1.3- sa:v ratio

• As cells increase in size, the continued decrease in the sa:v ratio places an upper limit on cell size.
• as cells increase in size, the cytoplasm may increase in volume as well as the surface area.
• the internal volumes of cells expand at a greater rate than the area of the plasma membrane.
• As the volume of a cell increase, the metabollic needs of the cell increase
• these needs can only be met by an increase of the inputs and outputs of materials across the plasma membrane.
• The increase of the SA (surface area) is always less than the increase in the volume.
• • Membranes are flexible due to their composition.
  • All membranes in a cell are made up of a double layer of phospholipid molecules 🡪 called the PHOSPHOLIPID BILAYER.
  • The phospholipid molecule is made up of: 
  • hydrophilic head (attracted to water)  
  • hydrophobic tail (repels water).
  • The hydrophilic heads are in contact with the watery environment of the cell
  • The hydrophobic tails face each other in the centre of the bilayer.
• Membrane composition

  • Phospholipid molecules:
  • Make up the double layer of the membrane & are free to move about within the layer; small molecules can pass through the membrane between the phospholipid molecules
  • Membrane proteins
  • Proteins provide channels for the movement of  larger              substances in and out of the cell.
  • Carbohydrate chains
  • Carbohydrate molecules attached to the outside of the membrane are involved in cell adhesion and cell recognition.
  • Cholesterol molecules 
  • Give stability to the membrane. Without them the membrane would break down and release its contents.