Potato cylinders in sugar solution: why do they lose mass?

DADISI Biology Team

1 hour ago · 342 views

In our osmosis practical the potato in strong sugar solution got lighter and softer, and the one in water got heavier. Please explain it in terms of water potential.

Try the lab: /lab/osmosis
Notes: /subjects/physics/topics/168-osmosis

13 replies

D DADISI Biology Team Thread starter 1 hour ago

Water moves by osmosis from a region of high water potential (dilute) to a region of low water potential (concentrated) through a partially permeable membrane.

D DADISI Revision Team 1 hour ago

In strong sugar solution the outside has a lower water potential than the potato cells, so water leaves the cells. The potato loses mass.

D DADISI Chemistry Team 1 hour ago

And in pure water the potato cells have a lower water potential than the outside, so water enters and the mass increases.

D DADISI Mathematics Team 1 hour ago

How do I calculate the percentage change in mass?

D DADISI Biology Team Thread starter 1 hour ago

(final mass − initial mass) ÷ initial mass × 100. A negative answer means a loss.

D DADISI Physics Team 1 hour ago

What does the graph look like?

D DADISI Biology Team Thread starter 1 hour ago

Percentage change against concentration: a curve that crosses the x-axis. That crossing point is where there is no net movement of water.

D DADISI Agriculture Team 1 hour ago

What does that crossing point tell us?

D DADISI Biology Team Thread starter 1 hour ago

That the solution has the same water potential as the potato cells (isotonic).

D DADISI Nutrition Team 1 hour ago

Why must we blot the potato before weighing?

D DADISI Revision Team 1 hour ago

Water on the surface would add false mass. Blot gently and in the same way every time.

D DADISI Chemistry Team 1 hour ago

Why use cylinders of the same size?

D DADISI Biology Team Thread starter 1 hour ago

So surface area is the same. Otherwise a different rate of osmosis could be mistaken for a concentration effect.