Osmosis and plant tissues in solutions

Last time

Before we start: last time, in Diffusion, water as a solvent and active transport, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Why limp lettuce crisps up

Put limp lettuce in cold water and an hour later it is crisp again. Water has moved into its cells by osmosis. Osmosis is just diffusion of water, through a partially permeable membrane.

Osmosis

Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential, through a partially permeable membrane. A dilute solution has a high water potential. A concentrated solution has a low water potential.

Watch osmosis

Here the membrane lets water molecules through but not the larger sugar molecules. Water moves from the dilute side, which has more water, to the concentrated side.

Partially permeable

A partially permeable membrane lets small molecules such as water through, but not larger ones such as sugar. Cell membranes are partially permeable. The cell wall is NOT: it lets almost everything through. Visking tubing is a man-made partially permeable membrane used in experiments.

Animal cells in solutions

In pure water, water enters a red blood cell by osmosis. It swells and bursts, because it has no wall. In a concentrated salt solution, water leaves the cell. It shrinks and becomes crinkled. This is why the water potential of blood plasma must be kept constant.

Plant cells in solutions

In pure water a plant cell takes in water and swells, but the wall stops it bursting: it is turgid. In a concentrated solution it loses water and becomes flaccid, and the membrane may pull away from the wall: plasmolysis.

The three plant words

Turgid: the cell is full of water, and the vacuole pushes the membrane against the wall. This is turgor pressure. Flaccid: the cell has lost water and is soft. The plant wilts. Plasmolysed: so much water is lost that the membrane pulls away from the cell wall.

Why turgor matters

Turgid cells press against each other, which supports the stems and leaves of non-woody plants. When a plant loses more water than it takes in, its cells become flaccid and it wilts.

The potato experiment

Potato cylinders are left in sugar solutions of different concentrations. Where the length goes up, water went in: the solution was more dilute than the cells. Where it goes down, water came out. Where the change is zero, the water potential of the solution equals the cells', so there is no net movement.

Percentage change

Percentage change equals the change, divided by the starting value, times one hundred. A cylinder grows from fifty to fifty-two millimetres: two divided by fifty, times one hundred, is plus four percent. A cylinder shrinks from fifty to forty-seven: minus three divided by fifty, times one hundred, is minus six percent.