Osmosis vs. Diffusion

• Diffusion is the movement of solutes (e.g. salts, nutrients) across a semi-permeable membrane into regions of lower concentration; osmosis is the movement of water through a semi-permeable membrane into a region of higher solute concentration.
  
  

Lipid Molecules

• The plasma membrane is just two molecules thick. The lipid molecules forming most of the membrane assemble in a double-layer, with their hydrophobic tails enclosed inside the membrane, away from the watery cell interior and exterior.
  
  

Permeability

• The lipid molecules are permeable to water-soluble substances such as ions and glucose, and fat-soluble substances such as oxygen and carbon dioxide.
  
  

Carbohydrates

• Carbohydrates in the cell membrane promote adhesion between cells.
  
  

Receptor Proteins

• Receptor proteins receive signals from the outside, but do not let the signaling chemical in due to their size; instead, they send their own signal into the cell.
  
  

Transport Proteins

• Transport proteins in the cell membrane actively transport nutrients into the cell and waste back out through the membrane.