Osmosis
Osmosis is the The movement of molecules from an area of higher concentration to an area of lower concentration. of water molecules, from a region where the water molecules are in higher concentration, to a region where they are in lower concentration, through a Also called semi-permeable. A partially permeable membrane allows water and other small molecules to pass through, but not larger molecules such as starch. membrane.
A dilute Mixture formed by a solute and a solvent. contains a high concentration of water molecules and is said to have a high Water potential is a measure of the tendency of water molecules to move from one place to another. During osmosis, water moves through a partially permeable membrane from high water potential (low solute concentration) to low water potential (high solute concentration)., while a concentrated solution contains a low concentration of water molecules and so has a low water potential.
The slideshow shows an example of osmosis showing the direction of movement of water between two different concentrations of sugar solutions:
Image caption, Water molecules move from solution 1 to solution 2 by osmosis
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Water molecules move from solution one to solution two.
When the concentration of water is the same on both sides of the membrane, the movement of water molecules will be the same in both directions. There will be no The overall effect of something when all additions and deductions are calculated. movement of water molecules. In theory, the level of solution two will rise, but this will be opposed by gravity and will be dependent on the width of the container.
Similar observations will be made with solutions containing different The dissolved substance in a solution., for instance, salt instead of sugar.
Osmosis across living cells
Cells contain A dilute solution is one with a low concentration of solute. solutions of Electrically charged particle, formed when an atom or molecule gains or loses electrons., A simple carbohydrate that is sweet to the taste. and The building blocks that make up a protein molecule. so have a high water potential.
The A selectively permeable membrane surrounding the cell and controlling the entry and exit of materials. is partially permeable.
Water will move into and out of cells by osmosis.
Plant cells
Isolated plant cells placed in a dilute solution or water will take in water by osmosis. If the soil is wet or moist, A specialised cell that increases the surface area of the root epidermis to improve the uptake of water and minerals., will also take up water by osmosis. Leaf cells of land plants, unless it is raining or the The amount of water vapour in the atmosphere measured as a percentage. is high, will have a tendency to lose water.
Plant cells have a strong A carbohydrate. It forms the cell wall in plant cells. Outer structure which provides support and prevents the cell from bursting from the uptake of water by osmosis. Plant, fungal and bacterial cell walls have different structures and chemical compositions. outside the cell membrane. The cell wall is fully permeable to all molecules and supports the cell and stops it bursting when it gains water by osmosis.
If plant cells are placed in solutions of increasing solute concentration:
Pure water
In pure water, the cell contents - the The living substance inside a cell (not including the nucleus). and A space within the cytoplasm of plant cells that contains cell sap. - push against the cell wall and the cell becomes Enlarged and swollen with water. Having turgor. Description of a plant cell in which the vacuole has swollen due to water gain by osmosis..
Fully turgid cells support the stems of non-woody plants.
Concentrated solution
In a more concentrated solution (low water potential), the cell contents lose water by osmosis. They shrink and pull away from the cell wall. The cell becomes Lacking turgor. Lacking in stiffness or strength. Soft and floppy.. It is becoming Description of a plant cell in which the vacuole has shrunk and the membrane has pulled away from the wall due to water loss by osmosis..
Highly concentrated solution
In a very concentrated solution, the cell undergoes full plasmolysis as the cells lose more water.
Plants would be exposed to higher concentrations of The dissolved substance in a solution. if there was less water in the soil - for instance, if plants were not watered, or plants in drought conditions. Plant cells would then lose water by The movement of water molecules across a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration..
Within water. , freshwater plants placed in the sea, or a seaweed in a rock pool where the water evaporated in the Sun, would also lose water by osmosis.
Animal cells
Animal cells also take in and lose water by osmosis. They do not have a cell wall, so will change size and shape when put into solutions that are at a different concentration to the cell contents.
For example, red blood cells:
In animals, the concentration of body fluids - The liquid part of the blood containing useful things like glucose, amino acids, minerals, vitamins (nutrients) and hormones, as well as waste materials such as urea. and Fluid which is derived from blood plasma that passes through the walls of capillaries. - must be kept within strict limits - if cells lose or gain too much water by osmosis, they do not function efficiently.