how do carrier proteins facilitate passive transport of molecules across a membrane
Passive transport Facilitated diffusion, also called carrier-mediated osmosis, is the movement of molecules across the cell membrane via special transport proteins that are embedded within the cellular membrane. Large, insoluble molecules, such as glucose, vesicles and proteins require a carrier molecule to move through the plasma membrane.[2] Therefore, it will bind with its specific carrier proteins, and the complex will then be bonded to a receptor site and moved through the cellular membrane. Facilitated diffusion is a passive process: the solutes move down their concentration gradient and do not require the expenditure of cellular energy for this process. Carrier proteins and channel proteins allow for the diffusion of molecules across the cell membrane. Carrier proteins undergo conformational alterations to allow molecules to pass, while channel proteins form unblocked pores.[3]
Active transport Unlike passive transport, which uses the kinetic energy and natural entropy of molecules moving down a gradient, 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. If the process uses chemical energy, such as from adenosine triphosphate (ATP), it is termed primary active transport. Secondary active transport involves the use of an electrochemical gradient. Examples of active transport include the uptake of glucose in the intestines in humans and the uptake of mineral ions into root hair cells of plants.[1]