Osmosis is the net movement of water molecules from an area of higher water concentration (dilute solution) to an area of lower water concentration through a partially permeable cell membrane. In plants, osmosis drives water intake at the roots and maintains structural firmness.

Osmosis is the diffusion of water across membranes from low to high solute concentrations, an essential cellular activity. It allows cells to use water to maintain cellular integrity or to adapt to changes in the solute composition in the extracellular environment. Osmosis relies on the transport of water by aquaporins. The passive diffusion of water molecules can be demonstrated with artificial (e.g., dialysis) membranes. If solute concentrations are higher on one side of the membrane, free water will cross the membrane to equalize the solute concentrations on both sides of the membrane.

In effect, water movement is away from the side of a membrane where the free water concentration is higher (i.e., where the concentration of solute is lower) and goes to the side where the concentration of free water is lower (i.e., where the concentration of solute is higher).

In hypotonic solutions, water enters plant cells, moving into the cytosol and then into water vacuoles called tonoplasts. This results in higher osmotic pressure (water pressure) in the tonoplasts. The expanding tonoplast creates turgor pressure, compressing the cytosol against the cell wall. Rather than bursting, the cells and therefore the plant tissues stiffen and become turgid. Since water cannot enter plant cells indefinitely, water stops entering the cells when the osmotic pressure outside the cells and the turgor pressure inside the cells are at equilibrium. Turgor pressure stiffens the leaves and stems that then become brittle and easily snapped or broken.

In hypertonic medium, plant cells lose water. The resulting shrinkage of the plasma membrane away from the cell walls is called plasmolysis, in which bits of plasma membrane remain tightly attached to the plant cell wall at several points. You may have seen under-watered plants with floppy or droopy stems and leaves. These have become flaccid due to loss of water and thus the loss of turgor pressure needed to keep leaves and stems upright. Formally, osmotic or turgor pressure is defined as the force per unit area required to prevent the passage of water across a semipermeable membrane from a hypotonic to a hypertonic solution.

Plants need an energy source to grow. In seeds and bulbs, food is stored in polymers (such as starch) that are converted by metabolic processes into sucrose for newly-developing plants. Once green shoots and leaves begin to grow, plants can produce their own food by photosynthesis. The products of photosynthesis are called photosynthates, which are usually in the form of simple sugars such as sucrose.

Sources are the structures that produce photosynthates for the growing plant. The sugars produced in the sources, such as leaves, must be delivered to growing parts of the plant. These sugars are transported through the plant via the phloem in a process called translocation. The points of sugar delivery, such as roots, young shoots, and developing seeds, are called sinks. Seeds, tubers, and bulbs can be either a source or a sink, depending on the plant’s stage of development and the season.

The products from the source are usually translocated to the nearest sink through the phloem. For example, photosynthates produced in the upper leaves will travel upward to the growing shoot tip, while photosynthates in the lower leaves will travel downward to the roots. Intermediate leaves will send products in both directions. The multidirectional flow of phloem contrasts the flow of xylem, which is always unidirectional (soil to leaf to atmosphere). However, the pattern of photosynthate flow changes as the plant grows and develops. Photosynthates are directed primarily to the roots during early development, to shoots and leaves during vegetative growth, and to seeds and fruits during reproductive development. They are also directed to tubers for storage.

Individual plant cells respond to changes in solute concentrations. These changes are rapidly communicated to adjacent cells through plasmodesmata. These structures connect the plasma membranes of adjacent cells through their cell walls, allowing rapid, direct sharing of physical and chemical information. In this way, the effects on osmotic pressure in a few cells—effects created by changes in water availability—are transmitted to adjacent cells, affecting turgor pressure in those cells and, ultimately, in plant tissues.

Plant life depends on water. Plant cells require a continual supply of water for use in photosynthesis, to provide hydrogen to reduce CO2 to glucose. Photosynthesis, as well as the loss of excess water from plant tissues (especially leaves) by transpiration, lowers cellular osmotic pressure. As water moves up from the roots to replace water used and lost by leaf cells, the osmotic pressure drops in the fine root-hair cells (with their high surface area). This draws water into the root cells by osmosis. Thus, osmotic pressure is the main force driving water into plants and, defying gravity, moving it up from the roots to the rest of the plant.