Bk 2 Ch 2 Membranes And Transport Flashcards
Amphipathetic
Term describing molecules that are part hydrophilic and part hydrophobic; e.g. membrane lipids.
Fluid- Mosaic model
A model of membrane structure, which visualises the membrane as a fluid lipid bilayer with membrane proteins embedded in, spanning or attached to this bilayer. Key to this model is the idea that the lipids and proteins of the membrane are highly mobile in the plane of the membrane.
Leaflet
Lipid layer 2 of which form the lipid bilayer of a cell membrane
Glycocalyx
Tangled layer of carbohydrate groups on the extracellular surface of cells, which are covalently attached to membrane lipids and proteins; has an important role in cell–cell recognition and adhesion.
Signal transduction
The process by which an extracellular signal is communicated or transmitted from the exterior of the cell to inside the cell and converted into a form that can bring about a specific cellular response.
Glycoprotein
A protein with one or more covalently attached carbohydrate groups (usually short sugar chains). Addition of such groups to proteins, termed glycosylation, is a form of post-translational modification.
Glycosylation
A type of post-translational protein modification where sugar residues are added to proteins to produce glycoproteins.
Saturated
In relation to an organic compound, particularly a fatty acid, this term indicates that it contains only single bonds between the carbon atoms.
Unsaturated
In relation to an organic compound, particularly a fatty acid, this term indicates that it contains one or more C=C double bonds.
Integral proteins
Membrane proteins that have one or more portions that are hydrophobic, through which they associate with the hydrophobic interior of the lipid bilayer. They may span the membrane, in which case they are called transmembrane proteins, or be embedded in the membrane on one side only.
Signal transduction
The process by which an extracellular signal is communicated or transmitted from the exterior of the cell to inside the cell and converted into a form that can bring about a specific cellular response.
Transmembrane proteins
Integral membrane protein which interacts with and spans the hydrophobic core of the lipid bilayer; may be described as single-pass or multipass, depending on whether the polypeptide crosses the membrane once or more than once.
Peripheral proteins
Membrane proteins that are not associated with the hydrophobic interior of the lipid bilayer; instead, they either associate with the membrane indirectly via non-covalent interactions with other membrane proteins, or directly via interactions with the hydrophilic head groups of the lipids. The proteins that mediate attachment of integral membrane proteins to the cell cytoskeleton are examples of peripheral proteins.
Lipid linked proteins
Are anchored in the bilayer by means of a lipid linkage at the cytosolic surface or the extracellular surface
Exocytosis
The release of materials, which may be dissolved or particulate and are enclosed within a vesicle, from a cell. Vesicles are transported to the cell surface where the cell membrane and vesicle membrane fuse, releasing the vesicle contents to the outside.
Lipid rafts hypothesis
According to this hypothesis, membranes contain microdomains (rafts) which are highly ordered. Their particular lipid composition means that lipid rafts are more cohesive and less fluid than the rest of the membrane. Thus lipid rafts can be envisaged as ‘islands’ in a sea of fluid membrane.
Caveolae
A very specific subtype of lipid raft containing the transmembrane proteins caveolin. They form flask-shaped invaginations (inward folds) of the cell membrane and are involved in receptor-mediated endocytosis.
Simple diffusion
Movement of a solute across a membrane down a concentration gradient without the assistance of any transport protein. This movement does not require energy.
Osmolarity (moles per litre of solution)
The total concentration of all solutes in a solution.
Osmosis
The movement of water across a differentially permeable membrane from a region of low solute concentration to a region of high concentration.
Carriers
Also called transporters, these multipass transmembrane proteins transfer small specific polar molecules and ions across membranes. The lipid impermeability to the transported substance is masked by combining it with the membrane-soluble carrier protein. They bind to a solute molecule (or ion) and release it on the other side of the membrane. Carrier-mediated transport can be either passive or active.
Channels
Membrane proteins that permit the transfer of specific small polar solutes (usually ions) across membranes. Channels provide an aqueous pore in the membrane through which solutes can pass. Channels allow only passive movement of solute.
Uniport
Type of carrier protein that transports only one type of solute across the membrane.
Cotransporters
A carrier protein that transports two different solutes across a membrane at a time; depending on whether the two solutes are transported in the same or in opposite directions, this transport is known as, respectively, symport or antiport. For many cotransporters, one of the two solutes that are transported is an ion.
Symport
Cotransport of two different solutes across a membrane at the same time and in the same direction. (Note that the term is used to describe both the type of transport and the carrier protein responsible.)
Antiport
Cotransport of two different solutes across a membrane at the same time in opposite directions. (Note that the term is used to describe both the type of transport and the carrier protein responsible.)
Facilitated diffusion
Diffusion of a solute across a membrane, down its concentration gradient (or, in the case of ions, down the electrochemical gradient), via either a carrier or channel protein. As for passive diffusion, no energy is required for facilitated diffusion to occur.
Anion
Ion with a negative charge
Cation
Ion with a positive charge
Membrane potential
Difference in voltage (electrical potential) across the cell membrane. The magnitude of this voltage difference, termed the membrane potential, ranges from -10 to -100 millivolts (mV), with a value of -70 mV being typical of many mammalian cell types. Note that, by convention, extracellular voltage is defined as zero and, since the inside of the cell is negative with respect to the outside, the membrane potential has a negative value.
Electrochemical gradient
The transmembrane gradient that determines the tendency of a particular ion to move passively from one side of the membrane to the other. The electrochemical gradient for a particular ion depends on both its concentration gradient and the existing membrane electrical potential.
Depolarising a neuron
Causes its membrane potential to become less negative
Active transport
Transport of a substance across a biological membrane against an electrochemical or concentration gradient. The energy required is provided, either directly or indirectly, by ATP.
Exocytosis - constitutive and regulated
The release of materials, which may be dissolved or particulate and are enclosed within a vesicle, from a cell. Vesicles are transported to the cell surface where the cell membrane and vesicle membrane fuse, releasing the vesicle contents to the outside.
Endocytosis - constitutive
The general term applied to the uptake of materials into cells, by invagination of and engulfment by the cell membrane. When solid particles are taken up the process is called phagocytosis.
Pinocytosis
Literally, ‘cell drinking’. The pinching off of small vesicles, containing extracellular fluid, from the cell membrane into the cell. This constitutive process recycles membrane material.
Lysosomes
Small spherical organelles with a single membrane. Lysosomes contain degradative enzymes and break down old organelles and also ingested material. They have a low internal pH.
Receptor - mediated endocytosis (RME)
The selective uptake of particular macromolecules and macromolecular complexes into the cell. RME involves specific receptor proteins at the cell surface that recognise and bind the macromolecule. The occupied receptors become clustered together in clathrin-coated pits, which then pinch off to become vesicles in the cytosol, i.e. they are endocytosed. RME thus transports the ligand and the receptor together into the interior of the cell. An example of RME is the uptake of low-density lipoproteins (LDLs) by animal cells.
Phagocytosis
The internalisation of particulate matter by endocytosis. Phagocytosis is similar to pinocytosis in that it involves invagination of an area of the cell membrane and the formation of a vesicle; however, the vesicles formed are much larger. Phagotrophic protists and some animal cells engulf particles of food; some phagocytes in vertebrates have both a scavenging role, ingesting dead cells and cell debris; and other types function in defence, engulfing and destroying invading microbes.
Membrane potential
The membrane potential is the electrical potential (voltage) gradient across cell membranes, with the inside negative with respect to the outside. The membrane potential therefore enhances the entry of positive ions and impedes the entry of negative ions. The tendency of a particular ion to be passively transported across the membrane thus depends on the net effect of its concentration gradient (it tends to move down the gradient of concentration) and the existing membrane potential (a positively charged ion tends to move inwards, a negatively charged ion tends to move outwards). This net driving force is the electrochemical potential of the ion.
Active transport
Transport of a substance across a biological membrane against an electrochemical or concentration gradient. The energy required is provided, either directly or indirectly, by ATP.
Sodium pump
A membrane carrier protein that uses the energy of ATP hydrolysis to pump sodium (Na+) ions out of the cell and at the same time pump in potassium (K+) ions, hence its alternative name: Na+–K+ ATPase. The sodium pump maintains a high intracellular K+ concentration and a low intracellular Na+, relative to the external medium.
