![]() These channels are always open and thus always "leaking" potassium out of the cell. The cell membrane contains many K "leak" channels, which allow the high intracellular K to flow down its chemical gradient and deposit extracellularly. Based on natural diffusion, molecules will always travel from a high concentration to a low concentration (chemical gradient). This process causes the pooling of K intracellularly and Na extracellularly. ![]() The Na/K pump uses ATP to transport three Na ions extracellularly and two K ions intracellularly. Sodium/potassium (Na/K) electrogenic pumps One of the typical examples of active transport is the Na/K ATPase pump, which helps restore a high extracellular Na and high intracellular potassium by using one ATP to pump three sodium (Na) ions extracellularly and two potassium (K) ions intracellularly. ![]() Active transport, like facilitated diffusion, uses a protein carrier, but active transport requires energy expenditure because it must move a molecule against its electrochemical gradient. This transport through a channel protein is an example of facilitated diffusion. Tiny, uncharged, nonpolar substances with high lipid solubility, like oxygen and carbon dioxide, move quickly through the cell membrane via passive diffusion, while water, though small, is insoluble in lipids and thus needs to travel through a membrane channel protein to flow into the cell. Simple and facilitated diffusion occurs without the use of energy expenditure and depends on a molecule's electrochemical gradient and the intrinsic size and solubility of the molecule. Transport across the cell membrane takes the form of simple diffusion, facilitated diffusion, and active transport. These lipids also inhibit the passive diffusion of hydrophilic substances. The protein component is responsible for transporting hydrophilic substances such as water, glucose, and ions. The lipid component (phospholipids, cholesterol, and glycolipids) allows for the high permeability of lipid-soluble compounds such as steroid hormones, carbon dioxide, and oxygen. Peripheral proteins loosely attach to the outside of the cell membrane by electrostatic forces these peripheral proteins serve multiple functions in the cell membrane, including signaling, recognition, membrane trafficking, cell division, and cell structure. Examples of essential proteins include any membrane channels, membrane pores, membrane receptors, and adhesion anchors. The protein component of the cell membrane includes either integral proteins (embedded within the membrane) and peripheral proteins (outside of the membrane). An additional lipid component is cholesterol, which prevents excessive membrane fluidity at elevated temperatures and ironically allows for membrane fluidity at decreased temperatures by preventing lipids from packing together. At the basic level, the cell membrane is a collection of lipids (namely phospholipid) and protein components. The phospholipid structure forms from a hydrophilic phosphorylated glycerol "head" and two hydrophobic fatty acid "tails." The cell membrane possesses a phospholipid bilayer, with the hydrophilic heads facing outwards and the hydrophobic tails facing inwards.
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