Features of biological membranes-Topic 5: ASSEMBLY OF ORGANIC MOLECULES IN CELLS (lecture 7)1. Understand the basic structure of cell membranes with respect to phospholipids and“dissolved” proteins. 2. Understand the concept of selective permeability and characteristics of substancesthat determine their permeability across biological membranes.3. Understand the basic determinants of membrane fluidity.4. Know the three basic types of protein fibers that make up the cytoskeleton and howthey can be differentiated.Biological membranesCells contain a complex series of external and internal membranes. These membraneshave similar structural features; they are absolutely essential for cellular function. Allmembranes display a number of features in common.Features of biological membranes-(1) selective permeability- they are selective in terms of which solute particles passthrough the membrane.*lipid soluble solutes are typically more permeable than polar solutes;permeability lipid solubility*smaller solutes are typically more permeable than larger solutes (relates tomolecular volume); permeability 1/molecular volume[NOTE: symbol means proportionality; X means directly proportional towhile 1/X means inversely (indirectly) proportional to](2) membranes consist of mostly lipid and protein (minor amounts of carbohydrate;typically attached to proteins); the ratio varies. Some membranes such as themembranes associated with cells that detect light may have up to 65% protein.(3) in addition to functioning as barriers, membranes also play a variety of otherroles including detecting and amplifying signals, packaging proteins, synthesizingchemical energyLipids in membranes- the predominant lipids in membranes are phospholipids. Theselipids are amphipathic (amphipathic = molecule has both hydrophobic and hydrophilicregions). If you add phospholipids to water and agitate them, you will form very tinydroplets. In effect, this is what happens when you add detergent to water. The detergentforms tiny droplets which trap fatty material (note: detergents are more effective at hightemperature because hydrophobic interactions are stabilized as temperature increases).Under certain circumstances, due to their amphipathic nature, phospholipids can formmonolayers and bilayers on water interfaces (Fig. 8.1).Membrane structure models (Fig. 8.2)- 1(1) Davson-Danielli - protein sandwich model espoused in the 50’s; places proteinlayers on the surface with lipid in the middle; based primarily on early electronmicrographs.(2) Singer-Nicholson- “fluid mosaic” model; based on freeze fracture electronmicrographs and protein mobility studies. The membrane is a phopholipid bilayer inwhich the proteins are dissolved. peripheral proteins- attached to the surface by polar interactions; interact with waterintegral proteins- which pass all the way through the membrane; stabilized in the interior by hydrophobic interactions and at either surface by polar interactions.Membrane fluidity- phospholipids are capable of lateral mobility; the degree of mobilityreflects fluidity which is a function of temperature and the qualitative nature of the lipidsin the membrane (Fig. 8.4)-(1) increasing the degree of unsaturation, increases fluidity(2) increasing cholesterol content, increases fluidity at low temperatures butreduces fluidity at moderate temperatures(3) increase temperature, increase fluidity and vice versaFig. 8.6- global view of a typical plasma membrane- some integral proteins may be anchored to proteins present in the interior of thecell (components of the cytoskeleton)- some peripheral proteins may have carbohydrate attached (glycoprotein) andsome phospholipids may also have carbohydrate (glycolipids); thesestructures are involved in cell recognition and also attachment to theextacellular matrixFig. 8.7- alpha helical regions penetrating the membraneVarious functions of membrane proteins (Fig. 8.9)- transport, catalysis (enzymes),receptors, intercellular junctions, cell-cell recognition and attachment to fixed structures.Multi-molecular assemblies of proteins.Cytoskeleton- network of protein fibers in the cell which plays a role in mechanicalsupport and motility ( movement of materials within the cell and the cell itself). There are three major kinds of fiber systems present in cells (Table 7.2).(1) microtubules- are hollow tubes made up of - and tubulin dimers; they canbe quite long ( up to 25 m [ 1 m = 10-6 meter]). Play a variety of roles in cellstructure, motility and internal transport.2(2) microfilaments- are made up of two intertwined molecules of f-actin (f = fibrous);g-actin (g = globular) polymerizes into a helically-arranged fiber known as f-actinwhich interacts with another f-actin molecule to form microfilaments (actin filaments).These filaments are very thin but can be quite long. Often microfilaments exist inassociation with motor proteins (such as myosin) promoting motility/internalmoevment; microfilaments also play a role in maintenance and changes in cellshape. Actin filaments may undergo rapid depolymerization/polymerzation cycles.Requires ATP.(3) intermediate filaments- are large assemblanges of protein filaments consistingof proteins in the keratin ( hair protein) family. They are more or less fairly permanentfixtures in cells that provide structure support.Microtubules can be packaged into very complex functional structures; for instance, lookat the typical flagellum or cilium from an advanced cell (Fig. 7.24). The microtubles arearranged in a complex scaffolding; upon the scaffold are attached motor proteins knownas dynein ATPases. Under appropriate conditions, portions of the dynein molecules bindto adjacent microtubules. This interaction ultimately results in bending of the
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