Cells and cell organelles are separated from the rest of the world by a membrane A membrane controls what goes in and out of the cell Like an animal’s skin

Membrane structure Cell membranes are composed of –Phospholipids –Transmembrane proteins (imbedded inside the lipid layer) –External proteins (attached to transmembrane proteins)

Diffusion Certain molecules can diffuse freely in or out of the cell Other molecules cannot get through the phospholipid layer Main determinants of diffusion –Size ( small molecules diffuse better than big ones) –Solubility in lipids (only molecules that are soluble in lipids can diffuse through the membrane –Charge (molecules with a charge cannot diffuse through the membrane_

Direction of diffusion Without any force or energy, molecules automatically diffuse from high concentration to low concentration Diffusion occurs down a concentration gradient Concentration gradient: different concentrations of an object in space

Gradient examples A color gradient A light gradient A molecular gradient

Movement of molecules DOWN a concentration gradient requires no energy –Molecules move from high to low concentration –Like a ball rolling downhill - natural Movement of molecules UP a concentration gradient requires energy –Moving molecules from low to high concentration –Pushing a ball uphill – unnatural, requires extra force

Going Down the gradient Some molecules move freely across the membrane down the gradient (O 2, H 2 O, N 2 ) Most molecules can’t get through the lipid bilayer of the membrane (proteins, sugars, ions) Passive transport aka facilitated diffusion: Proteins within the membrane act as channels for transporting molecules down the concentration gradient bXPY&NR=1

Going UP the gradient Sometimes cells need to maintain an uneven concentration of a molecule across the membrane But moving molecules UP their concentration gradient requires ENERGY

Active transport Some proteins in the membrane act like molecule pumps –Use ATP for energy –Move molecules against concentration gradient (in or out of the cell) – L4&feature=related

Active Transport Example –Na+ / K+ pump: animal cells need to have more sodium on the inside, and more potassium on the outside. This pump maintains an uneven gradient of each ion. – wkhttp:// wk

Summary A higher concentration of a molecule inside or outside of the cell creates a concentration gradient of that molecule across the membrane No energy is required for a molecule to move down the concentration gradient (high to low) Some molecules can diffuse right across the membrane down the gradient Others require protein channels to diffuse into the cell (passive transport aka facilitated diffusion) Moving molecules up the concentration gradient (low to high) requires energy, and must be done with special protein pumps in the membrane ATP provides the energy for protein pumps to move molecules from low to high concentrations across the membrane