1.3 Membrane structure


All cells, prokaryotic or eukaryotic, are surrounded by a cell surface membrane. Additionally eukaryotic cells contain membrane-bound organelles. The cell surface membrane maintains the integrity of a cell and is a controlling barrier to the entry and exit of substances. Organelle membranes share these functions but in addition have other specialised functions, such as for the reactions photosynthesis and cell respiration.


These pages are scanned from Biology for the IB Diploma, Second Edition, by C.J. Clegg



The biology in this unit is not so very much. But just as important as the biology are the different aspects of scientific understanding:

  • Why is it necessary to create a MODEL for the structure of cell membranes?
  • How were these models devised? On what evidence and with what knowledge?
  • How was one model superseded by another – in particular how was the Davson-Danielli model replaced by the Singer-Nicolson Fluid Mosaic Model?

This is all covered well enough in the pages scanned above.

Here is an interactive ppt (from Board works) about the theories of membrane structure. (You will probably receive a security warning when you try to open this ppt. There is no risk and you should activate the macros.)  the-fluid-mosaic-model-development-of-ideas


How can you quickly but accurately draw the cell surface membrane? Firstly, draw it in two dimensions – don’t ever try to draw it in 3-D! Make sure that your drawing includes all the necessary molecules and structures in the minimum of space. Don’t overload the number of phospho-lipid molecules. Label what your are expected to know and show. Here is a good example of such a drawing. Practice from this drawing but add some missing labels:

  • Cholesterol
  • Label for the Hydrophobic (non polar) fatty acid ‘tails
  • Label for the Hydrophilic (polar) phospho heads
[Not sure that you need to refer to exoplasm (E face) or cytoplasm (C Face) but the idea is simple enough – one side of the membrane is the outside (exo) and one side is the inside (cytoplasm) of the cell.]

Eyyy! edited small