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Substances that can move through the membranedissolved gases such as CO2, O2, NO; small, non-polar moleculesWhat substances can pass directly through the membrane?Plasma membrane transport mechanismsPassive: do not require energy for substance movement; protein channels (aquaporins:water), can be gated; protein facilitated carriers (glucose); substance moving requires a concentration gradient and a specific protein to act as a channel or facilitated carrier
Active: require energy input; protein utilizes ATP to move substance against concentration gradientConsider: Protein carrier mediated transport of molecules exhibit specificity, competition, and saturation. To increase facilitated diffusion of a molecule into a cell, the concentration of molecule outside the cell would need to be increased.cell signalingrequires target cell with specific receptor protein for chemical signal/messenger;Categories of cell signaling molecules:paracrine: cells secrete regulatory molecules that diffuse to nearby target cellsautocrine: cells secrete regulatory molecules that act on secreting cell itselfsynaptic: 1 neuron sends neurotransmitter messages to another cell (neuron dendrites or effector [muscle or gland]) via synapsesendocrine: cells secrete hormones (chemical regulators) that move through blood stream to target cells throughout body...some (lipophilic) can directly enter its target cell and attach to a receptor in the nucleus or cytosol...some (such as lipophobic protein/peptide epinephrine and acetylcholine) cannot enter the cell and interact with external receptor on a transmembrane protein and a second messenger inside the cell (cAMP)What are the categories of cell signaling molecules? How does each work? How do epinephrine and acetylcholine interact with their target cell? Consider: Cyclic AMP, made from ATP, is a second messenger for polar regulatory molecules and it activates enzymes inside the cell.bulk transportmoves large molecules and particles across plasma membrane through endocytosis (enterning) and exocytosis (exiting)gap junctionsprotein channels that directly connect one cell's cytoplasm to another cell's thereby allowing molecules and/or ions to freely move between the cells