<P> 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 . 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 . </P> <P> Facilitated diffusion may be achieved as a consequence of charge gradients in addition to concentration gradients . Plant cells create an unequal distribution of charge across their plasma membrane by actively taking up or excluding ions . Active transport of protons by H ATPases alters membrane potential allowing for facilitated passive transport of particular ions such as Potassium down their charge gradient through high affinity transporters and channels . </P> <P> Filtration is movement of water and solute molecules across the cell membrane due to hydrostatic pressure generated by the cardiovascular system . Depending on the size of the membrane pores, only solutes of a certain size may pass through it . For example, the membrane pores of the Bowman's capsule in the kidneys are very small, and only albumins, the smallest of the proteins, have any chance of being filtered through . On the other hand, the membrane pores of liver cells are extremely large, but not forgetting cells are extremely small to allow a variety of solutes to pass through and be metabolized . </P> <P> Osmosis is the movement of water molecules across a selectively permeable membrane . The net movement of water molecules through a partially permeable membrane from a solution of high water potential to an area of low water potential . A cell with a less negative water potential will draw in water but this depends on other factors as well such as solute potential (pressure in the cell e.g. solute molecules) and pressure potential (external pressure e.g. cell wall). There are three types of Osmosis solutions: the isotonic solution, hypotonic solution, and hypertonic solution . Isotonic solution is when the extracellular solute concentration is balanced with the concentration inside the cell . In the Isotonic solution, the water molecules still moves between the solutions, but the rates are the same from both directions, thus the water movement is balanced between the inside of the cell as well as the outside of the cell . A hypotonic solution is when the solute concentration outside the cell is lower than the concentration inside the cell . In hypotonic solutions, the water moves into the cell, down its concentration gradient (from higher to lower water concentrations). That can cause the cell to swell . Cells that don't have a cell wall, such as animal cells, could burst In in this solution . A hypertonic solution is when the solute concentration is higher (think of hyper - as high) than the concentration inside the cell . In hypertonic solution, the water will move out, causing the cell to shrink . </P>

Describe the factors that affect the rate of passive transport of molecules across a membrane