AS-LEVEL AQA BIOLOGY NOTES 
TOPIC 5: transport across cell membranes

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The roles of membranes are:

1. To act as partially (and sometimes selectively) permeable membranes between:
   
   1. A cell and its environment
      
   2. An organelle and the cytoplasm which surrounds it
      
   3. Within organelles (e.g. mitochondria/chloroplasts)
      
2. Sites of chemical reactions
   
3. Sites of cell communication and signalling
   

• The Fluid Mosaic Model
  

Biological membranes are thought of in a ‘fluid mosaic model’. By this, we mean that the membrane is constructed from several different components including phospholipids, proteins, and modified proteins, and that these membrane constituents (phospholipids and are continually moving relative to each other, i.e. not fixed. The basic structure of membranes is the same throughout living organisms.
The membrane’s components are:

• Phospholipids, which form the bilayer and the bulk of the membrane surface area
  
• Cholesterol, which regulate membrane mobility and flexibility
  
• Proteins, which are involved in more complex functions such as carrying out chemical reactions or regulating cross-plasma transport
  
  • Can be intrinsic/integral (span the entire membrane)
    
  • Can be extrinsic (embedded in one half of the membrane)
    
  • Can form cross-membrane channels
    

Glycoproteins and glycolipids, which can be antigens or receptor molecules

Substances may move across the membrane by:    

1. Diffusion    
2. Osmosis    
3. Facilitated diffusion
4. Active transport                    

                                         
Diffusion

• Movement of molecules or ions from an area of high concentration to an area of lower concentration
• Until the concentrations of the two regions are equal
• Dynamic equilibrium established
• Passive process
• The rate of diffusion is proportional to the concentration gradient the distance over which the diffusion must occur
• Substances capable of diffusion across the cell surface membrane include
  • Oxygen
  • carbon dioxide
  • Steroids
  • the fat-soluble vitamins (A,D,E,K)
  • Glycerol
  • alcohols and
  • ammonia.

                    
Osmosis

• Specialised form of diffusion
• Diffusion of water molecules from an area of high water potential (high concentration) to an area of low water potential (low concentration)
• Through a partially permeable membrane
• Passive process

                    
Facilitated diffusion

• Faster than normal diffusion
• Used to transport molecules such as
  • Glucose
  • Fructose
  • non fat-soluble vitamins
  • urea and
  • many ions

across the membrane.

• There are four key steps involved in facilitated diffusion:

1. Glucose binds with transport protein molecules on the cell surface. Different cells have different types of glucose transporter.    
2. The transport protein changes shape.    
3. The glucose is transported through the membrane to the inside of the cell.
4. The glucose detaches from the transporter protein and the protein reverts to its original shape. The glucose is then immediately phosphorylated, so the concentration of glucose inside the cell remains low and the concentration gradient across the membrane is maintained.

• The rate of facilitated diffusion is proportional to the concentration gradient and to the number of channels or transporter proteins that are available.
• Occurs down the concentration gradient
• Passive process

                    
Active Transport

• Ions such as
  • sodium (Na+)
  • potassium (K+)
  • chloride (Cl-)
  • hydrogen (H+) and
  • molecules such as amino acids and glucose

require active transport across the cell surface membrane

• ATP is required
• Active process
• This is because the movement of these ions/molecules is against the concentration gradient
• All of these substances can cross the membrane by facilitated diffusion if the concentration gradient is the right way round and if transporter channels are available.
• Commonly involves a protein which acts as a Na+/K+ pump.

Up to a threshold, temperature has little effect on the structure or permeability of a membrane. However beyond the threshold, the high temperature can denature some of the components of the membrane, such as the proteins and glycoproteins, changing their effect and reducing their ability to control transport of molecules across the membrane. Therefore, at high temperatures, the membrane becomes denatured and more permeable.
pH has a similar effect, by denaturing the proteins in the membranes that are in pH other than the optimal pH for that cell or organelle.

Cell specialisation

• Cells are specialised for optimal transport across their internal and outer membranes.
• E.g. root hair cells have a large surface area to increase rate of osmosis across the outer cell membrane

The effects of solutions of different water potentials on animal and plant cells

Solution	Animal cell	Plant cell
Hypotonic	Water potential outside cell higher than inside Water moves in by osmosis Cell swells may burst (Haemolysis)	Solution has higher water potential than vacuole Water flows in by osmosis Cell swells, becomes turgid Cannot burst because of cell wall
Isotonic	Water potential both inside and outside are equal Water flows in and out at an equal rate	Neither turgid nor plasmolysed Water moving in and out at same rate
Hypertonic	Water potential outside cell lower than inside Water moves out of the cell by osmosis Cell shrinks (Crentation)	Solution has lower water potential that vacuole Water flows out by osmosis Cell shrinks, becomes plasmolysed Cell wall rigid and does not collapse