Cell Theory (Edexcel International A Level Biology): Revision Note

The Cell as a Unit of Life

• Until microscopes became powerful enough to view individual cells, no-one knew for certain what living organisms were made from

• A scientist called Robert Hooke is thought to be the first person to view cells, using the term 'cell' to describe these newly discovered structures

• Matthias Schleiden and Theodor Schwann were two other scientists who studied animal and plant cells

  • In 1837 they came up with the idea that all living organisms are made of cells

  • This idea is known as ‘cell theory’

  • The cell theory is a unifying concept in biology, meaning that it is universally accepted

• Cell theory includes three main ideas

  • All living organisms are made up of one or more cells

  • Cells are the basic functional unit in living organisms

  • New cells are produced from pre-existing cells

• The cells of all living organisms share some common features

  • Cell surface membrane

  • Cytoplasm

  • DNA

  • Ribosomes

• Beyond these common features different cell types contain different structural elements and combinations of organelles, e.g.

  • Prokaryotic cells have no internal membranes and smaller ribosomes

  • Eukaryotic cells have several internal membrane-bound organelles and larger ribosomes

• When examined under a microscope it is possible to see the ultrastructure of different cell types

  • Cell ultrastructure refers to the internal structures of the cell