Levels of Organisation in Multicellular Organisms
Senior 1 · Biology
Levels of Organisation in Multicellular Organisms
Lesson 2 of 5
20 min
The largest cell in the human body is the egg cell: at about 0.1 mm across, it is just big enough to see without a microscope, like the full stop at the end of this sentence. One of the smallest is the sperm cell, whose head is only about 5 µm long. These two very different cells have the same purpose: to join together at fertilisation, so that a new human life can begin. Each is shaped for its own part in that job.
Definition
Gamete: a sex cell. The male gamete is the sperm, made in the testes; the female gamete is the egg (ovum), made in the ovaries.
Fertilisation: the joining of the nucleus of a sperm with the nucleus of an egg. The single cell formed is a zygote, which divides again and again to form an embryo.
Flagellum: a long, whip-like tail that moves a cell through liquid. A sperm has one.
Figure 1: A sperm cell and an egg cell. The large sperm at the top is enlarged about three times more than the egg; the small sperm beside the egg is drawn to the same scale as the egg.
The sperm cell must travel through the female reproductive system to the egg in the oviduct, a journey of about 15 to 20 cm, and then get inside it.
The egg cell must survive until it is fertilised and then feed the new embryo in its first days.
| Feature | Sperm cell | Egg cell |
|---|---|---|
| Made in | testes | ovaries |
| Size | about 0.05 mm long, head about 5 µm | about 0.1 mm across |
| Movement | swims using its tail | cannot move by itself |
| Food store | almost none | large store in the cytoplasm |
| Number made | millions every day, from puberty onwards | usually one released about once a month |
Warning
The egg does not swim. Students often write that the egg "moves to meet the sperm". It has no tail. After it is released from the ovary it is carried along the oviduct by the beating of the ciliated cells that line it (lesson 1) and by gentle contractions of the oviduct wall.
Worked example 1: how much bigger is the egg?
Treat the egg (100 µm across) and a sperm head (about 5 µm long) as balls. How many times greater is the volume of the egg?
Answer: the egg has about 8000 times the volume of a sperm head. Nearly all of that extra volume is cytoplasm and stored food: the egg supplies the food for the new embryo, the sperm supplies only a nucleus.
Worked example 2: a faulty middle piece
In a laboratory test, a man's sperm have normal tails but middle pieces with very few mitochondria. Predict the effect on his fertility, and explain.
Answer: fewer sperm will reach the egg, so his fertility is likely to be reduced. The example shows why structure and function go together.
Sperm cells can be frozen in liquid nitrogen at −196 °C and stay alive for years. This is how artificial insemination works in cattle breeding: sperm collected from a bull with good milk-producing daughters is stored frozen, and a trained inseminator later places it in a cow at the right time in her cycle. In Rwanda this service helps farmers, including families who received a cow through the Girinka programme, to breed calves that give more milk.
GCSE Biology: Specialised Cells, Hippomedics. Watch the sections on the sperm cell and the egg cell, and compare each adaptation with Figure 1.
Tip
Memory hook: the sperm is built to travel light (tail, mitochondria, enzymes, almost no cytoplasm); the egg is built to feed and protect (large food store, jelly coat).
Practice: sperm and egg cells
Key points
Sources and credits
Figure 1, the table, the worked examples and the practice questions are written by Ganzaa.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
