Osmosis is a special example of diffusion. It is the diffusion of water through a partially permeable membrane from a more dilute solution to a more concentrated solution – down the water potential gradient) Note: diffusion and osmosis are both passive, i.e. energy from ATP is not used. A partially permeable membrane is a barrier that permits the passage of some substances but not others; it allows the passage of the solvent molecules but not some of the larger solute molecules. Cell membranes are described as selectively permeable because not only do they allow the passage of water but also allow the passage of certain solutes. The presence of particular solutes stimulates the membrane to open specific channels or trigger active transport mechanisms to allow the passage of those chemicals across the membrane. Some major examples of osmosis
• Absorption of water by plant roots.
• Re-absorption of water by the proximal and distal convoluted tubules of the nephron. • Re-absorption of tissue fluid into the venule ends of the blood capillaries. • Absorption of water by the alimentary canal — stomach, small intestine and the colon.
Osmoregulation is keeping the concentration of cell cytoplasm or blood at a suitable concentration. (a) Amoeba, living in freshwater, uses a contractile vacuole to expel the excess water from its cytoplasm (thus need more respiration/O2/ATP than isotonic (marine) Amoebae). (b) The kidneys maintain the blood (thus, whole body) at the correct concentration. Osmosis and Plant Cells
(a) Plant Cells in a hypotonic (= weaker) solution – cells have lower water potential • the plant cells gain water by osmosis.
• the vacuole and cytoplasm increase in volume.
• the cell membrane is pushed harder against the cell wall causing it to stretch a little. • the plant tissue becomes stiffer (= turgid).
(b) Plant Cells in a hypertonic (=stronger) solution – cells have higher water potential • the plant...