13 September 2014

#24 Cell membranes - Fluid Mosaic Model of the plasma menbrane

The fluid-mosaic model describes the plasma membrane that surrounds animal cell. The membrane has 2 layers of phospholipids (fats with phosphorous attached), which at body temperature are like vegetable oil (fluid).

1. Fluid Mosaic Model
  • Because cells reside in a watery solution (extracellular fluid), and they contain a watery solution inside of them (cytoplasm), both layers of phospholipids (1)  have the hydrophilic heads (2) facing outwards into the water and the hydrophobic tails (3) facing inwards, avoiding contact with water.
  • Cholesterol molecules are among the phospholipids.
  • Protein molecules (4) float in the phospholipid bilayer. 
  • Many of the phospholipids and proteins have short chains of carbohydrates (5) attached to them, on the outer surface of the membrane. They are known as glycolipids (6) and glycoproteins (7). There are also other types of glycolipid with no phosphate groups.

The fluid mosaic model of membrane structure.

This is called the fluid mosaic model of membrane structure:

'fluid' because the membran is fluid (molecules within the membrane can move around within their own layers)

'mosaic' because the protein molecules are mosaiclly aranged 

'model' because no-one has ever seen a membrane looking like the diagram - the molecules are too small to see even with the most powerful microscope. 

The role of the components of cell membrane 

  • Have a hydrophobic head and a fatty acid tail --> form a bilayer separating the cell from the outside. 
  • They are fluid --> components can move around freely. 
  • Permeable to small and/or non-polar molecules
  • Impermeable to large molecules and ions --> prevent these substances from passing thorough.


  • Maintains the fluidity of the membrane
  • Increases the stability of the membrane: sits between fatty acid tails --> making the barrier more complete, preventing molecules like water and ions from passing through the membrane. Without cholesterol the membrane would easily split apart.

Proteins and glycoproteins
(Protein molecules + carbohydrates)
  • Channel proteins allow the movement of some substances, such as the large molecule sugar, into and out of the cell as they can’t travel directly through the cell surface membrane. The channels can be opened or closed to control the substances’ movement.

  • Carrier proteins actively move substances across the cell surface membrane, using energy from ATP.

  • Cell surface receptors are glycoproteins responsible for the binding of an extracellular signalling molecule (hormones and cell surface antigens) and transduction of its messages into one or more intracellular signalling molecules, which changes the cell’s behaviour.
  • Help to interact with other cells. 
  • Help to recognise cells: glycoproteins are specific for cells from a particular individual or a particular tissue 

(Phosopholipid molecules + carbohydrates)
  • Cell signalling
  • Cell surface antigens 
  • Cell adhesion (adhese to neighbouring cells to form tissues).

2. Roles of cell surface membranes  
  • Structural, keeping the cell contents together.
  • Separate cell components from the outside environment
  • Allows cells to communicate with each other by cell signalling.
  • Allows recognition of other external substances.
  • Allows mobility in some organisms, e.g. amoeba.
  • Selectively permeable barrier.
  • Regulating the transport of materials into or out of cells
  • The site of various chemical reactions.

 Syllabus 2015
(a) describe and explain the fluid mosaic model of membrane structure, including an outline of the roles of phospholipids, cholesterol, glycolipids, proteins and glycoproteins;

(b) outline the roles of cell surface membranes;

Syllabus 2016  - 2018

4.1 Fluid mosaic membranes 

The structure of cell surface membranes allows movement of substances between cells and their surroundings and allows cells to communicate with each other by cell signalling.

a) describe and explain the fluid mosaic model of membrane structure, including an outline of the roles of phospholipids, cholesterol, glycolipids, proteins and glycoproteins

b) outline the roles of cell surface membranes including references to carrier proteins, channel proteins, cell surface receptors and cell surface antigens

c) outline the process of cell signalling involving the release of chemicals that combine with cell surface receptors on target cells, leading to specific responses

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