Diffusion and Osmosis.docx

Movement through Cell Membranes Diffusion is the movement of molecules from a region of high concentration to a region of low concentration (until the two concentrations are the same). - Diffusion is passive i.e., no energy required. - Rate of diffusion depends on factors such as temperature, concen...

Movement through Cell Membranes Diffusion is the movement of molecules from a region of high concentration to a region of low concentration (until the two concentrations are the same). - Diffusion is passive i.e., no energy required. - Rate of diffusion depends on factors such as temperature, concentration difference, distance, surface area, type of medium and mass of substance. - Faster diffusion if the temperature is high, short distance, big concentration difference, gas medium. Examples of diffusion - gaseous exchange in alveoli of lungs - diffusion of oxygen into a cell for respiration and carbon dioxide out of mitochondrion - gaseous exchange in leaf e.g., carbon dioxide for photosynthesis. - the smell of perfume, bread baking and stink bombs - sugar in tea - transpiration of water vapour through stomata of leaf - absorption of food in villi Osmosis is the movement of water molecules from a region of higher water concentration to a region of lower water concentration across a semi-permeable membrane. Examples of Osmosis - water absorption by roots - water movement from cell to cell - water reabsorption by nephron (in kidney) Selectively permeable membranes allow some but not all substances to go through e.g., biological membranes, cellophane, visking tubing, dialysis tubing. They allow substances such as water, oxygen, and carbon dioxide to pass freely but do not allow sugars, proteins, and salts to pass through easily. Turgor or turgor pressure is the pressure of the cytoplasm and vacuole against the cell wall of a plant. A plant cell is turgid (firm) when no more water can enter the cell by osmosis i.e., outward pressure of vacuole (turgor pressure) versus resistance of cell wall. When a cell is fully turgid it can support its own weight. When cells lose turgor a plant wilts as cells become flaccid and cannot support their own weight. Application of High Salt or Sugar Concentration in Food Preservation When jam is made by boiling sugar and fruit, the high temperature reached kills all microorganisms present. The high sugar concentration removes water from any new microorganisms that land on it by osmosis, which results in their death, thus preventing reinfection and preserving the jam. Salting fish and beef kills all microorganisms present by extracting water from them by osmosis. It then kills any new microorganisms that land on it afterwards, in the same way. Mandatory Practical – Demonstrating Osmosis **Know in detail** (9) - Soften two 40 cm strips of visking tubing by soaking them in water. - Tie a knot at one end of each strip. - Using a clean funnel, half-fill one piece of tubing with sucrose solution and the other with distilled water. - Eliminate as much air as possible from the tubes and tie a knot at the open end of each tube. - In the case of each tube, tie the two ends, making a loop. - Wash off any excess sucrose solution from the outside of the sucrose tube and pat dry both tubes with the paper towels. - Observe and record the turgidity of each tube. - Find the mass of each tube and record. - Suspend the tube containing the concentrated sucrose solution, by means of a glass rod, in a beaker of distilled water and label it ‘sucrose solution’. - Similarly, place the tube containing the distilled water into a second beaker of distilled water and label it ‘distilled water’. This acts as the control. - Allow the tubes to stand for at least 15 minutes. - Remove the tubes and dry as before. - Observe and record the turgidity of each tube. - Again, find the mass of each tube and record. - Replicate the investigation or cross reference your results with other groups. Both the mass and turgidity of the Visking tubing containing the concentrated glucose solution will have increased, as deionised water has moved into the glucose solution, demonstrating osmosis.