cell membrane

• Cell membrane - a layer in the cell envelope appearing just beneath the cell wall
• The most widely accepted model for membrane structure is the fluid mosaic model of Singer and Nicholson, which proposes that membranes are lipid bilayers within which proteins float
• The fluid mosaic model of the cell membrane is how scientists describe what the cell membrane

  looks and functions like, because it is made up of a bunch of different molecules that are distributed across the membrane.
• there’s a pattern of different types of molecules put together, also known as a mosaic.
• These molecules are constantly moving in two dimensions, in a fluid fashion, similar to icebergs

  floating in the ocean. The movement of the mosaic of molecules makes it impossible to form a completely impenetrable barrier.
• Cell membrane holds the different components of the cell together and to protect it from the environment outside the cell
• The most important function of the cell membrane, is to act as a barrier between the inside and outside of the cell, regulating transport—the passage of nutrients into the cell and the discharge of wastes
• Although water and small uncharged molecules can diffuse across the membrane unaided, the membrane is a selectively permeable structure with special carrier mechanisms for passage of most molecules
• The membranes of bacteria are an important site for a number of metabolic activities. For example, most enzymes that handle the energy reactions of respiration reside in the cell membrane
• Enzyme structures located in the cell membrane also help

  synthesize structural ​macromolecules to be incorporated into the cell envelope and appendages.
• Other products (enzymes and toxins) are ​secreted by the membrane into the extracellular

  environment.
• Small, nonpolar molecules (e.g. oxygen and carbon dioxide):

  These molecules can pass through the lipid bilayer and do so by

  squeezing through the phospholipid bilayers. They don't need

  proteins for transport and can diffuse across quickly
• Small, polar molecules (e.g. water): These molecules can also

  pass through the lipid bilayer without the help of proteins, but

  they do so with a little more difficulty than the molecule type

  above.
• Large, nonpolar molecules (e.g. carbon rings): These rings can

  pass through but it is also a slow process.
• Large, polar molecules (e.g. simple sugar - glucose): The size and charge of large polar molecules make it too difficult to pass through the nonpolar region of the phospholipid membrane without help from transport proteins.
• Ions (e.g. Na+): Similarly, the charge of an ion makes it too difficult to pass through the nonpolar region of the phospholipid membranewithout help from transport proteins