deck_3738940 Flashcards
list the functions of membranes
1) Transport- Getting molecules into or out of the cell or organelles2) Enzyme activity- Concentrations of enzymes at the membrane allows catalysis of certain reactions3) Signal transduction- Cell communication e.g. hormones, growth factors etc4) Intercellular joining- Cell-cell junctions in tissues5) Cell recognition- The immune system6)Attachment to extracellular matrix- Formation of tissues
describe the structure and composition of a membrane
1) membranes are sheet like structures 2 molecules thick2) form closed boundaries (compartments)3) most membranes 5 – 10 nm thick4) Lipid molecules much smaller than proteins so approximately 50 lipid molecules per protein in a plasma membrane5) Membrane proteins vary in structure, association with the membrane and function.6)Membranes are asymmetrical and contain proteins as well as lipids. evidence: surface tension of membranes did not match that of lipid only model, but does if add proteins.7) Fluid Mosaic Model (proteins float around in a lipid bilayer) evidence: fused mouse and human cells8) Many different types of protein are found in the membrane, allowing membranes to carry out a diverse range of functions.9)500 – 1000 different lipid species identified in membranes. lots of variation due to combinations of head groups, hydrocarbon chain lengths and saturation states-Enveloped viruses such as HIV and influenza are enclosed by membranes, usually derived from the host cell. Bacteria also possess membranes that have a different composition to animal cell membranes
what is a phospholipid?
1) Phospholipids are phosphate containing lipids.2) They are the most abundant membrane lipid.3) Can be phosphoglycerides (glycerol based) or sphingolipids (sphingosine based).3) Variations in lipid composition of cell membranes of many species appear to be adaptations to specific environmental conditions- more saturated hydrocarbon tails makes the membrane more viscous while unsaturated hydrocarbon tails makes the membrane more fluid
what is cholesterol?
1)Common component of animal cell membranes (~20%).2) Modulates lipid bilayer properties (permeability and fluidity).3) Also a precursor of steroid hormones and bile acids.
what does membrane fluidity depend on?
1) Fluidity depends on: non-covalent bonds, membrane composition ( cholesterol) and temperature.2) Lateral movement occurs ~10 ^7 times per second.Flip-flop movement occurs
explain why membranes are electrically charged
1) Inside cells are proteins and ions that at neutral pH have a net negative charge2) Ions will travel in and out of cells to try to balance this charge, causing a gradient of charge or an electrochemical gradient.3) The voltage across a membrane is called the membrane potential. Ranges from -50 to -200 millivolts.4) The membrane potential favours the transport of cations into the cell and anions out of the cell.
the membrane is described as being selectively permeable. what does this mean?
1) some molecules can pass through the membrane while others cant 2) Hydrophobic (nonpolar) molecules, such as hydrocarbons, can dissolve in the lipid bilayer and pass through the membrane rapidly.3)Polar molecules, such as sugars, do not cross the membrane easily.-The plasma membrane is therefore able to regulate the cell’s molecular traffic
what are transport proteins ?
1) transport proteins allow passage of hydrophilic substances across the membrane. Some transport proteins, called channel proteins, have a hydrophilic channel that certain molecules or ions can use as a tunnel.2) Channel proteins called aquaporins facilitate the passage of water (a billion water molecules per second).3) Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane4) A transport protein is specific for the substance it moves.
A typical animal cell devotes almost 1/3 of its energy to running the Na+/K+ pump. explain Why?
1) Cells contain many solutes trapped within the cell by the plasma membrane: Proteins, sugars, amino acids, nucleotides, ions.2) Therefore water will move into the cell by osmosis. 3) If this is unregulated the cell will burst. - animal cells and bacteria control their intracellular osmolarity by actively pumping out inorganic ions, such as Na+ , so that their cytoplasm contains a lower total concentration of inorganic ions than the extracellular fluid, thereby compensating for their excess of organic solutes.
outline the steps of receptor mediated endocytosis
1) ligand binds to membrane receptor 2) receptor-ligand migrates to clathrin coated-pit 3) endocytosis 4) vesicle loses clathrin coat 5) receptors and ligands separate 6) ligands go to lysosomes or golgi for processing -There are several types of coated vesicles: Clathrin, COPI and COPII. Each is specialised for mediating different transport steps. Each concentrates a specific protein for internalisation.
define the following:1) phagocytosis 2) pinocytosis 3) receptor-mediated endocytosis
1) the process by which a cell—often a phagocyte or a protist—engulfs a solid particle to form an internal vesicle known as a phagosome.2) a mode of endocytosis in which small particles are brought into the cell, forming an invagination, and then suspended within small vesicles.3) is a process by which cells absorb molecules (endocytosis) by the inward budding of plasma membrane vesicles containing proteins with receptor sites specific to the molecules being absorbed.
what is exocytosis?
1) Secreted proteins are transported to the plasma membrane in vesicles, which then fuse with the plasma membrane releasing their contents2) Secreted proteins may be soluble inside the vesicle or be embedded in the vesicle membrane3) Secreted proteins are synthesised in the rough ER (signal peptide) and packaged by the Golgi body.
explain the difference between constitutive and regulated secretory pathway.
The secretory pathway involves the flow of material from the Golgi to the cell surface.1) constitutive secretion in which proteins are sorted into vesicles in the Golgi and move directly to the cell surface and fuse with the plasma membrane (exocytosis), resulting in the release of soluble proteins to the exterior, or incorporation of membrane proteins into the plasma membrane. 2) The second pathway, regulated secretion, differs in that the proteins are stored in secretory vesicles that later fuse with the plasma membrane in response to a specific signal.
How does the cell know where to send proteins?
1) Signal sequences determine where a particular cargo should go2) Post-translational modifications of proteins:- Glycosylation (in Golgi) targets proteins to plasma membrane- Phosphorylation can alter protein-protein interactions leading to re-distribution to other compartments (using adaptor proteins)- Ubiquitination sends proteins for breakdown in the proteasome.
name the three ways to transport protein around the cell
1) Gated transport e.g. nuclear pore (between compartments)2) Transmembrane transport e.g. mitochondria (between compartments e.g from cytosol to ER)3) Vesicular transport e.g. motor proteins (within compartments)
