Botany · Ch 1 — Transport in Plants
Osmosis
Osmosis
Osmosis — Diffusion of Water Across a Selective Membrane
A plant cell is bounded by a plasma membrane and, outside that, a cell wall; the wall itself is freely permeable and offers no real barrier to water or dissolved substances, so the plasma membrane and the vacuole's own membrane (the tonoplast) are what actually control what enters or leaves. Osmosis is simply the name given to diffusion of water specifically across such a selectively (differentially) permeable membrane — water still moves from higher water potential to lower water potential, driven by both the concentration gradient and any pressure gradient, until the two sides reach equilibrium.
Two classic demonstrations illustrate this:
- The potato osmometer — a potato tuber with a hollowed-out cavity filled with concentrated sugar solution is placed in water; water enters the cavity by osmosis and its level rises, showing net inward movement along the water-potential gradient.
- The thistle-funnel experiment (Figure 11.4) — a thistle funnel filled with sucrose solution, its wide mouth sealed with a selectively permeable membrane (such as one taken from an egg after removing its contents and dissolving the shell in dilute acid), is inverted into a beaker of pure water. Water diffuses in and the solution level rises in the narrow stem until equilibrium. Applying mechanical pressure at the top of the stem can stop this inward movement — and the exact pressure needed to halt it is, by definition, the solution's osmotic pressure. A more concentrated sucrose solution needs a greater counter-pressure to stop it.
Osmotic pressure and osmotic potential describe the same underlying quantity but with opposite sign: osmotic pressure is expressed as a positive value (the pressure that must be applied to stop inflow), while osmotic potential is expressed as a negative value (equivalent to solute potential). A companion reasoning exercise (Figure 11.3, two chambers of differing solute concentration separated by a membrane) is used to practise predicting which chamber has the lower water potential, which has the lower solute potential, and which way osmosis will proceed — the chamber with more dissolved solute always has the lower (more negative) water potential and draws water in. …
What this figure shows. A horizontal rectangle divided into two square chambers labelled 'A' (left) and 'B' (right) by a vertical dashed line labelled (via a leader from below) 'Selectively permeable membrane'. Both chambers are filled with teal/cyan shading representing water, with small maroon/purple dots representing dissolved 'Solute molecule' (leader line to a dot in chamber A) and the teal background itself labelled 'Water' (leader line). Chamber B visibly contains many more solute dots than chamber A (denser, more concentrated solution on the right), illustrating the question set t …
What this figure shows. Two side-by-side glass-beaker setups, each showing a thistle (inverted bulb-top) funnel standing in a beaker of pale-blue water, with a bulging membrane (drawn as a scalloped/wavy disc, labelled 'Membrane') sealing the wide mouth of the funnel where it sits in the beaker, and the funnel's narrow stem filled with pale-blue 'Sucrose solution' with small dots (sucrose molecules) inside it. Panel (a) (left, marked below): curved upward arrows at the membrane and a vertical double-headed arrow up the funnel stem show water diffusing in and the solution level rising; labelled 'water' at the membrane. Panel (b) (right, marked below): a metal rod/plunger is inserted down the funnel stem from the top with a downward arrow labelled 'Pressure', pushing against the risen solution to stop further water entry; the same 'Sucrose solution' and 'Membrane' labels point to the corresponding parts. …