Chapter 5: The Dynamic Cell Membrane
Chapter 5: The Dynamic Cell Membrane 1305
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Chapter 5: The Dynamic Cell Membrane What is the Structure of a biological membrane? -Some membrane proteins can move freely within the bilayer, while some are anchored to a specific region. -Some can be anchored by cytoskeleton elements, or by… Lipid rafts- lipids in semisolid state. -Membranes are dynamic and are constantly forming, transforming, fusing, and breaking down. Diffusion- the process of random movement toward equilibrium *Diffusion is net movement form regions of greater concentration to regions of lesser concentration Transmembrane proteins- have different domains on either side of the membrane; the two sides of the membrane can have very different properties Membranes have carbohydrates on the outer surface that serve as recognition sites for other cells and molecules. Glycolipids- carbohydrate lipid Glycoproteins- carbohydrate protein Membranes have selective permeability—some substances can pass through, but not others Passive transports- no outside energy required; diffusion Active transports- energy required -Active transport requires the input of energy to move substances against their concentration gradients. -Active transport is used to overcome concentration imbalances that are maintained by proteins in the membrane. -ATP (adenosine trisphosphate) is often the energy source for active transport -Active transport is directional and moves a substance against its concentration gradient. -A substance moves in the direction of the cell’s needs, usually by means of a specific carrier protein. Two types of Active Transport: 1. Primary active transport- involves hydrolysis for ATP for energy. 2. Secondary active transport- uses the energy from an ion + concentration gradient, or an electrical gradient; Na , moving with this concentration gradient, drives the transport of glucose against its concentration gradient. Simple Diffusion- small molecules pass through the lipid bilayer. -Liquid soluble molecules can diffuse across the membrane, as can water. -electrically charged and polar molecules cannot pass through easily. Osmosis- the diffusion of water *Osmosis depends on the number of solute particles present, not the type of particles. Facilitated Diffusion (passive)- polar molecules can cross the membrane through channel proteins and carrier proteins. Channel Proteins- have a central pore lined with polar amino acids. Ion channels: important channel proteins *Most are gated—can be close or open to ion passage. -Gate opened when protein is stimulated to change its shape. Stimulus can be a molecule (ligand-gated) or electrical charge resulting from many ions (voltage-gated). How Diffusion works: 1. The carrier protein has a glucose binding site 2. Glucose binds to carrier protein 3. Glucose changes shape when binded to carrier protein 4. The glucose is released from the carrier protein 5. The carrier protein returns to its original shape, ready to bind another glucose. Sodium-Potassium (Na —K ) Pump- an integral membrane protein that pumps Na out of a cell and K in. *One molecule of ATP moves two K and three Na ions.+ -in active transport, energy is used to move a solute against its concentration gradient. *For each molecule of ATP used, 2 K are pumped into the cell and 3 Na are pumped out of the cell. “What about those large, clunky, unwieldy, hard-to-handle Macromolecules???” Endocytosis- brings molecules into the cell The Three types of Endocytosis: 1. Phagocytosis (cellular eating)- part of the membrane engulfs a large particle or cell. Ex: a food vacuole(phagosome) forms and usually fuses with a lysosome, where contents are digested. 2. Pinocytosis (cellular drinking)- vesicles form. -The vesicles are smaller and bring in fluids and dissolved substances, as in the endothelium near blood vessels. 3. Receptor-mediated endocytosis- depends on receptors to bind to specific molecules (their ligands). -The receptors are integral membrane proteins located in regions called coated pits -The cytoplasmic surface is coated by another protein (often clathrin) -When receptors bind to their ligands, the coated pit invaginates and forms a coated vesicle. -The clathrin stabilizes the vesicle as it carries the macromolecules into the cytoplasm. -Once inside, the vesicle loses its clathrin coat and the substance is digested. In all three, the membrane invaginates, or folds around the molecules and forms a vesicle. *The vesicle then separates from the membrane. Exocytosis- moves materials out of the cell in vesicles. -The vesicle membrane fuses with the plasma membrane and the contents are released into the cellular environment. -Exocytosis is important on the secretion of substances made in the cell. Cells can respond to many signals if they have a specific receptor for that signal. Signal Transduction Pathway- a sequence of molecular events and chemical reactions that lead to a cellular response, following the receptor’s activation by a signal. - Signal transduction pathways involve multiple steps— enzymes may be either activated or inhibited by other enzymes. - Inhibition—protein kinase A inactivates glycogen synthase through phosphorylation, and prevents glucose storage. - Activation—Phosphorylase kinase is activated when phosphorylated and is part of a cascade that results in the liberation of glucose molecules. - Signal transduction ends after the cell responds—enzymes convert each transducer back to its inactive precursor - The balance between the regulating enzymes and the signal enzymes determines the cell’s response. Cells are exposed to many signals and may have different responses: Autocrine- affect the same cells that release them Paracrine- diffuse to and affect nearby cells Hormones- travel to distant cells Ligands are generally not metabolized further, but their binding may expose an active site on the receptor. Binding is reversible and the ligand can be released, to end stimulation. Inhibitor (or Antagonist)- can bind in place of the normal ligand Cytoplasmic Receptors- have ligands, such as estrogen, that are small or nonpolar and can diffuse across the membrane. Membrane Receptors- have large or poplar ligands, such as insulin, that cannot diffuse and must bind to a transmembrane receptor at an extracellular site. Receptors are also classified by their activity: 1. Ion Channel receptors- or gated ion channels, change their three-dimensional shape when a ligand binds. Ex: The acetylcholine receptor, a ligand-gated sodium channel, binds acetylcholine to open the channel and allow Na to diffuse into the cell. 2. Protein Kinase receptors- change their shape when a ligand binds. -the new shape exposes or activates a cytoplasmic domain that has catalytic (protein kinase) activity. 3. G Protein—linked receptors- expose a site that can bind to a membrane protein, a G protein. -The G protein is partially inserted in the lipid bilayer, and partially exposed on the cytoplasmic surface. -The Activated G protein—linked receptor exchanges a GDP nucleotide bound to the G protein for a higher energy GTP. -The Activated G protein activates the effector protein, leading to signal amplification. A second messenger is an intermediary between the receptor and the cascade of responses. + Examples: cAmp, NO, Ca2 , Inositol triphosphate, diacylglyceride Cells can alter the balance of enzymes in two ways: 1. Synthesis or breakdown of the enzyme 2. Activation or inhibition of the enzymes by other molecules Cell functions change in response to environmental signals: -opening of ion channels -alterations in gene expression -alteration of enzyme activities