Botany · Ch 11 — Transport in Plants
Osmosis
Osmosis
Osmosis is a specific, restricted form of diffusion in which only the solvent - water, in a living plant - crosses a selectively permeable membrane, always moving from the side with higher water potential (a dilute, hypotonic solution) toward the side with lower water potential (a concentrated, hypertonic solution), continuing until the two sides reach an isotonic balance and net movement stops. When this water movement is directed into a cell or system from a surrounding hypotonic medium it is called endosmosis - a dry raisin swelling when placed in water is a simple example - and when water instead moves out of a cell or system into a surrounding hypertonic medium it is called exosmosis, which in a living plant cell leads directly to plasmolysis: the protoplasm shrinking and the cell membrane pulling progressively away from the cell wall, passing through recognisably different reversible (incipient, evident) and finally irreversible (final) stages, as summarised in Table 11.2. A plasmolysed cell that is transferred back into water can, while still in a reversible stage, take up water again by endosmosis and regain its original shape and size - a recovery called deplasmolysis. Both the thistle-funnel experiment and the potato-osmoscope activity demonstrate endosmosis directly, by tracking a solution's level rising once it is separated from pure water by a semipermeable membrane. Reverse osmosis deliberately runs this whole process backward: by applying external pressure to th …
What this figure shows. A thistle funnel whose wide mouth is tied over with a goat-bladder membrane acting as a semipermeable barrier, filled with concentrated sugar solution and inverted into a beaker of plain water, with the initial sugar-solution level marked on the funnel's stem; over time the level rises up the narrow stem as water diffuses in through the bladder fas …
What this figure shows. A diagram comparing a cell or system placed in three solution types - a hypertonic (concentrated, low-Ψ) solution, from which the cell loses water; a hypotonic (dilute, high-Ψ) solution, into which the cell gains water; and an isotonic solution, where the concentrations match and there is no net water movement - shown with arrows indicating the net direction of wate …
What this figure shows. A peeled potato tuber hollowed into a cup shape, its cavity filled with concentrated sugar solution up to a marked initial level, the whole tuber then stood in a beaker of pure water; after about ten minutes the sugar-solution level inside the cavity has risen visibly above the original mark because water has entered the tuber tissue by endosmosis faster than sugar …
What this figure shows. A membrane apparatus with salt water on one side and pure water on the other, separated by a selectively permeable membrane; an externally applied pressure on the salt-water side is shown by an arrow forcing water molecules to move from the salt (hypertonic) side to the pure-water (hypotonic) side - the reverse of the direction wa …