Osmosis

• Osmosis is the movement of water molecules across a selectively permeable membrane from a region of higher water concentration to a region of lower water concentration.
• Diffusion is the movement of molecules from an area of higher concentration to an area of lower concentration.
• Active transport is the movement of molecules against their concentration gradient.
• A partially permeable membrane allows water and other small molecules to pass through, but not larger molecules such as starch.
• A dilute solution contains a high concentration of water molecules, while a concentrated solution contains a low concentration of water molecules.
• Osmosis refers to the movement of water molecules only.
• 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.
• 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 solutes.
• Cells contain dilute solutions of ions, sugars, and amino acids.
• The cell membrane is a selectively permeable membrane surrounding the cell and controlling the entry and exit of materials.
• Fully turgid cells support the stems of non-woody plants.
• In a more concentrated solution, the cell contents lose water by osmosis, shrink and pull away from the cell wall, causing the cell to become flaccid.
• Root hair cells, 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 if the soil is wet or moist.
• 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.
• Plant cells have a strong cellulose cell wall which provides support and prevents the cell from bursting from the uptake of water by osmosis.
• If plant cells are placed in solutions of increasing solute concentration, in pure water, the cell contents push against the cell wall and the cell becomes turgid.
• In a very concentrated solution, the cell undergoes full plasmolysis as the cells lose more water.
• For example, red blood cells could lose water and shrink or gain water, swell and burst in a more dilute solution.
• Aquatic plants, such as 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.
• In animals, the concentration of body fluids, such as blood plasma and tissue fluid, must be kept within strict limits.
• Isolated plant cells placed in a dilute solution or water will take in water by osmosis.
• Leaf cells of land plants, unless it is raining or the humidity is high, will have a tendency to lose water.
• Plant cells are partially permeable to water and take in water by osmosis.