 |
| Physical Processes |
|
| |
| The physical processes involving movement of materials in and out of a cell are diffusion and osmosis. Both these movements involve movement along the concentration gradient. Hence, there is no expenditure of energy. |
| |
|
| It is a process, which involves movement of a substance from a region of its higher concentration to a region of its lower concentration. |
| |
 |
| |
| fig. 15.3 - Diffusion |
| |
|
| Molecules of any substance are in constant random movement in all fluids. This movement is called Brownian movement. |
| |
| Apart from the three states of matter, diffusion can also occur through semipermeable membranes. |
| |
| The molecules of the substance undergoing diffusion exert a pressure in the available space. This pressure is known as diffusion pressure. Higher the diffusion pressure, higher is the rate of diffusion. The rate of diffusion is decided by factors like concentration of the molecules undergoing diffusion, space available for diffusion and temperature of the medium. |
| |
 Small molecules of non-polar substances (like uncharged polar carbon dioxide) rapidly diffuse through the lipid bilayer. |
| |
 Some essential charged particles may diffuse into the cell through specific channels or process formed temporarily by tunnel proteins. |
| |
 Some times a carrier molecule, almost always a protein, facilitates the movement of certain substances like amino acids or nucleotides, across the membrane. This process is called facilitated diffusion. |
| |
|
| It is a process involving net diffusion of water molecules from a region of higher water concentration to a region of lower water concentration through a semipermeable membrane. |
| |
 The osmotic movement of water from a surrounding medium into a cell is called endosmosis. Such a medium is described as hypotonic. |
| |
 The diffusion pressure exerted by water molecules on the semipermeable membrane is called as osmotic pressure. |
| |
| Demonstration of Osmosis |
| |
| A thistle funnel, covered at the broad end by a differentially permeable membrane, contains a 10% sugar solution. The beaker contains a 5% sugar solution as shown in fig. a. The solute is unable to pass through the membrane, but the water passes freely through in both directions. The net movement of water towards the inside of the thistle funnel occurs because the thistle funnel has a lower water concentration. |
| |
 |
| |
| fig. 15.4 - Osmosis |
| |
| The level of the solution rises in the thistle funnel until hydrostatic pressure increases to the level of osmotic pressure. |
| |
| Physiological Experiment to demonstrate Osmosis |
| |
| Osmosis in a cell can be demonstrated by a potato osmoscope experiment. |
| |
| Peel the skin of a potato and remove the inner contents to form a cup shaped hollow. Place the potato in a container with water. Pour 5% sugar solution in to the potato cup up to a particular level. Pierce a pin to mark the initial level. Leave the experimental set up undisturbed for about 30 minutes. |
| |
| At the end of the duration check the level of sugar solution in the potato cup. You will be able to record an increase in level caused by osmotic movement of water. |
| |
| You can repeat the experiment by reversing the situation. Take sugar solution in the container and water in the potato cup. Mark the initial level. At the end of 30 minutes note the difference. This time you will be able to record a decrease in the level of water inside the potato cup. |
| |
