Edexcel GCSE Biology

Eukaryotic and prokaryotic cells

Living things are built from cells, and cells come in two basic designs. Eukaryotic cells, which make up animals, plants, fungi and protists, keep their genetic material inside a nucleus. Prokaryotic cells, the bacteria, have no nucleus, and their DNA lies free in the cytoplasm. Prokaryotic cells are also much smaller, often only a thousandth of a millimetre or two across, whereas a typical animal cell is around ten to a hundred times wider.

Every structure inside a cell has a job, and the shape of a cell usually tells you what it does.

Animal cells

Cells that use a lot of energy, such as muscle cells, contain many mitochondria. Cells that make a lot of protein, such as those of the pancreas that produce digestive enzymes, contain many ribosomes.

Plant cells

A plant cell has all of the structures above, plus three more.

Cells in the roots have no chloroplasts, because they are underground and light does not reach them.

Bacterial cells

A bacterium has a cell wall (not made of cellulose), a cell membrane, cytoplasm and ribosomes. Its main DNA is a single chromosomal loop lying free in the cytoplasm. Many bacteria also carry small rings of extra DNA called plasmids, which hold genes that are not needed all the time, such as genes that make the bacterium resistant to an antibiotic. Some bacteria have a flagellum, a whip-like tail that rotates and pushes the cell through liquid.

Bacteria have no mitochondria or chloroplasts, so respiration takes place in the cytoplasm and at the cell membrane.

Specialised cells

Most cells in a multicellular organism are specialised: their structure is adapted to one function. Three examples are needed.

Sperm cells

A sperm cell has one job, which is to reach an egg and pass its genetic material into it.

• The tail moves the cell towards the egg.
• Many mitochondria, packed in the middle section behind the head, release the energy needed for the tail to keep beating over a long distance.
• The head contains the nucleus, which is haploid: it holds only one set of chromosomes (23 in humans), half the number in a body cell, so that fertilisation restores the full number.
• The acrosome, at the front of the head, is a small store of digestive enzymes. When the sperm reaches the egg the enzymes are released and break down the egg's outer layers so that the sperm can get through.

Egg cells

An egg cell (ovum) is one of the largest cells in the human body, and its structure fits what happens after fertilisation.

• The cytoplasm contains nutrients, mostly lipids and proteins, which feed the embryo during the first days of development before it can obtain food from the mother.
• The nucleus is haploid, so it contributes one set of chromosomes.
• After one sperm has entered, the cell membrane changes and forms a barrier that other sperm cannot cross. This makes sure only one nucleus fuses with the egg's, so the zygote has the right number of chromosomes.

Ciliated epithelial cells

These cells line surfaces such as the trachea and bronchi. Their upper surface is covered in cilia, tiny hair-like projections that move together in a wave. In the airways, the goblet cells between them make sticky mucus that traps dust and bacteria, and the cilia sweep the mucus up towards the throat where it can be swallowed. The cells therefore keep the lungs clean and protect them from infection. Cilia in the oviduct move the egg along towards the uterus in the same way.