Membranes

What is the relationship between the mosaic structure of the cell membrane and it’s function?  Osmosis and diffusion effects on biological organisms  Integral protein  Cholesterol  Cell to cell communication  Active/passive transport  Exocytosis/ endocytosis

Cell membrane  Happened early in evolution.  Separate cell from its surroundings  Selective permeability: allows only certain things to cross

Cell membranes  Membranes: fluid mosaics of lipids and proteins  Amphipathic molecule: phospholipids have both a hydrophilic (head) and hydrophobic (tail) region also membrane proteins  Hydrophobic portions are hidden in the middle of the bilayer membrane

Lateral movement  Proteins and lipids can move laterally within the membrane  Seem to move with specific direction….driven along cytoskeletal fibers by motor proteins.

 Membranes must be fluid to work properly.  Salad oil  Cholesterol hinders closeness of the phospholipids making membrane slightly less fluid but maintaining fluidness into lower temperatures.

Membranes differ  Membranes with different functions differ in their makeup.  Mitochondria have more embedded proteins

Membrane transport  Hydrophobic not charge can dissolve + partially cross  Hydrophilic: more difficult

Cell Wall: plants  Cell wall: –Cellulose –Proteins –Polysaccharides –Pectin (starch)

Cell Membrane  Cell Membrane (animal) –Phospholipid bilayer –Imbedded proteins: protein channels –Cholesterol –Glycoproteins: cell to cell recognition

Membrane transport  Hydrophobic: no charge, non polar  If small: CO2, O2 can dissolve and cross easily  Hydrophilic: polar: more difficult H2O, glucose

Passive transport  Passive transport = diffusion, osmosis, facilitated diffusion (uses transport protein)  Diffusion = tendency of molecules to spread out due to their kinetic energy.  Substances will diffuse down their concentration gradient  Unaffected by the concentration of other substances.

Membrane transport: Facilitated  Facilitated diffusion: H2O, glucose  Transport Proteins: may be hydrophilic inside  Carrier: may physically move substrate  Aquaporins: for H2O  May be substrate specific or may just span the membrane

Membrane Transport: facilitated  Transport protein: –Protein channel:  Ion channels usually gated. Need chemical or electrical stimulus –Protein carrier

Osmosis  Osmosis = diffusion of water: passive  Water down its concentration gradient  Hypotonic to Hypertonic less solute(more water) more solute

 Cell wall: lets in only so much water until cell is turgid (healthy for plant)  Animal cell with no wall will burst if put in hypotonic solution.

Active transport  Sodium – potassium pump  Pumping solutes against their concentration gradient.  Cell: inside K+ high Na+ low  Must use energy to maintain this balance  ATP: phos. Group might bind directly to the transport protein

Na+ K+ pump  Na+ binds with protein: causes ATP to ADP  The added phosphate changes protein shape  So, protein spits Na+ out the other side and grabs K+  K+ binding releases the Phosphate  3 Na+ out and 2 K+ in

Voltage  Cell membrane have voltage  Membrane potential = electrical potential energy due to the charges in and out of cell ( - inside + outside)  Animal: electrogenic pump (Na+, K+)  Plants: use proton pump

 2 forces drive diffusion  Membrane potential = electrical energy  Concentration gradient = chemical energy

Cotransport  Plant pumps H+ out  When it diffuses back in it can carry sucrose with it. (even against concentration gradient)

Exocytosis  Exocytosis: vesicle fuses with cell membrane to release it secretions outside

Endocytosis  Membrane forms vesicle with something from outside  1. phagocytosis: pseudopodium then fuse with lysosome  2. pinocytosis: wrap around drop of extracellular fluid  3. receptor mediated: humans- cholesterol ligand: any molecule that binds to receptor