active transport
THE MOVEMENT OF IONS OR MOLECULES ACROSS A CELL MEMBRANE INTO A REGION OF HIGHER CONCENTRATION, ASSISTED BY ENZYMES AND REQUIRING ENERGY.ACTIVE TRANSPORT USUALLY HAPPENS ACROSS THE CELL MEMBRANE.ACTIVE TRANSPORT USES CELLULAR ENERGY TO MOVE THEM AGAINST A GRADIENT, POLAR REPULSION, OR OTHER RESISTANCE. ACTIVE TRANSPORT IS USUALLY ASSOCIATED WITH ACCUMULATING HIGH CONCENTRATIONS OF MOLECULES THAT THE CELL NEEDS, SUCH AS IONS, GLUCOSE AND AMINO ACIDS.ACTIVE TRANSPORT USUALLY HAPPENS ACROSS THE CELL MEMBRANE. THERE ARE THOUSANDS OF PROTEINS EMBEDDED IN THE CELL'S LIPID BILAYER. THOSE PROTEINS DO MUCH OF THE WORK IN ACTIVE TRANSPORT. THEY ARE POSITIONED TO CROSS THE MEMBRANE SO ONE PART IS ON THE INSIDE OF THE CELL AND ONE PART IS ON THE OUTSIDE. ONLY WHEN THEY CROSS THE BILAYER ARE THEY ABLE TO MOVE MOLECULES AND IONS IN AND OUT OF THE CELL. THE MEMBRANE PROTEINS ARE VERY SPECIFIC. ONE PROTEIN THAT MOVES GLUCOSE WILL NOT MOVE CALCIUM (CA) IONS. THERE ARE HUNDREDS OF TYPES OF THESE MEMBRANE PROTEINS IN THE MANY CELLS OF YOUR BODY.MANY TIMES, PROTEINS HAVE TO WORK AGAINST A CONCENTRATION GRADIENT. THAT TERM MEANS THEY ARE PUMPING SOMETHING (USUALLY IONS) FROM AREAS OF LOWER TO HIGHER CONCENTRATION. THIS HAPPENS A LOT IN NEURONS. THE MEMBRANE PROTEINS ARE CONSTANTLY PUMPING IONS IN AND OUT TO GET THE MEMBRANE OF THE NEURON READY TO TRANSMIT ELECTRICAL IMPULSES.