Cells and cell organelles are separated from the rest of the world by a membrane

2. 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 • http://www.youtube.com/watch?v=owEgqrq51zY

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

4. 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_

5. 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

6. Gradient examples • A color gradient • A light gradient • A molecular gradient

7. 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

8. Going Down the gradient • Some molecules move freely across the membrane down the gradient (O2, H2O, N2) • 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 • http://www.youtube.com/watch?v=s0p1ztrbXPY&NR=1

9. 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

10. 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) • http://www.youtube.com/watch?v=STzOiRqzzL4&feature=related

11. 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. • http://www.youtube.com/watch?v=LSm2jPgxcwk

12. 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