Cell-surface Membrane

• What is the phospholipid bilayer?

  Has a hydrophobic tail and a hydrophilic head

  Acts as a barrier to diffusion of many substances

  Small non-polar (without charge) molecules can pass through by diffusion due to the middle being hydrophobic
• What is facilitated diffusion?

  Diffusion that is assisted by proteins in the membrane

  Ions and small polar molecules are transported by channel proteins that act like pores in the membrane

  Some of these can open and close; they are called gated channels

  Larger polar molecules use carrier proteins

  These are specific to the substance they are transporting

  The substance binds to the inside of the protein, and the protein then changes shape, transferring the substance to the other side
• What is co-transport?

  The coupled movement of substances across a membrane

  One substance moves down a concentration gradient, driving the movement of the other substance in the same direction