This lab is really testing two things at once. First, can you explain why molecules move across membranes based on membrane structure? Second, can you use quantitative data, including percent change in mass and water potential calculations, to support claims about osmosis? It connects membrane structure directly to real, measurable outcomes.

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Why This Lab Matters for the AP Exam

Osmosis and membrane transport show up constantly on the AP Biology exam, not just as vocabulary questions but as data interpretation and free-response problems. You will be asked to calculate water potential, predict the direction of water movement, and explain what happens to cells in different solutions. This lab gives you the hands-on foundation for all of that. If you understand what you actually did and why, those exam questions become much more manageable.

CED Connections

This lab directly supports four topics in Unit 2: Cells.

Topic 2.4 - Membrane Permeability (LO 2.4.A, LO 2.4.B)

The lab demonstrates selective permeability in action. The dialysis tubing or potato tissue acts like a membrane that allows water to pass but restricts larger solute molecules. This connects to EK 2.4.A.1 (the hydrophobic interior creates selective permeability), EK 2.4.A.2 (small nonpolar molecules pass freely; polar and charged molecules need help), and EK 2.4.A.3 (nonpolar hydrocarbon tails block ions and large polar molecules). EK 2.4.B.1 connects here too, since plant cell walls protect against osmotic lysis, which you can observe when comparing plant tissue behavior to animal-like membrane bags.

Topic 2.5 - Membrane Transport (LO 2.5.A)

Osmosis is a form of passive transport, so this lab directly illustrates EK 2.5.A.1 (selective permeability creates concentration gradients) and EK 2.5.A.2 (passive transport moves molecules from high to low concentration without energy input).

Topic 2.6 - Facilitated Diffusion (LO 2.6.A)

While the lab focuses on osmosis, it builds context for EK 2.6.A.2 and EK 2.6.A.3. Water moving through aquaporins is a form of facilitated diffusion, and understanding that water needs a channel to move rapidly across membranes in large quantities is part of what this lab reinforces.

Topic 2.7 - Tonicity and Osmoregulation (LO 2.7.A, LO 2.7.B)

This is the heaviest CED connection. The lab gives you direct evidence for EK 2.7.A.1 (water moves from hypotonic to hypertonic regions, from high water potential to low water potential) and EK 2.7.B.2 (osmoregulation depends on controlling solute concentration and water potential). You also use the water potential equations that appear directly on the AP exam.

What You Need to Be Able to Do

These are the concrete skills this lab builds. Expect all of them to appear on the exam in some form.

Calculate percent change in mass for dialysis bags or potato cores placed in solutions of different concentrations
Use the water potential equation ( $\psi = \psi_p + \psi_s$ ) to predict the direction of water movement
Calculate solute potential using $\psi_s = -iCRT$ with correct units and temperature conversion
Identify the isotonic point for a tissue by finding where mass change equals zero on a graph
Interpret a graph of percent mass change vs. solute concentration to draw conclusions about the solute concentration of the tissue
Distinguish between hypertonic, hypotonic, and isotonic conditions and predict cell behavior in each
Design a controlled experiment by identifying independent variables, dependent variables, and controls
Write a claim-evidence-reasoning (CER) response connecting your data to membrane transport mechanisms

Core Concepts

Selective Permeability and Membrane Structure

The plasma membrane is a phospholipid bilayer. The inside of that bilayer is hydrophobic, meaning it repels water and charged particles. This is what makes the membrane selectively permeable: it lets some things through and blocks others.

Small nonpolar molecules like O2, CO2, and N2 slip right through the hydrophobic core without any help. Small polar uncharged molecules like water (H2O) can cross in small amounts on their own, but water moves much faster through aquaporins, which are specialized channel proteins embedded in the membrane. Larger polar molecules and ions cannot cross the hydrophobic interior at all without help from transport proteins.

The fluid mosaic model describes the membrane as a flexible, moving structure with proteins embedded throughout. Some of those proteins are channel proteins that form pores for specific ions or molecules. Gated ion channels only open in response to a specific signal. Glucose transporters (like GLUT1) carry glucose across the membrane by facilitated diffusion. All of these proteins allow substances to move down their concentration gradient without requiring energy.

Concentration Gradient and Osmosis

A concentration gradient exists when there is a difference in the amount of a substance between two regions. Molecules naturally move from where they are more concentrated to where they are less concentrated. This movement is called diffusion, and it requires no energy input.

Osmosis is a specific type of diffusion. It is the movement of water across a selectively permeable membrane, from a region of lower solute concentration (higher water concentration) to a region of higher solute concentration (lower water concentration). Water moves down its own concentration gradient, which is also described as moving from high water potential to low water potential.

Water Potential

Water potential ( $\psi$ ) is a measure of the tendency of water to move from one place to another. The equation is:

$\psi = \psi_p + \psi_s$

Pressure potential ( $\psi_p$ ) is the physical pressure on a solution. In an open beaker, this is zero. In a plant cell with a rigid cell wall, turgor pressure pushes outward and adds positive pressure potential.
Solute potential ( $\psi_s$ ) is always zero or negative. Adding solutes lowers water potential because solute particles attract water molecules and reduce their tendency to move freely.

Pure water has a water potential of zero. Any solution with solutes dissolved in it has a negative water potential. Water always moves toward the more negative (lower) water potential.

To calculate solute potential, use:

$\psi_s = -iCRT$

Where:

$i$ = ionization constant (1 for glucose, 2 for NaCl since it splits into two ions)
$C$ = molar concentration of the solute (mol/L)
$R$ = 0.0831 L-bar/mol-K
$T$ = temperature in Kelvin (add 273 to Celsius)

Tonicity

Tonicity describes the relative solute concentration of a solution compared to the inside of a cell.

A hypertonic solution has a higher solute concentration than the cell. Water leaves the cell by osmosis. Animal cells shrink (crenation). Plant cells undergo plasmolysis, where the membrane pulls away from the cell wall.
A hypotonic solution has a lower solute concentration than the cell. Water enters the cell. Animal cells can burst (osmotic lysis). Plant cells become turgid, but the cell wall prevents bursting by exerting pressure back on the cell.
An isotonic solution has the same solute concentration as the cell. There is no net movement of water.

Active Transport and Other Transport Mechanisms

Not all transport is passive. Active transport moves molecules against their concentration gradient, from low to high concentration, and it requires energy (usually ATP). The Na+/K+ ATPase pump is a classic example. This is different from everything you observe directly in the osmosis lab, but it is part of the broader picture of how cells maintain concentration gradients.

For very large molecules or bulk amounts of material, cells use endocytosis (taking material in by folding the membrane inward to form a vesicle) and exocytosis (releasing material by fusing a vesicle with the plasma membrane). These processes also require energy.

Osmoregulation in Different Organisms

Different organisms have evolved structures to handle osmotic challenges. The contractile vacuole in freshwater protists pumps out excess water that constantly enters by osmosis. The central vacuole in plant cells stores water and contributes to turgor pressure, which keeps plant tissues firm. The cell wall in plants, bacteria, fungi, and archaea provides structural support and prevents osmotic lysis when cells are in hypotonic environments.

How the Lab Works

The lab uses two main setups to investigate osmosis, and understanding the logic behind each one matters more than memorizing steps.

Dialysis Bag Setup

A dialysis bag is made of a membrane with pores small enough to block large molecules (like starch or sucrose) but large enough to allow water to pass through. You fill bags with solutions of different sucrose concentrations, seal them, record their initial mass, and place them in beakers of distilled water or sucrose solutions.

After a set amount of time, you remove the bags, dry the outside, and record the final mass. If the bag gained mass, water moved in (the solution inside was hypertonic relative to the outside). If it lost mass, water moved out (the solution inside was hypotonic relative to the outside). The bag with no mass change is at the isotonic point.

This setup models what happens to a cell in different environments. The dialysis membrane represents the plasma membrane, and the sucrose solution inside represents the cell's cytoplasm.

Potato Core Setup

Potato cores are cut to uniform size and placed in sucrose solutions of increasing concentration. After soaking, you measure the change in mass (or length) of each core. Potato cells already have a solute concentration inside them, so the direction of water movement depends on how the external solution compares to that internal concentration.

By graphing percent change in mass against sucrose concentration, you can identify the concentration at which the potato neither gains nor loses mass. That point is the isotonic concentration, which tells you the approximate solute concentration of the potato cells themselves. This is a real quantitative finding, not just an observation.

Data and Analysis Moves

Calculating Percent Change in Mass

Always use percent change, not raw change in mass. This controls for differences in initial size between samples.

$\% \text{ change} = \frac{\text{final mass} - \text{initial mass}}{\text{initial mass}} \times 100$

A positive value means the sample gained water (it was in a hypotonic solution). A negative value means it lost water (it was in a hypertonic solution).

Graphing

Plot percent change in mass (y-axis) against sucrose concentration (x-axis). The line should cross zero somewhere along the x-axis. That crossing point is the isotonic concentration of the tissue. Draw a best-fit line through your data points, not a connect-the-dots line.

Identifying Controls and Variables

Independent variable: the sucrose concentration of the external solution
Dependent variable: the percent change in mass of the bag or potato core
Controls: initial mass of each sample, temperature, time in solution, surface area of tissue, type of tissue

Water Potential Calculations

On the AP exam, you may be given a temperature and a sucrose concentration and asked to calculate water potential. Here is the process:

Calculate $\psi_s$ using $\psi_s = -iCRT$ . For sucrose, $i = 1$ because it does not ionize.
Determine $\psi_p$ . For an open dialysis bag or a flaccid cell, $\psi_p = 0$ , so $\psi = \psi_s$ .
Compare the water potential inside and outside the membrane. Water moves toward the lower (more negative) value.

Predicting Direction of Water Movement

Once you have water potential values, the rule is simple: water moves from high water potential to low water potential. High water potential means more free water molecules (fewer solutes or more pressure). Low water potential means fewer free water molecules (more solutes or less pressure).

Error Sources to Acknowledge

Common sources of error in this lab include: unequal surface area of potato cores, not fully drying the outside of dialysis bags before massing, temperature fluctuations, and variation in the initial solute concentration of potato tissue between different potatoes. When writing up results, you should be able to name these and explain how they could affect your data.

Common Mistakes

Confusing the direction of water movement with solute movement. Water moves toward higher solute concentration (lower water potential). Solutes move toward lower solute concentration. These are opposite directions. On the exam, questions often try to get you to mix these up.

Saying water moves from high concentration to low concentration without specifying what is concentrated. Be precise. Water moves from where water is more concentrated (fewer solutes) to where water is less concentrated (more solutes). Or just say: water moves from high water potential to low water potential.

Forgetting that solute potential is always negative. When you calculate $\psi_s = -iCRT$ , the negative sign is built into the formula. The result should always be zero or negative. If you get a positive number, check your setup.

Using Celsius instead of Kelvin in the water potential equation. The formula requires Kelvin. Always add 273 to your Celsius temperature before plugging it in.

Confusing osmosis with active transport. Osmosis is passive. It requires no energy. Active transport moves solutes against their gradient and requires ATP. The dialysis bag and potato experiments show passive movement only.

Thinking the dialysis bag is a perfect model of a cell. It models selective permeability, but it does not have channel proteins, aquaporins, or active transport mechanisms. Real cell membranes are far more complex.

Misidentifying the isotonic point on a graph. The isotonic point is where the line crosses zero on the y-axis (zero percent change in mass), not where the line is steepest or flattest.

Ignoring the cell wall when explaining plant cell behavior. A plant cell in a hypotonic solution gains water and becomes turgid, but it does not burst because the cell wall pushes back. This is different from what happens to an animal cell in the same situation. The cell wall is the reason.

Quick Review Checklist

Selective permeability comes from the hydrophobic interior of the phospholipid bilayer, which blocks ions and large polar molecules.
Osmosis is the passive movement of water from high water potential (low solute concentration) to low water potential (high solute concentration).
Water potential equals pressure potential plus solute potential: $\psi = \psi_p + \psi_s$
Solute potential is calculated with $\psi_s = -iCRT$ and is always zero or negative.
Percent change in mass tells you the direction and magnitude of water movement in your samples.
The isotonic concentration of a tissue is found where percent change in mass equals zero on your graph.
Plant cells have a cell wall that prevents osmotic lysis in hypotonic solutions; animal cells do not.
Aquaporins are channel proteins that allow rapid water transport across membranes by facilitated diffusion.
Active transport moves molecules against their concentration gradient and requires energy; osmosis does not.