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### Course: AP®︎/College Biology>Unit 2

Lesson 7: Mechanisms of transport: tonicity and osmoregulation

# Water potential example

Example calculating the water potential of potato squares based on placing them in various concentrations of sucrose solutions.

## Want to join the conversation?

• That lesson comes out of absolutely nowhere.
Perhaps the course was updated and it all got taken out except for that practical question part?

On the biology course up to this point 'water potential' has never even been mentioned and now suddenly a video about solving a question on it and full of 'as we have talked before' when even the theme is completely new how much less all the supposedly already-known equations, symbols and concept.
• I know this comment was a year ago but, for anyone else having the same issue, I found the video explaining water potential on YouTube: https://youtu.be/O_eFNOz5WtY. Not sure why it's not in this playlist, though.
• why did he choose 0% change as the point of determining the molarity?
• Im confused as well so forgive me if I end up being wrong but I tried to think about it like this: the molarity is supposed to represent the molarity of all the potatoes. it doesn't make sense to use the molarity of a potato that changed mass, because a change in mass means that water went in or out, which means that its molarity changed in the beaker. so that means that in the beakers, all the potatoes have the same molarity because they are in the same sucrose solution, so they will go through different degrees of mass changes to gain an equilibrium of concentration in the soln. we need to find the potato with the molarity that had no mass change because that represents the molarity of all the potatoes after changing mass; in other words, after adjusting their molarities.
• If a student cut three potato pieces weighing 10.0g each and placed each in a different beaker containing different salt concentrations. Beaker A has 5% salt, Beaker B has 20% salt, and beaker C just has distilled water. The physiological concentration of the potato cell is 4.6%.
After 2 hours, the student removed one of the potato pieces form one of the beakers and weighed it.
The potato piece weighed 12.2g. How do I calculate which beaker was this piece taken from?
• there's no calculation involved here. you can immediately see that beakers A and B are more concentrated than the potato (leading to water osmosizing OUT of the cube, and losing mass), whereas beaker C is LESS concentrated (leading to water osmoizing INTO the cube). since the new mass of the cube is higher than the starting mass, it must be C
• If there was a pressure potential, would it be provided in the problem? Can I assume that problems without indication of container status are open? If not, how would pressure potential be calculated?
• How do you calculate water potential of the cytoplasm?
• The label on the y-axis of the graph says "% change in Mass of Potato Cubes." How can you have a negative % change?
• A % change indicates what percentage of the potato's mass has changed. The change can be positive or negative because the potato can gain or lose mass.