Osmosis

• Osmosis is the diffusion of water molecules across a partially permeable membrane (allows some small molecules like water through, but not big molecules like starch) from high to low concentration. For example, water to roots or reabsorbtion to kidneys. Diffusion is the net movement of particles from a high to low concentration until equal distribution.
• Factors which affect osmosis are: concentration gradient, surface area, temperature and distance travelled.

  Concentration gradient is the difference between the two concentrations, the steeper, the quicker diffusion. Surface area the bigger, the quicker. Distance the water has to travel, the smaller, the quicker. Temperature the higher, the quicker.
• Osmosis is a passive process which requires no energy
• • Osmosis in Plant cells - Roots cells take up water from soil and leaf cells lose water if the humidity is low. When plant cells gain water through osmosis, the cell organelles push against the cell wall, becoming turgid. When they lose water, the cell organelles pull away, becoming flacid - this is called plasmolysis. So when a plant is not watered, water diffuses out as the plant has a higher water concentration than the soil. The cells do not change shape due to the cell wall's support.
• • Osmosis in Animal cells - Animal cells do not have a cell wall so if blood cells lose water, the cell shrinks. If it gains too much water, they can burst. So the concentrations of blood plasma (inside the blood cell with the water) and tissue fluid (outside the blood cell with water) are kept within strict limits. For example, if there was more blood plasma than usual, the water concentration would be low meaning osmosis would bring in too much water, causing the blood cell to burst.
• Water concentration - amount of water compared to other molecules of solutes dissolved in it
• Solute - any substance that is dissolved in another substance
• When concentration is the same on both sides, movement will be the same in both directions.
• The cell membrane is semi-permeable.