Chapter 6: Cell Membranes
gap junction
-a 2.7-nanometer gap between plasma membranes of two animal cells, spanned by protein channels -gap junctions allow chemical substances or electric signals to pass from cell to cell
secondary active transport
-a form of active transport that does not use ATP as an energy source -rather, transport is coupled to ion diffusion down a concentration gradient established by primary active transport
tight junctions
-a junction between epithelial cells in which there is no gap between adjacent cells
glycolipid
-a lipid to which sugars are attached
gated channel
-a membrane protein that changes in 3-d shape, and therefore its ion conductance, in response to a stimulus -when open, it allows specific ions to move across the membrane
uniporter
-a membrane transport protein that carries a single substance in one direction
symporter
-a membrane transport protein that carries two substances in the same direction
antiporter
-a membrane transport protein that moves one substance in on direction and another in the opposite direction
exocytosis
-a process by which a vesicle within a cell fuses with the plasma membrane and releases its contents to the outside
endocytosis
-a process by which liquids or solid particles are taken up by a cell through invagination of the plasma membrane
receptor protein
-a protein that can bind to a specific molecule, or detect a specific stimulus, within the cell or in the cell's external environment
glycoprotein
-a protein to which sugars are attached
aquaporin
-a transport protein in plant and animal cell membranes through which water passes in osmosis
primary active transport
-active transport in which ATP is hydrolyzed, yielding the energy required to transport an ion or molecule against its concentration gradient
diffusion across membranes
-all the solutes diffuse at rates determined by temp, their physical properties, and their concentration gradients -the membrane is said to be permeable to solutes that can cross it mor or less easily -the membrane is impermeable to substance that cannot move across it
selective permeability
-allowing certain substances to pass through while other substances are excluded -a characteristic of membranes
desmosome
-an adhering junction between animal cells
ion channel
-an integral membrane protein that allows ions to diffuse across the membrane in which its embedded
channel protein
-an integral membrane protein that forms an aqueous passageway across the membrane in which it is inserted through which specific solutes may pass
transmembrane protein
-an integral membrane protein that spans the phospholipid bilayer
sodium-potassium (NA+-K+) pump
-antiporter responsible for primary active transport -it pumps sodium ions out of the cell and potassium ions into the cell, both against their concentration gradients -also called a sodium-potassium AT-Pase
ligand
-any molecule that binds to a receptor site of another (usually larger) molecule
integral membrane proteins
-are at least partly embedded in the phospholipid bilayer -these proteins have both hydrophobic and hydrophilic regions
cell recognition
-binding of cells to one another mediated by membrane proteins or carbs
passive transport
-diffusion across a membrane -may or may not require a channel or carrier protein -contrast with active transport
phagocytosis
-endocytosis by a cell of another cell or large particle
pinocytosis
-endocytosis by a cell of liquid containing dissolved substances
receptor-mediated endocytosis
-endocytosis initiated by macromolecular binding to a specific membrane receptor
hypertonic
-having a greater solute concentration -said of one solution compared with another -contrast isotonic, and hypotonic
hypotonic
-having a lesser solute concentration -said of one solution in comparing with another -contrasts hypertonic and isotonic
hydrophilic regions
-having an affinity for water -contrasts hydrophobic
hydrophobic regions
-having no affinity for water -uncharged and nonpolar groups of atoms are hydrophobic
isotonic
-having the same solute concentration -said of two solutions
integrin
-in animals, a transmembrane proteins that mediates attachment of epithelial cells to the extracellular matrix
carrier protein
-in facilitated diffusion, a membrane protein that binds a specific molecule and transports it through the membrane
peripheral membrane proteins
-lack exposed hydrophobic groups and are not embedded in the bilayer -have polar or charged regions that interact with exposed parts of integral membrane proteins or with polar heads of phospholipid molecules
freeze-fracturing
-method of tissue preparation for transmission and scanning electron microscopy in which a tissue is frozen and a knife is then used to crack open the tissue -the fracture often occurs in the path of least resistance, within a membrane
osmosis
-movement of water across a differentially permeable membrane, from one region to another region where the water potential is more negative
facilitated diffusion
-passive movement through a membrane involving a specific carrier protein -dos not proceed against a concentration gradient -contrast with active transport, diffusion
heterotypic
-pertaining to adhesion of cells of different types
homotypic
-pertaining to adhesion of cells of the same type
diffusion
-random movement of molecules or other particles, resulting in even distribution of the particles when no barriers are present
simple diffusion
-small molecules pass through the phospholipid bilayer of the membrane
other functions of membranes
-some organelle membranes help transform energy -some membrane proteins organize chemical reactions -some membrane proteins process information
cell junctions
-specialized structures associated with the plasma membranes of epithelial cells -some contribute to cell adhesion, others to intercellular communication
cell adhesion
-the connection between the two cells is strengthened
membrane potential
-the difference in electrical charge between the inside and outside of a cell, caused by difference in the distribution of ions
active transport
-the energy-dependent transport of a substance across a biological membrane against a concentration gradient-that is, from a region of low concentration to one of high concentration
turgor pressure
-the hydrostatic pressure of an enclosed solution in excess of the surrounding atmospheric pressure
diffusion within cells and tissues
-within cells, solutes distribute themselves rapidly by diffusion -diffusion is rapid enough to distribute small molecules and ions almost instantaneously
fluid mosaic model
a molecular model for the structure of biological membranes consisting of a fluid phospholipid nilayer in which suspended proteins are free to move in the plane of the bilayer