passive transport & types simple diffusion,facilitated diffusion.|class 9th chapter cell and tissue|

We know that cell membranes act as barriers to most, but not all, molecules. That is why cell membranes are called semi-permeable membranes. Cell membranes maintain equilibrium inside cell as well as outside by exchanging matter with cells’ environment according to needs. Cell membranes do it through the phenomena of diffusion, facilitated diffusion, osmosis, filtration, active transport, endocytosis and exocytosis. Diffusion Diffusion is the movement of molecules from an area of higher concentration to the area of lower concentration i.e. along concentration gradient. The molecules of any substance (solid, liquid or gas) are in motion when that substance is above 0 degrees Kelvin or -273 degrees Centigrade. In a substance, majority of the molecules move from higher to lower concentration, although there are some that move from low to high. The overall (or net) movement is thus from high to low concentration. Eventually, the molecules reach a state of equilibrium where they are distributed equally throughout the area. Diffusion is one principle method of movement of substances within cells, as well as across cell membrane. Carbon dioxide, oxygen, glucose etc. can cross cell membranes by diffusion. Gas exchange in gills and lungs occurs by this process. Movement of glucose molecules from small intestine lumen into the blood capillaries of villi is another example of diffusion. Because a cell does not expend energy when molecules diffuse across its membrane, the diffusion is type of passive transport. Facilitated diffusion Many molecules do not diffuse freely across cell membranes because of their size or charge. Such molecules are taken into or out of the cells with the help of transportproteins present in cell membranes. When a transport protein moves a substance from higher to lower concentration, the process is called facilitated diffusion. The rate of facilitated diffusion is higher than simple diffusion. Facilitated diffusion is also a type of passive transport because there is no expenditure of energy in this process #PassiveTransport #simplediffusion