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:

1. The more solute added, the lower the water potential.
2. The water potential of a solution must always be less than 0.
3. 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:

1. 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.

2. 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.