Cambridge Syllabus Notes

3.2 Osmosis: Water Potential & Water Movement in Organisms 🌱

What is Water Potential?

Water potential, denoted by the Greek letter Ψ, tells us how water will move. It is the sum of two components:

Solute potential (Ψ_s) – the effect of dissolved particles. More solutes = lower Ψ_s.
Pressure potential (Ψ_p) – the physical pressure on the water. Positive pressure pushes water out; negative pressure pulls it in.

Mathematically: $Ψ = Ψ_s + Ψ_p$.

Why is Water Potential Important?

Think of water potential like a traffic system. 🚗 If the road (cell membrane) is open, water will flow from the “high traffic” side (higher Ψ) to the “low traffic” side (lower Ψ) until the traffic levels equalise. This movement is called osmosis.

Analogy: Imagine a crowded room (high Ψ) next to an empty room (low Ψ). People (water molecules) will naturally move to the empty room until both rooms have the same number of people. That’s osmosis in a nutshell!

Osmosis in Plants 🌿

- Uptake of water occurs when the soil’s water potential (usually high) is greater than the plant’s root cells. Water moves into the roots, then up the stem via the xylem, driven by negative pressure (tension) created by transpiration. - Loss of water happens when the air around leaves has a lower water potential than the leaf cells. Water evaporates from stomata, pulling more water from the plant.

Location	Water Potential (Ψ)	Resulting Movement
Soil	High (e.g., $-0.2$ MPa)	Water moves into roots (uptake)
Leaf air	Low (e.g., $-1.5$ MPa)	Water evaporates from leaves (loss)

Osmosis in Animals 🐾

- In freshwater fish, the surrounding water has a higher Ψ than their body fluids. Water rushes into the fish, so they excrete large volumes of dilute urine to maintain balance. - In marine fish, the surrounding water has a lower Ψ. They drink seawater and actively excrete excess salts through specialized cells, while retaining water via concentrated urine.

Exam Tip: When answering questions about water movement, always:

Identify the direction of water flow (from high to low Ψ).
Explain the role of solute concentration and pressure.
Use the term osmotic pressure when discussing the force that drives water movement.
Remember to mention the turgor pressure in plant cells and how it keeps cells firm.

Key Take‑Away Points

Water potential is a measure of the tendency of water to move.
Osmosis is the passive movement of water across a semi‑permeable membrane.
Plants rely on osmosis for water uptake and to drive transpiration.
Animals manage water balance by adjusting solute concentrations and excreting urine.
Understanding Ψ helps predict how organisms respond to drought, salinity, and other environmental stresses.

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