ACTIVE TRANSPORT -> Requires Energy.
There are thousands of proteins surrounded 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. The liquids inside and outside of cells have different concentration. Now in this section, I will talk about the active transport which describes what happens when a cell uses energy (ATP) to maintain a proper balance of molecules and to transport them in and out of the cell from low to high concentration, as it shown in figure 1 ̽. Also, it happens only when they cross the bilayer.
The membrane proteins are very specific. One protein that moves glucose will not move calcium (Ca) ions. There are two types of active transport, one is primary and the other is secondary active transport. The primary active transport has three important examples, and I will talk about the most important one. The first example is sodium pumps (Na+ -K+), as it shown in figure 2 ̽. Their function is to pump (3) sodium ions out of the cell in concert with pumping (2) potassium ions into the cell. As a result , it will maintain an intracellular environment that has low sodium and high potassium, the opposite of extracellular fluid. In neurons, which frequently gain sodium during action potentials, it is more likely two-thirds of the ATP usage goes toward fueling this single transporter.