Molecular Biology of the Cell Chapter 11 PDF

Chapter 11 Small-Molecule Transport and Electrical Properties of Membranes Copyright © 2022 W. W. Norton & Company, Inc. Principles of membrane transport Inorganic Ion Concentrations Inside and Outside a Typical Mammalian Cell* Protein-free Lipid Bilayers Are Impermeable to I The relative perme...

Chapter 11 Small-Molecule Transport and Electrical Properties of Membranes Copyright © 2022 W. W. Norton & Company, Inc. Principles of membrane transport Inorganic Ion Concentrations Inside and Outside a Typical Mammalian Cell* Protein-free Lipid Bilayers Are Impermeable to I The relative permeability of a synthetic lipid bilayer to different classes of molecules. The smaller the molecule and, more importantly, the less strongly it associates with water, the more rapidly the - molecule Molecules diffuses diffuse down across their the bilayer. concentration across a protein free lipid bilayer. - The diffusion rate depends on the molecule size and its hydrophobicity. - The smaller the molecule and, more important, the less strongly it associates with water, the more rapidly the molecule diffuses across the bilayer.  Protein-free Lipid Bilayers Are Impermeable to I Permeability coefficients for the passage of various molecules through synthetic, protein- free lipid bilayers. The lipid bilayers are essentially impermeable to charged molecules (ions), no matter how small: the charge and high degree of hydration of such molecules prevent them from entering the hydrocarbon phase of the bilayer  There Are Two Main Classes of Membrane Transport Proteins: Transporters and Channels There Are Two Main Classes of Membrane Transport Proteins: Transporters and Channels. A) A transporter alternates between two conformations so that the solute-binding site of the transporter is sequentially accessible on one side of the bilayer and then on the other. Bind to specific solute to be transported and undergo a series of conformational changes B) a channel protein forms a pore across the bilayer through which specific solutes Transport can channels through passivelyoccurs diffuse. at a much faster rate than Opens mediated transport by conformational by changes, channels form continuous pores transporters that extend across the lipid bilayer. Active Transport Is Mediated by Transporters Coupled to an Energy Source Different forms of membrane transport and the influence of the membrane. Passive transport down a concentration gradient. Diffusion& facilitated diffusion: occurs spontaneously, either through the lipid bilayer directly or through channels or passive transporters. Active transport involves movement of the solute against its concentration or electrochemical gradient and hence requires an input of metabolic energy. The electrochemical gradient of a A conformational change in a transporter mediates the passive movement of a solute. The transporter is shown in three conformational states: the outward-open state, the binding sites for solute are exposed on the outside. the occluded state, the same sites are not accessible from either side; the inward-open state, the sites are exposed on the inside. The transitions between the states are reversible and do not depend on whether the solute-binding site is occupied. Three ways of driving active transport. 1. Coupled transporters harness the energy stored in concentration gradients to couple the uphill transport of one solute across the membrane to the downhill transport of another. 2. ATP-driven pumps couple uphill transport to the hydrolysis of ATP. Active Transport Can Be Driven by Ion- Concentration Gradients Uniporters: Some transporters simply facilitate the passive movement of a single solute from one side of the membrane to the other. Symporters (co-transporters)the transfer of one solute strictly depends on the transport of a second. Coupled transport involves either the intimately coupled transfer of a second solute in the same direction. ATP-driven pumps : are often called transport ATPases; they hydrolyze ATP to ADP and phosphate and use the energy released to pump ions or other solutes across a membrane. 1. P-type pumps: are structurally and functionally related to multipass transmembrane proteins. They are called “P-type” because they phosphorylate themselves during the pumping cycle. This class includes many of the ion pumps that are responsible for setting up and maintaining gradients of Na+, K+, H+, and Ca2+ across cell membranes. 2. ABC transporters (ATP-binding cassette transporters): primarily pump small organic molecules across cell membranes. 3. V-type pumps are turbine-like protein machines constructed from multiple different subunits. The V-type proton pump transfers H+ into organelles, such as lysosomes, synaptic vesicles, and plant or yeast vacuoles to acidify the interior of these organelles. 4. F-type ATPases in mitochondria and chloroplasts normally CHANNELS AND THE ELECTRICAL PROPERTIES OF MEMBRANES> Most channels in the plasma membrane of animal and plant cells that connect the cytosol to the cell exterior necessarily have narrow, highly selective pores that can open and close rapidly. Because these proteins are concerned specifically with inorganic ion transport, they are referred to as ion channels. For transport efficiency, ion channels have an advantage over transporters, in that they can pass up to 100 million ions through one open channel each second—a rate 10^5 times greater than even the fastest transporter. Channels cannot be coupled to an energy source to perform active transport, so the conductance they mediate is always passive (downhill). The function of ion channels is to allow specific inorganic ions—primarily Na+, K+, Ca2+, or Cl–—to diffuse rapidly down their electrochemical gradients across the lipid bilayer. The role of aquaporins in fluid secretion Aquaporins (water channels): allow water to move more rapidly. are particularly abundant in animal cells that must transport water at high rates, such as the epithelial cells of the kidney or exocrine cells that must transport or secrete large volumes of fluids. must solve a problem that is opposite to that facing ion channels. To avoid disrupting ion gradients across membranes, they have to allow the rapid passage of water molecules while completely Ion Channels Are Ion-selective and Fluctuate Between Open and Closed States Two important properties distinguish ion channels from aqueous pores. 1. Ion selectivity, permitting some inorganic ions to pass, but not others, their pores must be narrow enough in places to force permeating ions into intimate contact with the walls of the channel so that only ions of appropriate size and charge can pass. 2. Gated channels; between ion channels and aqueous are not continually open. Instead, they are gated, which allows them to open briefly and then close again. The gating of ion channels. voltage-gated channels: change in the voltage across the membrane mechanically gated channels: a mechanical stress ligand-gated channels: binding of a ligand The ligand can be either an extracellular mediator—specifically, a neurotransmitter (transmitter-gated channels)—or an intracellular mediators such as an ion (ion- gated channels) or a nucleotide (nucleotide- gated Mechanically gated channels: Example: bacterial mechanosensitive channels. The Membrane Potential in Animal Cells Depends Mainly on K+ Leak Channels and the K+ Gradient Across the Plasma Membrane A membrane potential arises when there is a difference in the electrical charge on the two sides of a membrane because of a minute excess of positive ions over negative ones on one side and a minute deficit on the other side. Such charge differences can result both from active electrogenic pumping. Model for the mechanism of voltage-gating.