NSB31 Recap

Cells: specialised cells and organisation

Why cells specialise

In a multicellular organism, cells do not all look the same. Each type is specialised — its shape and contents are adapted to one particular job. This lets a large organism carry out many tasks efficiently, but it also means the cells depend on each other: a nerve cell cannot make its own food and a root hair cell cannot think.

Specialised animal cells

  • Red blood cell — carries oxygen. It is a flattened disc with a dip on each side for a large surface area, and it has no nucleus, leaving more room for haemoglobin.
  • Nerve cell (neuron) — carries electrical messages. It is very long with branching ends so it can connect to many other cells.
  • Muscle cell — contracts to cause movement. It is long, and packed with mitochondria for energy.
  • Sperm cell — carries the father’s DNA to the egg. It has a tail for swimming and many mitochondria to power it.
  • White blood cell — fights infection. It can change shape to squeeze out of blood vessels and engulf microbes.

Specialised plant cells

  • Root hair cell — absorbs water and minerals from soil. Its long, thin extension gives a large surface area. It has no chloroplasts.
  • Palisade cell — the main site of photosynthesis. It is tall and packed with chloroplasts, and sits at the top of the leaf near the light.
  • Guard cell — pairs of these surround each stoma (pore) on the leaf. They change shape to open and close the pore, controlling gas exchange and water loss.
  • Xylem cell — hollow, dead tubes stacked end to end that carry water up the plant.

Levels of organisation

Specialised cells are arranged in a hierarchy:

cell→tissue→organ→organ system→organism\text{cell} \rightarrow \text{tissue} \rightarrow \text{organ} \rightarrow \text{organ system} \rightarrow \text{organism}
  • A tissue is a group of similar cells working together — muscle tissue, or the palisade layer in a leaf.
  • An organ is several tissues working together for one job — the heart, the stomach, a leaf.
  • An organ system is a group of organs sharing a task — the digestive system (mouth, stomach, intestines) or the circulatory system (heart, blood vessels, blood).
  • An organism is the whole living thing.

Plants have organs too: roots, stems, leaves and flowers.