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When we see healthy plants, we often associate their vigor with plenty of sunlight, balanced nutrients, and just the right amount of water. And while all these external factors are important to a plant’s survival, they’d be useless without osmosis, a simple biological process that allows plants to breathe and absorb water.
Osmosis, in its most basic sense, refers to a specific kind of movement. Most of the time, when we talk about osmosis, we are essentially talking about the movement of water across a semipermeable membrane, like a cell membrane.
Cell membranes have tiny openings which allow small molecules to pass through them (like water molecules). Large molecules, like salt, for example, cannot pass through this membrane without additional support from the cell.
Water, on the other hand, can pass across cell membranes passively, meaning it doesn’t cause the cell to use up any energy.
Osmosis is a type of diffusion, and water will usually move from areas of high concentration to areas of low concentration. Water molecules will move either into or out of the cell from areas where there is more water to areas where there is a lower concentration of water.
Whenever water moves, it aims to create an equilibrium. The best way to visualize this is to think about squeezing a water balloon. Wherever you squeeze the balloon, water will move in a way to create a balance and an equal amount of pressure (or water) on both sides of the membrane.
Water will also move in the direction of solutes. So, if there is an area with a higher concentration of solutes (like salt or sugar, for example), water will move in that direction.
Once we understand osmosis and what it does, it becomes a little easier to see how it might be useful to plants.
Osmosis is central to a plant’s ability to absorb water from soil via its roots.
When you water your plant, you essentially increase the concentration of water in the soil. Once the water concentration in the soil exceeds that in the plant’s roots, osmosis will occur and water will move passively from the soil into the roots.
Photosynthesis is the process by which plants are able to turn solar energy into chemical energy. In its most basic sense, plants use the sun to create proteins, sugars, and lipids which they can use to further grow, develop, and survive.
Successful photosynthesis requires 3 things: carbon dioxide, sunlight, and water. It primarily occurs on plant leaves, where there are plenty of stomata, a type of a plant pore that lives on the surface of leaves and is able to exchange gases CO₂ and O₂.
Stomata are guarded by a specific type of cells known literally as “guard cells.” These cells have vacuoles (tiny cellular sacs) that fill up with water and other fluids.
Osmosis causes guard cells to swell. When they do, they trigger the stomata to open and absorb CO₂ from the environment. Combined with the water taken up via the plant's roots and the sunlight absorbed via its leaves, the plant now has everything it needs to photosynthesize and grow.
There you have it—a simple introduction to osmosis. As you can see, this process is vital for the survival of your plants and, together with photosynthesis, is among the key processes behind your plants’ ability to grow and flourish.