Cell Biology

... from active transport to vesicles

concentration gradients

The cell membrane is more permeable to non-polar, hydrophobic molecules than to polar, hydrophilic molecules by virtue of the hydrophobic interior of the amphipathic lipids of the bilayer. As a result, some small non-polar molecules such as H2O and CO2 are able to diffuse directly across the cell membrane down a concentration gradient. This osmotic, chemical gradient limits both the rate of diffusion and the maximum concentration of the diffusing molecule in the cytosol or the extracellular fluid (ESF) in the case of waste products.

Cells also utilize energy to generate concentration gradients across cell membranes by means of protein pumps embedded in the cell membrane. Active transport of some substances against concentration gradients employs the ATPase-derived energy stored in ion gradients, such as proton (H+) or sodium (Na+) gradients, to drive transporter membrane proteins. In a symport, the ATPase transported molecule and the coupled, co-transported ion move in the same direction. Conversely, the ATPase transported molecule and the coupled, co-transported ion move in the opposite direction in an antiport.

Such concentration gradients are locally discharged when ion channels are opened by a conformational change elicited by specific molecules which bind to membrane bound receptor proteins or by receptor-specific neurotransmitter molecules.

Harvard University provides a wonderful video explaining operation of the F1-F0 ATPase.

• A • adhesion • C • cell membranescellular signal transductioncentrioleschemotaxischloroplastciliacommunicationconcentration gradientscytokine receptorscytoplasmcytoskeleton • E • energy transducersendoplasmic reticulumendosomesexosome • G • Golgi apparatusGPCRs • H • hormones • I • ion channels • L • lysosome • M • meiosismicrotubulesmitosismitochondrion • N • Nitric Oxideneurotransmissionneuronal interconnectionsnuclear membranenuclear pore • P • pinocytosisproteasomepumps • R •receptor proteinsreceptor-mediated endocytosis • S • second messengerssignaling gradientssignal transductionspindlestructure • T • transporttwo-component systems • V • vacuolevesicle

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