Cell Structures and Specialised Cells

Cells have structures that do different jobs

A cell is the basic unit of a living organism. Its sub-cellular structures are the smaller parts that work together to keep it alive. Some contain genetic instructions, some make proteins, and others control the movement of substances or release energy.

Animal and plant cells are eukaryotic: their DNA is enclosed in a nucleus. Bacterial cells are prokaryotic: they have no nucleus, so their chromosomal DNA lies in the cytoplasm. Bacteria are generally much smaller than animal and plant cells, but they still carry out the processes needed for life.

Structures shared by animal and plant cells

The nucleus contains DNA, the genetic material carrying instructions for making proteins. These proteins are needed to build cell structures and carry out cell processes, so the nucleus controls the cell's activities. A nuclear membrane encloses it.

The cell membrane forms the boundary of the cell and controls which substances enter and leave. This allows the cell to take in substances it needs and remove waste products. Inside the membrane is the cytoplasm, where many chemical reactions take place. It contains enzymes, which speed up these reactions.

Mitochondria are the sites of aerobic respiration reactions. Respiration releases energy for activities such as movement and building new molecules. Cells with high energy demands often contain many mitochondria.

Ribosomes are the sites of protein synthesis, meaning that they make proteins. This connects their role to that of the nucleus: DNA supplies the instructions, while ribosomes make the proteins.

Additional structures in plant cells

Plant cells have the shared structures above, together with features that help them make food and remain supported.

Chloroplasts are the sites of photosynthesis. They contain the green pigment chlorophyll, which absorbs the light needed for photosynthesis. This process makes food for the plant. Chloroplasts occur in photosynthesising plant cells; not every plant cell contains them.

A cell wall lies outside the cell membrane. It is made of cellulose and provides strength and support. The wall and membrane have different roles: the wall supports the cell, while the membrane controls the movement of substances.

The large permanent vacuole contains cell sap. It helps keep the cell firm, contributing to the support of the plant.

Plant cells also need mitochondria. Photosynthesis makes food, whereas respiration releases energy that the cell can use: these are different processes.

How a bacterial cell works without a nucleus

A bacterium has a cell membrane and cytoplasm, with the same basic roles as in animal and plant cells. It also has ribosomes, which make proteins. However, it does not have a nucleus, mitochondria or chloroplasts.

Its main genetic information is carried by chromosomal DNA, usually a single circular DNA molecule in the cytoplasm. This supplies instructions for making proteins and carrying out cell activities, without being enclosed in a nucleus.

Bacteria may also contain plasmids. These are small rings of DNA separate from the chromosomal DNA and carry additional genes.

Some bacteria have one or more flagella. A flagellum is a long, tail-like structure that enables the bacterium to move. Bacteria also have a supporting cell wall, but it is not made of cellulose like a plant cell wall.

The key comparison is therefore not whether a cell has DNA: all three cell types do. It is whether that DNA is enclosed in a nucleus.

Specialised cells have structures suited to their roles

In a multicellular organism, different cells perform different jobs. A specialised cell has features adapted to its particular function. Specialisation can involve a distinctive shape, additional surface structures or larger numbers of particular organelles.

Sperm cells and egg cells are specialised for reproduction. Ciliated epithelial cells are specialised to move material along a surface.

Sperm cells: reaching and fertilising an egg

A sperm cell must move towards an egg and deliver its genetic material. Its structures support different stages of this task.

• The tail enables the sperm to swim.
• The middle region contains many mitochondria, which release energy by respiration for swimming.
• The acrosome, a cap on the head, contains digestive enzymes. These help the sperm penetrate the egg's outer layers.
• The head contains a haploid nucleus, carrying half the normal number of chromosomes found in body cells.

The egg also has a haploid nucleus. When the two nuclei fuse during fertilisation, their chromosome sets combine, restoring the normal number in the fertilised egg.

Egg cells: supporting the start of development

An egg cell contributes genetic material and supports the early stages of development after fertilisation.

Its haploid nucleus contains half the normal number of chromosomes, so it can combine with the sperm's nucleus. Its large amount of cytoplasm contains nutrients needed for the growth of the early embryo.

After fertilisation by one sperm, the egg's cell membrane changes to prevent further sperm from entering. This ensures that only one sperm contributes genetic material to the fertilised egg.

Ciliated epithelial cells: clearing the airways

Epithelial cells form linings and coverings in the body. Ciliated epithelial cells line parts of the airways, including the trachea and bronchi. Their exposed surfaces have tiny, hair-like projections called cilia.

Mucus traps particles and microorganisms in the airways. The cilia beat to move this mucus towards the throat, where it can be coughed out or swallowed. Swallowed mucus passes to the stomach. This helps remove trapped material from the airways rather than allowing it to remain near the lungs.

Here, the adaptation is a moving surface: the cilia move the mucus, while the mucus carries the trapped particles.

Animal, plant and bacterial cells

• Eukaryotic: animal and plant cells; DNA enclosed in a nucleus.
• Prokaryotic: bacteria; no nucleus, with circular chromosomal DNA in the cytoplasm.