Biology Campbell Chapter 7 Def. And Concepts

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Chapter 6

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Membrane Structure and Function

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Overview - Life at the Edge

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The plasma membrane is the edge of life, the boundary that separates the living cell from its surroundings. It controls traffic into and out of the cell it surrounds.

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Selective Permeability

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A property of biological membranes that allows some substances to cross it more easily than others.

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Figure 7.1 How do cell membrane proteins help regulate chemical traffic?

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Provides a channel for a stream of potassium ions (K+) to exit the cell at a precise moment after nerve stimulation.

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Concept 7.1

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Cellular membranes are fluid mosaics of lipids and proteins

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Staple Ingredients of Membranes

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Lipids and proteins and some carbohydrates. The most abundant lipids are phospholipids.

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Phospholipids

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A lipid made up of glycerol joined to two fatty acids and a phosphate group. The hydrocarbon chains of the fatty acids act as nonpolar, hydrophobic tails, while the rest of the molecule acts as a polar, hydrophilic head. Phospholipids form bilayers that function as biological membranes.

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Amphipathic

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Having both a hydrophilic region and a hydrophobic region.

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Fluid Mosaic Model

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The membrane is a fluid structure with a "mosaic" of various proteins embedded in or attached to a double layer (bilayer) of phospholipids.

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Membrane Models: Scientific Inquiry

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Sub-Heading

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Scientist E. Gorter and F. Grendel

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Reasoned that cell membranes must be phospholipid bilayers.

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Scientist Hugh Davson and James Danielli

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Proposed a sandwich model; a phospholipid bilayer between two layers of proteins.

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Problems with Davson-Danielli Sandwich Model

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First, the generalization that all membranes of the cell are identical. Second, the protein placement. Membrane proteins are not very soluble in water because they are amphipathic. If such proteins were layered on the surface of the membrane, their hydrophobic parts would be in aqueous surroundings.

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Scientist S.J. Singer and G. Nicolson

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Proposed that membrane proteins are dispersed, individually inserted into the phospholipid bilayer with their hydrophilic regions protruding.

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The Fluidity of Membranes

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Sub-Heading

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Movement of Membrane

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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.

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Lateral Movement of Lipids

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Adjacent phospholipids switch positions about 10⁷ times per second. Like partygoers elbowing their way through a crowded room.

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Flip-Flop Movement of Lipids

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It is quite rare for a molecule to flip-flop transversely across the membrane, switching from one phospholipid layer to the other; to do so, the hydrophilic part of the molecule must cross the hydrophobic core of the membrane.

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Movement of Proteins

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Proteins are much larger than lipids and move more slowly, but some membrane proteins do drift. And some proteins seem to move in a highly directed manner, perhaps driven along cytoskeletal fibers by motor proteins connected to the membrane proteins' cytoplasmic regions. However, many other membrane proteins seem to be held virtually immobile by their attachment to the cytoskeleton.

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Membrane Solidity and Temperture

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A membrane remains fluid as temperature decreases until finally the phospholipids settle into a closely packed arrangement and the membrane solidifies, much as bacon grease forms lard when it cools. The temperature at which a membrane solidifies depends on the types of lipids it is made of. unsaturated lipids = fluid at lower temperature; saturated lipids = fluid at hight temperature

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Effects of Cholesterol

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Can be thought of as a "temperature buffer" for the membrane, resisting changes in membrane fluidity that can be caused by changes in temperature.

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Membrane Fluidity

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Must be fluid to work properly; they are usually as fluid as salad oil. When solidified, its permeability changes.

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Membrane Proteins and Their Functions

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Sub-Heading

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Membrane Proteins

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Determine most of the membrane's function. Different types of cells contain different sets of membrane proteins, and the various membanes within a cell each have a unique collection of proteins. There are two major populations of them: integral and peripheral proteins.

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Integral Proteins

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Penetrate the hydrophobic core of the lipid bilayer. Many are transmembrane proteins, which span the membrane; other integral proteins extend only partway into the hydrophobic core.

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Peripheral Proteins

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Not embedded in the lipid bilayer at all; they are appendages loosely bound to the surface of the membrane, often to expose parts of integral proteins.

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Six Major Functions Performed by Proteins of the Plasma Membrane

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A. Transport - particule may pass from outside the cell to inside and vice versa.
B. Enzymatic Activity - May cause particals to change physically or chemically.
C. Signal Transduction - May Have a binding site with a specific shape that fits the shape of a chemical messenger, such as a hormone.
D. Cell-Cell Recognition - Some glyco-proteins serve as identification tags that are specifically recognized by membrane proteins of other cells.
E. Intercellular Joining - Membrane proteins of adjacent cells may hook together in various kinds of junctions, such as gap junctions or tight junctions.
F. Attachment to the cytoskeleton and extracellular matrix (ECM) - Microfilaments or other elements of the cytoskeleton may be noncovalently bound to membrane proteins, a function that helps maintain cell shape and stabilizes the location of certain membrane proteins.

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The Role of Membrane Carbohydrates in Cell-Cell Recognition

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Sub-Heading

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Cell-Cell Recognition

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A cell's ability to distinguish one type of neighhoring cell from another.

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Glycolipid

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A lipid with covalently attached carbohydrate.

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Glycoproteins

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A protein with one or more carbohydrates covalently attached to it.

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Role of Membrane Carbohydrates

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The carbohydrates on the extracellular side of the plasma membrane vary from species to species, among individuals of the same species, and even from one cell type to another in a single individual. 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. Ex. A, B, AB, and O blood types differ in the carbohydrates on the surface of red blood cells.

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Synthesis and Sidedness of Membranes

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Sub-Heading

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Concept 7.2

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Membrane structure results in selective permeability

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The Permeability of the Lipid Bilayer

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Sub-Heading

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Transport Proteins

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Transport Proteins

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A transmembrane protein that helps a certain substance or class of closely related substances to cross the membrane.

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Aquaporin

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A channel protein in the plasma membrane of a plant, animal, or microorganism cell that specifically facilitates osmosis, the diffusion of water across the membrane.

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Concept 7.3

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Passive transport is diffusion of a substance across a membrane with no energy investment

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Diffusion

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The spontaneous movement of a substance down its concentration gradient, from a region where it is more concentrated to a region where it is less concentrated.

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Concentration Gradient

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A region along which the density of a chemical substance increases or decreases.

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Passive Transport

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The diffusion of a substance across a biological membrane with no expenditure of energy.

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Effects of Osmosis on Water Balance

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Sub-Heading

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Osmosis

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The diffusion of water across a selectively permeable membrane.

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Water Balance of Cells Without Walls

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Sub-Heading

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Tonicity

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The ability of a solution surrounding a cell to cause that cell to gain or lose water.

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Isotonic

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Referring to a solution that, when surrounding a cell, has no effect on the passage of water into or out of the cell.

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Hypertonic

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Referring to a solution that, when surrounding a cell, will cause the cell to lose water.

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Hypotonic

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Referring to a solution that, when surrounding a cell, will cause the cell to take up water.

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Osmoregulation

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Regulation of solute concentrations and water balance by a cell or organism.

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Water Balance of Cells with Walls

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Sub-Heading

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Turgid

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Swollen or distended, as in plant cells. A walled cell becomes turgid if it has a greater solute concentration than its surroundings, resulting in entry of water.

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Flaccid

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Limp. Lacking in stiffness or firmness, as in a plant cell in surroundings where there is no tendency for water to enter the cell.

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Plasmolysis

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A phenomenon in walled cells in which the cytoplasm shivels and the plasma membrane pulls away from the cell wall; occurs when the cell loses water to a hypertonic environment.

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Facilitated Diffusion: Passive Transport Aided by Proteins

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Sub-Heading

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Facilitated Diffusion

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The spontaneous passage of molecules or ions across a biological membrane with the assistance of specific transmembrane transport proteins.

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Ion Channels

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A transmembrane protein channel that allows a specific ion to flow across the membrane down its concentration gradient.

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Gated Channels

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A transmembrane protein channel that opens or closes in response to a particular stimulus.

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Concept 7.4

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Active transport uses energy to move solutes against their gradients

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The Need for Energy in Active Transport

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Sub-Heading

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Active Transport

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The movement of a substance across a cell membrane, with an expenditure of energy, against its concentration or electrochemical gradient; mediated by specific transport proteins.

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Sodium-Potassium Pump

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A transport protein in the plasma membrane of animal cells that actively transports sodium out of the cell and potassium into the cell.

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How Ion Pumps Maintain Membrane Potential

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Sub-Heading

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Membrane Potential

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The difference in electrical charge (voltage) across a cell's plasma membrane, due to the differential distribution of ions. Membrane potential affects the activity of excitable cells and the transmembrane movement of all charged substances.

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Electrochemical Gradient

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The diffusion gradient of an ion, which is affected by both the concentration difference of the ion across a membrane (a chemical force) and the ion's tendency to move relative to the membrane potential (an electrical force).

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Electrogenic Pump

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An ion transport protein that generates voltage across a membrane.

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Proton Pump

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An active transport protein in a cell membrane that uses ATP to transport hydrogen ions out of a cell against their concentration gradient, generating a membrane potential in the process.

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Contransport: Coupled Transport by a Membrane Protein

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Sub-Heading

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Cotransport

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The coupling of the "downhill" diffusion of one substance to the "uphill" transport of another against its own concentration gradient.

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Concept 7.5

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Bulk transport across the plasma membrane occurs by exocytosis and endocytosis

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Exocytosis

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Exocytosis

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The cellular secretion of biological molecules by the fusion of vesicles containing them with the plasma membrane.

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Endocytosis

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Endocytosis

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Cellular uptake of biological molecules and particulate matter via formation of new vesicles from the plasma membrane.

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Phagocytosis

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A type of endocytosis in which large particulate substances are taken up by a cell. It is carried out by some protists and by certain immune cells of animals (in mammals, mainly macrophages, neutrophils, and dendritic cells).

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Pinocytosis

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A type of endocytosis in which the cell ingests extracellular fluid and its dissolved solutes.

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Receptor-Mediated Endocytosis

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The movement of specific molecules into a cell by the inward budding of membranous vesicles containing proteins with receptor sites specific to the molecules being taken in; enables a cell to acquire bulk quantities of specific substances.

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Lingands

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A hard material embedded in the cellulose matrix of vascular plant cell walls that provides structural support in terrestrial species.