A good diagram answer links each structure to its function and, where the question asks, to why its features suit that function. In reproduction this means the organs of the male and female systems, and the placenta, umbilical cord and amniotic sac around the fetus.
This lesson follows the sequence of fertilisation and implantation and prepares you for precise explanations in human reproduction and development.
How do you turn a label into an explanation?
Use a three-part sentence: name, job, reason.
- Name: the structure, spelled correctly.
- Job: what it does in this process.
- Reason: the feature that makes it suited, or what would fail without it.
For example: “The oviduct carries the egg from the ovary towards the uterus and is where fertilisation usually happens, because sperm meet the egg here.” That is three parts in one sentence.
Key structures and their jobs
| Structure | Job |
|---|---|
| Testis | Makes sperm and the hormone testosterone |
| Sperm duct | Carries sperm towards the urethra |
| Prostate gland | Adds fluid to sperm, forming semen |
| Ovary | Produces and releases egg cells; makes hormones |
| Oviduct | Carries egg; site of fertilisation |
| Uterus | Lining receives the embryo; muscular wall holds the fetus |
| Cervix | Ring of muscle at the lower end of the uterus |
| Placenta | Exchange of substances between maternal and fetal blood |
| Umbilical cord | Carries fetal blood to and from the placenta |
| Amniotic fluid | Cushions and protects the fetus from knocks |
Worked example
Question: The diagram shows a fetus inside the uterus with the placenta and amniotic sac labelled. Explain how the placenta is adapted for its function.
Step 1. State the function. The placenta is where substances are exchanged between the mother’s blood and the fetus’s blood.
Step 2. List the features from the diagram. It is large and flat, it is full of tiny blood vessels, and the barrier between the two bloods is very thin.
Step 3. Link each feature to a reason.
- Large, flat surface: a bigger area for diffusion.
- Thin barrier: a short distance, so diffusion is faster.
- Rich blood supply: maintains steep concentration gradients, because blood keeps carrying substances away.
Step 4. Name what moves. Oxygen, glucose and amino acids diffuse or are transported into fetal blood. Carbon dioxide and urea pass out to maternal blood.
Model answer: “The placenta exchanges substances between maternal and fetal blood. It has a large surface area and a thin barrier, so diffusion is fast, and a rich blood supply keeps concentration gradients steep. Oxygen and nutrients pass to the fetus, and carbon dioxide and urea pass to the mother.”
The mistake to watch for
Mistaken answer: “The mother’s blood flows into the baby through the placenta.”
The bloods do not mix. Substances cross the thin barrier by diffusion and transport, but the two circulations stay separate. Mixing could be harmful, because the higher blood pressure of the mother could damage the fragile fetal capillaries.
The correction is to write what moves, in which direction, and that it crosses a barrier between separate bloods.
Check yourself
1. State one function of the amniotic fluid.
Show answer
It cushions and protects the fetus from mechanical shock.
2. Name two substances that move from fetal blood to maternal blood across the placenta.
Show answer
Carbon dioxide and urea (waste products made by the fetus).
3. A student says, “The testis carries sperm to the urethra.” Correct the statement.
Show answer
The testis makes sperm. The sperm duct carries them towards the urethra.
Where this leads next
Hormones control the timing of these structures, which is the focus of interpreting hormone-cycle data. When you are ready, test your diagram answers in the mixed practice set.
If you know the labels but the explanation sentences do not flow, our teachers can build that habit in online one-to-one Biology tuition.