Section 4- Transport across cell membranes

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Molecules within membranes

Phospholipids- Hydrophilic head of both layers point to the outside of the cell. Hydrophobic tails of both layers point into the middle, repelled by the water on the outside of tyhe layer.

Lipid soluable molecuels move through the fluid mosaic layer

Funtions of the phosphlipds is to- Allow lipid soluable substances to enter.

Prevent water soluable substances from entering.

Make membrane flexiable and self-sealing.

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Section 4- Transport across the cell membrane.

Proteins- They are embeded into the phospholipid bilayer in two main ways. 

In the surface of the bilayer. They act either to give mechanical supportto the membrane, or in conjunction with glycolipids, as cell receptorsfor molecuels such as hormones. 

Some span the entire length of the protein channel:

  • Protein channels- Form water filled tubes to allow water soluable ions to diffuse across into the membrane.
  • Carrier proteins- Bind to ions or molecules like gluecose and amino acids. Then change the shape in order to move these molecuels across the membrane.

Functions of proteins: Provide structeral support, act as channels for transporting water-soluable substances across the membrane, allow active transport across the membrane through carrier proteins, form cell-surface receptors for identifying cells, help cells adhere together, act as receptors


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Section 4- Transport across cell membranes


Colesterol molecuels add strength to the membranes.

Colesterol is extremely hydrophobic-playing an important role in preventing water loss and dissolved ions. 

Pull together the fatty acid tails of the phospholipid moelcuels limiting their movement. 

Functions of colesterol in the membrane:

  • Reduce lateral movement of other molecuel. 
  • Help maintain stability of memebrane 
  • Help cells attach to each other to form tissues. 
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Section 4- Transport across the cell membrane


Glycolipids are made up of an carbydrate covalently bonded with a lipid. 

Carbohydrate portion extends beond the bilayer where it acts as an cell surface receptor for specific chemicals. 


  • Act as recgonition sites. 
  • Help maintain stability of membrane
  • Help cells attach to one another to help form tissue
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Section 4- Transport across cell membranes


Carbohydrates are attached to many extrinsic proteins on the outer surface of the cell membrane. 

Also act as cell-surface receptors, more specifically for hormones and nuerotransmitters


  • Act as recognition sites
  • Help cells attach to one and another and so form tissue. 
  • Allows cells to regonise each other. 
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Section 4- Transport across cell surface membranes

Permeability of the cell surface membrane

Cell surface membrane controlles the movement of substances in and out of the cell

In general most molecules don't freely diffues across it because:

  • Not soluable in lipids
  • To large to pass through the protein channels. 
  • Same charge as the protein- Even if they are small enough they are repelled. 
  • Electrostatically charged- Find difficulty passing through the non-polar hydrophobic tails.
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Section 4- Transport across the cell membrane

The arrangement is known as a fluid-mosaic model:

  • Fluid- Because the individual phospholipid molecules can move relative to one another. This gives the membrane a flexiable structure constanly changing shape. 
  • Mosaic- Because proteins that are embedded in the phospholipid bilayer vary in shape, size and patten. 

Image result for phospholipid bilayer

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