BIOL 158: BIOLOGICAL CHEMISTRY II Lecture 1: Membranes Lecturer: Christopher Larbie, PhD
Introduction
Introduction Cells and Organelles have membranes Membranes contain lipids, proteins and polysaccharides in a bilayer
Membranes Proteins Three Types are present Integral Proteins Peripheral Proteins Transmembrane proteins Positioning of the protein depends on the hydrophobicity of the protein
Membrane Proteins serve six (6) different functions. Transport Enzymatic Activity
Signal Transduction Cell-cell recognition
Intercellular Joining Attachment to the cytoskeleton and extracellular matrix
Surface Proteins and Diseases, e.g. HIV/AIDS Infection CD Cluster of Differentiation (Type of T-cell receptor), CCR5 Chemokine Receptor type 5 (Receptor on WBCs)
Membrane Carbohydrates and their function Carbohydrates present in membranes are oligosaccharides covalently attached to proteins to form glycoproteins and to a lesser extent to lipids to form glycolipids. They are mainly gangliosides which contain galactose, mannose, glucose, N-acetyl glucosamine and N-acetyl galactosamine and are found on the exterior side of the plasma membrane. Roles of the membrane carbohydrates include cell-cell recognition, adhesion and receptor action.
Cell-cell recognition, a cell s ability to distinguish one type of neighbouring cell from another, is crucial to the functioning of an organism. It is important in the sorting of cells It is also the basis for the rejection of foreign cells by the immune system
The diversity of the molecules and their location on the cell s surface enable membrane carbohydrates to function as markers that distinguish one cell from another. For example, the four human blood types designated A, B, AB, and O reflects variation in the carbohydrate part of glycoproteins on the surface of red blood cells
Membrane Lipids and Function A primary role of lipids is in the formation of the permeability barrier of cells and subcellular organelles in the form of a lipid bilayer. Lipids are glycerol-based phospholipid Five kinds of phospholipid predominate: phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, phosphatidylglycerols, and sphingomyelin.
Membrane Fluidity Membranes are not static sheets of molecules locked rigidly in place. A membrane is held together primarily by hydrophobic interactions, which are much weaker than covalent bonds. Most of the lipids and some of the proteins can shift about laterally.
A membrane remains fluid as temperature decreases until finally the phospholipids settle into a closely packed arrangement and the membrane solidifies. The temperature at which a membrane solidifies depends on the types of lipids it is made of.
Cholesterol is wedged between phospholipid molecules in the plasma membranes of animal cells. At relatively high temperatures at 37 C, the body temperature of humans, for example cholesterol makes the membrane less fluid by restraining phospholipid movement. However, because cholesterol also hinders the close packing of phospholipids, it lowers the temperature required for the membrane to solidify.
The Fluid Mosaic Model of Biological Membranes
Transport across membranes
Passive Diffusion The diffusion of a substance across a biological membrane is called passive transport because the cell does not have to expend energy to make it happen. The concentration gradient itself represents potential energy and drives diffusion.
Membranes are selectively permeable and therefore have different effects on the rates of diffusion of various molecules. In the case of water, aquaporins allow water to diffuse very rapidly across the membranes of certain cells.
Diffusion in gaseous exchange
Osmosis and Water Balance of Cells
Movement of fluids by diffusion is described by Fick s first law of diffusion; J = kd 1 C 2 C 1 L Where J = net rate of transport in moles/cm 2 /sec. (C 2 C 1 ) = concentration difference across membrane in mol/cm 3 L = thickness of the membrane in cm D 1 = diffusion coefficient of the diffusing substance in membrane in cm 2 /s k = partition coefficient for the diffusing material between lipid and water i.e. ratio of solubilities of the substance in lipid and water. For ions and hydrophilic substance, K is very small.
Facilitated Diffusion The plasma membranes of both prokaryotic and eukaryotic cells have mechanisms for the translocation of various substances including sugars, amino acids, metabolic intermediates, inorganic ions and even H +.
It is now known that these membranes contain transport systems that play important role in the uptake of nutrients, maintenance of ionic concentrations and control of metabolism. These systems involve intrinsic or trans-membrane proteins and are classified on the basis of the mechanisms of translocation of substances across membranes and the energetics of the system. They all involve protein transporters.
Membrane transport systems There are two main modes of facilitated diffusion, passive and active mediated transport systems (transporter, translocase, permease, pump) The protein transporters for the two modes have a number of common characteristics; Each facilitates the movement of a molecule or molecules through the lipid bilayer at a significantly faster rate than can be explained for by passive or simple diffusion. Transporters demonstrate saturation kinetics, i.e. as the concentration of the substance to be translocated increase, the rate of transport increases but it reaches a maximum when the substance saturates the transporter. Most transporters have a high degree of structural and stereospecificity for the substance transported.
Mechanism of Transport of solute molecules Recognition and Binding Translocation or transport Dissociation or Release Recovery
Mechanisms of transport Uniport Symport Antiport
Types of Membrane Translocation Systems Channels are selective for specific inorganic cations and anions Pores are not selective but permit organic and inorganic molecules to pass through membrane.
Gap junctions etc
Passive Mediated Transport Systems of Glucose and The Na + -K + Active Transport System
Passive Mediated Transport Systems - Cl - and HCO 3 - Antiport Mechanism
Active Mediated Transport Systems Active transporters are classified as either primary active transporters if they utilize ATP directly or secondary active transporters if a trans-membrane chemical gradient of Na + or H + is utilized. The transporters that utilize ATP are referred to as ATPases because the translocation is linked to the hydrolysis of ATP to ADP and inorganic phosphate.
ATPases are classified as P, V, or F type transporters. P-type translocases are phosphorylated and dephosphorylated during the transport activity. V-type is found in the membranes of lysosomes, Golgi vesicles and secretory vesicles and is responsible for the acidification of the interior of these vesicles. F-type is found in the mitochondria and chloroplasts and is involved in the synthesis of ATP.
The Na + -K + Active Transport System
Ca 2+ Translocation Active Transport System
Transportation by Chemical Modification Transport of Amino Acids
Another group translocation mechanism is used for the uptake of sugars in bacteria. The pathway involves phosphorylation of the sugar using PEP as the phosphate donor (phosphoenol pyruvate phosphotransferase system [PTS]).
Ionophores These are antibiotics of bacterial origin that facilitates the movement of monovalent and divalent inorganic ions across biological membranes. They are divided into two major groups; Mobile carriers which are ionophores that diffuse back and forth across the membrane carrying the ions from one side of the membrane to the other (e.g. valinomycin). This mode of transport is affected by the fluidity of the membrane Channel formers that form a channel that traverses the membrane through which ions can diffuse (e.g. gramicidin A). Both types translocate ions by passive mediated transport mechanism.
Valinomycin: Produced by the Streptomyces sp and consists of an inner hydrophobic cavity which can accommodate K + ions but not others.
Gramicidine A: Two molecules of gramicidin A form a channel and the dimmer is in constant equilibrium with the free monomer form.
Bulk transport across the plasma membrane occurs by exocytosis and endocytosis Water and small solutes enter and leave the cell by diffusing through the lipid bilayer of the plasma membrane or by being pumped or moved across the membrane by transport proteins. However, large molecules, such as proteins and polysaccharides, as well as larger particles, generally cross the membrane in bulk by mechanisms that involve packaging in vesicles. Like active transport, these processes require energy
Exocytosis Examples, the cells in the pancreas that make insulin secrete it into the extracellular fluid by exocytosis. neurons (nerve cells) use exocytosis to release neurotransmitters that signal other neurons or muscle cells. when plant cells are making walls, exocytosis delivers proteins and carbohydrates from Golgi vesicles to the outside of the cell
Endocytosis