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http://www.teachnet.ie/mamond Movement of Water through membranes There are three types of membranes Permeable allows all substances to pass through’ Impermeable allows no substances to pass through’ Semi-permeable allows only some molecules to pass through eg Cell Wall eg Cell Membrane more correctly referred to as a PARTIALLY PERMEABLE solution Components of a solution Solute – the solid molecules dissolved in a liquid (usually water) Solvent – the liquid in which the solids are dissolved There are various types of solution Concentrated: lots of solute in very little solvent (a strong solution) Dilute: very little solvent in a lot of solution (a weak solution) Isotonic: when two solutions have the same amount of solute and solvent, they are said to be isotonic to each other Methods of moving molecules Diffusion: the movement of molecules from where they are plentiful to where they are scarce e.g. the movement of O2 from the air in the alveoli into the blood capillaries, the movement of digested food across the villi... Diffusion is passive – it uses no energy Water molecules are scarce in concentrated solutions and plentiful in dilute solutions Osmosis: a special case of diffusion - it is the diffusion of water molecules Osmosis is the diffusion of water molecules from a dilute solution to a concentrated solution across a partially permeable membrane To check Leaving Certificate Biology syllabus, log on to http://nbsstralee.ie http://www.teachnet.ie/mamond What happens to animal cells in an isotonic solution? • Water molecules diffuse freely in and out through the partially permeable lipoprotein membrane all the time • There is no change in the volume of the cytosol • OSMOREGULATION ensures that body cells are isotonic to the fluids which surround them - this is part of the body's control system (see homeostasis) What happens to animal cells in a strong solution? • Water molecules diffuse out of the cytosol towards the stronger solution • The cytosol loses volume and the lipoprotein membrane shrinks • If this happens in red blood cells, they are said to be CRENATED What happens to animal cells in a dilute solution? • Water molecules diffuse into the cytosol towards the stronger solution • The cytosol volume swells and pushes against the lipoprotein membrane • If this continues, the lipoprotein membrane may rupture, the cell bursts • If this happens in red blood cells, it is called HAEMOLYSIS What happens to plant cells in an isotonic solution? • Water molecules diffuse freely in and out through the semi permeable lipoprotein membrane and permeable cell wall all the time • There is no change in the volume of the cytosol What happens to plant cells in a strong solution? • Water molecules diffuse out of the cytosol towards the stronger solution • The cytosol loses volume, the lipoprotein membrane shrinks away from the cell wall • The cell is said to be FLACCID – this process is called PLASMOLYSIS • Plasmolysis can cause the plant to wilt What happens to plant cells in a dilute solution? • Water molecules diffuse into the cytosol towards the stronger solution • The cytosol volume swells and pushes against the lipoprotein membrane • The strong cellulose cell wall prevents the membrane from bursting • The cell is said to be TURGID • If all cells in a plant are turgid, it gives strength to support the plant upright To check Leaving Certificate Biology syllabus, log on to http://nbsstralee.ie
