precision point on wording. 'Testes' is plural; one is a testis. And 'sperm duct' is the syllabus term — if you write 'tube from the testis' you are describing rather than naming, and a naming question wants the name.

Part 2 of 9

The female reproductive system, following the route of an egg

Five parts, and the same trick works: they lie along the path an egg cell takes, from where it is made to where it would leave the body.
Ovaries. Two of them, one on each side of the lower abdomen. Their function: to produce egg cells, the female gametes, and to release one about once a month. (They also produce oestrogen and progesterone — again, 16.5.)
Oviducts. One tube on each side, running from close to an ovary to the uterus. The end nearest the ovary is a funnel that catches the released egg.
The oviduct has two functions and you need both. It carries the egg cell from the ovary to the uterus, and it is the place where fertilisation normally happens. That second point is examined constantly.
Uterus. The muscular, pear-shaped organ in the middle, with an oviduct entering at each upper corner. Its function: this is where the embryo implants and where the fetus develops. Its thick muscular wall contracts during birth to push the baby out.
Cervix. A ring of muscle at the lower end of the uterus — the neck of the uterus, where it meets the vagina. Its function: it keeps the uterus closed and holds the fetus in place during pregnancy, and it widens during birth to let the baby through.
Vagina. A muscular tube running from the cervix to the outside. Its function: it receives the penis and the semen during intercourse, and it is the birth canal through which the baby leaves.
Now put the two systems together, because that is the geography a question really tests. The egg travels ovary to oviduct to uterus. The sperm travel vagina to cervix to uterus to oviduct. They are moving in opposite directions and they meet in the oviduct.
The egg and the sperm travel in opposite directions and meet in the oviduct — which is where fertilisation happens.
The commonest anatomy error in this whole topic is saying fertilisation happens in the uterus. It does not. The uterus is where the embryo implants, several days later. Fertilisation happens in the oviduct.
The second commonest is writing 'ovary' when you mean 'oviduct'. They differ by three letters and by a whole function: the ovary makes the egg, the oviduct carries it and hosts fertilisation.

Part 3 of 9

Fertilisation: the fusion of two nuclei

The syllabus definition is exact, and the exactness is the point: fertilisation is the fusion of the nuclei from a male gamete (sperm) and a female gamete (egg cell).
Read what that sentence commits you to. Not the cells — the nuclei. Not meeting, not touching, not entering — fusing, joining to become one.
So the sperm swimming to the egg is not fertilisation. The sperm digesting its way through the jelly coat is not fertilisation. The sperm entering the egg cell is not fertilisation. All three are steps that come before it. Fertilisation is the single moment when the two nuclei join.
What a vaguer word costs. 'The sperm meets the egg' contains no joining at all and scores nothing. 'The sperm enters the egg' describes the step before. Only 'the nucleus of the sperm fuses with the nucleus of the egg cell' contains what the mark scheme is looking for.
The word for the cell produced is zygote. One cell, with one nucleus, made from two. If a question asks what fertilisation produces, 'a new cell' will not do — the answer is a zygote.
Why the fusion has to be of nuclei rather than anything else: the nucleus holds the chromosomes. Each gamete nucleus carries half the number of chromosomes a body cell has — that half number is called haploid — so fusing two of them restores the full number, which is called diploid.
You meet haploid and diploid properly in Topic 17, but they explain this topic. Half plus half equals whole is the entire reason gametes exist, and it is why exactly one sperm must fertilise the egg. Two sperm nuclei would give one and a half times the chromosome number, and the zygote would not develop.
And say where: fertilisation normally takes place in the oviduct, because that is where a swimming sperm meets an egg that has just been released.

Part 4 of 9

Why a sperm is shaped the way it is

A sperm has one job — reach an egg cell in the oviduct and get its nucleus inside. The syllabus limits you to three adaptations, and each one solves a specific part of that job.
Picture it in three regions. A head at the front, carrying the nucleus with a cap over the tip of it. A short middle section behind the head. A long thin flagellum trailing behind that.
The flagellum. This is the long tail. It beats from side to side, which pushes the sperm forward, so the sperm can swim from the vagina up through the cervix and uterus and into the oviduct to reach the egg cell.
Use the word flagellum. 'Tail' will usually be accepted, but flagellum is the syllabus term and it is the one that is never queried.
The mitochondria. The middle section is packed with them. Mitochondria are where aerobic respiration takes place, and aerobic respiration releases energy — the energy the flagellum needs in order to keep beating over a long journey.
That answer has three links and you must write all three: many mitochondria, therefore a lot of aerobic respiration, therefore energy released for the flagellum to beat. Writing only 'mitochondria give it energy' misses the middle link and typically scores one mark instead of two.
The acrosome. This is the cap over the tip of the head, and it contains enzymes.
Those enzymes digest a path through the jelly coat surrounding the egg cell, so the sperm can get through it and its nucleus can enter the egg cell. 'Digest' is the verb that earns the mark; 'breaks through' is vaguer and weaker.
Notice the shape of a full adaptation answer, because it is the same every time: name the feature, say what it does, then say what that achieves. Feature, mechanism, purpose. An answer missing the third part is a description, and Explain marks are not released by descriptions.

Part 5 of 9

Why an egg cell is built the way it is

The egg cell has the opposite job. It does not travel to anything. It has to be found, to admit exactly one sperm, and then to supply everything the new organism needs until it can be fed by the mother. Two adaptations, and the syllabus limits you to these.
Energy stores. The cytoplasm of an egg cell contains a large store of food, and that store is why the egg cell is so much bigger than a sperm.
What it is for: after fertilisation the zygote divides again and again, and all of that division needs energy and raw materials. For several days there is nowhere else to get them — the embryo has not implanted yet, so it is not yet connected to the mother. The store carries it through that gap.
The jelly coat. This is a layer surrounding the egg cell, and it has two things to say about it.
Before fertilisation, it protects the egg cell.
At fertilisation, it changes — and the change makes it impossible for any further sperm to get through. So only one sperm nucleus ever fuses with the egg nucleus.
That second point is the one the syllabus singles out, and it links straight back to chromosome number: one sperm gives a diploid zygote, two would not, and the zygote would fail. The jelly coat's change is what guarantees the arithmetic works.
When you write about the jelly coat, say what the change achieves, not just that it happens. 'The jelly coat changes' is half an answer. 'The jelly coat changes so that no more sperm can enter' is the whole one.

Part 6 of 9

Comparing the two gametes

The syllabus names the four things to compare — size, structure, motility and numbers — which is unusually helpful, because it tells you exactly what a comparison answer should contain and in what order.
Size. The egg cell is much larger than a sperm. The sperm is one of the smallest cells in the body; the egg cell is one of the largest.
Structure. The sperm has a flagellum, a middle section packed with mitochondria, an acrosome containing enzymes, and very little cytoplasm. The egg cell has no flagellum, a large volume of cytoplasm holding its energy stores, and a jelly coat around the outside.
Motility. Motile means able to move itself. The sperm is motile — it swims, using its flagellum. The egg cell is not motile; it is moved along the oviduct by the oviduct itself, not by any effort of its own.
That is a distinction worth being careful about. The egg cell does travel. It does not move itself. A question asking about motility is asking about the second thing.
Numbers. Sperm are produced continuously and in enormous numbers, hundreds of millions at a time. Egg cells are released in very small numbers — about one per menstrual cycle.
How to write it so it scores. Every sentence must name the same feature on both sides. 'The egg cell is much larger than the sperm' compares. 'The egg cell is large and the sperm has a flagellum' does not compare anything, because there is no shared feature in the sentence.
The word 'whereas' is your friend here: it forces both halves into one sentence and makes it impossible to write half a comparison by accident.

Part 7 of 9

Zygote, embryo, implantation — three words in a fixed order

This is a short objective with three words in it that must not be swapped, so take them one at a time.
Zygote. The single cell produced by fertilisation, in the oviduct.
The zygote then divides by mitosis — into two cells, then four, then eight, and so on. Because it is mitosis, every one of those cells is genetically identical to the zygote and to each other.
Embryo. That ball of cells is the embryo. So an embryo is not a stage in time so much as a description: a ball of cells produced by the dividing zygote.
While all this dividing is going on, the ball of cells is travelling down the oviduct towards the uterus.
Implantation. The embryo then sinks into the thick lining of the uterus and becomes embedded in it. That is implantation.
Be exact about the second half of that sentence. The embryo implants into the lining of the uterus — not 'into the uterus', not 'onto the wall', not 'attaches to the womb'. The lining is where the blood supply is, and the lining is what the syllabus says.
'Implants' is the load-bearing verb. 'Sticks to' and 'lands on' describe something sitting on a surface; implantation means sinking in and becoming embedded, which is what makes a placenta possible.
Note the geography again, because it mirrors the plant lesson exactly. Fertilisation happens in the oviduct; implantation happens in the uterus; several days separate them. Two events, two places, two names.
Later in the pregnancy the developing organism is called a fetus, once the organs have formed. Zygote, then embryo, then fetus, in that order — using 'fetus' for a ball of cells, or 'embryo' for something at seven months, is a straightforward loss of a mark on a question that names the stage.

Part 8 of 9

What surrounds and supports the fetus

Four structures to name and give a function for. They fit together like this, working outwards from the fetus.
The amniotic sac is a membrane that completely surrounds the fetus, forming a closed bag inside the uterus. Its function: it encloses the fetus and holds in the fluid.
Amniotic fluid is the liquid inside that sac. The fetus floats in it.
Its function: it supports the fetus and acts as a shock absorber, cushioning it against bumps and pressure from outside. It also keeps the temperature around the fetus steady, and lets the fetus move.
'Protects the fetus' on its own is thin. Say how it protects: by cushioning it against knocks. That is the difference between a stated function and a scored one.
The placenta is a disc-shaped organ attached to the wall of the uterus. It is the only point of contact between mother and fetus. Its function: it is where substances are exchanged between the mother's blood and the fetus's blood.
The umbilical cord joins the fetus to the placenta. It contains blood vessels, and its function is to carry the fetus's blood to the placenta and back again.
So the connection runs: fetus, then umbilical cord, then placenta, then the wall of the uterus. Three of those four are inside the amniotic sac; the placenta is attached to the uterus wall.
The word for the whole period of development inside the uterus is gestation. You are more likely to meet it in the wording of a question than to be asked to define it, but you should not be stopped by it.

Part 9 of 9

How the placenta actually works

Start with the fact that makes the whole thing make sense, and that candidates most often get wrong: the mother's blood and the fetus's blood do not mix. They are brought very close together and separated by a thin barrier, and substances cross that barrier.
Everything the fetus needs crosses by diffusion. Nothing crosses by the two blood supplies meeting.
The placenta has finger-like projections that push into the lining of the uterus, and the mother's blood flows around them. This gives a very large surface area, and the barrier between the two blood supplies is very thin.
Large surface area plus short diffusion distance is the same recipe you met in the lungs and the small intestine. It means substances cross quickly — and the process by which they cross is diffusion, down a concentration gradient in each case.
From mother to fetus: oxygen, and dissolved nutrients such as glucose, amino acids, vitamins and mineral ions. Each is at a higher concentration in the mother's blood, so each diffuses into the fetus's blood.
From fetus to mother: carbon dioxide, and excretory products, chiefly urea. Each is at a higher concentration in the fetus's blood, so each diffuses into the mother's blood, and the mother's lungs and kidneys then get rid of them.
The umbilical cord carries all of it. One blood vessel in the cord carries blood from the fetus to the placenta, loaded with carbon dioxide and urea. Another carries blood from the placenta back to the fetus, loaded with oxygen and dissolved nutrients.
Two phrasings to avoid. 'The placenta feeds the fetus' names no mechanism. 'The fetus breathes through the placenta' is wrong — the fetus does not use its lungs at all, and oxygen reaches it by diffusion. Use the words diffuses across the placenta and both problems disappear.
If you are asked to suggest why it matters that the two blood supplies stay separate, a sensible answer is that the mother's much higher blood pressure would damage the delicate vessels of the fetus, or that the two blood groups might not be compatible.
Now the consequence of that thin barrier. Because it is thin enough to let dissolved substances across quickly, it does not block everything harmful. Some pathogens and some toxins can pass across the placenta from the mother's blood into the fetus's blood, and affect the fetus.
Pathogens that can cross include viruses such as rubella and HIV. Toxins that can cross include nicotine and alcohol, and some other drugs.
The effects vary, and the syllabus does not ask you to list them in detail — damage to the developing fetus, a low birth mass, or infection of the fetus are enough. The examinable point is the crossing itself.
There is a neat symmetry to hold on to. The very features that make the placenta a good exchange surface — thin, and enormous in area — are exactly what make it possible for something harmful to get through. The strength and the weakness are the same structure.

Worked examples

Example 1. Sperm are released into the vagina. Name, in order, the structures through which a sperm passes from the testis in which it was made until it reaches an egg cell, and state where fertilisation takes place.

Show the working

Deal with the male system first, in the order the sperm travels. It is made in the testis and passes into the sperm duct.
The sperm duct leads to the urethra, and it is at that junction that the prostate gland adds fluid, making semen.
The urethra carries the semen down through the penis and out into the vagina. That is the handover point between the two systems.
Now the female system, still in travel order. From the vagina the sperm swims through the cervix, which is the ring of muscle at the base of the uterus.
Through the cervix it enters the uterus, swims across it, and enters an oviduct.
That is where it stops, because that is where the egg is. Fertilisation takes place in the oviduct — not the uterus, which is where the embryo will implant several days later.

Answer: Testis, sperm duct, urethra, penis, vagina, cervix, uterus, oviduct. Fertilisation takes place in the oviduct.

Example 2. Explain how three features of a sperm cell adapt it for reaching an egg cell and fertilising it.

Show the working

Take the three features the syllabus names, and for each one write feature, then mechanism, then purpose. That three-part shape is what turns a description into an explanation.
Feature one: the flagellum. Mechanism: it beats from side to side and pushes the sperm forward. Purpose: so the sperm can swim from the vagina to the oviduct to reach the egg cell.
Feature two: many mitochondria in the middle section. Mechanism: they carry out aerobic respiration. Purpose: this releases the energy the flagellum needs to keep beating.
Do not shortcut that middle link. 'Mitochondria for energy' skips aerobic respiration, which is the marking point in between.
Feature three: the acrosome at the tip of the head. Mechanism: it contains enzymes that digest the jelly coat of the egg cell. Purpose: so the sperm can get through the coat and its nucleus can enter the egg cell.
Check the finished answer: three features, and three purposes. If a sentence stops after the mechanism, it has stopped one clause too early.

Answer: The flagellum beats from side to side, propelling the sperm so it can swim through the uterus to the oviduct and reach the egg cell. The many mitochondria in the middle section carry out aerobic respiration, releasing the energy needed for the flagellum to keep beating. The acrosome at the tip of the head contains enzymes that digest a path through the jelly coat of the egg cell, so the sperm can enter and its nucleus can fuse with the egg nucleus.

Example 3. Compare a human sperm cell with a human egg cell in terms of size, structure, motility and numbers.

Show the working

The question hands you the four headings. Use them as four sentences, and put both cells into every sentence — that is what makes it a comparison.
Size: the egg cell is much larger than the sperm.
Structure: the sperm has a flagellum, a middle section full of mitochondria and an acrosome, with very little cytoplasm, whereas the egg cell has no flagellum but a large volume of cytoplasm containing energy stores, surrounded by a jelly coat.
Motility: the sperm is motile and swims using its flagellum, whereas the egg cell is not motile and is carried along the oviduct.
Numbers: sperm are produced continuously in very large numbers, whereas usually only one egg cell is released per menstrual cycle.
Read it back and check the word 'whereas' or 'but' appears in each line. If a sentence mentions only one of the two cells, it is a description that has wandered into a comparison answer and will not score.

Answer: The egg cell is much larger than the sperm. The sperm has a flagellum, many mitochondria and an acrosome and very little cytoplasm, whereas the egg cell has no flagellum, a large volume of cytoplasm containing energy stores, and a jelly coat. The sperm is motile and swims using its flagellum, whereas the egg cell is not motile and is moved along the oviduct. Sperm are produced continuously in enormous numbers, whereas about one egg cell is released each menstrual cycle.

Example 4. Describe how a fetus obtains the oxygen and glucose it needs, and how it removes the carbon dioxide and urea it produces.

Show the working

Begin with the structure, because every later sentence depends on it. At the placenta the mother's blood and the fetus's blood are brought very close together but do not mix.
Say why that arrangement works: the placenta has a very large surface area and the barrier between the two blood supplies is very thin, so substances cross quickly.
Name the process. Everything here crosses by diffusion, down a concentration gradient.
Inwards: oxygen and glucose are at a higher concentration in the mother's blood, so they diffuse across into the fetus's blood.
Outwards: carbon dioxide and urea are at a higher concentration in the fetus's blood, so they diffuse across into the mother's blood, and her lungs and kidneys remove them.
Finish with the transport link, which is the part most often left out. The umbilical cord contains blood vessels that carry the fetus's blood to the placenta and back again — without the cord, none of this exchange reaches the fetus.

Answer: At the placenta the mother's blood and the fetus's blood are brought close together but do not mix, across a large, thin surface. Oxygen and glucose are at a higher concentration in the mother's blood and diffuse across into the fetus's blood. Carbon dioxide and urea are at a higher concentration in the fetus's blood and diffuse across into the mother's blood, to be removed by her lungs and kidneys. The blood vessels in the umbilical cord carry the fetus's blood to the placenta and back again.

Where people go wrong

Mistake: Defining fertilisation as 'when the sperm meets the egg' or 'when the sperm enters the egg'.

Instead: Fertilisation is the fusion of the nucleus of the sperm with the nucleus of the egg cell, producing a zygote.

Meeting and entering are both steps that happen before fertilisation. The syllabus definition is about nuclei fusing, so an answer without the word fuse and without the word nuclei has not said what fertilisation is.

Mistake: Saying fertilisation happens in the uterus.

Instead: Fertilisation happens in the oviduct. The uterus is where the embryo implants, several days later.

The sperm swim up to the oviduct and the egg is released into it, so that is where they meet. Placing fertilisation in the uterus also collapses two separate events into one and loses the implantation mark as well.

Mistake: Writing that the embryo 'attaches to the uterus' or 'sticks to the wall'.

Instead: The embryo implants into the lining of the uterus.

The verb 'implants' means sinking in and becoming embedded, which is what puts the embryo next to the mother's blood supply. Sitting on a surface would not allow a placenta to develop, so the vaguer wording describes something that would not work.

Mistake: Saying the mother's blood and the fetus's blood mix at the placenta.

Instead: The two blood supplies are brought close together but stay separate. Substances diffuse across a thin barrier between them.

It is the single most-penalised statement in this subtopic, and it contradicts the rest of a correct answer. Diffusion across a barrier is what the whole structure of the placenta exists to do.

Mistake: Writing that the placenta 'feeds' the fetus, or that the fetus 'breathes' through the placenta.

Instead: Dissolved nutrients and oxygen diffuse from the mother's blood across the placenta into the fetus's blood.

Neither word names a mechanism, and the fetus does not use its lungs at all. Examiners are looking for diffusion and for a named substance moving in a named direction.

Mistake: Answering a Compare question with a list of features of one gamete followed by a list for the other, with no shared features.

Instead: Put both gametes into every sentence, along the four axes named by the syllabus: size, structure, motility, numbers.

A comparison mark requires a common feature and a stated difference. Two separate descriptions may contain all the correct facts and still score nothing on a Compare question.

Check yourself

State the function of the scrotum and the function of the prostate gland.
Answer
The scrotum holds the testes outside the body, keeping them at a temperature slightly below body temperature, which is needed for sperm production. The prostate gland adds fluid containing nutrients to the sperm, forming semen.
Give the two functions of the oviduct, and state one function of the cervix.
Answer
The oviduct carries the egg cell from the ovary to the uterus, and it is where fertilisation normally takes place. The cervix keeps the uterus closed and holds the fetus in place during pregnancy, and widens during birth.
Define fertilisation, name the cell it produces, and state where in the body it normally happens.
Answer
Fertilisation is the fusion of the nuclei from a male gamete (sperm) and a female gamete (egg cell). It produces a zygote. It normally happens in the oviduct.
Explain why a sperm cell contains many mitochondria, using all three links in the chain.
Answer
The many mitochondria carry out aerobic respiration, which releases energy, and that energy is used to make the flagellum beat so the sperm can swim to the egg cell.
State the two adaptive features of an egg cell and explain what each is for.
Answer
Energy stores in its cytoplasm supply the energy and materials for the zygote and embryo to divide and grow before the embryo has implanted and can obtain nutrients from the mother. The jelly coat protects the egg cell and changes at fertilisation so that no further sperm can enter, ensuring only one sperm nucleus fuses with the egg nucleus.
Compare male and female gametes in terms of motility and numbers.
Answer
The sperm is motile and swims using its flagellum, whereas the egg cell is not motile and is moved along the oviduct. Sperm are produced continuously in very large numbers, whereas about one egg cell is released per menstrual cycle.
Put these in order and name each stage: a ball of cells sinks into the lining of the uterus; two nuclei fuse; one cell divides by mitosis.
Answer
Two nuclei fuse — fertilisation, forming a zygote. The zygote divides by mitosis, forming an embryo, which is a ball of cells. The embryo sinks into the lining of the uterus — implantation.
State the function of the amniotic fluid and the function of the umbilical cord.
Answer
The amniotic fluid supports the fetus and acts as a shock absorber, cushioning it against bumps and pressure. The umbilical cord contains blood vessels that carry the fetus's blood to the placenta and back again.
Name two substances that pass from the mother's blood to the fetus's blood at the placenta and two that pass the other way. Then state what else can cross the placenta and harm the fetus.
Answer
Mother to fetus: oxygen and dissolved nutrients such as glucose or amino acids. Fetus to mother: carbon dioxide and urea. Some pathogens (for example rubella virus or HIV) and some toxins (for example nicotine or alcohol) can also cross the placenta and affect the fetus.

Exam questions

Try each part first. Open the mark scheme after you have written your answer.

Question 1. A couple are trying to conceive. A doctor explains the path taken by the sperm and the place where fertilisation occurs.

(a) State the function of the prostate gland and the function of the scrotum. 2 marks No calculator

Mark scheme

B1 prostate gland adds fluid (containing nutrients) to the sperm / forms semen
B1 scrotum holds the testes outside the body, keeping them below body temperature, which is required for sperm production

(b) Identify, in order, the structures a sperm passes through from the testis until it reaches an egg cell. 2 marks No calculator

Mark scheme

B1 sperm duct, then urethra, then penis (in that order)
B1 vagina, then cervix, then uterus, then oviduct (in that order)

(c) Describe what happens at fertilisation and name the structure in which it normally occurs. 3 marks No calculator

Mark scheme

B1 the nucleus of the sperm fuses with the nucleus of the egg cell. CON if the answer says only that the sperm enters or meets the egg
B1 forming a zygote
B1 in the oviduct. CON if uterus is named

Question 2. The diagram-free description below concerns human gametes. A sperm cell has a head containing a nucleus with a cap over its tip, a middle section, and a long flagellum. An egg cell is a much larger, rounded cell with a great deal of cytoplasm and an outer jelly coat.

(a) Explain how the flagellum and the mitochondria of a sperm cell adapt it for reaching an egg cell. 4 marks No calculator

Mark scheme

B1 the flagellum beats / moves from side to side, propelling the sperm
B1 so that the sperm can swim (through the uterus) to the oviduct to reach the egg cell
B1 the many mitochondria carry out aerobic respiration
B1 releasing the energy needed for the flagellum to beat / for swimming

(b) State what the cap at the tip of the sperm head is called, and what its contents do. 2 marks No calculator

Mark scheme

B1 acrosome, containing enzymes
B1 the enzymes digest (a path through) the jelly coat so the sperm can enter the egg cell

(c) Explain why the jelly coat of the egg cell changes at fertilisation. 2 marks No calculator

Mark scheme

B1 so that no further sperm can enter the egg cell
B1 so only one sperm nucleus fuses with the egg nucleus / so the zygote has the correct number of chromosomes, owtte

(d) Compare male and female gametes in terms of size and numbers. 2 marks No calculator

Mark scheme

B1 the egg cell is much larger than the sperm, ORA
B1 sperm are produced in far greater numbers than egg cells (about one egg cell per cycle), ORA

Question 3. A zygote is formed in an oviduct. Six days later a ball of cells is embedded in the lining of the uterus.

(a) Describe what happens to the zygote during those six days. 3 marks No calculator

Mark scheme

B1 the zygote divides by mitosis (repeatedly)
B1 forming an embryo, which is a ball of cells
B1 the embryo travels along the oviduct to the uterus and implants into the lining of the uterus

(b) Suggest why the embryo can survive for those six days without being connected to the mother's blood supply. 2 marks No calculator

Mark scheme

B1 the egg cell contained energy stores / a food store in its cytoplasm
B1 which supply the energy and materials for cell division until the embryo implants and can obtain nutrients from the mother, owtte

Question 4. A fetus develops in the uterus, enclosed by the amniotic sac and connected to the placenta by the umbilical cord.

(a) State the function of the amniotic fluid. 1 mark No calculator

Mark scheme

B1 supports the fetus and acts as a shock absorber / cushions it against bumps and pressure, owtte. 'Protects the fetus' alone is insufficient

(b) Describe how oxygen and glucose reach the fetus from the mother. 3 marks No calculator

Mark scheme

B1 at the placenta the mother's blood and the fetus's blood are brought close together but do not mix
B1 oxygen and glucose diffuse across the placenta from the mother's blood into the fetus's blood, down a concentration gradient
B1 the blood vessels in the umbilical cord carry them from the placenta to the fetus

(c) State one excretory product that passes from the blood of the fetus to the blood of the mother. 1 mark No calculator

Mark scheme

B1 urea (accept carbon dioxide, or another named excretory product)

(d) A pregnant woman is advised not to smoke and to avoid people with rubella. Suggest why. 2 marks No calculator

Mark scheme

B1 some toxins (nicotine) and some pathogens (rubella virus) can pass across the placenta
B1 into the blood of the fetus, where they can damage or infect it, owtte

How this is examined

How the placenta gets substances into and out of a fetus without the two blood supplies ever mixing.: A three- or four-mark Describe or Explain, often after a stem naming the umbilical cord, and frequently followed by a one-mark Suggest about smoking, alcohol or an infection during pregnancy. Four things, in this order: the mother's blood and the fetus's blood are close together but do not mix; the placenta gives a large surface area and a thin barrier; named substances diffuse in a named direction (oxygen and dissolved nutrients in, carbon dioxide and urea out); the umbilical cord's blood vessels carry the fetus's blood to the placenta and back. Saying the bloods mix contradicts the rest of the answer and cancels marks. For the follow-up: pathogens and toxins can cross the placenta and affect the fetus.
Each sperm adaptation written as feature, then mechanism, then purpose.: A four-mark Explain, usually attached to a labelled sperm and often asking for a stated number of adaptive features. The syllabus limits it to three, so the question cannot go beyond flagellum, mitochondria and acrosome. 'The flagellum beats from side to side so the sperm can swim to the oviduct'; 'the many mitochondria carry out aerobic respiration, releasing energy for the flagellum to beat'; 'the acrosome contains enzymes that digest a path through the jelly coat so the sperm can enter the egg cell'. Dropping the middle link — writing 'mitochondria for energy' without aerobic respiration — costs one of the two marks.
Fertilisation as a fusion of two nuclei, in the oviduct, producing a zygote.: A two- or three-mark Describe near the start of the human reproduction question, often paired with 'name the structure in which this occurs'. The word fuses (or fusion) applied to the nucleus of the sperm and the nucleus of the egg cell; the word zygote for the product; the oviduct for the place. 'The sperm meets the egg' and 'the sperm enters the egg' both describe earlier steps and score nothing, and naming the uterus loses the location mark.
Zygote to embryo to implantation — the words, and the fact that implantation is into the lining of the uterus.: A three-mark Describe of what happens in the first week after fertilisation, or a one-mark 'name the process' after a stem describing a ball of cells embedding in the uterus lining. The zygote divides by mitosis; this forms an embryo, a ball of cells; the embryo implants into the lining of the uterus. Both load-bearing words must be exact — 'implants' rather than 'attaches', and 'the lining of the uterus' rather than 'the uterus'. Calling a ball of cells a fetus loses the stage mark.
The four axes of the gamete comparison, and the egg cell's two adaptations.: A two- or three-mark Compare, sometimes as a table with a column for each gamete, sometimes as continuous prose. The syllabus names the four axes, so the question rarely strays from size, structure, motility and numbers. Both gametes in every sentence, joined by 'whereas': larger egg cell; sperm has flagellum, mitochondria and acrosome whereas the egg cell has cytoplasm with energy stores and a jelly coat; sperm is motile whereas the egg cell is moved along the oviduct; sperm in huge numbers whereas about one egg cell per cycle. For the jelly coat, say what its change achieves — no further sperm can enter.
The function of each named reproductive structure, said as a job rather than a position.: A labelled diagram of the male or female system with two or three labels to complete, then 'State the function of X' for one mark each. A verb that names the job: 'the testes produce sperm', 'the scrotum keeps the testes below body temperature for sperm production', 'the prostate gland adds fluid to the sperm to make semen', 'the oviduct carries the egg to the uterus and is where fertilisation occurs', 'the cervix holds the fetus in place and widens at birth'. Saying where a structure sits earns nothing.

Words to know

Gamete: A sex cell. In humans the male gamete is the sperm and the female gamete is the egg cell.
Fertilisation: The fusion of the nuclei from a male gamete (sperm) and a female gamete (egg cell), normally in the oviduct.
Zygote: The single cell formed when the nuclei of two gametes fuse.
Embryo: The ball of cells formed when the zygote divides by mitosis in early development.
Implantation: The sinking of the embryo into the lining of the uterus, where it becomes embedded.
Fetus: The developing organism after the organs have formed, from the embryo stage onwards.
Gestation: The period of development of the fetus inside the uterus.
Semen: Sperm together with the fluid added by the prostate gland.
Acrosome: The cap at the tip of a sperm head, containing enzymes that digest the jelly coat of the egg cell.
Flagellum: The long tail of a sperm, which beats to propel it forward.
Motile: Able to move itself. Sperm are motile; egg cells are not.
Placenta: The organ attached to the wall of the uterus at which substances are exchanged between the mother's blood and the fetus's blood, without the two mixing.
Amniotic fluid: The liquid inside the amniotic sac, which supports the fetus and cushions it against bumps.

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