AP Biology · Unit 2 · Topic 2.7
Water potential decides it, and you can calculate it. Set the solutions, work out the numbers, and watch the cell swell or shrink.
Learning objectives 2.7.A Explain how concentration gradients affect the movement of molecules across membranes. 2.7.B Explain how osmoregulatory mechanisms contribute to the health and survival of organisms.
Narrated walkthrough of the “Put a cell in it” tab. Captions are on by default.
Set the solution and the tool substitutes into the equation in front of you, one factor at a time. Cover the answer, work it out yourself, then check.
Set the concentration to 0 and the pressure to 0. That is pure water in an open container, and its water potential is exactly zero — the reference point everything else is measured against. Every solution you can make from here is more negative.
R is 0.0831 L bar/mol K and it never changes. If your answer comes out in the wrong order of magnitude, the temperature is the first thing to check.
Set the solute concentration inside the cell and in the solution around it. The tool works out both water potentials and then tells you the direction — and you can check it against the tonicity terms. Both potentials are worked out for a non-ionizing solute (i = 1) at 22 °C, like the sucrose on the first tab, with no pressure potential.
The tonicity terms and the water potential equation are two ways of saying the same thing, and they never disagree. Tonicity compares solute concentrations; water potential compares Ψ values. More solute always means a lower water potential, so the two predictions line up every time.
One line per solute, all at the same temperature. The slope of each line is −iRT, so a solute that breaks into more particles gives a steeper line.
Osmosis does not stop because an organism would rather it did. Each of these lives somewhere that pushes water one way, and each has a way of coping.
Answer these in your own words before opening the answers. Show your working for the calculation.
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