Steps in the process of receptor-mediated endocytosis of cholesterol.
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Cholesterol is carried in the blood by LDL. The uptake of LDL with its associated cholesterol, is mediated by a specific LDL-receptor protein on the cell surface. Once internalized, the cholesterol is removed from the LDL-receptor complex and used by the cell. The LDL receptors are sent back to the cell surface to bind more LDL.
impermeable to most polar and charged molecules. Transport of small water-soluble molecules is achieved by specialized transmembrane proteins called transporter proteins. Most membrane transporters are highly specific—a different transporter protein is required for each type of molecule to be transported. Only lipid-soluble molecules can permeate the lipid bilayer directly by simple diffusion.
Membrane transport proteins are of three basic kinds: ATP-driven pumps, carriers, and channel proteins. Channel proteins are the simplest of the three, forming a water-filled pore through the lipid bilayer. These pores are able to open and close to allow ions to pass through the membrane. The particular structure of the protein channel ensures that only ions of a certain size and charge can move through the membrane. Pumps and carrier proteins, however, bind to the solute to be transported and move it through the membrane by undergoing a structural, or conformational, change. Pumps and carriers have a transport maximum that is much lower than that of channels because they must bind to the molecules to be transported and then move them through the membrane. Pumps and carriers, which transport ions and nonelectrolyte molecules (e.g., glucose and amino acids), are also highly specific for the substances they transport.
Lipid-soluble particles can cross the lipid bilayer directly by simple diffusion through the hydrophobic lipid portion of the membrane. Polar or charged molecules must cross the membrane via protein channels or carriers. Transport through membrane proteins may be a passive or an active process. Passive transport through membrane proteins is called facilitated diffusion. Diffusion of ions occurs passively because of an electrochemical gradient. The electrochemical gradient exists because of differences in intracellular and extracellular charge and/or concentration of chemicals and is governed by laws of physics. Channel proteins only allow particles to move down their electrochemical or concentration
gradients. Some carriers are passive, but others use the movement of one ion flowing down its concentration gradient (usually Na+ moving into the cell) to move another substance uphill against its gradient. This process is called secondary active transport because ATP is not used directly; however, ATP is necessary to run the pumps that maintain the sodium gradient. The lipid bilayer is fairly impermeable to water because of its polar structure. Water moves across the plasma membrane through channels called aquaporins. Nearly all cells have aquaporins present in their cell membranes at all times, with the exception of a few specialized cells in the kidney tubules. Net movement of water across a membrane (osmosis) occurs in response to differences in osmotic pressure on either side of the membrane and is a passive process.