Osmosis moves water but not solutes because it is defined as the net movement of water across a selectively permeable membrane. The membrane provides a route through which water can pass, while the relevant dissolved particles cannot cross at the same rate. Water therefore moves from a region of higher water potential to a region of lower water potential until equilibrium is reached or an opposing pressure prevents further net movement.
The key idea is selective permeability. Water can cross biological membranes directly through the phospholipid bilayer and, much more rapidly in many cells, through water-channel proteins called aquaporins. Ions and most polar solutes cannot pass freely through the hydrophobic interior of the bilayer, although some can move through their own specific transport proteins. If a solute crosses a membrane, that movement is diffusion, facilitated diffusion, or active transport, not osmosis.
This single-concept explanation develops the membrane mechanism behind osmosis. For broader coverage of definitions, tonicity, cell responses, and exam applications, see RevisionDojo's topic-wide IB Biology osmosis explained exam guide.
The short explanation: why osmosis moves water
Imagine two solutions separated by a membrane that allows water through but prevents sucrose from crossing. Water molecules move randomly on both sides and cross the membrane in both directions. However, the difference in water potential produces a greater movement in one direction than the other, creating net movement toward the side with lower water potential.
Under equal pressure, adding a non-penetrating solute such as sucrose lowers the water potential of a solution. Water consequently moves from the more dilute side, where water potential is higher, to the more concentrated side, where water potential is lower.
The sucrose remains behind because it has no suitable route through the membrane. Osmosis is therefore not the membrane pushing water toward solute, nor is water consciously trying to equalize concentrations. It is the statistical outcome of random molecular movement combined with a membrane that is much more permeable to water than to the solute.
A strong IB Biology explanation includes four linked points:
Water molecules are in constant random motion.
A difference in water potential exists across the membrane.
The membrane is selectively permeable, allowing water to cross more readily than the relevant solute.
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There is net water movement from higher to lower water potential without direct ATP expenditure.
Selective permeability is the essential condition
A biological membrane is not simply a sheet containing holes of one fixed size. It consists primarily of a phospholipid bilayer containing proteins, cholesterol, and carbohydrate-associated components. Its structure makes it permeable to some substances and poorly permeable or impermeable to others.
The phospholipid heads face the aqueous environments, while the fatty-acid tails form a hydrophobic interior. Small non-polar molecules such as oxygen and carbon dioxide dissolve in this interior and cross readily. Charged ions and many polar molecules interact unfavourably with the non-polar core, so they generally require channel or carrier proteins.
Water is polar, which means the hydrophobic membrane interior still resists its movement. However, an individual water molecule is very small, so some water crosses directly through the bilayer. Many cells greatly increase their water permeability by incorporating aquaporins into the membrane.
Substance
Typical ability to cross the phospholipid bilayer directly
Main reason
Oxygen and carbon dioxide
High
Small and non-polar
Water
Limited but significant
Very small, although polar
Sodium, potassium and chloride ions
Extremely low
Charged and surrounded by hydration shells
Glucose and amino acids
Low
Relatively large and polar
Proteins and polysaccharides
Negligible
Very large, often polar or charged
This is why explaining permeability only in terms of molecular size is incomplete. Polarity, charge, lipid solubility, molecular size, membrane composition, and the availability of transport proteins all affect whether a substance can cross.
How aquaporins allow rapid water movement
Aquaporins are membrane channel proteins that provide hydrophilic pathways through the bilayer. Water molecules pass through these channels in single file, avoiding direct contact with the membrane's hydrophobic core. Their presence can make a membrane far more permeable to water than it would be through the lipid bilayer alone.
Aquaporins are selective because the channel has a narrow internal structure and specific chemical properties. These features permit water to move rapidly while excluding hydrated ions and many solutes. A channel is therefore not an unrestricted opening through which everything smaller than its diameter can pass.
Aquaporin-mediated movement remains passive transport. The channel does not pump water and does not hydrolyse ATP to force water through. It lowers the barrier to movement, while the water-potential difference determines the direction of net flow.
The current IB Biology course explicitly includes the movement of water across membranes by osmosis and the role of aquaporins within B2.1 Membranes and membrane transport. Students can review the complete syllabus context through RevisionDojo's B2.1 membranes and membrane transport lessons.
Why solutes do not move during the osmotic process
Solutes do move in biological systems, but their movement is not called osmosis. The phrase “osmosis moves water but not solutes” describes a situation in which the membrane is permeable to water but effectively impermeable to the solute being considered.
There are several reasons why a solute might not cross:
Ions are charged, so entering the hydrophobic core is energetically unfavourable.
Larger polar molecules such as glucose do not dissolve readily in the non-polar membrane interior.
The membrane may lack a channel or carrier that recognizes the solute.
An available transport protein may be closed, saturated, or regulated.
The solute may be too large for existing pathways.
For example, sodium ions cannot cross a typical phospholipid bilayer freely. A sodium ion in water is also associated with a hydration shell, increasing the effective barrier to its passage through the non-polar membrane interior. Sodium can cross through appropriate ion channels or transporters, but this is a separate transport process.
Glucose provides another useful example. It is uncharged but relatively large and polar, so it generally requires a specific carrier. If glucose moves down its concentration gradient through that carrier, it undergoes facilitated diffusion. Water moving in response to the resulting water-potential difference is undergoing osmosis.
Does osmosis literally mean that no solute can cross?
Not every real membrane is perfectly impermeable to every solute. The scientifically important question is whether the solute crosses sufficiently slowly relative to water to maintain an osmotic gradient. Such a solute is often described as non-penetrating or effectively impermeant over the relevant timescale.
If both water and a solute can cross, two processes may occur simultaneously:
Water moves according to the water-potential gradient.
The solute moves according to its own electrochemical or concentration gradient.
As the solute redistributes, the water-potential difference may decrease, so the osmotic effect can weaken. A permanently maintained osmotic gradient generally requires solute particles that cannot readily cross or a biological mechanism that continually maintains unequal solute distributions.
This distinction matters when considering tonicity. Tonicity describes how a surrounding solution changes cell volume and depends mainly on solutes that cannot readily cross the cell membrane. A solute that rapidly enters the cell may increase measured osmolarity but produce only a temporary change in cell volume.
Water potential explains the direction more precisely
Students often learn that water moves from a dilute solution to a concentrated solution. This is useful under simple conditions, but water potential is the more complete explanation.
Water potential represents the tendency of water to move from one region to another. Water moves passively from a region of higher water potential to a region of lower water potential when a suitable pathway exists. Pure water under standard reference conditions has a water potential of zero, while dissolved solute makes solute potential negative.
A simplified relationship used in biological contexts is:
water potential = solute potential + pressure potential
A concentrated solution normally has a more negative solute potential than a dilute solution. However, pressure can oppose the osmotic movement of water. This is particularly important in plant cells, where water entering the cell increases turgor pressure against the cell wall.
At equilibrium, water molecules still cross the membrane in both directions. The rates are equal, so there is no net movement. Equilibrium does not mean that molecular movement has stopped.
The IB specimen materials use water potential, solute potential, pressure potential, and the idea that equilibrium is reached when opposing effects balance. RevisionDojo's D2.3 water potential questionbank can help students apply these ideas to unfamiliar data.
Osmosis compared with other membrane transport
Process
Substance moved
Membrane required?
Transport protein required?
ATP required directly?
Typical direction
Osmosis
Water
Yes
Not always, but often through aquaporins
No
Higher to lower water potential
Simple diffusion
Small molecules such as O₂ or CO₂
Not necessarily
No
No
Down a concentration gradient
Facilitated diffusion
Ions or polar solutes
Yes
Yes
No
Down an electrochemical or concentration gradient
Active transport
Ions or other solutes
Yes
Yes
Directly or indirectly
Against a gradient
The distinction is based on what moves and how it crosses, not merely on whether movement occurs through a membrane. Osmosis is specifically water movement. Diffusion is a broader process that can involve many kinds of particles, while active transport requires an energy source to establish or maintain gradients.
Suppose an animal cell contains a solution with a lower water potential than the surrounding fluid. The plasma membrane is highly permeable to water but does not allow the principal intracellular solutes to leave rapidly.
Water molecules cross in both directions, but the inward rate is greater than the outward rate. The cell therefore gains water and increases in volume. If the gradient is sufficiently large and sustained, the cell may lyse because an animal cell has no rigid cell wall.
Now suppose the surrounding solution has a lower water potential than the cytoplasm. Net water movement is outward, so the cell loses volume and shrinks. The dissolved cytoplasmic substances do not all leave with the water because the membrane's hydrophobic core and specific transport proteins regulate their passage.
In a plant cell, inward osmosis also expands the protoplast, but the cell wall resists further expansion. The resulting pressure potential raises the cell's water potential until the net movement can reach zero. This produces a turgid cell rather than unrestricted swelling.
Common misconceptions to avoid in IB exams
“Water moves from low concentration to high concentration”
This statement is ambiguous because it does not identify what is being measured. Water moves from higher water concentration to lower water concentration in simplified systems, corresponding to lower solute concentration toward higher solute concentration. The safest expression is from higher water potential to lower water potential across a selectively permeable membrane.
“Solute attracts or pulls water through the membrane”
This wording suggests an active pulling force. Dissolved particles lower the chemical potential or water potential of the solution, producing a gradient. Random molecular movement then creates a net flux of water through the available pathway.
“Water is the only substance that can cross cell membranes”
Cell membranes permit many substances to cross, either through the bilayer or through proteins. Osmosis refers only to the water component of membrane transport. Solute movement is classified separately.
“Aquaporins use energy to pump water”
Aquaporins are passive channels, not ATP-powered pumps. They increase the rate at which water can move, but they do not determine a new direction against the water-potential gradient.
“At equilibrium, water stops moving”
Water continues to cross in both directions because molecules remain in random motion. Equilibrium means that the two opposing rates are equal, producing no overall change.
How to write an exam-quality explanation
For a question asking why water moves but sucrose, salt, or another solute does not, organize the answer as a causal chain rather than listing disconnected definitions:
State that the membrane is selectively permeable.
Explain that water crosses through the bilayer and/or aquaporins.
Explain why the named solute cannot cross freely, referring to charge, polarity, size, or the absence of a suitable transport protein.
Identify the difference in water potential.
Give the correct direction of net water movement.
State that the process is passive and does not directly require ATP.
A concise model answer could read:
Water molecules move by osmosis from the side with higher water potential to the side with lower water potential. The membrane is selectively permeable, and water can cross through aquaporins, whereas the dissolved ions cannot pass through the hydrophobic bilayer without specific transport proteins. Water moves in both directions, but the difference in water potential creates net movement toward the lower water potential. No ATP is used directly because osmosis is passive.
Avoid claiming that every solute is simply “too large.” That may be appropriate for starch or a large protein, but it does not explain why small ions are blocked. For targeted preparation, use the B2.1 membrane transport questionbank and B2.1 membrane transport flashcards. Jojo AI can then help identify whether an explanation contains the required causal links rather than only a memorized definition.
Conclusion
Osmosis moves water but not the relevant solute because the separating membrane is much more permeable to water. Water crosses the bilayer and, in many cells, travels rapidly through aquaporins, while charged, polar, or large solutes require their own specific transport pathways. A difference in water potential produces net water movement from higher to lower water potential without direct ATP use.
Solutes can cross membranes, but when they do, their movement is classified as diffusion, facilitated diffusion, or active transport rather than osmosis. For IB Biology revision, connect the definition to membrane structure, aquaporin selectivity, water potential, and dynamic equilibrium. RevisionDojo's B2.1 lessons, Flashcards, Questionbank, and Jojo AI are useful for practising that complete explanation in exam-ready language.
Sarah holds a PhD in Cell Biology and taught IB Biology across Europe and Asia for 18 years, latterly as a science department lead. Outside of the papers, her focus lies with the Biology EE, especially with its new format, closing the gap between understanding and application.
