Anatomy Chapter 3.docx

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University of Toronto Scarborough
Biological Sciences
Connie Soros

Chapter 3 Plasma Membrane & Membrane Potential • Basics - selectively permeable - controls what gets in and out of cell - Membrane Structure 1. basic membrane structure is a phospholipid bilayer a. hydrophilic (water loving) heads are polar, face outward toward water b. hydrophobic (water fearing) tails are nonpolar, face inward away from water (1) barrier to diffusion - stops water soluble molecules from passing through (2) water itself is small enough to get through c. responsible for membrane fluidity 2. cholesterol a. between phospholipids b. contributes to fluidity and stability 3. proteins a. some span the membrane (1) selective channels to transport substances across membrane (e.g., ions), opening filled with water, channel is specific (2) carrier proteins also transport specific molecules across membrane b. some on one side of membrane (1) receptors - bind with molecules on outer surface and initiate changes in cell (chemicals in blood only influence cells with the right receptors) (2) membrane bound enzymes – chemical reactions at inner or outer membrane surface (3) filamentous meshwork on inner side bind with cytoskeleton to maintain cell shape and for movement (4) cell adhesion molecules (CAMs) - stick out from outer surface and secure cell to other cells (some span the membrane), also cell communication (growth, defense responses) (5) some allow cells to recognize "self" and interact with one another (often glycoproteins) 4. carbohydrates a. on outer surface, bound to membrane proteins and lipids (glycoproteins, glycolipids) b. important in recognition of cells of same type and tissue organization c. involved in tissue growth (cells won't overgrow) - structure known as fluid mosaic model • Cell to cell adhesions - carbohydrates on membrane surface help arrange cells into groups, which are held together in various ways 1. CAMs 2. extracellular matrix (connective tissues) a. cells may not be joined directly to other cells, but embedded in matrix of carbohydrates and protein fibers (1) collagen - resists tension (e.g., skin) (2) elastin - stretch and recoil (e.g., skin, lungs) (3) fibronectin - holds cells in position (all over body) b. substances diffuse through, going between blood and tissues c. important in normal cell functioning 3. specialized junctions a. desmosomes - cells join at particular spots, found all over body, particularly where stretch occurs (e.g., skin, muscle) b. tight junctions - impermeable barrier, common in epithelial sheets where they prevent leakage c. gap junctions - cells linked by protein tunnels, allows small molecules to pass between cells, important in some cells that transmit electrical activity (e.g., cardiac muscle) • Membrane Transport (see table Methods of Membrane Transport) - two factors influencing transport - solubility of the substance in lipid, and size of substance 1. small, uncharged or nonpolar molecules move through lipid bilayer (e.g., O CO , 2atty2acids) 2. ions and small polar molecules (like glucose) can move through channels or by carrier proteins if the right transporter exists 3. substances too big or without a special protein transporter need special mechanisms to get through the membrane - Passive Transport (no ATP used) 1. diffusion - molecules move down their concentration gradient (greater  lesser concentration), charged particles move down electrochemical gradients a. simple diffusion - substance moves through lipid bilayer or protein channels (e.g., O , 2 CO , some ions) 2 b. osmosis - water moves down its concentration gradient c. facilitated diffusion uses a carrier protein that binds to the molecule to be transported and brings it to the other side of the membrane (e.g., glucose) 2. Filtration - water and solutes forced through membrane by pressure (e.g., in kidneys) - Active Transport (ATP used) 1. carrier proteins transport substance against its concentration gradient (needs ATP to change conformation) a. primary active transport - energy from ATP used directly to transport a substance (e.g. + + Na -K pump, in all cells) b. secondary active transport - driven by gradients set up by primary active transport + (1) in the digestive tract glucose and amino acids are "dragged along" with Na + + + diffusing into cell (Na gradient set up by Na -K pump) 2. vesicular transport (bulk transport) - large molecules or multimolecular substances enclosed in pieces of membrane a. endocytosis (pino/phagocytosis) (1) fuse with lysosomes which break down substance and release products to cell (e.g., bacteria) (2) vesicle travels to opposite side of cell and releases contents (cells lining capillaries) b. exocytosis (1) secretion of large polar molecules like hormones and enzymes (2) adding components to membrane • Intercellular communication and signal transduction - cells must communicate so they can coordinate their activities (maintain homeostasis, control growth and development) - 3 types of intercellular communication 1. gap junctions a. small molecules and ions directly exchanged between cells b. important in spread of electrical signals (cardiac and smooth muscle, very rarely neurons) 2. signal molecules on cell surface allow direct interaction a. phagocytes (body defense cells) recognize and kill invading cells 3. chemical messengers a. a specific chemical is made by special cells (1) acts on target cells, which then respond appropriately b. 4 types (1) paracrines - act locally (e.g., histamine in inflammatory response) (2) neurotransmitters - act locally; nerve cells release them to other nerve cells, muscles, or glands (3) hormones - acts over long distances, released into blood by endocrine glands (4) neurohormones - act over long distances, release
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