Membrane transport How things get in and out of the cell

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 –______________ –Transmembrane proteins (__________ _________________) –External 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 ( __________________________) –Solubility in lipids (__________________________________________ ________________________________________) –Charge (__________________________________________ __________________________________________)

Direction of diffusion Without any force or energy, molecules automatically diffuse from _____________________________ ______________________ Diffusion occurs down a concentration gradient Concentration gradient: _____________________________________________ __________________________________________

Gradient examples A color gradient A light gradient A molecular gradient

____________________ ____________________ ____________________ –Molecules move from high to low concentration –Like a ball rolling downhill - natural ____________________ ____________________ ____________________ –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 (__________________) Most molecules can’t get through the lipid bilayer of the membrane (__________________ ___________________________) Passive transport aka facilitated diffusion: __________________________________ __________________________________ __________________________________ 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 _____________________________________________ _____________________________________________ No energy is required for a molecule to move down the concentration gradient (__________________) Some molecules can diffuse right across the membrane down the gradient Others require protein channels to diffuse into the cell (________________________________) Moving molecules up the concentration gradient(___________________) requires energy, and must be done with special protein pumps in the membrane ATP provides the energy for protein pumps _____________________________________________ _____________________________________________ across the membrane