Investigation 4: Diffusion and Osmosis

Water crosses a selectively permeable membrane by osmosis, moving toward the side with more dissolved solute, because that side has relatively less free water. A dialysis bag filled with concentrated sugar solution and dropped into plain water gains mass: water rushes in to dilute the sugar inside, even though the sugar itself never leaves the bag. The membrane's pores are small enough to block sucrose but not water, so it is selectively permeable, not fully permeable.

To find an unknown solute concentration inside living tissue like a potato, you soak identical potato cores in a range of known sucrose concentrations, weigh each core before and after, and calculate percent mass change: (final mass - initial mass) / initial mass x 100. Cores in solutions weaker than the potato's internal concentration gain mass as water enters; cores in stronger solutions lose mass as water leaves. Plotting percent mass change against external sucrose concentration gives a line that crosses zero at the one concentration where no net water movement happens in either direction — that point equals the potato's internal solute concentration, since at that concentration the solution is isotonic to the tissue.

This investigation connects diffusion and osmosis to the broader science practices of this unit: designing a controlled experiment with one manipulated variable (sucrose concentration), collecting quantitative data, graphing it, and extracting a value algebraically from the line. The same x-intercept logic — set y to zero, solve for x — reappears anywhere a linear model estimates an unknown by finding where an effect disappears.