Biochemistry 8 Biological Membrane

Biochemistry 8: Biological Membrane Fluid Mosaic Model Accounts for the presence of lipids, protein, and carbohydrates in a dynamic, semisolid plasma membrane that surrounds cells Phospholipid Each phospholipid has a hydrophilic head and hydrophobic Bilayer: tail. They are arranged so the heads are facing outward and the tails make up the inside of the membrane. Proteins are embedded in the bilayer Lipid Rafts: Lipids move freely in the plane of the membrane and can assemble into lipid rafts Flippases: Specific membrane proteins that maintain the bidirectional transport of lipids between the layers of the phospholipid bilayer in cells Proteins & May also move within the membrane, but are slowed by Carbohydrates: their relatively large size Membrane Transport Concentration All transmembrane movement is based on Gradients: concentration gradients. The gradient tells us whether the process is passive or active Osmotic A colligative property. It is the pressure applied to a Pressure: pure solvent to prevent osmosis and is used to express the concentration of the solution. It can be conceptualized as a “sucking” pressure in which a solution is drawing water in, proportional to its concentration p!=𝑖!𝑀!R!𝑇 Passive Does not require energy because the molecule is moving Transport: down its concentration gradient. Simple Diffusion: A form of passive transport. Small, nonpolar molecules passively move form an area of high concentration to an area of low concentration until equilibrium is achieved Osmosis: A form of passive transport. Describes the diffusion of water across a selectively permeable membrane Facilitated A form of passive transport. Uses transport proteins to Diffusion: move impermeable solutes across the cell membrane Membrane Components Lipids: The primary membrane component, both by mass and mole fraction Triacylglycerols & Act as phospholipid precursors and are found in Fatty Acids: low levels in the membrane Glycerophospholipids: Replace one fatty acid with a phosphate group, which is often linked to other hydrophilic groups Cholesterol: Is present in large amounts and contributes to membrane fluidity and stability ¯temp = INCREASE fluidity #temp = DECREASE fluidity Waxes: Present in very small amounts, if at all; they are most prevalent in plants and function in waterproofing and defense Transmembrane A type of integral protein that spans the entire Proteins: membrane. They are often glycoproteins. Embedded Proteins: Are most likely part of a catalytic complex or involved in cellular communication Membrane-Associated May act as recognition molecules or enzymes Proteins: Carbohydrates: Can form a protective glycoprotein coat and also function in cell recognition Ligands: Extracellular ligands can bind to membrane receptors, which function as channels or as enzymes in second messenger pathways Gap Junctions: Allow for rapid exchange of ions and other small molecules between adjacent cells Tight Junctions: Prevent solutes from leaking into the space between cells via a paracellular route, but do not provide intercellular transport Desmosomes & Desmosomes bind adjacent cells by anchoring to Hemidesmosomes: their cytoskeletons. Hemidesmosomes are similar, but their main function is to attach epithelial cells to underlying structures Active Transport: Requires energy in the form of ATP or an existing favorable ion gradient Primary Active Uses ATP or another energy molecule to directly power Transport: the transport of molecules across a membrane Secondary Active “Coupled transport”. Harnesses the energy released by Transport: one particle going down its electrochemical gradient to drive a different particle up its gradient. Symport: Both particles flow the same direction Antiport: The particles flow in opposite directions Endocytosis & Methods of engulfing material into the cells or releasing Exocytosis: material out of the cell. Pinocytosis: Ingestion of liquids via vesicles Phagocytosis: Ingestion of larger solid materials Specialized Membranes Membrane (Vm) Maintained by the Na+/K+ pump and leak channels. Potential: Resting potential of most cells is between -40 and -80 mV Nernst The electrical potential created by one ion can be calculated Equation: using the Nernst Equation. (!) =>.@ [012]456789: [012]456789: 𝐸 =! !ln!( ) =! !log!( ) *!+ [012]8;789: * [012]8;789: Goldman- Resting potential of a membrane at physiological temp can Hodgkin-Katz be calculated using the Goldman-Hodgkin-Katz Voltage Voltage Eq: Equation, which is derived from the Nernst equation. 𝑉D = 61.5!log!( HIJK ×![MNK]456789: O!HPK ×![QK ]456789:OHRST !×![UVT]8;789: ! HIJK ×![MNK ]8;789:O!HPK ×![QK ]8;789:OHRST !×![UVT ]456789: ) Mitochondrial The outer mitochondrial membrane is highly permeable to Membranes: metabolic molecules and small proteins. The inner mitochondrial membrane surrounds the mitochondrial matrix, where the citric acid cycle produces electrons used in the ETC. The inner mito membrane does not contain cholesterol.