BISC 101 Cell Membrane Structure & Function STUDENT COPY PDF

Dr. Onkar S. Bains BISC 101  Concept A: Cellular membranes are fluid mosaics of lipids and proteins Phospholipids are the most abundant lipid in the plasma membrane Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions The fluid mosaic model...

Dr. Onkar S. Bains BISC 101  Concept A: Cellular membranes are fluid mosaics of lipids and proteins Phospholipids are the most abundant lipid in the plasma membrane Phospholipids are amphipathic molecules, containing hydrophobic and hydrophilic regions The fluid mosaic model states that a membrane is a fluid structure with various proteins embedded in it  Fluidity of membranes Phospholipids in the plasma membrane can move within the bilayer Most of the lipids, and some proteins, drift laterally or rotate Rarely does a molecule flip-flop transversely across the membrane  As temperatures increase, membranes move from a solid (gel) state to a more fluid state Membranes with shorter fatty acid chains are more fluid than those with longer fatty acid chains – Shorter fatty acid chains means less surface area to allow for stabilizing van der Waal’s or hydrophobic interactions to occur between fatty acid chains of neighboring phospholipid molecules = hydrophobic or van der Waal’s interactions  Membranes rich in unsaturated fatty acids are more fluid that those rich in saturated fatty acids – Kinks (or bends) in unsaturated fatty acids prevent tight packing between phospholipids  In animal cells, the steroid cholesterol has different effects on membrane fluidity at different temperatures (i.e., cholesterol acts as a fluidity buffer against temperature extremes) – At warm temperatures, cholesterol restrains movement of phospholipids (in this case, cholesterol acts to decrease membrane fluidity) – At cool temperatures, cholesterol prevents tight packing between phospholipids (in this case, cholesterol acts to increase membrane fluidity)  Membrane proteins and their functions A membrane is a collage of different proteins embedded in the fluid matrix of the lipid bilayer Proteins determine most of the membrane’s specific functions  Peripheral proteins are bound to the surface of the membrane Integral proteins penetrate the hydrophobic core Integral proteins that span the membrane are called transmembrane proteins The hydrophobic regions of an integral protein consist of one or more stretches of non-polar amino acids, often coiled into alpha helices  Six major functions of membrane proteins: 1. Transport 2. Enzymatic activity 3. Signal transduction 4. Cell-cell recognition 5. Intercellular joining 6. Attachment to the cytoskeleton and extracellular matrix  Role of membrane carbohydrates in cell-cell recognition Cells recognize each other by binding to surface molecules, often carbohydrates, on the plasma membrane Membrane carbohydrates may be covalently bonded to lipids (forming glycolipids) or more commonly to proteins (forming glycoproteins) Carbohydrates on the external side of the plasma membrane vary among species, individuals, and even cell types in an individual  Concept B: Membrane structure results in selective permeability A cell must exchange materials with its surroundings, a process controlled by the plasma membrane Plasma membranes are selectively permeable, regulating the cell’s molecular traffic Hydrophobic (non-polar) molecules can dissolve in the lipid bilayer and pass through the membrane rapidly – Water molecule: only polar molecule that can pass easily Polar molecules, such as sugars, do not cross the membrane easily Charged substances (ions) are virtually unable to pass through a lipid bilayer  Transport proteins 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 (span whole membrane) Channel proteins called aquaporins facilitate majority passage of water Other transport proteins, called carrier proteins, bind to molecules and change shape to shuttle them across the membrane A transport protein is specific for the substance it moves  Concept C: Passive transport is diffusion of a substance across a membrane with no energy investment Diffusion is the tendency for molecules to spread out evenly into the available space Although each molecule moves randomly, diffusion of a population of molecules may exhibit a net movement in one direction At dynamic equilibrium, as many molecules cross one way as cross in the other direction  Substances diffuse down their concentration gradient, the difference in concentration of a substance from one area to another No work must be done to move substances down the concentration gradient The diffusion of a substance across a biological membrane is passive transport because it requires no energy from the cell to make it happen  Effects of osmosis on water balance Osmosis is the diffusion of water across a selectively permeable membrane Water diffuses across a membrane from the region of lower solute concentration to the region of higher solute concentration – This also means that water moves from high H2O to low H2O concentration  Water balance of cells without cell walls Tonicity is ability of a solution to cause a cell to gain or lose water Isotonic solution: Solute concentration is the same as that inside the cell; no net water movement across the plasma membrane Hypertonic solution: Solute concentration is greater than that inside the cell; cell loses water Hypotonic solution: Solute concentration is less than that inside the cell; cell gains water (cell lyses!) With animal cells, what is the proper scientific term for cell shriveling in hypertonic solutions? Crenation  Hypertonic or hypotonic environments create osmotic problems for organisms Osmoregulation, the control of water balance, is a necessary adaptation for life in such environments The protist Paramecium, which is hypertonic to its pond water environment, has a contractile vacuole that acts as a pump to force water out Water balance of cells with cell walls Cell walls help maintain water balance A plant cell in a hypotonic solution swells until the wall opposes uptake; the cell is now turgid (firm) If a plant cell and its surroundings are isotonic, there is no net movement of water into the cell; the cell becomes flaccid (limp), and the plant may wilt Water balance of cells with cell walls In a hypertonic environment, plant cells lose water; eventually, the membrane pulls away from the wall, a usually lethal effect called plasmolysis Facilitated diffusion: passive transport aided by proteins In facilitated diffusion, transport proteins speed the passive movement of molecules across the plasma membrane Channel proteins provide corridors that allow a specific molecule or ion to cross the membrane Channel proteins include: – Aquaporins, for facilitated diffusion of water – Ion channels that open or close in response to a stimulus (gated channels) Carrier proteins undergo a subtle change in shape that translocates the solute-binding site across the membrane EXTRACELLULAR FLUID Channel protein Solute CYTOPLASM (a) A channel protein Carrier protein Solute (b) A carrier protein  Concept D: Active transport uses energy to move solutes against their gradients Active transport moves substances against their concentration gradient Active transport requires energy, usually in the form of ATP Sodium-potassium pump = example of active transport  Molecules Molecules moving from moving from HIGH to LOW LOW to HIGH concentration concentration Both simple diffusion and facilitated diffusion are types of passive Active transport move transport molecules against its concentration gradient (i.e., Both simple diffusion and facilitated diffusion move molecules along (or from low to high with or down) its concentration gradient (i.e., from high to low concentration) concentration) Active transport requires Both simple diffusion and facilitated diffusion do not need energy! energy! Facilitated diffusion needs membrane-bound transport proteins (either Active transport requires a carrier or channel proteins) to move molecules across membrane while membrane-bound transport simple diffusion does not need membrane-bound transport proteins proteins (only carrier proteins…there are NO Simple diffusion is mainly involved in the movement of lipid soluble channel proteins for active molecules (including gases) while facilitated diffusion is for movement transport) of water soluble molecules (including ions)  Co-transport: coupled transport by a membrane protein Simultaneous movement of two distinct molecules across a biological membrane by one membrane transport protein – If two molecules transported in same direction, it is called symport – If two molecules are transported in opposite directions, it is called antiport  Concept E: Bulk transport across the plasma membrane occurs by exocytosis and endocytosis In exocytosis, transport vesicles migrate to the membrane, fuse with it, and release their contents In endocytosis, the cell takes in macromolecules by forming vesicles from the plasma membrane Endocytosis = reversal of exocytosis There are three types of endocytosis: – Phagocytosis (“cellular eating”); Pinocytosis (“cellular drinking”); Receptor-mediated endocytosis  In phagocytosis, a cell engulfs a solid particle in a vacuole In pinocytosis, fluids are taken up in a vacuole  In receptor-mediated endocytosis, binding of ligands to receptors triggers vesicle formation A ligand is any molecule that binds specifically to a receptor site of another molecule