Cells is a Miniature Chemical Factory
Due to its incessant metabolic processes, a living cell earns the attribute of a miniature chemical factory metabolites and free oxygen are its most essential raw materials life is its ultimate product and Co2 H2O and nitrogenous wastes are its main by products.
Passive and Active Transport : If transportation across the plasma membrane by any of the above pathways id due to normal kinetic motion of molecules from the region of their higher concentration to the region of their lower concentration it is called passive transport or diffusion. Diffusion through interstices of lipids bilayer or through watery channels of channel proteins is called simple diffusion, diffusion by way of carrier proteins is, on the other hand called facilitated diffusion, Certain substances are taken in or extruded forcibly against concentrations gradients i.e. from the region of lower to the region of higher concentrations .The process always occur through the carrier proteins and a lot of energy of a ATP is spent on it. It is therefore ,called active transport.
By maintaining fixed concentration gradients in the cytoplasm and ECF, cells render the passive transport of certain solute molecules unidirectional .For instance. O2 and useful metabolites. Such as glucose and most of the amino acids are used up in metabolism as soon as these enter into a cells Hence their passage throu7gh the plasma membrane is always towards the cytoplasm from ECF. Similarly metabolic by products continuously form in the cell Hence there is a continuous traffic of these products from the cell into the ECF.
Sodium potassium pump This is the most important example of carrier mediated active transport system of cells ,Normally, the plasma membrane is far less permeable to sodium ions Na+ than to potassium K+ ions . K+ ions are required in the cells for the activity of many enzyme , and protein synthesis .Hence cells in general maintain a high internal concentration of k+ ions ,but a low concentration of Na+ ions . Conversely , the ECF always has a high concentration Na + and a low concentration of k+ ions. Due to their concentration gradients k+ ions keep escaping out of cells and Na+ ions keep entering into them .cells therefore have to forcibly extrude Na + and take in K+ against their concentration gradients through carrier proteins by spending about 10% to40% of their total energy output. This is called sodium potassium pump. The carrier protein of plasma membrane to operate this pump is an enzyme called Na+ K+ ATP ase .This enzyme molecule has three receptor sites on its inner surface for binding three sodium ions and two receptor sites onits outer surfade for binding two potassium ions. As sodium and potassium ions bind at the respective sites with this molecule an ATP molecule gets degraded to ADP releasing energy which triggers a conformational change in the molecule. As a result of this released into the ECF and potassium ions into the cytoplasm. The precise mechanism of this conformational change in the Na+ k+ ATP ase molecule in not yet known.
One of the most important of Na+ K+ pump is to control the volume of the cells. Due to the presence of a large number of nondiffusible protein molecules and other organic compounds in the cytoplasm water always tends to enter into the cells from the ECF by osmosis. This might cause the cells to swell and ultimately burst. By pumping out three Na+ k+ ions from the cells for every two k+ ions pumped into the cells the Na + K+ pump maintains a continual net loss of ions by the cells and resultant reverse tendency of water out of the cells.
Another important of the Na+ k+ pump is that it creates an electric potential across the cell membrane ,i.e. it is electrogenic. Continual net loss of one positive ion in each revolution of the pump maintains positivity on the outside and negativity on the inside of plasma membrane.