PL1003 Cell Membranes Learning Outcomes PDF

Topic 5 (Cell Membranes) Learning outcomes The cell membrane is described as a ‘fluid mosaic.’ Essentially this means that the membrane is a very thick, viscous layer of liquid (the ‘fluid’) with proteins inserted into it (the ‘mosaic’). The fluidity of the membrane is due to the fact that it is mad...

Topic 5 (Cell Membranes) Learning outcomes The cell membrane is described as a ‘fluid mosaic.’ Essentially this means that the membrane is a very thick, viscous layer of liquid (the ‘fluid’) with proteins inserted into it (the ‘mosaic’). The fluidity of the membrane is due to the fact that it is made up of a lipid (fat) bilayer. Many different fatty acids can be found within this bilayer, and it is the chemical and structural nature of these fats that determine how fluid the membrane is. Membrane fluidity is important because it affects the function of the proteins that are inserted into it. Many of these proteins are ion channels, and it is the activity of these ion channels that allows an electrical potential difference to exist across the membrane. This polarity (and the transient reversal of it) is what accounts for the electrical activity necessary for the propagation of impulses within both the nervous system and heart. Movement through ion channels requires them to change shape within the membrane, and this shape change is easier when the membrane is more fluid. In the first subtopic, we will look at the basic anatomy of a fatty acid, and describe the functional differences between the fatty acid types that may be found within the membrane. We will also explore the idea that fatty acids are not merely inert structures, but can also be broken down to form eicosanoids, which function as bioactive signalling molecules that have a profound effect on cellular function in both health and disease. In the second subtopic, the functional role of the cell membrane as a selective, permeable barrier will be discussed. This selectivity enables the cell to regulate day-to-day functions and to respond to changes in its immediate environment. Charged particles cannot passively diffuse through the plasma membrane. Consequently, cell membranes possess selective channels and transporters that facilitate the movement of ions (K+, Na+, Ca2+ and Cl-) both into and out of the cell. By maintaining different concentrations of ions inside the cell relative to the extracellular space, an electrical potential is established. The selective movement of +’ve and –‘ve charged ions into and out of the cell will alter this electrical potential and thereby modulate cell function. For example, changes in cell permeability to Ca2+ will affect muscle contractility and the ability of a nerve cell to conduct an electrical signal depends on Na+ influx. However, increases in Cl- permeability reduce conduction and decrease muscle contractility (thereby accounting for much of the effects found after drinking alcohol!). We will identify the ions that are important and the relevant transmembrane proteins will be discussed, with particular emphasis on ion channels. Their role in maintaining the electrochemical gradient will be highlighted. Learning outcomes By the end of subtopic 1, you should be able to: describe the fluid mosaic model of the cell membrane and explain the role of the lipid bilayer in maintaining membrane fluidity.  describe a typical fatty acid with reference to the length of the carbon backbone, the a-, b- and w-carbon atoms and the number and position of any double bonds. understand the effect that the presence of a double bond in the backbone of a fatty acid will have on its melting temperature and the effect that will have on membrane fluidity. describe stearic, oleic, arachidonic, eicosapentaenoic and docosahexaenoic acids in terms of their double bonds. describe how membrane lipids act as a store of bioactive signalling molecules. describe (in general terms) the eicosanoid pathway, and briefly explain the physiological effects of some of the eicosanoids (namely leukotriene B4, thromboxane A2 and A3, prostacyclin and prostaglandin D2). describe the differences between w-6 and w-3 fatty acids in terms of their ‘eicosanoid profiles.’ NB In order to understand and be able to describe the physiological effects of eicosanoids, you will need to understand their receptors. Most of these receptors are either G-protein coupled receptors or nuclear hormone receptors, which are covered later in this module. When you have completed the work on the general principles of these types of receptors, it is essential that you re-visit this section so that you can explain the interaction of eicosanoids in terms of the interaction with their receptors. Additional notes will be provided on the virtual learning environment to support this application of your knowledge. By the end of subtopic 2, you should be able to: define what is meant by the term “electrochemical gradient”. differentiate between passive, facilitated and active transport. correctly state the concentrations of intracellular K+, Na+, Ca2+ and Cl- and their corresponding extracellular concentrations. describe the term “membrane potential”. differentiate between depolarisation and hyperpolarisation with respect to the movement of each ion species.