Edexcel GCSE Combined Science

Eukaryotic and prokaryotic cells

Animals and plants are made of eukaryotic cells, which keep their genetic material in a nucleus surrounded by a membrane. Bacteria are prokaryotic: they have no nucleus, and their DNA lies loose in the cytoplasm. Prokaryotic cells are also much smaller, usually a few micrometres long, while typical animal and plant cells are tens of micrometres across.

Each part inside a cell, called a sub-cellular structure, has a job, and a cell works because those jobs fit together.

Animal cells

Plant cells

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

Chloroplasts are found in the cells of leaves and green stems but not in root cells, which sit in the dark and have no light to absorb.

Bacterial cells

A bacterium has a cell wall (not made of cellulose), a cell membrane, cytoplasm and ribosomes, and its genetic material is arranged differently from ours.

• Chromosomal DNA is a single loop that floats in the cytoplasm, since there is no nucleus to hold it.
• Plasmid DNA is one or more small extra rings of DNA, separate from the chromosome, which often carry genes such as those for antibiotic resistance. Plasmids can be passed from one bacterium to another.
• A flagellum (plural flagella) is a long whip-like tail that rotates and moves the cell through liquid. Not all bacteria have one.

Specialised cells

In a many-celled organism, cells become specialised: their structure is adapted to a particular function. Each adaptation can be explained by what the cell has to do.

Sperm cells

A sperm cell has to reach an egg and get its genetic material inside.

• The tail moves the sperm through fluid towards the egg.
• Many mitochondria in the middle section release the energy the tail needs to keep moving.
• The acrosome at the head holds digestive enzymes, which break down the outer layers of the egg so that the sperm nucleus can enter.
• The nucleus is haploid, meaning it holds one copy of each chromosome (23 in a human sperm), so that fertilisation restores the full set of 46.

Egg cells

An egg cell is much larger than a sperm, because it has to support the first stage of a new organism.

• The cytoplasm holds a store of nutrients, which feed the embryo until it can obtain food in other ways.
• The nucleus is haploid, carrying the mother's half of the chromosomes.
• After one sperm has entered, the cell membrane changes so that no other sperm can get in. If a second sperm fused with the egg the embryo would have too many chromosomes and would not develop properly.

Ciliated epithelial cells

Ciliated epithelial cells line the airways, including the trachea and bronchi. Their surface is covered with cilia, tiny hair-like projections that beat together in a wave. Mucus made by other cells in the lining traps dust and bacteria, and the cilia sweep this mucus up towards the throat, where it is swallowed or coughed out. This keeps the lungs clear and cuts the risk of infection. Smoking damages cilia, which is one reason smokers cough more.

Ciliated cells are found in other tubes too, such as the oviduct, where the cilia help move the egg cell along.