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 substances that 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 how they can be differentiated. Biological membranes Cells contain a complex series of external and internal membranes. These membranes have similar structural features; they are absolutely essential for cellular function. All membranes display a number of features in common. Features of biological membranes- (1) selective permeability- they are selective in terms of which solute particles pass through the membrane. *lipid soluble solutes are typically more permeable than polar solutes; permeability a lipid solubility *smaller solutes are typically more permeable than larger solutes (relates to molecular volume); permeability a 1/molecular volume [NOTE: a symbol means proportionality; a X means directly proportional to while a 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 the membranes 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 other roles including detecting and amplifying signals, packaging proteins, synthesizing chemical energy Lipids in membranes- the predominant lipids in membranes are phospholipids. These lipids are amphipathic (amphipathic = molecule has both hydrophobic and hydrophilic regions). If you add phospholipids to water and agitate them, you will form very tiny droplets. In effect, this is what happens when you add detergent to water. The detergent forms tiny droplets which trap fatty material (note: detergents are more effective at high temperature because hydrophobic interactions are stabilized as temperature increases). Under certain circumstances, due to their amphipathic nature, phospholipids can form monolayers and bilayers on water interfaces (Fig. 8.1). 1Membrane structure models (Fig. 8.2)- (1) Davson-Danielli - protein sandwich model espoused in the 50’s; places protein layers on the surface with lipid in the middle; based primarily on early electron micrographs. (2) Singer-Nicholson- “fluid mosaic” model; based on freeze fracture electron micrographs and protein mobility studies. The membrane is a phopholipid bilayer in which the proteins are dissolved. peripheral proteins- attached to the surface by polar interactions; interact with water integral 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 mobility reflects fluidity which is a function of temperature and the qualitative nature of the lipids in the membrane (Fig. 8.4)- (1) increasing the degree of unsaturation, increases fluidity (2) increasing cholesterol content, increases fluidity at low temperatures but reduces fluidity at moderate temperatures (3) increase temperature, increase fluidity and vice versa Fig. 8.6- global view of a typical plasma membrane - some integral proteins may be anchored to proteins present in the interior of the cell (components of the cytoskeleton) - some peripheral proteins may have carbohydrate attached (glycoprotein) and some phospholipids may also have carbohydrate (glycolipids); these structures are involved in cell recognition and also attachment to the extacellular matrix Fig. 8.7- alpha helical regions penetrating the membrane Various 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 mechanical support 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 a- and b tubulin dimers; they can be quite long ( up to 25 mm [ 1 mm = 10-6 meter]). Play a variety of roles in cell structure, motility and internal transport. 23(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-actin which interacts with another f-actin molecule to form microfilaments (actin filaments). These filaments are very thin but can be quite long. Often microfilaments exist in association with motor proteins (such as myosin) promoting motility/internal moevment; microfilaments also play a role in maintenance and changes in cell shape. Actin filaments may undergo rapid depolymerization/polymerzation cycles. Requires ATP. (3) intermediate filaments- are large assemblanges of protein filaments consisting of proteins in the keratin ( hair protein) family. They are more or less fairly permanent fixtures in cells that provide structure support. Microtubules can be packaged into very complex functional structures; for instance, look at the typical flagellum or cilium from an advanced cell (Fig. 7.24). The microtubles are arranged in a complex scaffolding; upon the scaffold are attached motor proteins known as dynein ATPases. Under appropriate conditions, portions of the dynein molecules bind to adjacent microtubules. This interaction ultimately results in bending of the whole
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