Active Transport – Sometimes the movement of substances across the cell-membrane takes place against the concentration gradient (uphill movement) i.e. from a region of lower concentration to a region of higher concentration. This type of uphill movement of substances requires the expenditure of energy and is called active transport.
12.2.3 Osmosis
As you all know that a living cell is enclosed in the cell membrane, which allows some substances to pass through it but prevents the passage of others. Such membrane, which is selective in the passage of substances, is called a selectively permeable or differentially permeable membrane. The solvent molecules, which in most cases are water molecules, can readily pass through the cell membrane. The water molecules can easily diffuse through the cell membrane. The movement of water (solvent) through a differentially permeable membrane is a special kind of diffusion called osmosis.
Practical work: to demonstrate osmosis
Take a thistle funnel and tie a cellophane or egg membrane (both are differentially permeable) around the wide end or the mouth of the funnel. These two membranes are permeable to water molecules but not to dissolved solute molecules such as source (sugar).
Fill the thistle funnel, upto a marked level with concentrated sugar solution and place the funnel, with its mouth end immersed in water. You will see that the water would move both into and out of the funnel. But because of the grater water concentration outside the funnel, the movement of water into the funnel would be more rapid than its movement to the outside of the funnel into the beaker. Consequently, the level of sugar solution will rise in the tube of the funnel. As the movement of water into the funnel goes on, the sugar solution in the funnel will become more and more dilute.
The movement of water molecules into the funnel was the quickest at the time when it was first immersed into the beaker because at that time there was a concentration gradient greater difference between the water in the beaker and the sugar solution is the funnel. At that time, the movement of water out of the funnel was the slowest. The movement of water into and out of the funnel went on until the equilibrium of water molecules on both sides of the membrane was reached.
12.2.4 Turgor
If a cell is surrounded by water or a solution more dilute than its contents (i.e. having lesser concentration of solutes and higher concentration of water), the water tends to move into a cell vacuole by osmosis. As the water enters the vacuole, it increases in size and pushes the cell contents against the wall. The plant cell does not burst as the cell wall is fairly strong and relatively inelastic. However, the cell wall cannot expand beyond a certain limit. The cell wall, thus prevents over-expansion of the cell by exerting an inwardly directed or an opposing pressure which prevents the entry of more water into the cell. The cell in this state becomes turgid. The condition thus produced in the cell is called turgor and the internal pressure exerted on the cell wall is called turgor pressure (fig. 12.4a).
When all the cells in a leaf and stem are turgid, the plant structures like leaves and young branches will be stiff, firm and upright. If the cells some how lose water, the cell will lose turgor and will become flaccid (soft and limp). The leaves with flaccid cells become limp and the stem droops. Thus plant cells with reduced turgor due to loss of water is said to be wilting (fig. 12.4b).
Importance of turgor in plants
Turgor plays an important role in maintaining the shape of the soft tissues in plants. The movements of certain plant parts also result from the changes in their turgor e.g., changes in the turgor of the guard cells of the stomata cause the opening and closing of the stomata. Some flowers open during the day time and close at night or vice versa. In these the bending movements of their petals is due to changes in turgidity of the cells. The closing of leaves of some plants e.g. touch-me-not, is also due to turgor changes in the leaf-lets.