Human Reproduction: WJEC A Level Biology Cram

Lesson 1 of 21

Which organs make up the male reproductive system?

Cram notes for topic 4.1, Sexual reproduction in humans, in WJEC A level Biology Unit 4.

The diagram below shows the male reproductive system.

Labelled diagram of the male reproductive system. A cream bladder sits at the top centre, with a purple seminal vesicle on each side below it and a pink prostate gland beneath. A pink penis runs down the middle with a gold urethra inside it. On each side, a rounded scrotum contains a pink testis drawn with wavy lines for the seminiferous tubules, a dark pink crescent for the epididymis on its outer side, and a gold vas deferens curving up from it to the seminal vesicle. Labels name the bladder, seminal vesicle, prostate gland, vas deferens, testis, epididymis, seminiferous tubules, scrotum, ejaculatory duct and urethra. — The male reproductive system seen from the front. Original BioWelsh diagram.

Testis: seminiferous tubules make ; secrete .
Scrotum: holds the testes outside the body at about 34 °C, the best temperature for making .
: mature and are stored.
: carries to the urethra.
adds fructose and mucus; the adds alkaline fluid. Together with the this makes .

Lesson 2 of 21

What can you see in a section of a testis?

The diagram below shows a in transverse section.

Labelled diagram of a circular tubule. The wall holds four concentric rings of cells: large purple circles at the outer edge, then blue, then pink, then small cream ones. Six pale green wedges run through the wall from the outer edge to the centre. A cream circle in the middle is the lumen, with dark sperm heads and curved tails pointing into it. Groups of orange oval cells lie in the corners outside the tubule. Labels name the germinal epithelium, spermatogonia, primary spermatocytes, secondary spermatocytes, spermatids, spermatozoa, a Sertoli cell and interstitial cells. — A seminiferous tubule in transverse section. Original BioWelsh diagram.

The is the outer wall, and cells develop inwards towards the .
Order from the edge inwards: , primary spermatocytes, , , then in the .
Sertoli cells run through the wall and nourish the developing cells.
Interstitial (Leydig) cells lie between tubules and secrete .

Lesson 3 of 21

Which organs make up the female reproductive system?

The diagram below shows the female reproductive system.

Labelled diagram. In the centre a pear-shaped uterus is drawn with three nested layers: a dark pink outer band, a mid pink band and a white cavity. Below it a purple tube narrows at the cervix and widens into the vagina. From each upper corner a gold oviduct curves outwards to a cream funnel edged with short gold fimbriae, and below each funnel is a purple oval ovary containing small white circles. Labels name the oviduct, oviducal funnel with fimbriae, ovary, myometrium, endometrium, cervix and vagina. — The female reproductive system seen from the front. Original BioWelsh diagram.

: makes and secretes and .
(): ciliated, and the usual site of fertilisation; its funnel has .
Uterus: the embryo implants and develops here. Its wall is , (muscle) and (lining).
: a ring of muscle at the neck of the uterus, opening into the vagina.

Lesson 4 of 21

What can you see in a section of an ovary?

The diagram below shows the stages of development in the .

A large pale purple oval with a thick purple outline. Six structures are arranged clockwise round the inside, joined by purple arrows: a small pink circle with a dot, a larger pink circle with a white space inside, a large pink circle with a big white space and a small pale circle to one side, a broken pink C shape with a small round cell escaping above the outline, a yellow circle ringed with small yellow dots, and a pale yellow circle with a cross through it. Text labels name the germinal epithelium, primary follicle, the follicle growing and forming an antrum, the stroma, and the statement that it degenerates if there is no pregnancy. Labels on the right name the Graafian follicle, ovulation and the corpus luteum. — One cycle of follicle development in the ovary. Original BioWelsh diagram.

covers the surface; the fills the inside.
A primary is one in a ring of .
A is large, with a fluid-filled and the oocyte to one side.
At the bursts and the is released.
The remains become the , which secretes and degenerates if there is no pregnancy.

Lesson 5 of 21

What is gametogenesis, and why must gametes be haploid?

Gametogenesis has three stages: multiplication by mitosis, growth, then maturation by meiosis. Meiosis halves the chromosome number from 46 to 23, so the is again, and it shuffles so gametes vary.

Lesson 6 of 21

How does the testis make spermatozoa?

The flow diagram below shows .

Flow diagram of six rows of ring-shaped cells, each row joined to the next by downward arrows. Row 1 has one purple cell, row 2 two purple cells, row 3 two blue cells, row 4 four pink cells, row 5 four cream cells and row 6 four grey cells. Labels to the right of each row read germinal epithelium cell 2n, spermatogonium 2n, primary spermatocyte 2n, secondary spermatocyte n, spermatid n and spermatozoon n. Labels to the left of the arrows read mitosis, mitosis then growth, meiosis I, meiosis II and maturation. — Spermatogenesis: one primary spermatocyte makes four spermatozoa. Original BioWelsh diagram.

cell divides by mitosis to give .
Some keep dividing by mitosis, so the supply never runs out; others grow into primary spermatocytes (2n).
Meiosis I gives two (n); meiosis II gives four (n).
The mature into four equal , gaining an and a tail.

Lesson 7 of 21

How is a spermatozoon adapted to its job?

The diagram below shows a .

Labelled side view. At the left an oval head has a cream cap over its front half and a purple oval inside it. A small grey block with two bars follows. Then a cream rectangle crossed by nine curved gold lines. From it a long dark line waves to the right, its waves growing larger. Scale bars underneath read head about 5 micrometres, middle piece about 8 micrometres and tail about 50 micrometres. Labels name the acrosome, the haploid nucleus, the centrioles, the mitochondria in the middle piece and the tail. — The structure of a human spermatozoon. Original BioWelsh diagram.

: that digest the and .
: 23 chromosomes.
Middle piece: that make for swimming.
Tail: lashes from side to side. Total length about 60 µm.

Lesson 8 of 21

How does the ovary make a secondary oocyte?

The flow diagram below shows .

Flow diagram of five rows of ring-shaped cells joined by downward arrows. Row 1 has one purple cell, row 2 two purple cells, row 3 two blue cells, row 4 two pink cells each with a much smaller ring beside it, and row 5 two cream cells each with a much smaller ring beside it. Labels to the right read germinal epithelium cell 2n, oogonium 2n, primary oocyte 2n stopped at prophase I, secondary oocyte n and a small first polar body, and ovum n and a small second polar body. Labels to the left of the arrows read mitosis, mitosis then growth, meiosis I unequal and meiosis II unequal. — Oogenesis: one primary oocyte makes one ovum and two polar bodies. Original BioWelsh diagram.

Before birth: mitosis gives , which grow into and stop in prophase I.
From puberty, meiosis I finishes at , giving one and one small first .
The stops at metaphase II. It is the cell released at .
Meiosis II finishes only if a enters, giving one and a second .

Lesson 9 of 21

What surrounds a secondary oocyte?

The diagram below shows a just after .

Labelled diagram of a large circle. A ring of pale purple ovals radiates around the outside. Inside them is a pink ring, then a paler pink interior. Small dark red dots form a ring just inside the edge. Near the centre, four short purple bars sit in a line between two dashed curves that meet at points on either side. A small white circle containing a dark dot sits between the pink ring and the interior at the upper right. Labels name the corona radiata, the zona pellucida, the cortical granules, the chromosomes on the spindle at metaphase II, the first polar body and the cytoplasm. — A secondary oocyte just after ovulation. Original BioWelsh diagram.

: the outer layer of .
: a glycoprotein coat just inside the .
lie under the cell membrane, ready for the .
The chromosomes are held at metaphase II, and the first is inside the .

Lesson 10 of 21

How do spermatogenesis and oogenesis compare?

Spermatogenesis and oogenesis compared
Feature	Spermatogenesis	Oogenesis
Starts	At puberty	Before birth
Pattern		One about every 28 days
From one primary cell	Four equal gametes	One and two
Meiosis completed	Yes, before release	Only if a enters
	Small, about 60 µm, little cytoplasm	Large, about 120 µm, food reserves

Lesson 11 of 21

How do you work out the real size of a cell down a microscope?

Actual size = measured size ÷ . Convert both measurements to the same unit first: there are 1000 µm in 1 mm.

In the laboratory, calibrate an against a , as shown below.

Two large circles side by side, each representing the field of view. In the left circle a black scale numbered 0, 5 and 10 lies above a red scale, and text reads that 10 eyepiece units line up with 0.1 millimetres, which is 100 micrometres, so one eyepiece unit is 10 micrometres. In the right circle the same black scale lies above a pink circle with a purple centre, and a green bar beneath it is labelled the cell measures 6.4 eyepiece units, 6.4 multiplied by 10 equals 64 micrometres. — Calibrating an eyepiece graticule with a stage micrometer. Original BioWelsh diagram.

Line up the two scales and find how many fit a known length.
Here 10 epu = 0.1 mm = 100 µm, so 1 epu = 10 µm.
Measure the specimen in and multiply: 6.4 epu × 10 = 64 µm.
Calibrate again after changing the objective lens.

Calibrate the graticule against the , and repeat for every objective lens.
View the slide at low power first, then high power, focusing upwards.
Measure across the widest part, repeat on several examples and take a .
A shows tissue outlines only; a high power drawing shows a few cells in detail.

Lesson 12 of 21

What happens when a sperm meets a secondary oocyte?

The panels below show fertilisation in order.

Six panels in two rows. Each panel has a title, a drawing of a round pink oocyte and a short explanation. Panel 1, capacitation, shows a sperm approaching an oocyte ringed by pale purple ovals. Panel 2, acrosome reaction, shows small gold dots streaming from the sperm head towards the oocyte. Panel 3, sperm head entry, shows a purple circle inside the oocyte with an arrow pointing to it. Panel 4, cortical reaction, shows a thick dark red ring around the oocyte, red dots inside it and a second sperm blocked by a red cross. Panel 5, meiosis II, shows two small circles inside the oocyte and a small white circle at its edge. Panel 6, nuclei fuse, shows a single purple oval labelled 2n. — Fertilisation, from a capacitated sperm to a zygote. Original BioWelsh diagram.

in the removes cholesterol and glycoproteins from the membrane.
The reaction releases that digest the and .
The membranes fuse and only the head enters.
The forms the , preventing .
Entry triggers meiosis II, then the two pronuclei fuse to give a .

Lesson 13 of 21

How does the oocyte stop a second sperm getting in?

under the cell membrane rupture and release into the . The zona thickens and hardens into a , so no second can get through.

If two did enter, the cell would have 69 chromosomes and be , and the embryo would not survive. In , an immature oocyte has fewer , so the risk is higher.

Lesson 14 of 21

How does the blastocyst reach the uterus and implant?

The diagram below follows the embryo from to .

A curved gold tube runs from a purple oval at the far left to a thick pink uterus at the right. Along the tube, seven circles show the stages in order: a single cell with a purple dot, a single cell labelled fertilisation, then balls of 2, 4, 8 and about 14 cells, then a hollow circle with a small pink patch at one side. Inside the uterus a small hollow circle has short spikes radiating into the pink wall. Text reads cleavage, mitosis with no growth between the divisions, and labels name ovulation, the blastocyst and implantation. — From fertilisation to implantation, over about seven days. Original BioWelsh diagram.

Cilia and muscle in the move the cell towards the uterus.
is mitosis with no growth between divisions: 2, 4, 8 cells, then the solid .
A cavity forms, giving the : a outside, an and a .
About seven days after fertilisation, grow into the . This is .

Lesson 15 of 21

What happens in the ovary and the uterus over 28 days?

Days 1 to 5: , as the is shed. Days 6 to 13: a grows and the is rebuilt. Day 14: . Days 15 to 28: the keeps the thick, then degenerates if there is no pregnancy.

The graph below shows the thickness of the through one cycle.

Line graph with the day of the menstrual cycle from 0 to 28 on the horizontal axis and the thickness of the endometrium in millimetres from 0 to 10 on the vertical axis. A purple line starts at 6, falls steeply to 1 at day 5, rises steadily to 6 at day 14, rises more gently to a peak of 8 at about day 22, then drops sharply to 6 at day 28. Text along the top reads menstruation, rebuilt by oestrogen and maintained by progesterone, and ovulation is marked near day 14. — Thickness of the endometrium during one menstrual cycle. Original BioWelsh graph.

It falls to about 1 mm by day 5 during .
It rises to about 6 mm by day 14 under .
It reaches about 8 mm by day 22 under , then falls sharply at the end of the cycle.

Lesson 16 of 21

How do FSH, LH, oestrogen and progesterone control the cycle?

The graph below shows all four hormones through one cycle.

Line graph with the day of the menstrual cycle from 0 to 28 on the horizontal axis and relative concentration in the blood from 0 to 11 on the vertical axis. A green solid line for FSH starts at about 3, peaks at 4 near day 3, falls to 2, rises to a smaller peak of 5.6 near day 13, drops back to 2 and then rises again to about 2.9 at day 28. A red dashed line for LH stays near 1.6 then rises to a single sharp peak of 10 just before day 14 and falls back. A blue dotted line for oestrogen rises from 1 to a peak of 9 just before day 13, falls, then has a second broad peak of 4 near day 21. An orange dot-dash line for progesterone stays at 0.4 until day 14, rises to a broad peak of 9 near day 22, then falls sharply to 0.4 by day 28. A grey dashed vertical line marks day 14. — Hormone concentrations during one menstrual cycle. Original BioWelsh graph.

from the anterior stimulates to develop and to secrete .
rebuilds the . At low levels it inhibits by .
Very high just before day 14 reverses this and stimulates a surge of by , which triggers .
forms the , which secretes . maintains the and inhibits and .
If there is no pregnancy, falls, the is shed and rises again.

Lesson 17 of 21

How is the placenta built for exchange?

The diagram below shows the in the wall of the uterus.

Cross-section diagram in horizontal bands. At the top a dark pink band is labelled myometrium and below it a mid pink band is labelled endometrium. Below them a pale pink space contains scattered red ovals and four finger-shaped projections that point upwards from a cream band labelled chorion. Each projection contains a red and a blue line. A red vessel enters the space from the top left with an arrow pointing down, and a blue vessel leaves at the top right with an arrow pointing up. Below the chorion a narrow column containing one blue and two red lines is labelled umbilical cord to the foetus. Labels name the intervillous space, a chorionic villus, a foetal capillary, the maternal artery and vein, the umbilical vein and the two umbilical arteries. — The structure of the placenta in the wall of the uterus. Original BioWelsh diagram.

project into the , which is filled with the mother's blood.
Each villus contains foetal capillaries and has a wall one cell thick.
The carries two umbilical arteries and one umbilical vein.
The umbilical artery carries deoxygenated blood from the foetus; the umbilical vein carries oxygenated blood to it.

Lesson 18 of 21

Why must the blood of the mother and the foetus never mix?

The diagram below shows exchange across one .

A pale pink rectangle labelled maternal blood in the intervillous space contains scattered red ovals. In the centre a pale finger-shaped villus rises from the bottom edge, fringed with short pink lines along its whole outline. Inside it a narrow pale blue loop contains small blue ovals. A green arrow points from the left into the villus, labelled oxygen, glucose, amino acids and antibodies. A dark red arrow points from the villus to the right, labelled carbon dioxide, urea and water. A bracket between the villus surface and the blue loop is labelled only about 5 micrometres, and a label on the right reads microvilli increase the surface area. — Exchange across the wall of a chorionic villus. Original BioWelsh diagram.

To the foetus: oxygen, glucose, amino acids, water, ions and . To the mother: carbon dioxide, and water.
The distance is only about 5 µm, because the walls are one cell thick.
and the huge number of villi give a large surface area, and blood flows in opposite directions on the two sides.

Maternal blood pressure is much higher and would damage foetal capillaries.
The mother's immune system would recognise the foetus as foreign.
Different blood groups could cause .
The barrier keeps out many , and maternal hormones.

Lesson 19 of 21

Which hormones maintain a pregnancy, and how is one detected?

from the , and then the , maintains the , so stays high and the is kept. If falls, the degenerates, falls, the is shed and the embryo is lost.

The graph below shows the three placental hormones through a pregnancy.

Line graph with time from fertilisation in weeks from 0 to 40 on the horizontal axis and relative concentration from 0 to 11 on the vertical axis. A red solid line for hCG rises from near zero to a peak of about 9.8 at week 9, then falls steeply and stays near zero from week 20. A blue dotted line for oestrogen and an orange dot-dash line for progesterone both rise steadily from near zero, reaching about 8.4 and 6.3 by week 40. A grey dashed vertical line marks week 38. — Hormone concentrations in the mother's blood during pregnancy. Original BioWelsh graph.

rises steeply from about week 3, peaks at about week 9 and then falls.
and rise steadily and stay high until birth.
maintains the and stops the contracting; grows the uterus and the ; both inhibit and .

Lesson 20 of 21

What triggers birth and the release of milk?

The diagram below shows the that drives birth.

Four pink boxes arranged in a square and joined by arrows running clockwise. The boxes read the baby's head pushes on the cervix, stretch receptors send impulses to the hypothalamus, the posterior pituitary releases more oxytocin, and the myometrium contracts more strongly. Text in the centre reads positive feedback, each turn of the loop makes the next one stronger. — Oxytocin and birth: a positive feedback loop. Original BioWelsh diagram.

falls near the end of pregnancy, so the can contract.
The baby's head stretches the ; stretch send impulses to the .
The releases , which makes the contract more strongly.
Stronger contractions stretch the more, so more is released. This is .

from the anterior stimulates the to secrete milk, and released during suckling ejects it. also inhibits , so is less likely while breastfeeding.

Lesson 21 of 21

Which hormone does what? A summary

Every hormone in this topic
Hormone	Secreted by	Main effect
		and male reproductive system
	Anterior	development and secretion
	Anterior	Surge causes ; forms the
	, then the	Rebuilds the ; grows the uterus and
	, then the	Maintains the ; stops the contracting
	, then the	Maintains the
		Contraction of the ; milk ejection
	Anterior	Milk secretion; inhibits

releases a .
The secretes .
is mitosis with no growth in between.
The umbilical artery carries deoxygenated blood away from the foetus.
Maternal and foetal blood never mix.

Which organs make up the male reproductive system?

What can you see in a section of a testis?

Which organs make up the female reproductive system?

What can you see in a section of an ovary?

What is gametogenesis, and why must gametes be haploid?

How does the testis make spermatozoa?

How is a spermatozoon adapted to its job?

How does the ovary make a secondary oocyte?

What surrounds a secondary oocyte?

How do spermatogenesis and oogenesis compare?

How do you work out the real size of a cell down a microscope?

What happens when a sperm meets a secondary oocyte?

How does the oocyte stop a second sperm getting in?

How does the blastocyst reach the uterus and implant?

What happens in the ovary and the uterus over 28 days?

How do FSH, LH, oestrogen and progesterone control the cycle?

How is the placenta built for exchange?

Why must the blood of the mother and the foetus never mix?

Which hormones maintain a pregnancy, and how is one detected?

What triggers birth and the release of milk?

Which hormone does what? A summary

Worksheets

Worksheet 1: 4.1 Human reproduction (132 marks)

Check your understanding.

The testes are held in the scrotum, outside the body cavity. Which statement best explains why?

Sexual reproduction in humans: cram notes

Which organs make up the male reproductive system?

What can you see in a section of a testis?

Which organs make up the female reproductive system?

What can you see in a section of an ovary?

What is gametogenesis, and why must gametes be haploid?

How does the testis make spermatozoa?

How is a spermatozoon adapted to its job?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 2 of 7 WJEC exam series (2017 to 2025), 8 marks in total. This is a revision priority, not a prediction of your exam.

How does the ovary make a secondary oocyte?

What surrounds a secondary oocyte?

How do spermatogenesis and oogenesis compare?

How do you work out the real size of a cell down a microscope?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 3 of 7 WJEC exam series (2017 to 2025), 8 marks in total. This is a revision priority, not a prediction of your exam.

What happens when a sperm meets a secondary oocyte?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 2 of 7 WJEC exam series (2017 to 2025), 8 marks in total. This is a revision priority, not a prediction of your exam.

How does the oocyte stop a second sperm getting in?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 3 of 7 WJEC exam series (2017 to 2025), 8 marks in total. This is a revision priority, not a prediction of your exam.

How does the blastocyst reach the uterus and implant?

What happens in the ovary and the uterus over 28 days?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 3 of 7 WJEC exam series (2017 to 2025), 13 marks in total. This is a revision priority, not a prediction of your exam.

How do FSH, LH, oestrogen and progesterone control the cycle?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 3 of 7 WJEC exam series (2017 to 2025), 13 marks in total. This is a revision priority, not a prediction of your exam.

How is the placenta built for exchange?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 3 of 7 WJEC exam series (2017 to 2025), 16 marks in total. This is a revision priority, not a prediction of your exam.

Why must the blood of the mother and the foetus never mix?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 2 of 7 WJEC exam series (2017 to 2025), 11 marks in total. This is a revision priority, not a prediction of your exam.

Which hormones maintain a pregnancy, and how is one detected?BWRBioWelsh recommendedWe recommend revising this because it has appeared regularly in past papers. Asked in 2 of 7 WJEC exam series (2017 to 2025), 16 marks in total. This is a revision priority, not a prediction of your exam.

What triggers birth and the release of milk?

Which hormone does what? A summary

Worksheets

Worksheet 1: 4.1 Human reproduction (132 marks)

Check your understanding.

How is a spermatozoon adapted to its job?

How do you work out the real size of a cell down a microscope?

What happens when a sperm meets a secondary oocyte?

How does the oocyte stop a second sperm getting in?

What happens in the ovary and the uterus over 28 days?

How do FSH, LH, oestrogen and progesterone control the cycle?

How is the placenta built for exchange?

Why must the blood of the mother and the foetus never mix?

Which hormones maintain a pregnancy, and how is one detected?