Water as a solvent

A solvent is a substance that dissolves another substance to form a solution. A solute is the substance dispersed among solvent particles in a solution. A solution is a homogeneous mixture, with solute particles distributed through a solvent.

Solvation is the dissolving process where solvent molecules attract solute particles and surround them. If the solvent is water, the term hydration sometimes appears, but solvation is broader and is the term used here.

Water works so well as a solvent because each water molecule is polar. The oxygen has a partial negative charge, while the hydrogen atoms have partial positive charges. That polarity lets water interact with solutes in a few key ways.

• Polar molecules, such as sugars with hydroxyl groups, can form hydrogen bonds with water molecules.
• Positively charged ions are attracted to the partial negative oxygen side of water molecules.
• Negatively charged ions are attracted to the partial positive hydrogen side of water molecules.

In an ionic solid such as sodium chloride, positive and negative ions are held in a crystal lattice. When the solid dissolves, water molecules form shells around the separated ions. The combined attraction between many water molecules and an ion can keep that ion dispersed, rather than letting the ions clump back together.

This has a real biological payoff. Cytoplasm is not just “watery fluid”; it is an aqueous solution containing ions, sugars, amino acids and many other dissolved substances. Solubility differences also matter a lot in living organisms. Polar or charged substances tend to interact with water, while non-polar substances interact poorly with water. That contrast helps explain why the hydrophobic interior of membranes restricts many dissolved substances, and why some storage molecules are kept in less soluble forms.

Direction is described using solute concentration

A concentration is the amount of solute per unit volume of solution. In this topic, describe the direction of water movement using solute concentration: water has a net movement from the less concentrated solution to the more concentrated solution. In IB answers, avoid saying “from high water concentration to low water concentration”; it’s less precise, and the guide specifically wants solute concentration language.

Water molecules move randomly all the time. When two solutions are connected in a way that lets water move between them, water molecules travel in both directions. What matters is the net movement: the overall movement after comparing the two opposite flows. More water moves towards the solution with the higher concentration of osmotically active solutes.

An osmotically active solute is a dissolved particle that attracts water molecules strongly enough to affect net water movement. Ions such as sodium, potassium and chloride, and polar molecules such as glucose, are osmotically active.

Hypotonic, hypertonic and isotonic

These three words compare one solution with another. A solution is not simply “hypertonic” by itself; it is hypertonic compared with another solution.

A hypotonic solution has a lower concentration of osmotically active solutes than the solution it is being compared with. A hypertonic solution has a higher concentration of osmotically active solutes than the solution it is being compared with. An isotonic solution has the same concentration of osmotically active solutes as the solution it is being compared with.

So the tidy sentence is: water has a net movement from a hypotonic solution to a hypertonic solution. Between isotonic solutions there is no net movement, although water molecules still move both ways.

This is one example of a wider biological idea: variables such as concentration gradients, pressure differences and electrical gradients can set the direction in which materials move through tissues. In this topic, the key variable at SL is the concentration of osmotically active solutes.

Osmosis across plasma membranes

Osmosis is the net movement of water across a selectively permeable membrane, from the side with lower solute concentration to the side with higher solute concentration. A selectively permeable membrane allows some particles to cross more easily than others.

In most cells, the plasma membrane lets water through readily, but many solutes cross much less easily. So when the solute concentration differs between the cytoplasm and the surrounding fluid, water usually moves to correct the difference. The solutes don’t all simply diffuse until both sides are equal.

Osmosis is passive transport, meaning movement across a membrane that does not require direct energy input from ATP. Cells cannot “pump” water in the usual sense. Instead, they affect water movement by changing how permeable the membrane is to water, or by changing solute concentrations on one side of the membrane.

Predicting water movement

If the environment around a cell is hypotonic compared with the cytoplasm, water moves into the cell overall. If the environment is hypertonic compared with the cytoplasm, water moves out overall. In an isotonic environment, water still crosses the membrane in both directions, but at equal rates; this is dynamic equilibrium, a state in which opposite processes continue at equal rates so there is no overall change.

Learn that last point carefully. Isotonic does not mean “water stops moving”. It means “no net water movement”.

What happens to plant tissue

Plant tissue is made of cells with plasma membranes, cytoplasm, vacuoles and cell walls. Put pieces of plant tissue into solutions with different solute concentrations, and water may move into or out of the cells by osmosis. In a short experiment, a change in mass or length mostly shows that water has moved.

In a hypotonic solution, water enters the plant cells. The cells swell and become firm, so the tissue usually gains mass and may get slightly longer. In a hypertonic solution, water leaves the plant cells. The tissue loses mass, feels less firm and may become shorter or more flexible.

Measuring mass and length changes

A typical osmosis practical uses equal-sized pieces of a plant storage organ, such as potato. The independent variable is the solute concentration of the bathing solution. Useful dependent variables include percentage change in mass and percentage change in length. Percentage change works better than raw change if the starting pieces are not exactly identical.

Keep the other variables constant: type and region of tissue, starting dimensions, solution volume, time in solution, temperature, drying method before weighing, and balance or ruler used. Surface drying really matters. If one sample is left dripping wet while another is blotted carefully, the “mass change” may partly come from solution on the outside, not osmosis.

The isotonic solute concentration is the external solute concentration at which there is no net change in mass or length of the tissue. On a graph, estimate it where the curve crosses zero percentage change. Water is still moving in and out of cells at this point, but the net movement is zero.

Using standard deviation and standard error

Biological samples vary, so repeats are evidence, not decoration. A standard deviation is a statistic that measures how spread out individual results are around their mean. A larger standard deviation shows that the repeated pieces of tissue behaved less consistently.

A standard error is a statistic that estimates how precisely a sample mean represents the true mean of the population being sampled. It can be calculated as $SE = s/\sqrt{n}$, where $SE$ is the standard error of the mean (in the same unit as the measured variable), $s$ is the sample standard deviation (in the same unit as the measured variable) and $n$ is the number of repeats (no unit). You do not need to memorize this formula, but you should know what the statistic tells you.

Standard error can be plotted as error bars on a graph of mean percentage change against solute concentration. Smaller error bars suggest a more reliable estimate of the mean. Comparing standard deviation and standard error can help you judge whether length measurements or mass measurements gave the more reliable data in a particular osmosis investigation.

Animal cells and other cells without walls

A cell wall is a rigid extracellular layer that resists expansion of a cell. Animal cells don't have cell walls, so the plasma membrane forms the main boundary between the cytoplasm and the surrounding fluid. It’s flexible, but large changes in pressure or volume can damage it easily.

Place a cell without a wall in a hypotonic medium, and water enters by osmosis. The cell swells. If too much water enters, the membrane can’t tolerate the stretch and the cell bursts. Lysis is the bursting of a cell due to rupture of its plasma membrane.

In a hypertonic medium, the opposite happens: water leaves the cell by osmosis. The cytoplasm shrinks, and the membrane wrinkles. Crenation is the shrinkage of an animal cell into a wrinkled or notched shape after water loss in a hypertonic medium.

Osmoregulation without a wall

Freshwater unicellular organisms have a constant problem. Their cytoplasm is usually more concentrated than the surrounding freshwater, so water tends to enter continuously by osmosis. A contractile vacuole is a membrane-bound organelle that collects excess water from the cytoplasm and periodically expels it from the cell. This stops the cell from swelling and bursting, although it costs energy because the cell must maintain the solute gradients that make water enter the vacuole.

Multicellular animals handle the same basic problem at the tissue level. Tissue fluid bathes their cells, so the body has to keep that fluid close to isotonic with the cells. If tissue fluid became too hypotonic, cells could swell and lyse; if it became too hypertonic, cells would shrink and malfunction. Regulation of body fluids protects cell volume.

Turgor in hypotonic media

Plant cells have a cell wall outside the plasma membrane. In most young plant tissue, this wall lets water pass through freely, but it is strong and does not stretch easily. That changes everything.

When a plant cell is placed in a hypotonic medium, water enters by osmosis. The vacuole and cytoplasm expand, so the plasma membrane is pushed against the cell wall. Turgor pressure is the hydrostatic pressure exerted by the cell contents against the cell wall. A turgid cell is a plant cell in which internal pressure pushes the plasma membrane firmly against the cell wall.

The cell usually does not burst, because the wall resists further expansion. In non-woody plants, turgor helps hold up leaves and stems. Think of a firm celery stalk: much of that crispness comes from cells full of water pressing against their walls.

Plasmolysis in hypertonic media

In a hypertonic medium, a plant cell loses water by osmosis. The vacuole and cytoplasm shrink, and the pressure against the cell wall drops. A flaccid cell is a plant cell with too little turgor pressure to remain firm. At tissue level, many flaccid cells cause wilting.

If water loss continues, the cytoplasm and plasma membrane pull away from the cell wall. Plasmolysis is the separation of the plasma membrane and cytoplasm from the cell wall after water loss in a hypertonic medium. Plasmolysis is usually harmful and may become irreversible if severe.

Why medical fluids must be isotonic

Human cells can be damaged quickly if the surrounding fluid has the wrong osmotic balance. A hypertonic fluid pulls water out of cells, so they shrink. A hypotonic fluid pushes water into cells, which can make them swell and burst. In an isotonic fluid, water moves both ways across plasma membranes at equal rates, keeping cell volume stable.

That is why medical fluids are formulated so carefully. Normal saline is a sterile sodium chloride solution that is approximately isotonic with human blood plasma. Doctors commonly use it for intravenous fluids because it can enter the bloodstream without causing dangerous osmotic changes in blood cells and tissues.

Isotonic solutions are used when organs are prepared for transplantation too. Donor organs are bathed in preserving fluids that prevent osmotic swelling or dehydration of cells while the organ is being transported. Cooling slows metabolism, but isotonicity helps protect cell structure.

The same idea links back to solubility and membranes. Dissolved ions and polar solutes affect osmotic balance because they interact strongly with water, while membranes do not let every solute cross freely. So medical fluids must match the osmotic conditions cells normally experience, not just “contain water”.

Defining water potential

Water potential is the potential energy of water per unit volume, compared with pure water under standard reference conditions. It is written as $\psi_w$, where $\psi_w$ is water potential (kPa). In biology, it is often measured in kilopascals, kPa, or sometimes megapascals, MPa.

For cells, the absolute potential energy of water can’t be measured directly in a useful way, so water potential is stated relative to a reference point. Pure water at atmospheric pressure and $20 ^\circ\text{C}$ is assigned a water potential of $0$ kPa.

In living systems, two factors matter especially. Dissolved solutes lower the potential energy of water, so adding solute makes water potential more negative. Hydrostatic pressure also changes the potential energy of water: positive pressure raises it, while tension can lower it.

A useful mental anchor: pure water at atmospheric pressure sits at the top of the usual biological scale, at $0$ kPa. Cell solutions usually fall below that, with negative water potentials, because they contain dissolved solutes.

Potential energy explains the direction

Water has a net movement from higher water potential to lower water potential. The reason comes down to energy: when systems are free to change, they tend to move in ways that lower potential energy. Think of an object rolling downhill because its gravitational potential energy decreases.

Watch the negative numbers. A water potential of $-100\text{ kPa}$ is higher than $-500\text{ kPa}$ because it is less negative. So, if there’s a pathway, water moves from $-100\text{ kPa}$ to $-500\text{ kPa}$.

This is the water-potential version of the earlier SL rule. At SL, we say water moves from lower solute concentration to higher solute concentration. At HL, the same movement is explained more generally: water moves from higher $\psi_w$ to lower $\psi_w$. It also fits the wider linking idea that movement in tissues depends on gradients — concentration gradients, pressure gradients, electrical gradients and, in this case, water potential gradients.

The water potential equation

For plant cells with walls, water potential is usually worked out from solute potential and pressure potential:

$\psi_w = \psi_s + \psi_p$, where $\psi_w$ is water potential (kPa), $\psi_s$ is solute potential (kPa) and $\psi_p$ is pressure potential (kPa).

Solute potential is the part of water potential caused by dissolved solutes lowering the potential energy of water. Pure water has a solute potential of 0 kPa. When solute is added, solute potential becomes negative. It cannot be positive, because a solution cannot contain “less than no solute”.

Pressure potential is the part of water potential caused by hydrostatic pressure relative to atmospheric pressure. In turgid plant cells, pressure potential is usually positive, as the cell contents push against the wall. In xylem vessels transporting sap under tension, pressure potential can be negative because the water column is being pulled.

In many plant cells, the two effects work against each other: solutes lower $\psi_w$, while turgor pressure raises $\psi_w$. A fully turgid cell in pure water may reach $\psi_w = 0$ kPa because the positive pressure potential balances the negative solute potential.

Plant tissue in a hypotonic solution

Place plant tissue in a hypotonic solution, and the bathing solution has a higher water potential than the cell contents. Water enters the cells by osmosis, moving from higher $\psi_{w}$ to lower $\psi_{w}$.

As water enters, the cell sap is diluted, so solute potential becomes less negative. The cell contents also push harder against the wall, making pressure potential more positive. Together, these changes raise the water potential of the cell. Net water entry stops once the water potential of the cell equals the water potential of the bathing solution.

In pure water, the highest water potential the tissue can usually reach is $0\,\text{kPa}$. At this point the cell is fully turgid, with the positive pressure potential balancing the negative solute potential.

Plant tissue in a hypertonic solution

Put plant tissue in a hypertonic solution, and the bathing solution has a lower water potential than the cell contents. Water leaves the cells by osmosis.

As water leaves, pressure potential falls because the cell contents no longer press as strongly against the wall. When pressure potential reaches zero, the cell is flaccid. If water loss continues, the volume of the cytoplasm and vacuole decreases, and the plasma membrane pulls away from the wall: plasmolysis.

During this water loss, solute concentration inside the cell rises, so solute potential becomes more negative. Net water loss stops only when the water potential inside the cell equals that of the surrounding solution. In plasmolysed cells, pressure potential is usually zero, so solute potential mainly controls the comparison.

This is why HL explanations shouldn’t stop at “hypertonic means water leaves”. Add the mechanism: compare water potentials, then explain how $\psi_{s}$ and $\psi_{p}$ change as water enters or leaves.