The Plasma Membrane Gateway to the Cell

Functions of Plasma Membrane Protective barrier Regulate transport in & out of cell (selectively permeable) Allow cell recognition Provide anchoring sites for filaments of cytoskeleton

Functions of Plasma Membrane Provide a binding site for enzymes Interlocking surfaces bind cells together (junctions) Contains the cytoplasm (fluid in cell)

Membrane Components Phospholipids Proteins (peripheral and integral) The Plasma Membrane 4/20/2017 Phospholipids Proteins (peripheral and integral) Cholesterol Carbohydrates G. Podgorski, Biol. 1010

Composition of Membranes Water Lipid many different types Carbohydrate on lipids and proteins Simple to very complex Protein Many different types

Membrane lipids Glycerophospholipids (phosphoglycerides) Sphingolipids: sphingophospholipids sphingoglycolipids Cholesterol

Phosphoglycerides Simplest glycerophos-pholipids is phosphatidic acid Different molecules are added onto the phosphate group to make the other phosphoglycerides Phosphate Hydrophillic region Glycerol Ester linkage Fatty acids (saturated and unsaturated) Hydrophobic region

Phosphoglycerides (continued) Polar groups attached to phosphatidic acid Inositol Serine Glycerol Choline ethanolamine

Phosphoglycerides (continued) Hydrophilic region Hydrophobic region

Sphingolipids Based on Sphingosine Fatty acyl group Amide linkage Based on Sphingosine Sphingosine convertrd to cermide with the condensation with a long chain fatty acid Sphingomyelin derived from cerimides

Glycolipids Carbohydrate attached to lipid by way of beta glycosidic bond instead of phosphate ester Usually sphingosine derivative (by way of ceramide) Cerebroside Galactocerebroside Glucocerobroside Ganglioside More complex carbohydrate structure Contains sialic acid (an acidic carbohydrate)

Cholesterol Hydrophillic region Hydrophobic region

Membrane proteins Varies from membrane to membrane Amount Diversity Classes of membrane proteins Integral Peripheral Lipid anchored

Proteins Are Critical to Membrane Function The Plasma Membrane 4/20/2017 Proteins Are Critical to Membrane Function G. Podgorski, Biol. 1010

Membrane protein function •transport •enzymatic activity •signal transduction •intercellular joining •cell-cell recognition •ECM attachment

Fluid mosaic model FLUID MOSAIC MODEL The Plasma Membrane 4/20/2017 FLUID MOSAIC MODEL Fluid mosaic model FLUID- because individual phospholipids and proteins can move around freely within the layer, like it’s a liquid. MOSAIC- because of the pattern produced by the scattered protein molecules when the membrane is viewed from above. G. Podgorski, Biol. 1010

Movement of lipids in membrane Rotation Flexing of fatty acyl chain Lateral diffusion Transverse (flip-flop) from one side to other Frequently Infrequently

RBC plasma membrane

Semipermeable Membrane Small molecules and larger hydrophobic molecules move through easily. e.g. O2, CO2, H2O Ions, hydrophilic molecules larger than water, and large molecules such as proteins do not move through the membrane on their own.

Types of Transport Across Cell Membranes

Simple Diffusion Diffusion is a PASSIVE process which means no energy is used to make the molecules move Molecules move from area of HIGH to LOW concentration

Diffusion of H2O Across A Membrane High H2O potential Low solute concentration Low H2O potential High solute concentration

NO NET MOVEMENT OF H2O (equal amounts entering & leaving) Isotonic Solution Hypotonic Solution Hypertonic Solution NO NET MOVEMENT OF H2O (equal amounts entering & leaving) CYTOLYSIS PLASMOLYSIS

Three Forms of Transport Across the Membrane The Plasma Membrane Three Forms of Transport Across the Membrane 4/20/2017 G. Podgorski, Biol. 1010

Passive Transport Simple Diffusion Doesn’t require energy The Plasma Membrane 4/20/2017 Passive Transport Simple Diffusion Doesn’t require energy Moves high to low concentration Example: Oxygen or water diffusing into a cell and carbon dioxide diffusing out. G. Podgorski, Biol. 1010

Facilitated diffusion The Plasma Membrane 4/20/2017 Passive Transport Facilitated diffusion Doesn’t require energy Uses transport proteins to move high to low concentration Examples: Glucose or amino acids moving from blood into a cell. G. Podgorski, Biol. 1010

Types of Transport Proteins Channel proteins are embedded in the cell membrane & have a pore for materials to cross Carrier proteins can change shape to move material from one side of the membrane to the other

Channels that are proteins Cellular channels usually consist of large protein complexes with multiple transmembrane a-helices. Control of channel gating is a form of allosteric regulation. Conformational changes associated with channel opening may be regulated by: Voltage Binding of a ligand (a regulatory molecule) Membrane stretch (e.g., via link to cytoskeleton)

Ion Channels Channels cycle between open & closed conformations. When open, a channel provides a continuous pathway through the bilayer, allowing flux of many ions. Gramicidin is an example of a channel.

ionophores Antibiotics of bacterial origin facilitate the movement of ions across membranes called ionophores Ionophores : carriers and channels. valinomycin (a carrier) gramicidin (a channel).

Valinomycin is a carrier for K+. It is a circular molecule, made up of 3 repeats of the sequence shown above.

Valinomycin is highly selective for K+ relative to Na+. The ionophore valinomycin is a uniport carrier.

Gramicidin A

Gating (opening & closing) of a gramicidin channel is thought to involve reversible dimerization. An open channel forms when two gramicidin molecules join end to end to span the membrane.

Classes of carrier proteins Uniport (facilitated diffusion) carriers mediate transport of a single solute. An example is the GLUT1 glucose carrier.

Symport (cotransport) carriers bind two dissimilar solutes & transport them together across a membrane. Transport of the two solutes is obligatorily coupled. A gradient of one substrate, usually an ion, may drive uphill (against the gradient) transport of a co-substrate. It is sometimes referred to as secondary active transport. E.g:  glucose-Na+ symport, in plasma membranes of some epithelial cells

Facilitated Diffusion Some Carrier proteins do not extend through the membrane. They bond and drag molecules through the lipid bilayer and release them on the opposite side

Carrier Proteins Other carrier proteins change shape to move materials across the cell membrane

Active Transport Requires energy or ATP The Plasma Membrane 4/20/2017 Active Transport Requires energy or ATP Moves materials from LOW to HIGH concentration AGAINST concentration gradient G. Podgorski, Biol. 1010

Active transport Examples: Pumping Na+ (sodium ions) out and K+ (potassium ions) in against strong concentration gradients. Called Na+-K+ Pump

Sodium-Potassium Pump 3 Na+ pumped in for every 2 K+ pumped out; creates a membrane potential

Transport of macromolecules Exocytosis~ secretion of macromolecules by the fusion of vesicles with the plasma membrane Endocytosis~ import of macromolecules by forming new vesicles with the plasma membrane phagocytosis pinocytosis receptor-mediated endocytosis (ligands)

Exocytosis - moving things out. Molecules are moved out of the cell by vesicles that fuse with the plasma membrane. This is how many hormones are secreted and how nerve cells communicate with one another.

Pinocytosis Most common form of endocytosis. The Plasma Membrane Pinocytosis 4/20/2017 Most common form of endocytosis. Takes in dissolved molecules as a vesicle. G. Podgorski, Biol. 1010

Pinocytosis Cell forms an invagination Materials dissolve in water to be brought into cell Called “Cell Drinking”

Receptor-Mediated Endocytosis The Plasma Membrane 4/20/2017 Receptor-Mediated Endocytosis Some integral proteins have receptors on their surface to recognize & take in hormones, cholesterol, etc. G. Podgorski, Biol. 1010

Endocytosis – Phagocytosis The Plasma Membrane 4/20/2017 Endocytosis – Phagocytosis Used to engulf large particles such as food, bacteria, etc. into vesicles Called “Cell Eating” G. Podgorski, Biol. 1010

Exocytosis The opposite of endocytosis is exocytosis Exocytosis The opposite of endocytosis is exocytosis. Large molecules that are manufactured in the cell are released through the cell membrane. Inside Cell Cell environment

Specialized Transport Transport proteins Facilitated diffusion~ passage of molecules and ions with transport proteins across a membrane down the concentration gradient Active transport~ movement of a substance against its concentration gradient with the help of cellular energy

Movement of compounds across membranes Passive diffusion Affected by Size of molecule Composition of molecule Temperature Lipid composition Not mediated by proteins Molecules move down a concentration gradient Unimportant for most polar molecules S

Pore (a.k.a. Channel) Proteins Actively regulated to control flow across membranes Very specific for molecules able to pass through channel Rate of movement of solutes is usually greater than for transporters Fig. 9.30

Barriers, Pores, pumps and gates.mht