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From Cells to Organisms

≈ 18 min
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Cells rarely work alone. In any large organism they are organised into five levels, each built from the one before:

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 doing the same job — muscle tissue, nerve tissue, xylem tissue in a plant stem.
  • An organ is several different tissues working together as a unit — the heart, the stomach, a leaf, a root.
  • An organ system is a group of organs that together carry out one large function — the digestive system, the circulatory system, the respiratory system.
  • An organism is the complete living thing, with all its systems working at once.

Worked example — place the heart in the hierarchy.

Start at the smallest and build upwards:

  1. A single cardiac muscle cell — one cell that can contract.
  2. Many of them together form cardiac muscle tissue.
  3. That tissue, plus nerve tissue, blood vessels and connective tissue, forms the heart — an organ.
  4. The heart together with the arteries, veins and blood forms the circulatory system.
  5. That system, working alongside the digestive, respiratory and other systems, makes up the human being — the organism.

Do the same for a plant: palisade cell → palisade tissue → leaf → shoot system → maize plant.

Which sequence shows the levels of organisation in the correct order, from smallest to largest?

What word describes a group of similar cells working together to do the same job?

Which of the following is an organ?

An elephant is far bigger than a mouse. Which statement about their cells is correct?