Included in the illustration above is the sodium-potassium pump which is a vital cell process. Many crucial processes in the life of cells depend upon active transport. Since in the normal state of a cell, large concentration differences in K +, Na + and Ca 2+ are maintained, it is evident that active transport mechanisms are at work. If a molecule is to be transported from an area of low concentration to an area of high concentration, work must be done to overcome the influences of diffusion and osmosis. This is called osmosis, and the tendency to transport the solvent molecules is quantified in terms of osmotic pressure. and only the smaller solvent molecules like water molecules will move across the membrane. There are times when membranes are impermeable to some molecules because of their size, polarity, etc. For example, water molecules at 20☌ have an effective or rms speed of over 600 m/s or over 1400 miles/hr! This motion from areas of high concentration to low is called diffusion. Molecules at normal temperatures have very high speeds and random motions. There is a very strong tendency for molecules to move from higher concentration to low, just based on thermal energy. This is counter to what would be expected and is labeled 'active transport'. There are numerous situations in living organisms when molecules move across cell membranes from an area of lower concentration toward an area of higher concentration. Active Transport Across Cell Membranes Active Transport Across Cell Membranes

Concentration gradient