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IB Biology Osmosis: 7 Steps for the Potato Practical and an HL SAM8 Tip

IB Biology Osmosis: 7 Steps for the Potato Practical and an HL SAM8 Tip

13 min readOliver Kidd (Co-founder)4 Oct 2026

Osmosis is the net movement of water through a selectively permeable membrane towards the side with the higher solute concentration. Examiners expect you to use that exact phrasing, along with terms like water potential and hypertonic or hypotonic, rather than vaguer wording about water moving “to where there’s less of it”. Getting this right matters because a cell that gains or loses too much water can become turgid, flaccid, plasmolysed or, in animal cells, can lyse.

TL;DR:

  • Solutions with higher solute concentrations cause water to move into cells or solutions with lower water potential, leading to cell swelling or plasmolysis in plants.
  • Aquaporins are membrane proteins that facilitate faster water movement across cell membranes, significantly increasing osmosis rates.
  • The osmotic potential of a 0.15 M sucrose solution at 25°C is approximately -3.7 bars, roughly half the value of an equally concentrated NaCl solution.
  • Accurate calculations involve water potential components, with the negative solute potential indicating the strength of water draw into solutions.
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Table of Contents

What is osmosis and how does it differ from diffusion?

Osmosis is a special case of diffusion: water moves passively across a selectively permeable membrane from a region of higher water potential to one of lower water potential, continuing until equilibrium is reached or the membrane’s selectivity prevents further net flow. Only the solvent crosses in osmosis, while diffusion can involve any particle moving down its own concentration gradient, with or without a membrane present. Mixing these two up is one of the most common ways students lose marks in short-answer questions.

A few wording traps to avoid:

  • Never say water moves “towards higher water concentration”: always frame it as movement towards lower water potential.
  • Avoid “semi-permeable” when the syllabus term is “selectively permeable”.
  • Do not describe osmosis as “active”: it requires no metabolic energy, just a concentration gradient.

Water potential and osmotic potential: the maths behind the membrane

Water potential, given the symbol ψ, measures the tendency of water to move out of a solution. It has two main components: solute potential (ψS), which is always negative or zero, and pressure potential (ψP), which reflects turgor pressure pushing back against incoming water. Together, ψ = ψS + ψP.

Higher level students should know the iCRT form for solute potential:

  1. i is the ionisation constant: 1 for sucrose, 2 for NaCl because it dissociates into two ions.
  2. C is the molar concentration of the solution.
  3. R is the pressure constant (0.0831 litre bars per mole per kelvin).
  4. T is the temperature in kelvin.

A 0.15 M sucrose solution at 25°C has an osmotic potential of approximately −3.7 bars, and a 0.15 M NaCl solution at the same concentration and temperature has about twice the osmotic potential, because sodium chloride ionises and doubles the effective particle count, according to the College Board’s diffusion and osmosis lab manual. That negative sign matters: the more negative the value, the more strongly that solution draws water in.

Tonicity and what happens to plant and animal cells

Tonicity describes a solution’s solute concentration relative to the cytoplasm inside a cell. A hypotonic solution has a lower solute concentration than the cytoplasm, so water enters the cell; a hypertonic solution has a higher solute concentration, so water leaves; an isotonic solution causes no net water movement. Plant and animal cells respond very differently because only plant cells have a rigid cell wall to resist expansion.

  • In a hypotonic solution, plant cells become turgid as the cell wall prevents bursting, while animal cells can undergo lysis.
  • In a hypertonic solution, plant cells become flaccid and may show plasmolysis, where the membrane pulls away from the wall, while animal cells crenate, shrivelling as water leaves.
  • In an isotonic solution, both cell types show no net change in volume, though water still moves in both directions at equal rates.

Freshwater fish cells sit in a hypotonic environment and must constantly manage water gain, a useful example drawn from LibreTexts’ overview of osmosis/02%3A_Cell_Biology/2.01%3A_Osmosis) that pairs well with exam answers on tonicity.

How membranes and aquaporins move water, and a current research note

Water can cross the phospholipid bilayer by slow diffusion between lipid molecules, but most of it moves through aquaporins, channel proteins that let water through far faster than the bilayer alone allows, according to Wikipedia’s entry on osmosis. Osmotic pressure itself is a colligative property: it depends on the number of dissolved particles, not their identity, which is why NaCl and sucrose solutions of equal molarity produce different water potentials.

Aquaporin channel showing water movement

For HL extended responses, a 2026 study in Nature identified a protein called SAM8 that condenses inside plant cells when water potential drops, acting as an internal sensor that triggers changes in protein synthesis under osmotic stress. Mentioning SAM8 briefly, and labelling it clearly as recent research rather than syllabus content, can show examiners wider reading without overclaiming what is still an emerging finding.

Pro Tip: Name aquaporins whenever a question asks why water crosses membranes faster than other molecules: it shows you understand the mechanism, not just the outcome.

Practical guide: the potato osmosis experiment and the isotonic point

This practical is a staple of IB Biology coursework and a common IA starting point. The aim is to find the isotonic point, the sucrose concentration at which a potato cylinder’s mass does not change.

  1. Cut potato cylinders of identical diameter and length using a cork borer and ruler, then blot each dry before weighing.
  2. Prepare a concentration series of sucrose solutions, typically 0.0 M to 1.0 M in steps of 0.2 M, each at the same temperature.
  3. Place one cylinder in each solution, cover to limit evaporation, and leave for a fixed time, usually 30 to 60 minutes.
  4. Remove each cylinder, blot it dry in the same way, and reweigh it immediately.
  5. Calculate percentage change in mass using (final mass minus initial mass, divided by initial mass, times 100).
  6. Repeat each concentration at least three times and take a mean to reduce the effect of natural variation between potatoes.
  7. Plot concentration against mean percentage change in mass; the isotonic point is where the line crosses zero.
Sucrose concentration (M) Approximate percentage mass change
0.0 Positive change (mass gain)
0.2 Smaller positive change
— Close to zero
— Negative change (mass loss)
— Larger negative change
1.0 Largest negative change

Standard school practical method confirms this is how the isotonic point is located by plotting percentage mass change against concentration. Keep cylinder size, temperature and timing controlled across every sample, since any one of these can shift your line and distort the isotonic point you read off the graph.

Common sources of error include uneven blotting, which affects mass readings, and natural variation in starting water potential between potato cylinders. A realistic improvement is extending the incubation time or controlling temperature with a water bath, both of which reduce variability in your final dataset.

How to structure exam answers on osmosis for maximum marks

A reliable template for short-answer questions is claim, then mechanism, then consequence, then example: state what happens, explain why using water potential, describe the effect on the cell, and give a concrete case such as plasmolysis in a hypertonic solution.

  • Always use psi notation (ψ, ψS, ψP) when a question involves calculations, not just descriptive words.
  • Label graph axes with units and show working for percentage change calculations to the correct number of significant figures.
  • Use comparative language such as “higher water potential” rather than absolute statements, since osmosis is always about relative gradients.
  • Annotate diagrams of cells or membranes clearly: examiners check for labelled arrows showing direction of net water movement.

Why practical osmosis work builds real exam confidence

Working through the potato practical by hand, rather than just reading about it, is what makes water potential calculations click. Once you can link a definition, a labelled diagram and a calculation in the same answer, osmosis questions stop feeling like a memory test and start feeling like straightforward application. That connection between practice and understanding is what builds genuine readiness for paper 2 and paper 3.

— Oliver

How Tiber Tutor supports mastering osmosis

The platform offers notes, animated walkthroughs, practical templates and mock exams on osmosis and water transport, designed to align with syllabus outcomes. Resources are interconnected with related topics like membrane structure and active transport, and progress tracking helps identify which exam question types require further practice.

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Students can start with the free IB Biology tests or build a custom set with the IB Biology Test Builder, then move on to full mock exams once the fundamentals feel solid. Full access starts with a free 7-day trial, after which the All-Access Plan costs $19 per month or the Per-Subject Plan costs $9 per month, both on the pricing page.

FAQ

How difficult is IB Biology?

IB Biology is considered demanding because it combines detailed content with data-based and extended-response questions that require clear written explanations, not just recall. Topics like osmosis and water potential are often cited as challenging because they mix conceptual understanding with calculation, so regular practical and past-paper practice makes a real difference.

What are five examples of osmosis in biology?

Examples include water entering plant root hair cells from soil, red blood cells swelling in a hypotonic solution, freshwater fish managing water gain through their gills, potato cylinders losing mass in concentrated sucrose solutions, and water reabsorption in the kidney’s nephron. Each case involves water moving across a selectively permeable membrane towards lower water potential.

Can you explain osmosis to kids in simple terms?

Osmosis is when water moves on its own through a thin, selective barrier to even out how concentrated a solution is on each side. Think of a teabag left in water: the water moves in to balance things out, much like water moves into or out of a cell depending on what is dissolved around it.

What are five differences between diffusion and osmosis?

Diffusion moves any particle down its concentration gradient and does not require a membrane, while osmosis specifically moves water across a selectively permeable membrane. Diffusion can occur in gases, liquids or across membranes, whereas osmosis is restricted to solvent movement in liquid systems, and only osmosis is described in terms of water potential rather than simple concentration.

What is water potential in IB Biology?

Water potential, symbol ψ, measures how likely water is to move out of a solution or cell, combining solute potential and pressure potential. Water always moves from higher to lower water potential, which is the core principle examiners expect in osmosis answers.

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