The diagram shows a normal red blood cell and a shrunken red blood cell, both of which are in salt water. In which direction did the water move to make the red blood cell shrink? Why did water move in that direction?

Water moved from inside the red blood cell into the salt water.

This is because of the osmotic difference between the salt solution and the red blood cell. This means that there is difference in the solute (salt) concentration inside the red blood cell and the salt solution.

Explanation: The salt concentration in the solution is higher than the salt concentration inside the red blood cell, that is, the red blood cell has more water concentration that the salt solution, therefore there will be movement of water from the inside of the red blood cell into the salt solution thereby causing the red blood cell to reduce in size. The movement of water from the red blood cell into the salt solution is to create a balance between the water concentration in the two environments, hence the movement of water from an area of high water concentration to an area of low solvent concentration across the selectively permeable membrane of the red blood cell.