The key difference between water potential and osmotic potential is that water potential is the measure of the concentration of free water molecules while the osmotic potential is the measure of the tendency of a solution to withdraw water from pure water through a semi-permeable membrane via osmosis.
Water potential and solute potential or osmotic potential are two measures of the potential energy of water. Water potential describes how freely water molecules can move in a particular environment or system. Osmotic potential describes the potential energy of water molecules when there are solute molecules in the solution. Solute molecules attract water molecules and restrict their freedom to move. Both water potential and osmotic potential are measured in Mpa. Water potential can be zero or a negative value, but it cannot be a positive value. Moreover, pure water has zero water potential and zero osmotic potential.
CONTENTS
1. Overview and Key Difference
2. What is Water Potential
3. What is Osmotic Potential
4. Similarities Between Water Potential and Osmotic Potential
5. Side by Side Comparison – Water Potential vs Osmotic Potential in Tabular Form
6. Summary
What is Water Potential?
Water potential is a measure of the concentration of free water molecules. Free water molecules are the water molecules that are free to move. Therefore, it is the potential energy in water. The Greek letter ψ (psi) is used to denote water potential, and it is measured from the unit of pressure: megapascals (MPa). It is calculated using the equation of Ψ = Ψs + Ψp + Ψg + Ψm. Ψs stands for solute potential, Ψp for pressure potential, Ψg for gravitational potential, and Ψm for the matric potential.
Figure 01: Water Potential
Pure water has zero water potential. Hence, water potential is the difference between the potential in a given water sample and pure water. A given sample that contains solutes has a negative water potential. When there are lots of solute molecules, water molecules are less free to move; hence, there is negative potential energy. When the concentration of the solutes in a sample is higher, the movements of water molecules are lesser in a solution. Water generally moves from high water potential to the low water potential. Water potential does not take a positive value.
In plants, water potential is important in moving water to leaves in order to carry out photosynthesis. Moreover, water potential is needed in order to move water from the soil to the top of the plants.
What is Osmotic Potential?
Osmotic potential, also known as solute potential, is a component of water potential. It is a measure of water potential for the movement from a region of lower solute concentration to higher solute concentration. Osmotic potential in pure water is zero. A solution has a negative osmotic potential. The presence of solutes always makes the osmotic potential negative since water molecules will be less free to move due to solute molecules. Generally, osmotic potential decreases with the increase of solute concentration. Osmotic potential is denoted by Ψs and measured in Mpa.
Figure 02: Osmotic Potential
Osmotic potential can be defined as the potential of water molecules to move from a hypotonic solution to a hypertonic solution across a semi-permeable membrane. A hypotonic solution has a less solute concentration and more water while a hypertonic solution has a high solute concentration and less water. The difference between osmotic potential causes the movement of water from hypotonic to hypertonic solution. When the osmotic potentials of two regions or solutions are similar, there is no net movement of water molecules between them, and the solutions are called isotonic solutions.
What are the Similarities Between Water Potential and Osmotic Potential?
- Osmotic potential is a component of water potential.
- Water potential and osmotic potential are zero in pure water.
- Any solution has negative water potential and negative osmotic potential.
- They are measured in MPa.
- In the plant cells, both water potential and solute potential are negative.
What is the Difference Between Water Potential and Osmotic Potential?
Water potential is a measure of the potential energy of water in a system compared to pure water while the osmotic potential is the potential of water molecules to move from a hypotonic solution to a hypertonic solution across a semi-permeable membrane via osmosis. Thus, this is the key difference between water potential and osmotic potential. Furthermore, the water potential occurs due to the movement of free water molecules in a particular environment, while the osmotic potential occurs due to dissolved solutes.
Below table summarizes the differences between water potential and osmotic potential.
Summary – Water Potential vs Osmotic Potential
Water potential is the measure of the potential energy in the water while the osmotic potential is the part of the water potential that results from the presence of solute particles. Therefore, the osmotic potential is a result of dissolved solutes. Water potential (Ψ) is equal to pressure potential (Ψp) + solute or osmotic potential (Ψs). Osmotic potential is one of the two components of water potential. Water potential in pure water is zero. Similarly, osmotic potential in pure water is zero. Both water potential and osmotic potential become negative when solutes are dissolved in water. Thus, this summarizes the difference between water potential and osmotic potential.
Reference:
1. “Water Potential.” Biology Dictionary, 29 Apr. 2017, Available here.
2. “Transport of Water and Solutes in Plants.” Lumen – Boundless Biology, Available here.
Image Courtesy:
1. “Water potential updated” (CC BY-SA 3.0) via Commons Wikimedia
2. “Solute Potential” By Kade Kneeland – Own work (CC BY-SA 4.0) via Commons Wikimedia