Question

The cell \(A\) has an osmotic potential of \(-20\) bars and a pressure potential of \(+6\) bars. What will be its water potential? (a) \(-14\) bars (b) \(+14\) bars (c) \(-20\) bars (d) \(-26\) bars

Short answer

-14 bars

Step-by-step solution

Understand Water Potential

Water potential (\Psi) in a plant cell is calculated as the sum of osmotic potential (\text{OP}) and pressure potential (\text{PP}). The formula is \[ \Psi = \text{OP} + \text{PP} \]

Identify the Osmotic Potential and Pressure Potential values

From the given data, the osmotic potential (\text{OP}) is \[ \text{OP} = -20 \text{ bars} \] and the pressure potential (\text{PP}) is \[ \text{PP} = +6 \text{ bars} \].

Calculate Water Potential

Insert the given values into the water potential formula \[ \Psi = \text{OP} + \text{PP} \] \[ \Psi = (-20 \text{ bars}) + (+6 \text{ bars}) \] \[ \Psi = -14 \text{ bars} \]

Key concepts

Osmotic Potential

Osmotic potential, also known as solute potential, is a measure of the tendency of water to move by osmosis across a semi-permeable membrane. More simply, it represents how the solute concentration in a solution can influence the movement of water. In plant cells, which are often surrounded by solutions, osmotic potential can play a critical role in water uptake and retention.

Factors Affecting Osmotic Potential

• Solute concentration: A solution with a high concentration of solutes (like salts or sugars) will have a lower (more negative) osmotic potential.
• Temperature: Higher temperatures can increase the kinetic energy of water molecules, potentially impacting osmotic potential.
• Compartmentalization: Within plant cells, different compartments like the vacuole or cytoplasm can have varying osmotic potentials.

It's measured in units of pressure, such as bars or megapascals (MPa), and it's often a negative value because it works against the pure water potential (which is zero). When discussing a plant cell, a more negative osmotic potential is indicative of a higher solute concentration, and thus a greater potential for water to move into the cell.

Pressure Potential

Pressure potential is the physical pressure component of water potential. In the context of plant cells, pressure potential usually refers to the turgor pressure exerted by the cell wall against the cell membrane. This pressure is a result of water pushing outward on the cell wall and is a positive force that contributes to the overall water potential in the cell.

Understanding Turgor Pressure

• Cell Wall Rigidity: The rigidity of the cell wall influences the amount of pressure it can exert back against the water within the cell.
• Water influx: As water enters a cell due to osmosis, it can lead to an increase in pressure potential as the cell swells and the membrane pushes against the rigid cell wall.
• Wilting: A loss of turgor pressure, often due to water loss or excessive solute outside the cell, can result in plant wilting, where the leaves droop as a result of reduced pressure potential.

Pressure potential can vary from positive to zero, and in certain conditions such as plasmolysis, it can even be negative when the cell membrane pulls away from the cell wall during extreme water loss. It's a critical factor for plant structure and growth, helping to maintain the stiffness of plant tissues and aiding in the transport of water within the plant.

Plant Cell Water Potential

Water potential in plant cells is a crucial concept that integrates both osmotic potential and pressure potential. It's defined as the potential energy of water in a particular system compared to pure water, and it determines the direction of water movement. The total water potential within a plant cell dictates whether water will enter the cell, leave it, or stay at equilibrium.

Significance in Plant Physiology

• Water Uptake: Water potential gradients drive the uptake of water through the roots into the plant.
• Stomatal Functioning: The opening and closing of stomata are governed in part by changes in water potential within guard cells.
• Nutrient Transport: The flow of water within the plant affects nutrient transport, as nutrients often move along with water.

Since water always moves from an area of higher water potential to lower water potential, understanding the water potential within plant cells is key for studying how plants absorb water from the soil, distribute it through their tissues, and contend with environmental stresses such as drought or saline soils.

Water Potential Calculation

Calculating water potential is a quantitative way of predicting the movement of water. It's especially important in the study of plant physiology, agriculture, and horticulture. By using the formula

\[\begin{equation}\Psi = \text{OP} + \text{PP}\end{equation}\]
we can determine the tendency of water to move into or out of plant cells or tissues.

Applying the Formula

To apply the water potential formula, one must simply insert the known values of osmotic potential (OP) and pressure potential (PP) and calculate the sum. For example, if a plant cell has an osmotic potential of -20 bars and a pressure potential of +6 bars, the calculation would be:

\[\begin{equation}\Psi = (-20 \text{ bars}) + (+6 \text{ bars})\end{equation}\]
\[\begin{equation}\Psi = -14 \text{ bars}\end{equation}\]

The resulting water potential is -14 bars, indicating that water in this cell has less potential energy compared to pure water and that water would tend to move into a region with higher (less negative) water potential under normal circumstances.

Considerations for Accuracy

• Measurement Units: Consistency in units (such as bars or MPa) is essential for the accurate calculation of water potential.
• Standard Conditions: A reference point, usually pure water at standard conditions, is needed to make meaningful comparisons.
• Environmental Variables: Temperature and atmospheric pressure can influence water potential and should be considered depending on the specificity needed.

By understanding how to calculate water potential, researchers, farmers, and gardeners can make informed decisions about irrigation practices, crop selection, and management of plant stressors.