Specialisation of eukaryotic cells

In a complex multicellular organism, different cells perform different functions. A specialised cell has features that help it carry out a particular role. These features may include its shape, the abundance of a cell structure, or the way structures are arranged.

For example, a root hair cell absorbs water from the soil. Its long, thin extension gives it a large surface area in contact with the soil, allowing more water to enter the cell.

A sperm cell has a tail that propels it towards an egg. It also has many mitochondria, which release energy needed for the tail to move. Both features are suited to the sperm cell’s role in reaching the egg.

To explain a cell’s adaptation, name a specific feature and link it to the function it helps the cell perform. For a root hair cell, stating that it has a long extension identifies the feature; explaining that the extension increases the surface area for water absorption shows how the feature helps.

Sperm cell adaptations

A sperm cell is specialised to reach and fertilise an egg cell. Its structures help it swim to the egg, penetrate its outer layers and deliver genetic information.

Swimming towards the egg

The sperm’s tail beats to propel it through fluid towards the egg. The sperm contains many mitochondria, which carry out respiration and release energy to power the tail’s movement.

Entering the egg

At the head of the sperm is a cap called the acrosome. It contains enzymes that help digest through the outer layers of the egg, allowing the sperm nucleus to enter.

Delivering genetic information

The sperm’s nucleus is haploid: it contains one set of chromosomes, rather than the two sets usually found in body cells. The egg nucleus is also haploid. When the sperm and egg nuclei fuse, their chromosome sets combine, restoring the normal diploid number in the fertilised egg.

Nerve cell adaptations

A nerve cell, or neurone, is specialised to transmit electrical impulses through the nervous system. Its shape and coverings help it carry impulses over distances and communicate with other cells.

A neurone has a long axon that carries electrical impulses along the cell. Its branched ends connect with other cells, allowing impulses to be passed on.

Many neurones have a myelin sheath around the axon. This sheath insulates the axon and increases the speed at which impulses travel along it.

Muscle cell adaptations

Muscle cells are specialised to contract, producing a pulling force. When they contract, they shorten; when they relax, they can lengthen again.

Inside each muscle cell are protein filaments that slide past one another. This sliding shortens the cell. Muscle cells are elongated, so their contraction produces a pull along their length.

Contraction requires energy. Muscle cells contain many mitochondria, which release energy by respiration to support repeated contraction.

Muscle cells act together in muscle tissue. Their combined contractions can produce movement or change the shape of an organ.

Root hair cell adaptations

Root hair cells absorb water and mineral ions from the soil into plant roots. Their structure helps them take in these substances efficiently.

Each root hair cell has a long, hair-like extension that gives it a large surface area in contact with the soil. This provides more area for water and mineral ions to enter the cell.

The cell has a thin cell wall, giving water a short path into the cell. Water enters by osmosis: its net movement across a partially permeable membrane from a more dilute solution to a more concentrated solution.

The cell also contains mitochondria, which release energy by respiration. This energy is needed for the active uptake of mineral ions when their concentration is lower in the soil than inside the cell. The ions are taken up against their concentration gradient, from a lower concentration to a higher concentration.

Xylem cell adaptations

Xylem cells form vessels that carry water and dissolved mineral ions from the roots through the plant. Their structure also helps support the plant.

Hollow xylem vessels

As xylem cells develop, they die and lose their contents. Their end walls break down, joining the cells to form a continuous hollow tube. With no cell contents or end walls blocking the tube, water and dissolved mineral ions can flow through it.

Strengthened xylem walls

Xylem vessel walls are strengthened with lignin, a strong material that helps prevent the vessels from collapsing as water moves through them. The strengthened walls also provide structural support for the plant.

Phloem cell adaptations

Phloem transports dissolved sugars and other organic substances around a plant. This movement, called translocation, carries substances from regions where they are supplied to regions where they are used or stored.

Sieve tube elements

Living sieve tube elements join end to end to form tubes through which phloem sap can flow. Their end walls form sieve plates, which have pores that allow sap to pass from one element to the next. The elements have little internal cell contents, leaving more space for sap to flow with less obstruction.

Companion cells

Each sieve tube element is supported by a companion cell. Companion cells contain many mitochondria, which release energy for processes involved in loading substances into the phloem. The loaded substances can then be transported in the sap.