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

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: ______________________________________________________________________________________________________

• http://www.youtube.com/watch?v=s0p1ztrbXPY&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) – http://www.youtube.com/watch?

v=STzOiRqzzL4&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.

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

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