Membrane Structure and Ion Transport 2024 - PDF

1.1 Review: Membrane structure and ion transport Dr. Veronica Campanucci Objectives for Section 1.1: To review basic concepts and terminology on membrane structure and ion transport. Readings from Boron: Functional organization of the cell Chapter 2 Transport of solutes and water...

1.1 Review: Membrane structure and ion transport Dr. Veronica Campanucci Objectives for Section 1.1: To review basic concepts and terminology on membrane structure and ion transport. Readings from Boron: Functional organization of the cell Chapter 2 Transport of solutes and water Chapter 5 Refreshing your knowledge on the plasma membrane… What is the structure of biological membranes and how do ions cross them Phospholipids Phospholipids are “amphipathic”: they have hydrophilic head and hydrophobic tail groups. At low concentrations, phospholipids in water form a monolayer. At higher concentrations they form “micelles” and eventually lipid Phosphatidylethanolamine bilayers. Phospholipid membranes form an impermeable barrier to diffusion of ions. Cells need transport systems (pumps, channels, transporters) to control internal ion concentrations and the flow of ions between inside and outside the cell. The plasma membrane is considered semipermeable because it has transport system Figure 2 Phospholipids. Membrane fluidity Figure 2 Phospholipids. Cell membrane fluidity is a parameter describing the freedom of movement of proteins and lipids within the cell membrane. Fluidity can influence several cellular processes including the activity of membrane-associated enzymes. Lipid packing can influence the fluidity of the membrane (cholesterol). Simple diffusion The movement from one location to another as a result of random thermal motion Diffusion of glucose between two compartments of equal volume separated by a barrier permeable to glucose [] [] Fick’s Law JX = PX([X]o – [X]i) The simplest description of diffusion The flux of solute X (JX) will depend on the permeability coefficient of X (PX) multiplied by the difference of the external and internal concentrations ([X]o – [X]i). For uncharged particles only Why do solutes cross a permeable membrane The driving force that determines the passive transport of solutes across a membrane is the chemical, electrochemical gradient or potential energy difference acting on the solute between the two compartments. For charge solutes (e.g., Na+, Cl−), the electrochemical potential energy difference includes a contribution from the concentration gradient (or the chemical potential energy difference) and a contribution from any difference in voltage that exists between the two compartments (or the electrical potential energy difference). Transport of solute X across a cell membrane What are the driving forces for the net movement of X? 1) Concentration of X (higher outside [X]o than inside [X]i 2) If [X] has a charge, then the electrical potential energy on the outside (0) is not the same as on the inside (i), this voltage difference will also drive the net movement of X, provided X is charged. When no net driving force is acting on Figure 5-2 Uncoupled transport of a solute X, we say that X is at equilibrium across a cell membrane. across the membrane and there is no net transport of X across the membrane Types of transport across a membrane Simple diffusion Passive transport (by facilitated diffusion) Active transport (primary and secondary) Passive transport by facilitated diffusion Proteins enable solutes to cross biological membranes by moving them downhill. This type of transport is passive because it does not require energy. Facilitated transport depends on integral membrane proteins Some substances cross the membrane passively through intrinsic membrane proteins that can form pores, channels, or carriers: - Pores: channels that are always open - Channels: which can be opened or closed by the action of specific mechanisms - Carriers: which facilitate passive transport through membranes. These are all examples of facilitated diffusion. In the absence of proteins, cell membranes are practically impermeable to ions and water molecules. PORES Figure 5-5 Structure of the human AQP1 water channel. A, Top view of an aquaporin tetramer. Each of the four identical monomers is made up of 269 amino acids and has a pore at its center. B, Side view of aquaporin. The images are based on high-resolution electron microscopy at a resolution of 3.8 Å. Pores provide an aqueous transmembrane conduit that is always open - porins: in outer membranes of gram-negative bacteria and mitochondria - perforin: cytotoxic T lymphocytes kill their target cell - Nuclear pore complex (NPC): regulates traffic into/out of the nucleus - Aquaporins (AQPs): channels just large enough to allow water molecules to pass through. Figure 5-3 & 5-5. Pore  CHANNELS Channel functional components Channels are gated pores gate: determines whether the channel is open or closed (conformational change) sensors: can respond signals (changes in membrane voltage; or second- messengers; or ligands} selectivity filter: determines the classes of ions (e.g., anions or cations) or the particular ions (e.g., Na+, K+, Ca2+) that have access to the channel pore. Open channel pore: ions can flow through it passively by diffusion until the channel Figure 5-3. Channels closes again.  CARRIES They are never an open gate through the membrane They transfer a broad range of ions and organic solutes They have specific affinity for binding one or a small number of solutes The simplest passive carrier-mediated transporter is one that mediates facilitated diffusion. No ATP requirement Figure 5-3. Carriers  Flux across the membrane: simple vs. facilitated diffusion Through lipid face Any transport facilitated by proteins A, In simple diffusion, flux increases linearly with increases in [X]o, with no maximal rate of transport (O2, CO2, NO). B, In a cell membrane there is a fixed number of carriers or channels and each has a limited speed. When the extracellular X concentration is gradually increased the flux of X will eventually reach maximum once all the carriers have become loaded with X (saturation). Figure 5-6 Dependence of transport rates on solute concentration. Active transport Proteins can also enable ions to cross biological membranes by moving them across in an energy-dependent fashion through primary or secondary active transport. Example of PRIMARY active transport: the Na+/K+ pump (-1) Figure 5-8 Model of the sodium pump. A, Schematic representation of the alpha and beta subunits of the pump. B, The protein cycles through at least eight identifiable stages as it moves 3 Na+ ions out of the cell and 2 K+ ions into the cell. THE Na-K PUMP IS ELECTROGENIC  SECONDARY active transport: Symporters Co-transporters use an existing gradient (e.g. that of Na+ or K+) to move one ion across the membrane. CO-TRANSPORTERS (SYMPORTERS) SGLTs do not directly utilize ATP to transport glucose against its concentration gradient; rather, they must rely on the sodium concentration gradient generated by the sodium–potassium ATPase as a source of chemical potential. Figure 5-11-A Na/Glu cotransporter. Membrane vesicles generated from kidney epithelia used to study how the Na + gradient affects glucose uptake. In the absence of Na + in the experimental medium, glucose enters renal membrane vesicles slowly by facilitated diffusion until reaching an equilibrium (green). At this point, internal and external glucose concentrations are identical. In contrast, adding Na + to the external medium establishes a steep inwardly directed Na gradient, which dramatically accelerates glucose uptake (red). The result is a transient “overshoot” during which glucose accumulates above the equilibrium level. Thus, in the presence of Na + , the vesicle clearly transports glucose uphill. Figure 5-12 Most symporters use the Na grad. Figure 5-11(B-L). Representative cotransporter. Another class of SECONDARY active transport: Exchangers EXANGERS or ANTIPORTERS use an existing gradient (e.g. that of Na+ or K+) to move one ion to the side of the membrane of lower concentration in exchange for another ion (usually of the same charge) that is moving to the opposite side of the membrane where it is present in higher concentration. Figure 5-13-A Na/Ca exchanger. Functional difference between channels and carriers or pumps B A A. Ion channels have a continuous B. Ion pumps and transporters have aqueous pathway for ion conduction two gates in series that control ion across the membrane. This pathway flux. The gates are never open can be occluded by the closing of a simultaneously, but both can close to gate. trap one or more ions in. Comparison of facilitated transport across the membrane In a typical cell… Active transport is responsible for the maintenance of differences between the concentrations of key ions inside and outside of mammalian cells Figure 5-14 Ion gradients, channels, and transporters in a typical cell. Self-testing questions 1) Name 3 different transport mechanisms for Na+ to cross the membrane. 2) Carrier-mediated transporter is a form of secondary active transport. (T or F?) 1) Carrier-mediated transport is mediated by facilitated diffusion, just like voltage-gated ion channels. (T or F?) 2) Do carriers have a selectivity filter? 3) Describe the effect of ouabain on the Na-K pump. What is the most likely consequence (regarding ion composition) of ouabain on a given cell? 4) Explain why the Na-K pump is electrogenic. 5) Knowing that ion channels move ions using the concentration gradient generated by pumps as electromotive force, why aren’t they considered secondary active transport?