232%20Chap%203%20outline - Cell Theory The cell is the...

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Cell Theory The cell is the basic structural and functional unit of life Organismal activity depends on individual and collective activity of cells Continuity of life has a cellular basis Plasma Membrane (general) Selectively permeable barrier surrounding the cell Plays a dynamic role in cellular activity Provides biological markers for cell-cell recognition Fluid Mosaic Model Bilayer of lipids with imbedded, dispersed proteins Membrane lipids Phospholipids - have hydrophobic “tails” and hydrophilic “heads” Hydrophobic (nonpolar) tails make up the interior of the membrane Glycolipids - are lipids with bound carbohydrate chains Cholesterol helps stabilize the membrane fluidity Integral and Peripheral Proteins Integral proteins are firmly embedded in the membrane Most protrude from both sides of the membrane as transmembrane proteins Peripheral proteins are more loosely attached at the surface Glycoproteins have attached carbohydrate chains Functions of Membrane Proteins Transport Enzymatic activity
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Receptors for signal transduction Intercellular adhesion Cell-cell recognition Attachment to cytoskeleton and extracellular matrix Glycocalyx carbohydrate area on the cell surface (glycolipids and glycoproteins) provides biological markers for cell recognition and interaction Specializations of the Cell Membrane Microvilli - fingerlike projections of the cell membrane that increase membrane surface area Membrane Junctions Tight junction – impermeable (watertight) junction of interlocking proteins encircling the cell Desmosome – anchoring junction consisting of an internal protein plaque and external linker proteins Gap junction – collection of hollow protein channels (connexons) that allow substances to pass between cells Membrane Transport: Passive Processes Diffusion is the tendency for molecules or ions to disperse evenly A concentration gradient is a difference in concentration of a substance between two areas Simple diffusion Non-polar and lipid-soluble substances: Diffuse directly through the lipid bilayer Facilitated diffusion Small lipid- insoluble solutes: diffuse through channel proteins
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Large, polar molecules such as simple sugars Combine with protein carriers which move them across Osmosis -- Diffusion of water across a semipermeable membrane Occurs when the concentration of water is different on opposite sides of a membrane Osmolarity – total concentration of solute particles in a solution Osmotic Pressure and Tonicity Osmotic pressure – the pressure required to resist further water entry into a cell In plant cells this creates internal pressure and stops net water movement
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