IGCSE Biology 0610 - 3.2 Osmosis

Objective: Explain the importance of water potential and osmosis in the uptake and loss of water by organisms.

1. Introduction to Water Potential

Water potential (Ψ) is a measure of the tendency of water to move from one area to another. It is defined as the difference in water potential between a pure solvent (usually water) and a solution.

1.1 Defining Water Potential

Water potential is typically expressed in units of Pascals (Pa) or kilopascals (kPa). Pure water has a water potential of zero (Ψ = 0). Solutions with solutes have a negative water potential.

1.2 Factors Affecting Water Potential

Several factors can influence water potential:

• Solute Concentration: The more solutes in a solution, the lower the water potential.
• Pressure: Increasing pressure decreases water potential.
• Temperature: Increasing temperature generally increases water potential.

2. Osmosis: The Movement of Water

Osmosis is the net movement of water across a semi-permeable membrane from a region of high water potential to a region of low water potential.

2.1 Semi-Permeable Membranes

Semi-permeable membranes allow the passage of water molecules but restrict the passage of larger solute molecules. Cell membranes are examples of semi-permeable membranes.

2.2 Water Potential Gradient

Water moves down the water potential gradient – from areas of higher water potential (less negative) to areas of lower water potential (more negative).

2.3 Osmotic Pressure

Osmotic pressure is the pressure required to prevent the flow of water across a semi-permeable membrane. It is directly related to water potential.

3. Osmosis in Plants

Plants require water for photosynthesis, turgor pressure (which provides structural support), and nutrient transport. Osmosis plays a crucial role in the uptake and loss of water in plants.

3.1 Water Uptake by Roots

Water is absorbed by the roots from the soil via osmosis. The soil typically has a higher water potential than the root cells (due to dissolved minerals). This causes water to move into the root cells.

3.2 Turgor Pressure

The pressure exerted by the cell contents against the cell wall. Turgor pressure is maintained by the influx of water through osmosis. When turgor pressure is high, the plant cells are firm and the plant is rigid.

3.3 Water Loss by Plants (Transpiration)

Water is lost from the plant primarily through transpiration, which is the evaporation of water from the leaves. This creates a water potential gradient between the leaves (high water potential) and the atmosphere (low water potential), driving water movement out of the leaves.

3.4 Factors Affecting Transpiration Rate

Several factors influence the rate of transpiration:

• Temperature: Higher temperatures increase transpiration.
• Humidity: Low humidity increases transpiration.
• Wind: Wind increases transpiration.
• Light Intensity: Higher light intensity increases transpiration.
• Soil Water Availability: Low soil water availability decreases transpiration.

4. Osmosis in Animals

Osmosis is essential for maintaining the water balance in animal cells.

4.1 Isotonic Solutions

An isotonic solution has the same water potential as the cell. There is no net movement of water across the cell membrane.

4.2 Hypotonic Solutions

A hypotonic solution has a higher water potential than the cell. Water moves into the cell, causing it to swell and potentially burst (lysis).

4.3 Hypertonic Solutions

A hypertonic solution has a lower water potential than the cell. Water moves out of the cell, causing it to shrink (plasmolysis).

5. Summary Table

Concept	Description
Water Potential (Ψ)	Measure of water tendency to move. Zero in pure water, negative in solutions.
Osmosis	Net movement of water across a semi-permeable membrane from high to low water potential.
Turgor Pressure	Pressure of cell contents against the cell wall, maintained by osmosis.
Transpiration	Water loss from plants through evaporation from leaves.
Isotonic Solution	Same water potential as the cell; no net water movement.
Hypotonic Solution	Higher water potential than the cell; water moves into the cell.
Hypertonic Solution	Lower water potential than the cell; water moves out of the cell.

Suggested diagram: A diagram showing a plant cell in an isotonic, hypotonic, and hypertonic solution, illustrating plasmolysis and turgor pressure.