Transport across cell membranes

• The cell membrane is made of proteins and phospholipids. Proteins can either pass through the membrane or float on the surface.
• The membrane is fluid – it is flexible and the lipids can

  move within the membrane. The membrane is a mosaic – there are proteins embedded which also move.
• Membranes that allow ALL molecules to pass through = PERMEABLE
• Selectively permeable membranes allow:

  
  

  -Water

  
  

  -Soluble substances (CO2, O2, sugar)

  
  

  to pass through.
• Passive transport - Movement of a substance down a

  concentration gradient and does not

  require energy. Small molecules can pass easily

  through a cell membrane by passive

  transport.
• Diffusion - Movement of molecules from an area

  of HIGH concentration to an area of LOW concentration, down a concentration gradient.
• Active transport involves the movement of

  molecules and ions against the concentration

  gradient. Energy is required for this process.