Osmosis

Water can move across a membrane that allows water molecules through but prevents some dissolved substances from crossing. Such a membrane is called partially permeable. The diffusion of water through it is called osmosis.

When the solutions on either side have different concentrations, there is a net movement of water from the more dilute solution to the more concentrated solution. Water molecules move in both directions, but more move towards the concentrated solution than in the opposite direction.

A cell membrane is partially permeable, so water can move into or out of a cell by osmosis. If the solution outside is more dilute than the solution inside, there is a net movement of water into the cell. If the solution outside is more concentrated, there is a net movement of water out of the cell.

Percentage Change in Plant Tissue Mass

A plant tissue sample may gain or lose mass. To express the change relative to its starting mass, calculate the percentage change in mass.

First subtract the initial mass from the final mass. Then divide the change by the initial mass and multiply by 100%:

A positive result means the tissue gained mass; a negative result means it lost mass. A result of zero means its mass did not change.

Comparing changes in tissue mass

For example, if a sample starts at $$5,\text{g}$$ and ends at $$4,\text{g}$$:

The sample has lost 20% of its initial mass. If another sample starts at $$10,\text{g}$$ and also loses $$1,\text{g}$$, its percentage change is $$-10%$$. Both samples lose the same mass, but the loss is a smaller proportion of the second sample’s starting mass.

Using the initial mass as the reference therefore allows changes in tissues with different starting masses to be compared.