🔹 Water Potential of Pure Water

This is important:

Pure water (meaning no solutes like salt or sugar are dissolved in it) has the highest possible water potential.

👉 By definition, its water potential is:

• Ψw = 0 (zero)

Any solution that has something dissolved in it, like sugar or salt, will always have:

• Ψw < 0 (a negative value)

So:

• Pure water = Ψw = 0
• Saltwater or sugary water = Ψw = – something (negative)

🧠 Think of it this way: The more stuff (solute) you add to water, the less free the water molecules are, so the lower the water potential becomes.

🧪 Definition of Osmosis

This is a key concept in biology. Osmosis is defined as:

The movement of water molecules from an area of higher water potential (Ψw) to an area of lower water potential (Ψw) through a partially permeable membrane.

Let’s break this down:

🧵 What is a Partially Permeable Membrane?

It’s like a filter — a thin barrier that only allows certain molecules (like water) to pass through but blocks larger molecules (like sugar or salt).

📌 Example: The cell membrane of a plant cell is partially permeable — it allows water in or out but blocks other substances.