Drawing the Fluid Mosaic Model
Membranes
- Membranes are vital structures found in all cells
- The cell surface membrane creates an enclosed space separating the internal cell environment from the external environment
- Intracellular membranes (internal membranes) form compartments within the cell, such as organelles (including the nucleus, mitochondria and RER) and vacuoles
- Membranes not only separate different areas but also control the exchange of materials passing through them; they are partially permeable
- Membranes form partially permeable barriers between the cell and its environment, between cytoplasm and organelles and also within organelles
- Substances can cross membranes by diffusion, facilitated diffusion, osmosis and active transport
- Membranes play a role in cell signaling by acting as an interface for communication between cells
Membranes formed from phospholipid bilayers help to compartmentalise different regions within the cell, as well as forming the cell surface membrane
Fluid Mosaic Model
- The fluid mosaic model of membranes was first outlined in 1972 by Singer and Nicolson and it explains how biological molecules are arranged to form cell membranes
- The fluid mosaic model also helps to explain:
- Passive and active movement between cells and their surroundings
- Cell-to-cell interactions
- Cell signalling
- The fluid mosaic model describes cell membranes as ‘fluid’ because:
- The phospholipids and proteins can move around via diffusion
- The phospholipids mainly move sideways, within their own layers
- The many different types of proteins interspersed throughout the bilayer move about within it (a bit like icebergs in the sea) although some may be fixed in position
- The fluid mosaic model describes cell membranes as ‘mosaics’ because:
- The scattered pattern produced by the proteins within the phospholipid bilayer looks somewhat like a mosaic when viewed from above
- The fluid mosaic model of membranes includes four main components:
- Phospholipids
- Cholesterol
- Glycoproteins and glycolipids
- Transport proteins
The main components of cell membranes. The distribution of the proteins within the membrane gives a mosaic appearance and the structure of the proteins determines their position in the membrane.
Examiner Tip
When drawing the fluid mosaic model remember to include (and label) the phospholipid bilayer (making it clear which part is the phosphate head and which parts are the hydrocarbon tails), the thickness of the membrane (7 - 10 nm), integral proteins (show then embedded in the phospholipid bilayer and include a couple of different types e.g. channel/carrier), peripheral proteins (do not extend the protein into the hydrophobic region), glycoprotein (with a carbohydrate attached) and finally cholesterol (ensure the orientation is correct, OH group next to the phosphate heads and the rest positioned next to the tails).