Membrane Structure and Function

Fluid Mosaic Model The plasma membrane is composed of a phospholipid bilayer with embedded proteins. Carbohydrates are also attached to the outer surface of the membrane. The accepted model of the plasma membrane is that the membrane is a fluid structure composed in the above form with a “mosaic” of proteins

Fluid Mosaic Model Membranes are not static Held together by hydrophobic interactions Lipids and proteins can shift within the membrane (usually laterally)

Membrane Proteins Two major types Integral Proteins Many are transmembrane proteins (span through the membrane) others only extend midway Have a hydrophobic region composed of one or more nonpolar amino acids, usually in α helices Many also have hydrophilic channel for hydrophilic substances to pass

Membrane Proteins Peripheral Proteins Not imbedded in bilayer, but attached to the surface of the membrane – via the cytoskeleton or ECM Often attached to exposed areas of integral proteins

Protein Functions

Membrane Carbohydrates Cellular recognition often relies on carbohydrates because these are often binding sites Usually short, branched chains and can be covalently bonded to lipids (glycolipids) or proteins (glycoproteins) Glycoproteins are more common Carbohydrate variations on cell surfaces allow for marking of cells Example: Blood typing of A, B, AB, O is designated by carb. variation

Synthesis of Membranes Membrane proteins and lipids begin in the ER. Carbs are added to proteins. In GA, glycoproteins undergo carb modification. Carbs added to lipids. Transmembrane proteins, glycolipids, and secretory proteins are transported via vesicles to the membrane Vesicles fuse with the membrane, realeasing the secretory proteins and positioning the glycoproteins and glycolipids outside the membrane

Selective Permeability Nonpolar molecules are hydrophobic and can therefore dissolve in the lipid bilayer without membrane proteins Includes: hydrocarbons, carbon dioxide, and oxygen Ions and polar molecules (ie. hydrophilic) are blocked and/or slowed by the hydrophobic core of the membrane therefore they often use transport proteins Water often uses a specialized channel protein called an aquaporin

Transport Proteins Channel Proteins – transport proteins with a hydrophilic channel that allow certain polar molecules and ions to pass Carrier Proteins – change shape to specifically hold whatever is crossing **Transport proteins are specific to each substance and will not allow other substances via that route

Passive Transport Diffusion – the movement of molecules of any substance so that they spread out evenly in an available space In the absence of other forces, a substance will diffuse from where it is more concentrated to where it is less concentrated It diffuses DOWN the concentration gradient and does not use energy

Passive Transport Osmosis – The diffusion of water across a selectively permeable membrane Water diffuses across the membrane from the region of lower solute concentration to that of higher solute concentration

Water Balance (No Walls) Tonicity – the ability of a solution to cause a cell to gain or lose water; related to the concentration of solutes that cannot cross membrane Isotonic – no NET movement of water Hypertonic – cell loses water to the environment; greater solute concentration outside the cell Hypotonic – cell gains water from the environment; greater solute concentration inside the cell

Water Balance (Walls) Cell wall protects against excess water uptake Cells can be turgid (full of water) or flaccid (limp) Plasmolysis – in a hypertonic environment, a plant cell can lose water, causing the plasma membrane to pull away from the cell wall and the plant to wilt

Passive Transport Facilitated Diffusion – diffusion using transport proteins Used by water and other polar molecules May also include gated channels that are opened via a chemical or mechanical stimulus

Active Transport The pumping of solute across a membrane AGAINST the concentration gradient, thus using energy (ATP) Enables cells to maintain solute concentrations that are different from their environment Example: Sodium-Potassium Pump

Active Transport Ion Pumps – The inside of the cell is more negative in relation to the outside of the cell, favoring the transport of cations into the cell and anions out of the cell This can be a form of diffusion; however, if movement goes against the electrochemical gradient it can be active

Active Transport Cotransport – the transport of several solutes driven by a single ATP-powered pump The 2nd step in this is actually diffusion, but indirectly relies on the ATP from step 1 Ex: Plant Cells use a single proton pump for hydrogen ions but this also drives the transport of amino acids, sugars and nutrients

Active Transport Exocytosis - Cell secretions are released via vesicles Endocytosis – Cell takes in items from outside the cell by creating a vesicle Phagocytosis – cellular eating Pinocytosis – cellular drinking; the extracellular fluid engulfed also contain a variety of solutes Receptor-Mediated Endocytosis – receiving specific substances via binding proteins in the plasma membrane which forms a vesicle (Ligand with specific receptor)