Understanding Osmosis and Water Potential
Osmosis is a fundamental concept in A Level Biology, particularly when studying transport across cell membranes. This page delves into the definitions, principles, and key components of osmosis and water potential.
Definition: Osmosis is the passage of water from a region where it has a higher water potential to a region where it has a lower water potential through a semi-permeable membrane.
To fully grasp osmosis, it's crucial to understand the following terms:
Vocabulary:
- Solute: Any substance dissolved in a solvent
- Solvent: The liquid in which a solute is dissolved
- Solution: Formed from the solute and solvent together
Water Potential (ψ)
Water potential is a critical concept in understanding osmosis:
- Represented by the Greek letter psi (ψ)
- Measured in units of pressure, typically kilo Pascals (kPa)
- Represents the pressure created by water molecules
Highlight: Pure water has a water potential of 0 (at standard temperature and pressure).
Key principles of water potential:
- The more solute added, the lower the water potential.
- The water potential of a solution must always be less than 0.
- Water will move by osmosis from higher water potential to lower water potential.
The Process of Osmosis
Osmosis occurs when:
- Water molecules diffuse from a solution of high concentration to low concentration.
- This movement happens down a water potential gradient.
- It occurs across a selectively permeable plasma membrane.
- The process continues until a dynamic equilibrium is established, and there is no net movement of water.
Example: In a plant cell placed in pure water, water molecules will move into the cell through osmosis because the cell has a lower water potential due to dissolved solutes inside it.
Hypotonic vs. Hypertonic Solutions
Understanding the difference between hypotonic and hypertonic solutions is crucial for grasping osmosis:
-
Hypotonic Solution:
- Water has more freedom as it has no attractions to solutes.
- Cells placed in hypotonic solutions tend to swell due to water influx.
-
Hypertonic Solution:
- Water has less freedom as it attracts to solutes (ions or polar molecules).
- Cells placed in hypertonic solutions tend to shrink due to water loss.
Vocabulary: Isotonic solution - A solution with the same solute concentration as the cell, resulting in no net movement of water.
Water Potential Calculation
The water potential of a solution can be calculated using the formula:
Definition: Solute potential (ψs) = -iCRT
Where:
- i = ionization constant
- C = molar concentration of solute
- R = gas constant
- T = temperature in Kelvin
This formula is essential for quantitative analysis in A Level Biology osmosis problems.
Highlight: Water will always flow from high potential to low potential across a semi-permeable membrane.
Understanding these concepts is crucial for answering Transport across cell membranes A Level Biology AQA exam Questions and tackling Water potential calculation in biology problems.
