Membrane transport. Pharmacy Dr. Szilvia Barkó

Transcription

1 Membrane transport Pharmacy Dr. Szilvia Barkó

2 Cell Membranes

3 Cell Membrane Functions Protection Communication Import and and export of molecules Movement of the cell

4 General Structure A lipid bilayer that contains 2 sheets of lipids interdispersed with proteins.

5 Fluid-Mosaic Model

6 Lipid Structure Most abundant lipid is the phospholipid Phospholipids have a PO 4 group in the 3 rd OH group of the glycerol instead of hydrocarbon This can attach a hydrophilic group Ethanolamine Serine Choline

7 Amphipathic Molecules The molecule contains both a hydrophilic and a hydrophobic portion Bilayer formation Other molecules: Steroids Glycolipids lipid with a sugar attached rather than a phosphate group

8 Membrane Fluidity Enables the membrane proteins to diffuse rapidly Simple means of distributing lipids and proteins Allows membranes to fuse with one another Evenly distributed during daughter cell formation

9 Membrane Fluidity Each C=C bond causes a kink or bend in the tail Cholesterol is added to areas that have lots of unsaturated lipids to help fill in the gaps between the tails Helps to stiffen and stabilize the bilayer Less fluid Less permeable Hydrocarbon tail determines the fluidity of the membrane just as it does in fats and oils 2 components are important Length of hydrocarbon chain 14 to 24 C but usually 18 to 20 C per tail Level of unsaturation (# of C=C bonds)

10 Amphipathic molecules in the membrane The hydrophilic head molecules interact with the aqueous solution The hydrophobic tails will interact with each other

11 Movement in the membrane Lipids cannot move from one layer to another without the aid of proteins Lipids can exchange places with neighbors Lipids can rotate around their axis

12 Membranes are Asymmetrical Inner surface is different from the outer surface Types of lipids in each layer Proteins in the bilayer have a specific orientation due to its function

13 New Membrane New lipids are added on one side of the membrane Enzyme called flippase used to put the lipid in the other half of the bilayer Flippase may be selective for the type of lipids that it puts on either surface

14 Membranes as Barriers Because of the hydrophobic interior of the bilayer Membrane is impermeable to ions and large charged molecules and require special membrane proteins to transport across

15 Transmembrane Proteins Protein has hydrophilic and hydrophobic portions Hydrophilic will interact with the aqueous solutions on either surface Hydrophobic will be in contact with the hydrophobic interior of the bilayer Also called integral membrane proteins

16 Plasma Membrane Proteins: different functions PROTEINS CAN MOVE IN THE MEMBRANE, TOO!

17 Diffusion through the cell membrane

18 Transmembrane transport proteins allow selective transport of hydrophilic molecules & ions 1. carrier protein Carrier protein Solute A carrier protein alternates between two conformations, moving a solute across the membrane as the shape of the protein changes. The protein can transport the solute in either direction, with the net movement being down the concentration gradient of the solute.

19 Transmembrane transport proteins allow selective transport of hydrophilic molecules & ions 2. channel protein EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM (a) A channel protein (purple) has a channel through which water molecules or a specific solute can pass. Note: channel proteins mediate only passive transport

20 Coupled Transporters

21 Example1: Glucose transporter GluT1 : (carrier-mediated facilitated diffusion-uniport) Glucose + ATP hexokinase glucose-6-phosphate + ADP

22 Example2: Na-glucose cotransport (carrier-mediated facilitated diffusionsynport) Mostly Na + and an other molecule (charged or neutral) Direction: from the extracellular space inside Driving force: gradient of Na + toward to inside The concentration of the transported molecule is going to be higher in the cell than out (secondary active transport)

23 Example3: Anion exchange protein 1 (carrier-mediated facilitated diffusionantiport) At the region of tissues capillaries: CO 2 : free diffusion inside the erythrocyte The carbonic anhydrase converts CO 2 to H 2 CO 3 H 2 CO 3 dissociates to H + and HCO 3 - AE1 changes HCO 3 - to Cl - At the region of lungs capillaries: same antiporter reverse function

24 The sodiumpotassium pump 1 Cytoplasmic Na + binds to the sodium-potassium pump. Na + Na + [Na + ] high [K + ] low Na + Na + Na + 2 Na+ binding stimulates phosphorylation by ATP. CYTOPLASM Na + [Na + ] low [K + ] high ADP P ATP EXTRACELLULAR FLUID Na + Na + Na + 3 K + is released and Na + sites are receptive again; the cycle repeats. K + K + P 4 Phosphorylation causes the protein to change its conformation, expelling Na + to the outside. K + K + P P i 5 Loss of the phosphate restores the protein s original conformation. K + K + 6 Extracellular K + binds to the protein, triggering release of the Phosphate group.

25 Passive transport Active transport Simple diffusion Facilitated diffusion No protein channel carrier protein protein carrier protein HIGH to low conc HIGH to low conc low to HIGH conc favorable favorable Unfavorable Add energy ATP

26 Carbohydrates on Cell Surface Many of the plasma membrane proteins have sugars attached to them Short oligosaccharides glycoproteins Long polysaccharides - proteoglycans Sugars on the surface make up the glycocalyx Keeps cells moist and slippery Used as cell recognition (lectins) and adhesion molecules

27 Glycocalyx Cell Coat