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plasma membrane forms the outer boundary of the cell, through which the cell interacts with its external environment nucleus directs cell activities, most of which take place in the cytoplasm; usually located near the center of the cell organelle specialized structure that performs a specific function intracellular substances inside the cell extracellular substances outside the cell membrane potential charge difference across the plasma membrane resulting from the regulation of ion movement glycocalyx collection of glycolipids, glycoproteins and carbohydrates on the outer surface of the plasma membrane phospholipids and cholesterol predominant lipids of the plasma membrane liquid bilayer double layer of phospholipid molecules hydrophilic "water-loving" hydrophobic "water-fearing" fluid-mosaic model model suggesting that the plasma membrane is neither rigid nor static in structure, but is highly flexible and can change its shape and composition through time cholesterol limits the movement of phospholipids, providing stability to the plasma membrane marker molecule cell surface molecule that allows a cell to identify other cells or other molecules glycoprotein protein with attached carbohydrate glycolipid lipid with attached carbohydrate integral (intrinsic) protein protein molecule that penetrates deeply into the lipid bilayer peripheral (extrinsic) protein protein molecule attached to either the inner or outer surface of the lipid bilayer attachment protein integral protein that allows a cell to attach to other cells or to extracellular molecules cadhedrin protein that attaches a cell to other cells integrin protein that attaches a cell to extracellular molecules transport protein integral protein that allows ions or molecules to move from one side of the plasma membrane to the other channel protein on or more integral proteins arranged so that they form a tiny channel through the plasma membrane leak ion channel channel protein that is always open and responsible for the plasma membrane's permeability to ions when the membrane is at rest gated ion channel channel protein that can be opened or closed ligand chemical signal ligand-gated ion channel channel protein that opens or closes in response to chemical signals voltage-gated ion channel channel protein that opens or closes when there is a change in charge across the plasma membrane carrier protein (transporter) integral membrane protein that moves ions or molecules from one side of the plasma membrane to the other; has specific binding sites to which ions or molecules attach on one side of the plasma membrane; change shape to move the bound ions or molecules uniporter carrier protein that moves one specific ion or molecule across the membrane symporter carrier protein that moves two different ions or molecules in the same direction across the plasma membrane antiporter carrier protein that moves two different ions of molecules in opposite directions across the plasma membrane ATP-powered pump transport protein that moves specific ions and molecules from one side of the plasma membrane to the other; fueled by the breakdown of ATP; has binding sites for the molecule and ATP receptor protein protein or glycoprotein in the plasma membrane that has an exposed receptor site; can act an intracellular communication system receptor site area of a receptor protein that attaches to specific chemical signals enzyme catalyzes chemical reactions on either the inner or outer surface of the plasma membrane selectively permeable allows only certain substances to pass through it diffusion movement of solutes from and area of higher concentration to an area of lower solute concentration concentration gradient concentration difference between two points, divided by the distances between the two points viscosity measure of how easily a liquid flows osmosis water diffuses across a selectively permeable membrane facilitated diffusion carrier proteins combine with substances and move them across the plasma membrane; no ATP is used; always moved from areas of higher concentration to lower concentration active transport ATP-powered pumps combine with substances and move them across the plasma membrane; ATP is used; substances can be move from areas of lower concentration to higher concentration secondary active transport ions are moved across the plasma membrane by active transport, which establishes and ion concentration gradient; ions then move back down their concentration gradient by facilitated diffusion endocytosis plasma membrane forms a vesicle around the substances to be transported; requires ATP phagocytosis "cell-eating;" solid particles are ingested and vesicles are formed pinocytosis "cell-drinking"; contain molecules dissolved in liquid; smaller vesicles receptor-mediated endocytosis receptor cites combine only with certain molecules exocytosis materials manufactured by the cell are packaged in secretory vesicles that fuse with the plasma membrane and release their contents in the outside of the cell cytosol fluid portion of the plasma of the cytoplasm; colloid, viscous substance cytoskeleton supports the cell and holds the nucleus and other organelles in place cytoplasmic inclusion aggregates of chemicals either produced or taken in by the cell cytoplasm cellular material outside the nucleus but inside the plasma membrane microtubules hollow tubes composed primarily of protein units called tubulin actin filaments (microfilaments) small fibrils that forms a bundles, sheets or networks in the cytoplasm; for structure, support for microvilli, contractility and movement intermediate filaments protein fibers that provide mechanical strength to cell