Most people know that plants need water to stay alive, but figuring out how often to water them can be tricky for botanists and plant enthusiasts alike. One simple trick is to mark the calendar when you water your plant, then wait until it begins to wilt to calculate how long to wait between watering sessions. The ideal timing is just before the plant would wilt.

The science behind why this works? Cell membranes and osmosis.

All cells need to move molecules into and out of the cell. Some of the mechanisms to accomplish this require the cell to use energy, such as setting up pumps in the cell membrane to transport molecules.

Diffusion is a way to move some molecules across a membrane for free – from areas of higher concentration of solutes to lower concentration – without requiring the cell to spend valuable energy. Osmosis is a lot like diffusion, but instead of moving the molecules, or solute, it moves the solvent, which is pure water.

Semipermeable membranes, like those found in animal and plant cells, separate the interior of the cell from what is outside the cell. The process of osmosis moves water molecules across the semipermeable membrane when there is a concentration gradient such that there are different concentrations of solute on each side of the biological membrane.

Osmotic pressure will simply move the water molecules across the membrane until the solute (the molecule dissolved in the water) reaches equilibrium. At this point, the amount of solute and solvent (water) are equal on each side of the membrane.

For example, consider a solution of salt water where salt is dissolved in water across a membrane. If there is a higher concentration of salt on one side of the membrane, the water moves from the less salty side across the membrane to the saltier side until both sides of the membrane are equally salty.

The process of osmosis can cause cells to shrink or expand (or stay the same) with the movement of the water molecules. Osmosis affects cells differently depending on the type of solution in question.

In the case of a hypertonic solution, there is more solute outside the cell than inside the cell. To equalize this, water molecules leave the cell, moving toward the side of the membrane with a higher solute concentration. This water loss causes the cell to shrink.

If the solution is a hypotonic solution, there is more solute inside the cell than outside the cell. To find equilibrium, water molecules move into the cell, causing the cell to expand as the water volume inside the cell increases.

An isotonic solution has the same amount of solute on both sides of the cell membrane, so this cell is already at equilibrium. It will remain stable, neither shrinking nor swelling.

A good model for understanding how the process of osmosis affects human cells is the red blood cell. The body works hard to maintain isotonic conditions so that your red blood cells stay at equilibrium, neither shrinking nor swelling.

Under highly hypertonic conditions, the red blood cells shrink, which may kill the red blood cell. Highly hypotonic conditions are no better since the red blood cells can swell until they burst, which is called lysis.

In a plant cell, which has a rigid cell wall outside the cell membrane, osmosis will draw water into the cell only to a certain point. The plant stores this water in its central vacuole. The plant’s internal pressure, called turgor pressure, prevents too much water from entering the cell for storage in the vacuole.

Remember that plant you needed to water? It wilts without enough watering because the plant loses turgor pressure.