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Unformatted text preview: Cell Membrane Structure and Function www.uic.edu Membrane Characteristics The plasma membrane is the "edge of life" (the outermost living limit of all cells. The membrane is permeable to x=x is permeant to the membrane. Selective permeability is a property of all membranes (not just plasma membranes). Small molecules, in general, are more permeant than are larger ones. Nonpolar molecules cross membranes more easily than do polar ones or ions. The (oversimplified) bilayer model of a membrane:
Membranes consist largely of two layers of phospholipids. What do you remember about this important group of compounds? (above model based on work of Davson and Danielli, 1935) Later studies revealed further information about membrane lipids The unsaturated nature of the hydrocarbon tails of the phospholipids is important. Cholesterol is often an important component of Diagrams from Campbell membranes.ed. & Reece, Biology, 7th Advances in electron microscopy and biochemistry led to a more complex model
Proteins as well as phospholipids are essential components of membranes: Diagram from Campbell & Reece, Biology, 7th ed. Functions of membrane proteins: Diagrams from Campbell & Reece, Biology, 7th ed. Our current "fluid-mosaic" model of the membrane also includes carbohydrates as important components. Diagram from Campbell & Reece, Biology, 7th ed. These carbohydrates are actually "oligosaccharides" Some are chemically bound to proteins and are called glycoproteins. Others ("glycolipids") are attached to the hydrophilic heads of phospholipids. They confer to cells the ability to recognize other cells, critical to an organism's ability to distinguish "self" from "foreign". They also allow cells to recognize other cells of How substances cross membranes
Concentration gradients are an important piece in this process: Diagram from Campbell & Reece, Biology, 7th ed. TWO IMPORTANT CAVEATS:
1. Diffusion (random movement down a concentration gradient i.e., from a region of higher concentration to a region of lower concentration) can and does occur in the absence of membranes! (Examples??)
( 2. The existence of a concentration gradient of a particular solute across a membrane does not guarantee that the solute will be able to diffuse across the membrane. WHY is this so? Factors that affect the rate of diffusion
Extrinsic factors: Temperature Presence of an partial barrier, as well as its selective permeability to the diffusing particle Intrinsic factors (features of the diffusing particle): Its molecular size Its polarity Whether or not it is charged Osmosis: The diffusion of a solvent (especially
water) across a selectively permeable membrane
Although polar, water molecules are relatively small. They move relatively easily across membranes*. The direction of movement of water is from regions of -- lower to higher solute concentration (hypotonic to hypertonic regions) -- higher to lower pressure potential -- (thus) higher to lower water potential * Some slipping through between phospholipid molecules and others moving through channels known as "aquaporins" Aquaporins Special protein channels through the unit membrane, discovered in the mid 1990's Two wonderful animations: http://nobelprize.org/nobel_prizes/chemistry/laur and http://nobelprize.org/nobel_prizes/chemistry/laur Water moves from an area of higher "free" water concentration to one of lower free water concentration: Diagram from Campbell & Reece, Biology, 7th ed. Plant and animal cells respond differently in a hypotonic environment! Diagram from Campbell & Reece, Biology, 7th ed. Maintaining water balance is a real challenge for all aquatic organisms.
Were it not for their contractile vacuoles, many protists would explode in their hypotonic world. Video: http://www.microscopy-uk.org.uk/mag/indexmag.html?http://www.microscopy-uk.org.uk/mag/articles/param1.html biology.umt.edu www.emc.maricopa.edu Transport of solutes across membranes can be either passive or active.
Passive Transport always Active Transport always "up" "down" (or "with") the solute's (or "against") the solute's concentration gradient concentration gradient Two types: 1. Simple diffusion through Characteristics: the bilayer (especially small Occurs through specialized nonpolar molecules) and specific protein channels 2. Facilitated transport, Is made possible through the aided by specific intrinsic expenditure of ATP (requires protein channels (e.g., some ENERGY) monosaccharides, amino Examples: sucrose (in some acids, . . . ) cases), Na+, K+, and Cl- Simple diffusion, facilitated diffusion, and active transport an overview: Passive transport. Down the concentration gradient, with no expenditure of energy. Rate increased by specialized transport proteins in the membrane. Active transport. Specific membrane proteins, aided by ATP, act as pumps, concentrating substances against their concentration gradient. Two different kinds of transport proteins that carry out facilitated diffusion: Diagrams for this slide and the previous slide are from Campbell & Reece, Biology, 7th ed. Active transport
An important example (the Na+/K+ pump)D Diagram from Campbell & Reece, Biology, 7th ed. An animation showing how Na+ ions are concentrated on one side of the membrane and K+ on the other side: cas.bellarmine.edu Another important example the "proton pump" Diagram from Campbell & Reece, Biology, 7th ed. A magic trick? Some substances can get from one side of a plasma membrane to another (leaving or entering a cell) without crossing any membrane. How do they do it? The answer is exocytosis and endocytosis.
Endocytosis can be of three different types: 1. Pinocytosis (literally, "cell drinking") 2. Phagocytosis (literally, "cell eating") 3. Receptor-mediated endocytosis Reverse phagocytosis is a special form of exocytosis. Phagocytosis in Animal Cells (Funny video: http://www.cellsalive.com/mac.htm )
The cell that is engulfing particles from the environment sends out "pseudopodia" that wrap around the particle. It becomes enclosed in a membrane-bound sac, which later fuses with a Above: TEM of an amoeba lysosome. Digestion follows. phagocytozing a bacterium
(from Campbell & Reece). Above: From Campbell & Reece Above: A colorized SEM of a leucocyte engulfing bacteria, from www.ecbody.com gak.med.kagawa-u.ac.jp Pinocytosis
In this process the cell "gulps" little droplets of fluid from the environment, forming tiny vesicles. This nonspecific process allows the cell to take molecules from its environment that were dissolved in the fluid it took in. Above diagram from Campbell & Reece Above: An endothelial cell (lining a capillary) takes in nutrients from the surrounding digestive cavity (TEM
from Campbell & Reece) Colorized TEM from http://www.denniskunkel.com/DK/DK/Medical/2194B.html ReceptorMediated Endocytosis
(d) are from your textbook. In (a) note the shallow coated pit, lined with specific receptor proteins facing outside. In (b) and (c) The pit deepens, and finally in (d) it pinches off as a coated vesicle, containing substances (e.g., cholesterol) that the cell has taken in.
(a) Specialized cell surfaces
You are on your own for this; it is largely descriptive and/or review. Make sure you understand the structure and function of each of the following (and know where they are occur): Desmosomes Tight junctions Gap junctions Plasmadesmata Cell walls NEXT: Energy flow in living organisms (Ch. 6) Segueing into metabolism (Ch. 8) www.metrodenver.org www.arenal.net www.hhmi.princeton.edu ...
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This note was uploaded on 04/09/2008 for the course BIOLOGY 101 taught by Professor Beitch during the Fall '07 term at Quinnipiac.
- Fall '07