Eduqas Biology for A Level Year 1 & AS: Student Bk

YOU SHOULD KNOW › › › ››› The definition of osmosis in terms of water potential ››› The water potential of pure water is zero ››› The addition of solutes reduces the water potential ››› The meaning of the terms turgor and plasmolysis in plant cells ››› The effect of osmosis on animal cells Key Terms Osmosis: The diffusion of water, from a region of higher water potential to a region of lower water potential, through a selectively permeable membrane. Water potential ( ψ ): A measure of the free energy of water molecules, measured in kilopascals (kPa). Examtip Learn the definition of osmosis by heart. Study point For pure water, ψ = 0. For solutions, the water potential is negative. Going further Water molecules travel across cell membranes through specialised channels called aquaporins. A cell may have thousands of aquaporins and transfer billions of water molecules each second. Osmosis Most cell membranes are permeable to water and to certain solutes. Osmosis is a special case of diffusion which involves the movement of water molecules only. Water potential Water potential ( ψ ) is a measure of the free energy of water molecules and is the tendency for water to move. It is measured in kilopascals (kPa). There is no tendency for water molecules to move into pure water, so pure water has a water potential of zero. The addition of a solute to pure water tends to bring water molecules in. As the force pulls inwards, it has a negative sign and so the addition of a solute to pure water lowers the water potential and gives it a negative value. The higher the concentration, the more strongly water molecules are pulled in, the lower, i.e. the more negative the water potential. This can be explained in terms of energy: where there is a high concentration of water molecules, i.e. a dilute solution, water molecules have a high potential energy because they are free to move. In a solution, water molecules are weakly bound to the solute so fewer are free to move. The system has a lower potential energy. External water molecules, with higher potential energy, will move down an energy gradient to the lower potential energy. This is the pulling force they experience, which is the osmotic pull inwards, i.e. the water potential. A more concentrated solution has even fewer free water molecules. Consequently, the pull on water molecules is greater so the water potential is more negative, i.e. lower. Water moves from a higher to a lower, more negative, water potential The diagram shows the movement of water between cells with different water potentials. Water moves from an area of higher water potential, e.g. –100 kPa, to an area of lower water potential, e.g. –200 kPa. Fig 36 –200 kPa –100 kPa –800 kPa water moves from a highWP to a lower or more negativeWP Eduqas AS / Yr 1 Biology: Chapter 3 58