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BIOL 130 Study Notes Unit VI Membranes

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BIOL 130
Richard Ennis

BIOL 130 Unit VI Study Notes Biological Membranes: • All cells have a plasma membrane which encloses the contents of the entire cell • Eukaryotic cells have membrane-bound organelles as well (creates compartmentalization) o Examples include:  Nuclear envelop  Double-membrane of mitochondria/chloroplasts  Endoplasmic reticulum  Golgi apparatus  Etc. Function of Membranes: Tip: Remember “P-TRECS”  • Permeable barrier for selectivity of things going in or out of cell o to prevent unrestricted exchange of molecules • Transports solutes o to exchange molecules across a membrane • Responds to external signals (i.e. signal transduction) o Communication with other cells • Energy transduction o to convert one form of energy into another • Compartmentalization (eukaryotes) o to create separate environments for different cellular activities • Scaffold (structure) for biochemical activities to take place in Membrane Phospholipids: • Recall from unit 2 that phospholipids are “amphipathic”, meaning that they are polar on one end and non-polar on the other • The general phospholipid is built as shown by the following schematic: (Hydrophilic Polar Head Group) – (Phosphate) – (Glycerol) – (Hydrophilic Fatty Acid Tail) • There are four prominent types of phospholipids, each with their own distinct polar head group: SECI o Phosphatidyl Serine (PS) – uses serine as the head group o Phosphatidyl Ethanolamine (PE) – uses ethanolamine as the head group o Phosphatidyl Choline (PC) – uses choline as the head group o Phosphatidyl Inositol (PI) - inositol as the head group Phospholipid Composition and Membrane Fluidity: • The phospholipid bilayer is not static and is constantly in motion o Allows them to break apart and readily be able to form into many different shapes • A membrane can be more fluid when: o It has more unsaturated kinks because kinks make it harder for the lipids to pack together, which would otherwise make them less fluid and stiff o The tails are freely moving • A membrane is less fluid when: o Cholesterol binds into an unsaturated kink making it stiffer  Cholesterol can make up to 50% of the plasma membrane in some animal cells o The tails are more tightly packed together o Decreases in temperature occur Temperature Effect on Membrane Fluidity: • Transition temperature refers to the temperature at which a given membrane solidifies or “gels”. • At or above room temperature, phospholipid membranes are fluid and freely moving • As the temperature drops, fluidity and permeability decrease as well • At very low temperatures, the phospholipids gel because the hydrophobic tails pack together very tightly How do Membranes Regulate Fluidity? • It is very important that a fluid state is maintained in a membrane otherwise normal cell function would be impeded • Membranes have a variety of strategies for maintaining fluidity by specifically changing their compositions: o Altering phospholipids may include:  Desaturate fatty acids (for cold temperatures) • This makes sense b/c making the membrane unsaturated would make it more fluid as opposed to keeping the membrane saturated, in which the membrane is less fluid and stiff  Change the length of the fatty acid chains • For colder temperatures, a shorter fatty acid chain is better since the lipid would be less stiff and more viscous; the smaller molecules size makes it more susceptible to changes in kinetic energy  Adjusting the amount of cholesterol (animals only) • Cholesterol prevents the membrane from becoming too fluid at high temperatures • But its presence also keeps membranes from packing too close together during cold temperatures to maintain fluidity Asymmetry of Lipid Bilayer: • Recall on the second page that there are 4 prominent types of phospholipid that use 4 different types of polar head groups: o Phosphatidyl choline (PC) o Phosphatidyl serine (PS) o Phosphatidyl ethanolamine (PE) o Phosphatidyl inositol (PI) • Specific types of heads are more likely to exist on one side of the membrane than on the other o Creates asymmetry inherent in the lipid bilayer • In the picture below it is clear that: o Phosphatidyl choline (PC) and sphingomyelin (SM) tends to be on the side of the lipid bilayer that points to the outside into the extracellular space o Whereas phosphatidyl serine, phosphatidyl ethanolamine, and phosphatidyl inositol tend to be on the side of the bilayer that is on the inside of the cell o Cholesterol is equally likely to appear on either side of the bilayer • Asymmetry is also preserved during membrane transport Human Red Blood Cells as Model Organisms for Studying the Plasma Membrane: • Human RBCs are a model organism for studying the plasma membrane b/c: o Inexpensive and abundantly available o Already present in single cell suspension within blood o Simplistic structure – no nucleus, no ER, no mitochondria, etc.  Allows for pure preps of plasma membranes  Simplistic structure well understood Various Membrane Proteins: • Transporters (e.g. Na+ pump) o Proteins that are channels, shuttles, or pumps that can help various materials go in and out of the cell • Anchors (e.g. integrins) o Important in determining cell shape by linking the membrane to the extracellular skeleton • Receptors (e.g. platelet-derived growth factor receptor) o Allows for things outside of the cell to bind on if it matches the receptor to send signals to the cell • Enzymes (e.g. adenylyl cyclase) o Catalyzes chemical reactions to lower activation energy in order to speed up biological reactions Protein Association with Membranes: Integral vs. Peripheral: • Proteins can either be integral relative to the membrane or peripheral • If a protein is integral, then it is embedded within the membrane itself o Specifically integral proteins can also be:  Transmembrane – integrated through the entire membrane  Monolayer –does not embed through the entire membrane just the top layer • If a protein is peripheral, then it is not directly embedded in the membrane but rather is bound on the outside of the membrane attached to something that is embedded in the membrane (like a lipid) The Many Different Shapes that Proteins Have within a Membrane: • If polypeptide chains cross a membrane, they usually do so as α-helices o These helices should be hydrophilic and are readily able to interact with things outside of the cell o Hydrophilic channels can be formed from several α-helices o It is found that an arrangement of 7 transmembrane α-helices is a very efficient signalling system • Proteins can also fold into pleated sheets, which can form pores in membranes Protein Movement in Membranes: • Membrane proteins have been proven to actually be moving within the lipid bilayer o Membrane proteins from a human cells and those from a mouse cell were combined into a hybrid cell all within a single membrane o After 40 minutes the human and mouse membrane proteins were found to have moved all over the place and were intermingled with one another: • However, cells can restrict the movement of membrane proteins o Tight junctions can be used to act as sort of a fence that separates one cell from another: • D) is an epithelial cell, and its specific protein distribution can be seen below o Notice that an Epithelium is fenced by its tight junctions between other Epitheliums o It has an upper apical plasma membrane with one type of protein on it o At the bottom, it has a basal plasma membrane with another type of protein o And between epithelial cells, they each have their own lateral plasma membranes just separated by the tight junction Eukaryotic Cells are coated with Sugars: • Epithelial cells of animal tissue often are covered in a coat of sugars, referred to as the “glycocalyx”. o The glycocalyx refers to an extracellular coating of glycolipids and glycoproteins on the surface of the tissue o The glycocalyx coating has many functions in:  Supporting the cell  Cell-to-cell communication and recognition • By extension the glycocalyx is a type of identifier that the body uses to distinguish between its own cells and foreign cells  Cell adhesion to guide movement and to allow cells to adhere to one another Overview of Membrane Transport: • Membrane transport is important because it allows the passage of certain materials in and out of a cell o E.g. certain gases, nutrients, and waste products • Lipid bilayers tend to block the passage of water soluble molecules o But for the substances that can enter, they can do so by…  Passing directly though  Being transported across the membrane either by protein channels or protein carriers  Being engulfed by the cell (phagocytosis) The Movement of Molecules by Diffusion: • Solutes that diffuse across a semi-permeable membrane do so from a high concentration to a low concentration spontaneously o It continues to do so until equilibrium is achieved, at which point despite that fact that there is still random movement of solute from either side, each side generally now has the same concentration (dynamic equilibrium) • If two solutes are present, then the same concept still applies. Both will spontaneously move from a region of high concentration to low concentration, until equilibrium is achieved relative to both individual concentrations: Osmosis: • The diffusion of water across a semi-permeable membrane down its concentration gradient that is always toward a higher solute concentration o This is because water’s flow is dependent on the concentration of substrate present on either side, and will always flow towards the side with higher concentration of solute to try and equalize the concentration of the solute. • If the solute concentration changes on either side, then osmosis will occur! • How osmosis works: o Isotonic scenario:  The concentration of solutes is equal on both sides, so therefore no net flux of water since water now randomly moves between both sides at equilibrium  Result: No net flow of water in or out of cell o Hypertonic scenario:  The concentration of solutes OUTSIDE of the cell is higher  This means that the water will flow out of the cell (toward the outside) since the concentration of solute is higher there  The osmotic flow of water out of the cell attempts to bring more solute into the cell to equalize solute concentration.  Result: Net loss of water causing cell to shrink o Hypotonic scenario:  The concentration of solute INSIDE the cell is higher  This means that water will flow into the cell (towards the inside) since the concentration of solute is higher within the cell  The osmotic flow of water into the cell attempts to move the high concentration of solute inside the cell to the outside to equalize solute concentration  Result: Net gain of water causing the cell to swell • Osmotic Effects on Cells o Isotonic scenario:  Solute concentration is equal on both sides, so no net flow of water in or out of the cell  Animal cells are most happy and normal in this state  Plant cells are actually not as happy as they could be; they are in a flaccid state o Hypertonic scenario:  Higher solute concentration on the outside of the cell, therefore water movement goes
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