Membrane Structure and Function PDF

Membrane Structure  The plasma membrane separates the living cell from its nonliving surroundings.  This thin barrier, 8 nm thick, controls traffic into and out of the cell.  Like other membranes, the plasma membrane is selectively permeable, allowing some substances to cross more easily than oth...

Membrane Structure  The plasma membrane separates the living cell from its nonliving surroundings.  This thin barrier, 8 nm thick, controls traffic into and out of the cell.  Like other membranes, the plasma membrane is selectively permeable, allowing some substances to cross more easily than others.  The most abundant lipids cell membrane are phospholipids.  Phospholipids and most other membrane constituents are amphipathic molecules.  Amphipathic molecules have both hydrophobic regions and hydrophilic regions.  The phospholipids and proteins in membranes create a unique physical environment, described by the fluid mosaic model.  A membrane is a fluid structure with proteins embedded or attached to a double layer of phospholipids. All membrane lipids are amphipathic—that is, they contain both a hydrophilic (water-loving) region and a hydrophobic (water-hating) region. Thus the most favorable environment for the hydrophilic head is an aqueous one, whereas the hydrophobic tail is more stable in a lipid environment. The amphipathic nature of membrane lipids means that they naturally form bilayers in which the hydrophilic heads point outward towards the aqueous environment and the hydrophobic tails point inward towards each other (Figure 2a). When placed in water, membrane lipids will spontaneously form liposomes, which are spheres formed of a bilayer with water inside and outside, resembling a tiny cell (Figure 2b). This is the most favourable configuration for these lipids, as it means that all of the hydrophilic heads are in contact with water and all of the hydrophobic tails are in a lipid environment  The molecules in the bilayer are arranged as hydrophobic fatty acid tails are sheltered from water while the hydrophilic phosphate groups interact with water.  Membrane proteins are amphipathic, with hydrophobic and hydrophilic regions. Fig. 8.1b  If at the surface, the hydrophilic regions would be in contact with water.  In this fluid mosaic model, the hydrophilic a regions of proteins and phospholipids are in contact with water and the hydrophobic regions are in a nonaqueous environment. Membranes are mosaics of structure and function • A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer. To work properly with permeability, membrane must be fluid about as fluid as oil. A)- The plasma membrane has a unique collection of proteins. • There are two populations of MEMBRANE PROTEINS. 1. Peripheral proteins are not embedded in the lipid bilayer at all. Instead, they are loosely bounded to the surface of the protein, often connected to the other population of membrane proteins. 2. Integral proteins penetrate the hydrophobic core of the lipid bilayer, often completely spanning the membrane (a transmembrane protein). Where they contact the core, they have hydrophobic regions with nonpolar amino acids, often coiled into alpha helices. Where they are in contact with the aqueous environment, they have hydrophilic regions of polar amino acids. Integral proteins Hydrophilic region Peripheral proteins Hydrophobic region The proteins in the plasma membrane may provide a variety of major cell functions. Aquaporins (channel proteins): are transport proteins that function by having a hydrophilic channel that facilitate the passage of water molecules through the membrane in certain cells. Without aquaporins, only a tiny fraction of water molecules would pass through the cell membrane. Carrier protein (glucose transporter): in the plasma membrane of red blood cells transports glucose across the membrane 50,000 time faster than glucose can pass through on its own. Nonpolar molecules, such as hydrocarbons, CO2, and O2, are hydrophobic and can therefore dissolve in the lipid and without the aid of bilayer of the membrane and cross it easily, membrane proteins. Thus, the selective permeability of a membrane depends on both the discriminating barrier of the lipid bilayer and the specific transport proteins built into the membrane Specific proteins facilitate passive transport • Many polar molecules and ions diffuse passively through the lipid bilayer with the help of transport proteins (gated channels ‫)قنوات ُم َبوبة‬. • The passive movement of molecules down its concentration gradient via a transport protein is called facilitated diffusion. • Many transport proteins simply provide channels allowing a specific molecule or ion to cross the membrane. Functions of cell membrane (Plasma membrane) 1- Selective permeability  A steady traffic of small molecules and ions moves across the plasma membrane in both directions.  For example, sugars, amino acids, and other nutrients enter a muscle cell and metabolic waste products leave it.  The cell absorbs O2 and expels CO2.  It also regulates concentrations of inorganic ions, like Na+, K+, Ca2+, and Cl-, by passing them across the membrane.  However, substances do not move across the barrier indiscriminately ‫ عشوائيا‬as membrane is selectively permeable.  Hydrophobic molecules, like hydrocarbons, CO2, and O2, can dissolve in the lipid bilayer and cross easily as described in the previous slide.  Ions and polar molecules like H2O and glucose pass through channel proteins as described in the previous slide.  Thus membrane proteins assist and regulate the transport of ions and polar molecules. Selective Permeability CO2 CO2 Nucleus O2 O2 The cell is able to take up particular molecules and exclude others 2- Passive transport Involves the movement of molecules across the cell membrane without an input of energy by the cell. No ENERGY is required to move substances across membrane (water, lipids, and other lipid soluble substances). Rather, the CONCENTRATION GRADIENT represents potential energy and drives diffusion Types of Passive transport: A. Diffusion B. Osmosis C. Facilitated Diffusion A)- Diffusion: Is the tendency of molecules of any substance to spread out in the available space randomly. • For example, a permeable membrane separating a solution with sugar molecules from pure water, sugar molecules will cross the barrier randomly. • The sugar molecules will cross the membrane until both solutions have equal concentrations of the sugar (dynamic equilibrium). Lump of sugar dynamic equilibrium • A substance will diffuse from where it is more concentrated to where it is less concentrated, down its concentration gradient.