Biological Membranes

The structure of membranes and their component functions. As well as the different processes involved such as active tansport, osmosis, facilitated and normal diffusion.



  • Separates the cell contents from the outside environment.

  • Separates the cell components from the cytoplasm within the cell.

  • Uses cell signalling and recognition to communicate.

  • Regulates the transport of materials and substances in and out of the cell.

  • And also holds components of metabolic pathways in place for the cell.

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The structure of phospholipids

   A phospholipid molecule has two parts:

  • A hydrophilic head (water loving) made of phosphate.

  • A hydrophobic pair of tails (water hating) made of fatty acid.

The properties of a molecule depends on the charge distribution:

  • NON- Even distribution means that the molecules can easily interact with water molecules.

  • Even distribution means that molecule cannot easily mix with water molecules because its even charge means that it repels water instead.

    If a phospholipid molecule was to mix with water then it would form a layer with other phospholipids at the water’s surface.

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Phospholipid Bilayer

 A phospholipid bilayer is created when phospholipid molecules are surrounded by water.

  • The phosphate heads are on either side of the bilayer

  • The fatty acid tails are mirror images of each other and so point towards each other and away from the water.

    In this state a phospholipid molecule can easily move freely like a fluid molecule.

  • RARELY a phosphate head can change from monolayer to monolayer but they find it hard to pass through the hydrophobic tail region/ layer because of the absence of water.

  • This provides the membrane with stability despite the phosphate heads not being bonded together.

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Phospholipid Bilayer

They all have the basic structure of a phospholipid bilayer.*The hydrophobic layer (fatty acid tails) form a barrier for the cell from the outside environment. This also helps to stop ions and molecules that aren’t needed from getting in.

 In watery environments some membranes form a layer of oil; this is the ideal boundary for some living systems. The metabolic reactions within the cell can take place within the layer.

  • Light microscopes cannot be used to see the cell surface membrane in detail because of its lower resolution.

  • However an electron microscope can be used to see the cell surface membrane in greater detail.

  • The hydrophilic layers can be seen or appear as dark tram lines whereas the hydrophobic tails can be seen in the paler regions.

     A membrane can vary in width but is usually 7- 10 nm. A cell surface membrane is 7.5 nm. Simple phospholipid bilayers are too FRAGILE and INCAPABLE of acting and functioning as a biological membrane, as they need other components to develop/ improve it. The specialisation of membranes are called differentiation.

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