Principles of organisation: Cells are the basic building blocks of all living organisms.

The idea

This statement establishes the cell as the fundamental unit of life: every living organism is built from cells, and all the processes of life occur within or are controlled by cells. Organisms may be unicellular, such as a single bacterium or Amoeba, or multicellular, such as a human or oak tree, but in every case the cell is the smallest unit that can be called living. Cells contain the structures needed for metabolism, and in multicellular organisms they become specialised for particular jobs. This idea underpins the whole of biology: tissues, organs and organ systems are all built from cells, so understanding cell structure and function explains how larger living systems work.

What earns the marks

• States that all living organisms are composed of cells, whether unicellular or multicellular.
• Recognises the cell as the smallest unit that shows all the characteristics of life, such as metabolism and response to stimuli.
• Uses examples such as a single-celled bacterium or Amoeba alongside multicellular organisms like humans or plants.
• Explains that in multicellular organisms cells may be specialised, and that groups of similar cells form tissues.
• Links the idea to the wider hierarchy: cells build tissues, tissues build organs, organs build organ systems.
• Applies the concept when interpreting diagrams or microscope images of cells from different organisms.

Common mistakes and how to put them right

• Thinking that only animals and plants are made of cells.

  Put it right: All living organisms, including bacteria and fungi, are made of cells.

• Believing that a cell is always visible to the naked eye.

  Put it right: Most cells require a microscope, although some large cells and cell clusters can be seen.

• Confusing 'basic building block' with 'smallest particle of matter'.

  Put it right: Cells are the basic unit of life, not atoms or molecules.

• Assuming all cells in an organism are identical.

  Put it right: Many cells are specialised in structure and function.

Exam tips

• Use the phrase 'basic building block of all living organisms' accurately when defining a cell.
• Give one unicellular and one multicellular example to show the idea applies across life.
• When describing levels of organisation, start at the cell and work upwards to tissue, organ and organ system.
• If asked to compare organisms, refer to cell number and specialisation rather than size alone.