Osmosis is the diffusion of water across a membrane, in fact, osmosis is just simple, passive diffusion. However, since water is so important to life and such an abundant molecule in cells, the diffusion of water has been given its own name - osmosis.
When explaining, discussing and answering questions about osmosis it is super important that you’re able to use the appropriate terminology (and understanding water potential is a fundamental expectation, since osmosis can be quantified using water potential; The short hand symbol for water potential is Ψ the Greek letter psi, pronounced “sy").
Water potential is simply the effective concentration of water which is measured in units of pressure i.e. measured in Pascals (Pa) (or usually kPa kilopascals). The rule is very simple, just like that of passive diffusion:-
“Water always "moves" from a high water potential to a lower water potential”.
The key thing to remember is that 100% pure water has a water potential (Ψ) of zero (0), and zero is the highest possible water potential. Thus, ALL solutions have a water potential (Ψ) Less than zero (0).
You cannot have a water potential (Ψ) great than zero (0).
Another term term you may come across when learning osmosis is osmotic pressure (OP).
You just need to know that the more concentrated a solution, the higher the osmotic pressure. Therefore, OP is the opposite of water potential (Ψ), meaning water will move from a low OP to a high OP.
Now that you know the difference between OP and Ψ you’ll be expected to apply your understanding of osmosis to cells. and once again use the appropriate terminology.
A Level Biology - An Overview of fundamentals of Osmosis
A Level Biology - Osmosis: Key terms you must know, understand and use when explaining Osmosis
A Level Biology - Osmosis and Aquaporins
A Level Biology - Osmosis in Plant cells - Explaining Stomatal regulation in terms of Osmosis
A Level Biology - Factors affecting the rate of Osmosis
Check Your Exam Specification
★ AQA Specification Reference: - 3.2.3 Transport across cell membranes. Movement across membranes occurs by: Students should be able to explain osmosis (explained in terms of water potential). Explain how surface area, number of channel or carrier proteins and differences in gradients of concentration or water potential affect the rate of movement across cell membranes.
★ CIE Specification Reference: - 4 Cell membranes and transport. 4.2 Movement of substances into and out of cells: a) describe and explain the processes of Osmosis. Explain the movement of water between cells and solutions with different water potentials and explain the different effects on plant and animal cells.
★ Edexcel (Biology A – Salters-Nuffield) Specification Reference: - Topic 2: Genes and Health: 2.3 Understand what is meant by osmosis in terms of the movement of free water molecules through a partially permeable membrane (consideration of water potential is not required).
★ Edexcel (Biology B) Specification Reference: - Topic 4: Exchange and Transport. 4.2 Cell transport mechanisms: Understand how passive transport is brought about by osmosis: CORE PRACTICAL 6: Determine the water potential of plant cells.
★ OCR (Biology A) Specification Reference: - Module 2: Foundations in biology: 2.1.5 Biological membranes. (e) the movement of water across membranes by osmosis and the effects that solutions of different water potential can have on plant and animal cells. Practical investigations into the effects of solutions of different water potential on plant and animal cells. Osmosis to be explained in terms of a water potential gradient across a partially permeable membrane.
★ OCR (Biology B) Specification Reference: - Module 2: Cells, chemicals for life, transport and gas exchange.
★ WJEC Specification Reference: - Core Concepts: 3. Cell membranes and transport. Overview: Cell membranes are essential in the control of the movement of substances into and out of the cell.