Cell Membrane And Transport Session III

Water potential is a concept that helps us understand and describe the movement of water in plants, organisms, and their surrounding environments. It is a measure of the potential energy stored in water molecules. It indicates the direction and rate of water movement. Water potential is denoted by the Greek symbol psi Ψ.

There are three primary factors that contribute to the determination of water potential. First is solute potential. Solute potential refers to the effect of dissolved solutes on the movement of water. To understand solute potential, we need to revisit the process of osmosis. As we know, osmosis is the movement of water molecules through semipermeable membrane from an area of lower solute concentration to an area of higher solute concentration. Water moves from area of higher water potential to area of lower water potential.

When solutes, such as salts, sugars, or ions, are dissolved in water, they take up space and interact with water molecules. These solute molecules attract water molecules through hydrogen bonding. This reduces the potential energy of the water molecules and lowers the overall water potential. So the solutions that have low concentration of salts have high water potential. And the solutions that have high concentration of salts have low water potential.

Solute potential is represented by the symbol Ψs. It is expressed in units of pressure, such as pascals. Solute potential and water potential are inversely related to each other. As the concentration of solutes rises in a solution, the solute potential rises. As a result water potential decreases.

Lets take an example to understand the water potential. When fruits or vegetables are preserved in a high concentration of solutes, such as sugar or salt solutions, it affects the water potential inside the food cells. The high solute concentration in the preserving solution creates a lower water potential compared to the water potential inside the cells of the fruits or vegetables. As a result, water from the cells of the food items moves out through osmosis, seeking to equalize the water potential between the food cells and the preserving solution. This outward movement of water helps to dehydrate the food which inhibits the growth of microorganisms. It preserves the food for longer periods.

Pressure potential is another important component of water potential. It refers to the physical pressure exerted by water on its surroundings. Pressure potential is denoted by Ψp. Pressure potential can be either positive or negative, depending on the conditions. Positive pressure potential occurs when water is under pressure, such as in a plant cell that has taken up water and expanded. Cell becomes turgid.

One common example of positive pressure potential is the turgor pressure in plant cells. When plant cells absorb water through osmosis, the cell wall resists the expansion of the cell. As a result internal pressure is created. This internal pressure is known as turgor pressure. Turgor pressure is essential for maintaining the rigidity and shape of plant tissues. It allows them to stand upright and support the overall structure of the plant.

Negative pressure potential, on the other hand, occurs when water is under tension or being pulled. This negative pressure potential is often referred to as tension or suction. Cell shrinks as a result. When a plant cell is placed in a solution that has high concentration of solutes, the water moves out of the cell. As a result negative pressure potential is created inside the cell.

A hypertonic solution is a solution that has a higher concentration of solutes compared to another solution. In other words, it has a lower water potential compared to the other solution. When a cell that has lower concentration of solutes is placed in hypertonic solution, the cell loses water and shrinks. This is because the water potential is greater inside the cell as compared to the solution in which it is placed. As a result water moves from higher water potential inside the cell to lower water potential outside the cell.

Plasmolysis is a phenomenon that occurs when a plant cell is placed in a hypertonic solution, causing water to move out of the cell. As a result, the cell membrane detaches from the cell wall, and the cytoplasm shrinks away from the cell wall. When a plant cell is in a hypertonic solution, the higher concentration of solutes in the solution creates a lower water potential outside the cell compared to the inside. As water moves out of the cell to equalize the concentration of solutes, the protoplast shrinks and pulls away from the cell wall. This process is known as plasmolysis.

A hypotonic solution is a solution that has a lower concentration of solutes compared to another solution. In other words, it has a higher water potential compared to the other solution. When a cell is placed in hypotonic solution, the cell absorbs water and becomes turgid. This is because the water potential is greater outside the cell as compared to inside of cell. As a result water moves from higher water potential outside the cell to lower water potential inside the cell.

Hypotonic solutions have various biological implications. In biological systems, Hypotonic solutions can affect the behavior and function of cells. For example, if our body cells are exposed to a hypotonic solution, water will enter the cells, causing them to swell. In plant cells, a hypotonic solution in the soil can facilitate water uptake by the roots. This ensures proper hydration and maintains cell turgidity.

An isotonic solution refers to a solution that has the same concentration of solutes as another solution or a reference solution. When a cell is placed in an isotonic solution, there is no net gain or loss of water. The cell maintains its normal shape and volume. This is because the concentration of solutes is same inside and outside of cell. In other words, the water potential inside the cell is equal to the water potential outside the cell. The cell becomes flaccid.

There is another term that we can use to describe the movement of water into and out of a cell. As we know, water moves from inside of cell to outside when placed in a hypertonic solution. This movement of water from inside of cell to outside is called Exosmosis. Can you tell whether the concentration of solutes is high or low inside the cell as compared to outside during Exosmosis?.

The movement of water from outside of cell to inside is called Endosmosis. This occurs when cell is placed in a hypotonic solution. Can you tell whether the concentration of solutes is high or low inside the cell as compared to outside during Endosmosis?.