Osmosis is important to plants because it allows for water uptake, photosynthesis and general stability. Osmosis ensures that all cells and structures within a plant have correct water pressure and volume.
Osmosis is a specific form of diffusion, the movement of molecules from areas of high concentration to areas of low concentration. Osmosis is the diffusion of water molecules through semipermeable membranes. A cell membrane surrounds plant cells, just like animal cells. Plant cells also have an additional outer layer, the cell wall, that is much more rigid than the cell membrane. As water enters a plant cell it collects in fluid-containing organelles, or vacuoles. Pressure inside the cell increases as vacuoles fill with water, lending rigidity to the cell wall and the entire plant. Plants that do not receive enough water wilt because there is not enough pressure in their cells to support the structure of the plant. Conversely, cells that fill with too much water begin to burst, causing brown spots on plant leaves. Osmosis is the control by which cells maintain just the right amount of water. If the concentration of water molecules is higher outside of the cells, water flows into the plant's cells. But, if the concentration is higher inside the cells, excess water flows out. Plants absorb water from the soil through osmosis in their root cells. When water concentrations around a plant's roots are higher than inside the plant, water flows into the roots and then, through continuing osmosis, up the stem into the rest of the plant. During photosynthesis osmotic pressure in the cells of plant leaves causes openings, or stomata, in the leaves to open. Once open the stomata begin to absorb the carbon dioxide that is vital for photosynthesis to occur.