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 neuron, is specialised to carry electrical impulses between parts of the body. Its shape and structures help impulses travel over distances and allow the neuron to communicate with other cells.

• Long axon: The axon is an extension that carries impulses along the neuron. Its length allows an impulse to travel a considerable distance within one cell.
• Branched extensions: These allow the neuron to connect with other cells. Branches at the end of the axon allow it to pass signals to multiple target cells.
• Myelin sheath: Many axons have a layer of myelin around them. This sheath acts as insulation and increases the speed at which impulses travel along the axon.

Muscle cell adaptations

Muscle cells are specialised to contract, producing a pulling force. They are elongated and contain filaments made of contractile proteins. When these filaments slide past one another, the cell shortens and produces force.

Contraction requires energy. Many muscle cells contain numerous mitochondria, where respiration releases energy to support repeated contractions.

Muscle cells work together in muscle tissue. Their combined contractions produce movement of the body or of structures within it.

Root hair cell adaptations

Root hair cells are specialised to absorb water and mineral ions from the soil. They help supply the rest of the plant with these substances.

Each root hair cell has a long, hair-like projection. This gives the cell a large surface area in contact with the soil, allowing more water and mineral ions to be absorbed. The cell also has a thin cell wall, so water and dissolved substances have a short path into the cell.

Water and mineral ion uptake

Water enters a root hair cell by osmosis. It moves across the partially permeable cell membrane from the more dilute soil solution into the more concentrated cell contents.

Mineral ions can enter by active transport when their concentration is lower in the soil than inside the cell. Transport proteins in the cell membrane move the ions against this concentration difference. The cell’s mitochondria release energy through respiration, providing the energy needed for active transport.

Xylem cell adaptations

Xylem vessel cells are specialised to carry water and dissolved mineral ions from the roots through the plant. Their structure also helps support the plant.

As xylem vessel cells mature, they die and their contents are lost, leaving hollow interiors. The end walls between cells break down, so cells joined end to end form a continuous vessel. Water and dissolved mineral ions can flow through this hollow tube.

The walls of xylem vessels are reinforced with lignin. This strengthens the vessels and helps prevent them from collapsing, keeping the pathway open for transport. The strengthened walls also help support the plant.

Phloem cell adaptations

Phloem transports dissolved sugars and other organic substances between parts of a plant. Its cells are adapted to form a pathway for sap to flow and to supply the energy needed to load sugars into that pathway.

Sieve tube elements

Living sieve tube elements join end to end to form a sieve tube. Their end walls form sieve plates, which contain pores. Sap can pass through these pores from one sieve tube element to the next.

Mature sieve tube elements have no nucleus and contain little cytoplasm. This leaves more space inside the cells for sap to flow.

Companion cells and sugar transport

Companion cells are associated with sieve tube elements and support them. They contain many mitochondria, which supply energy for activities including the active loading of sugars into the phloem.

Sugars are transported from a source, where they are supplied, to a sink, where they are used or stored. Sources and sinks can be in different parts of the plant.