Cell membrane

• Cell membrane structure diagram 


  diagram

  
• What do cell membranes do

  • Creates an enclosed space separating the internal cell environment from the external environment and intracellular membranes within the cell e.g. the nucleus allowing cellular processes to occur separately
  • Semi permeable- Control the exchange of substances from one side of a membrane to the other by diffusion and active transport allowing selected molecules to move in and out of the cell
  • Site for bio chemical reactions
  • Contain receptor proteins e.g for binding to hormones and antigens
  • Allow a cell to change/hold shape
• What are cell membranes formed of

  Cellular membranes are formed from a double layer or bilayer of phospholipids
• What do phospholipids consist of

  1. A molecule of glycerol- forms the backbone of the phospholipid
  2. A phosphate group- forms the hydrophilic phosphate head (polar)
  3. Lipid tail- 2 hydrophobic fatty acid tails (non polar)
• What do phospholipids form when exposed to water

  1. Bilayer
  2. Micelle

  
• 2 types of proteins in the cell membrane

  1. Integral(/intrinsic) -embedded in cell membrane 2 types

  • channel proteins- transports large or charged molecules
  • Carrier proteins- transports sugars and amino acids etc

  1. peripheral (/extrinsic) - help with cell Recognition and communication. found on outer of inner surface of The membrane
• Diffusion of molecules across Membranes

  • smaller and less polar molecules diffuse across membranes quickly and easily e.g. CO2, O2
  • Polar molecules diffuse much slower e.g. H2O
  • Charged ions cannot diffuse across by themselves e.g. Na+
• Cholesterol in cell membranes

  • a molecule which regulates membrane fluidity
  • increases fluidity of membrane at low temps stopping it becoming too rigid
  • decreases fluidity at high temps stabilising the membrane stopping it becoming too fluid
  • has higher melting point to stabilise cell at higher temps
  • Interacts/bonds to phospholipids hydrophobic tails. Less cholesterol means phospholipids movement is not restricted by bonds
  • spaces/stabalises phospholipids tails stopping them packing too closely together giving rigid shape
• Cholesterol main function in cell membranes

  Increases the mechanical strength and stability of membranes, without it membranes would breakdown and cells would burst
• Summarise cell components and functions

  • peripheral proteins- involved in cell recognition, signalling and attachment
  • integral proteins - transport molecules across cell membranes
  • Glycoprotein and glycolipids- transport molecules across the membrane or
  • Cholesterol- help control membrane fluidity
  • Phospholipids- act as a barrier to most molecules
• evidence for the fluid mosaic model

  When a and a human cell undergo cell fusion, immediately after a fusion the cell membranes and phospholipids fuse and join together. Then proteins move within the membrane. This results in mixed membrane proteins.
• What is a fluid membrane needed for?

  • Diffusion of substances membrane.
  • to move and change shape
• Why is it called the fluid mosaic model?

  Because the model describes the fluid nature of the lipid bilayer by diffusion and the mosaic arrangement of proteins and phospholipids within it.
• Factors affecting membrane fluidity

  1. temperature - ^ temp= ^ kinetic energy of phospholipids causing bonds to break, increasing fluidity and permeability
  2. PH - ^ the charges on the membranes proteins altering the structure of the membrane increasing the permeability and fluidity
  3. solvents - e.g. Alcohol/ detergents that lipid by layer harmful as cell work properly
  4. Cholesterol decreases membrane fluidity
  5. hormones affect permeability e.g. Insulin and glucose