Plants rely on the nutrients and water in soil to survive. In order for water uptake to occur, plant cells undergo a process called osmosis. Osmosis is the spontaneous movement of a solvent (water) through a cellular membrane. This is a special kind of diffusion that moves water molecules from a place of higher concentration to a place of lower concentration to create a stable and equal cellular environment. The process of osmosis is kind of like squeezing the middle of a water balloon. When you squeeze right in the middle the water displaces to either side equally. If you squeeze on one end all of the water (and weight) goes to one side or the other. Osmosis seeks to create a balance between the two sides of the water balloon like if you were to squeeze it in the center. Osmosis continues until there is an equal pressure of fluid on either side of the membrane.
The water creates a pressure that makes the balloon expand. In plants this pressure is called turgor pressure, or the pressure that pushes the cell membrane against the plasma wall to maintain the cell’s shape. Turgor pressure is affected by osmotic pressure, or the pressure differentials that cause osmosis to occur.
If one side of the membrane has a higher pressure, it will cause the other side of the cell to have low pressure which equals a not-well-supported plant structure. This difference in concentration is an osmotic pressure differential. The fuller the vacuoles, cellular sacs that hold fluid like water, the healthier the plant is and the more alive the plant looks. This also indicates successful and ongoing osmosis to make sure all of the cells have equal volume and pressure.
Osmosis is a vital function to the growth and stability of plant life. Without osmosis, photosynthesis would never occur and plants would wilt and die. A wilted plant looks wilted because the vacuoles of the cells do not have proper amounts of water. Although osmosis is still occurring, the lack of volume of water in the plant causes all of the cell walls to lose their turgor, and thus the plant loses its upright and healthy state. Osmosis distributes water through selectively permeable membranes to maintain this proper volume and pressure of all plant cells. Plant cell walls are incredibly tough and rigid which is necessary to uphold the integrity of the cell. It’s when there is adequate water that the pressure from the water can become too high in some places and through osmosis the water moves to a place of lower pressure and concentration.
Osmosis is especially crucial to photosynthesis. Photosynthesis is the process of converting solar energy into chemical energy. Basically, the plant uses the sun to create proteins, sugars and lipids that in turn become energy for the plant’s survival. Photosynthesis primarily occurs on plant leaves and requires a combination of carbon dioxide, sunlight and water to be successful. On every plant leaf there are many guard cells that literally guard the stomata. A stoma is a plant pore that lives on the plant leaf surface. Stomata are responsible for plant gas exchange which enables the process of photosynthesis.
During osmosis, the guard cells swell with water and the pressure triggers the stomata to open. When the stomata open they suck in carbon dioxide from the air which is then used in combination with the water from the roots and the sunlight absorbed in chlorophyll to produce plant energy. Plant energy is then used to feed and nurture the plant.
The roots of a plant are the plants lifeline. They reach out into the soil to establish an avenue for the transport of nutrients and water to the rest of the plant. The roots absorb water through osmosis. If the water concentration outside of the plant roots is greater than that of the water concentration in the roots osmosis occurs. The difference in pressure triggers the plant to bring in water through the root cell walls to create a pressure balance and thus providing necessary water to the plant. When a plant has adequate water uptake, it will flourish and grow.
a. Effects of Osmosis:
If the water concentration of the cells cytoplasm is lower than that of the medium (i.e. the medium is a hypotonic solution) surrounding the cell then osmosis will result in the cell gaining water. The water molecules are free to pass across the cell membrane in both directions, but more water molecules will enter the cell than will diffuse out with the result that water enters the cell, which will then swell up and could possibly burst.
If the water concentration inside the cell is the same as that in the surrounding medium (i.e. the medium is a isotonic solution) there will exist a dynamic equilibrium between the number of molecules of water entering and leaving the cell and so the cell will retain its original size.
b. Types of Osmosis
i. Reverse Osmosis
The osmotic pressure defines at what point a differential gradient between high and low solute triggers osmosis. In reverse osmosis, increased volumetric or atmospheric pressure will "push" the higher solute particles past the membrane, overcoming the gap that may exist when the osmotic pressure won't allow diffusion through the membrane. This process is often used to filter water of impurities when their concentrations are too low for regular osmosis, but cleaner water is still needed, as in desalination and pharmaceutical operations.
ii. Forward Osmosis
Unlike reverse osmosis, which goes from high-to-low concentrations, forward osmosis forces low solute particles to move to a higher solute in essence, the opposite of the normal osmotic process. Whereas reverse osmosis "pushes" particles, forward osmosis "draws" them in, resulting in cleaner water.