osmosis

An account of osmosis for GCSE biology students. A Definition of Osmosis An Explanation of the Definition An Explanation of Osmosis The Consequences of Osmosis in Plant Cells The Consequences of Osmosis in Animal Cells When you have read this page put the Osmosis and Homeostasis Revision Windows on your desktop   new There are also some TESTS but read this page first. Definition Osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration. The definition contains three important statements: Osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration. Osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration. Osmosis is the passage of water from a region of high water concentration through a semi-permeable membrane to a region of low water concentration. It does not matter too much which order you put these statements in. Nor does it matter if you write the definition as one sentence or three sentences. All that matters in your exam is that you make all three points when you explain what osmosis is. Now put the Osmosis and Homeostasis Revision Windows on your desktop   new Back to the top Explanation First the definition of osmosis: Semi-permeable membranes are very thin layers of material (cell membranes are semi-permeable) which allow some things to pass through them but prevent other things from passing through. Cell membranes will allow small molecules like Oxygen, water, Carbon Dioxide, Ammonia, Glucose, amino-acids, etc. to pass through. Cell membranes will not allow larger molecules like Sucrose, Starch, protein, etc. to pass through. A region of high concentration of water is either a very dilute solution of something like sucrose or pure water. In each case there is a lot of water: there is a high concentration of water. Some teachers use the definition which starts "Osmosis is the passage of water from a dilute solution to a......" this means exactly the same as the definition I have given. A region of low concentration of water is a concentrated solution of something like sucrose. In this case there is much less water. So you could use the definition "Osmosis is the passage of water from a dilute solution through a semi-permeable membrane to a more concentrated solution. Back to the top Now to explain osmosis: When you put an animal or plant cell into a liquid containing water one of three things will happen. If the medium surrounding the cell has a higher water concentration than the cell (a very dilute solution) the cell will gain water by osmosis. Water molecules are free to pass across the cell membrane in both directions, but more water will come into the cell than will leave. The net (overall) result is that water enters the cell. The cell is likely to swell up. If the medium is exactly the same water concentration as the cell there will be no net movement of water across the cell membrane. Water crosses the cell membrane in both directions, but the amount going in is the same as the amount going out, so there is no overall movement of water. The cell will stay the same size. If the medium has a lower concentration of water than the cell (a very concentrated solution) the cell will lose water by osmosis. Again, water crosses the cell membrane in both directions, but this time more water leaves the cell than enters it. Therefore the cell will shrink. Back to the top The Consequences of Osmosis Firstly what happens to plant cells: Plant cells always have a strong cell wall surrounding them. When the take up water by osmosis they start to swell, but the cell wall prevents them from bursting. Plant cells become "turgid" when they are put in dilute solutions. Turgid means swollen and hard. The pressure inside the cell rises, eventually the internal pressure of the cell is so high that no more water can enter the cell. This liquid or hydrostatic pressure works against osmosis. Turgidity is very important to plants because this is what make the green parts of the plant "stand up" into the sunlight. When plant cells are placed in concentrated sugar solutions they lose water by osmosis and they become "flaccid"; this is the exact opposite of "turgid". If you put plant cells into concentrated sugar solutions and look at them under a microscope you would see that the contents of the cells have shrunk and pulled away from the cell wall: they are said to be plasmolysed. When plant cells are placed in a solution which has exactly the same osmotic strength as the cells they are in a state between turgidity and flaccidity. We call this incipient plasmolysis. "Incipient" means "about to be". When I forget to water the potted plants in my study you will see their leaves droop. Although their cells are not plasmolsysed, they are not turgid and so they do not hold the leaves up into the sunlight. Back to the top And now for the animal cells: When animal cells are placed in sugar solutions things may be rather different because animal cells do not have cell walls. In very dilute solutions, animal cells swell up and burst: they do not become turgid because there is no cell wall to support the cell membrane. In concentrated solutions, water is sucked out of the cell by osmosis and the cell shrinks. In either case there is a problem. So animal cells must always be bathed in a solution having the same osmotic strength as their cytoplasm. This is one of the reasons why we have kidneys. The exact amount of water and salt removed from our blood by our kidneys is under the control of a part of the brain called the hypothalamus. The process of regulating the amounts of water and mineral salts in the blood is called osmoregulation. My insulin page will tell you more about other homeostatic mechanisms. Animals which live on dry land must conserve water; so must animals which live in the sea (the sea is very salty!), but animals which live in freshwater have the opposite problem; they must get rid of excess water as fast as it gets into their bodies by osmosis. Now put the Osmosis and Homeostasis Revision Windows on your desktop   new Back to the top If you do a search of the INTERNET for "osmosis" you will find lots of references to osmosis in sailing boats. When yachtsmen and yachtswomen talk about "osmosis" they mean that the the hull of their little boatie is being damaged by water but this is nothing to do with osmosis as we biologists use the term. Yacht people have stolen our word. So don't get confused by what you may read out there on the INTERNET.

absorption of water by plant roots animation

Plants absorb water through the entire surface - roots, stems and leaves. However, mainly the water is absorbed by roots. The area of young roots where most absorption takes place is the root hair zone. The root hairs are delicate structures which get continuously replaced by new ones at an average rate of 100 millions per day. The root hairs lack cuticle and provide a large surface area. They are extensions of the epidermal cells. They have sticky walls by which they adhere tightly to soil particles. As the root hairs are extremely thin and large in number, they provide enormous surface area for absorption. They take in water from the intervening spaces mainly by osmosis.

Water in the roots move by two pathways. They can be classified as1) Apoplast pathway2) Symplast pathway

Apoplast pathway

In this pathway the movement of water occurs exclusively through cell wall without the involvement of any membranes. Majority of the amount of water goes through the apoplast pathway. The cortex of the root does not oppose such movement of the water.

Symplast pathway

Here the movement of water molecules is from cell to cell through the plasmodesmata. The plasmodesmata forms a network of cytoplasm of all cells.

The Casparian strip separates the cortex and the endodermis. It is composed of a wax like substance called suberin, which blocks water and solute molecules through the cell wall of the endodermis. Now the water is forced to go through the cell membranes of different cells leading to a transmembrane pathway.

Areas of Root Involved in Absorption and Translocation of Water

Mechanism of Water Absorption

Water can be absorbed by two methods:

• Active absorption
• Passive absorption

Active Absorption

Water is absorbed due to activities going on in roots. Absorption of water occurs with the help of energy in the form of ATP, which is released due to metabolic activities of root cells such as respiration. Absorption takes place against concentration gradient - even when the concentration of cell sap is lower than that of soil water.

Passive Absorption

Passive absorption is by osmosis. Passive absorption takes place along the concentration gradient - when the concentration of cell sap is higher than that of soil water. Water is absorbed when transpiration rate is high or soil is dry. Due to high transpiration rate, water deficit is created in transpiring cells. Rapid transpiration removes water and reduces turgor pressure in living cells of root. The suction force thus developed is transmitted to root xylem. It pulls water from surrounding root cells to make up water deficit.