Passive Transport, Active Transport & Tonicity
Learn to predict how molecules and water cross a selectively permeable membrane — diffusion, facilitated diffusion, osmosis, active transport, and hypotonic vs. hypertonic vs. isotonic.
What you'll do in this lesson
A voice-first session with the Crimsora tutor on Passive Transport, Active Transport & Tonicity, then targeted practice and FRQs — with the tutor adapting to where you get stuck.
What this lesson covers
You already know the membrane is a fluid mosaic of phospholipids studded with proteins. Now comes the part that makes it matter: that mosaic decides what gets in, what gets out, and what stays put. A cell can be surrounded by sugar and starve, or sit in pure water and burst. Which one happens depends on concentration gradients and on whether the cell spends ATP.
In this lesson you will learn to trace a molecule's path across the membrane and say which transport process moved it, then look at a cell in a beaker of solution and predict which way water flows and what happens to the cell's volume. The reasoning is always the same two questions: is there a gradient, and does the substance need help (a protein, energy, or both) to follow it? Master those and tonicity problems stop being memorization.
In this lesson you will learn to trace a molecule's path across the membrane and say which transport process moved it, then look at a cell in a beaker of solution and predict which way water flows and what happens to the cell's volume. The reasoning is always the same two questions: is there a gradient, and does the substance need help (a protein, energy, or both) to follow it? Master those and tonicity problems stop being memorization.
Gradients and Selective Permeability: Why Anything Moves At All
Molecules in a solution are in constant random motion. If a substance is more concentrated in one region than another, that random motion spreads it out — not because molecules "want" to go anywhere, but because there are simply more of them starting on the crowded side. The difference in concentration between two regions is a concentration gradient, and movement from high to low concentration is movement down the gradient.
A selectively permeable membrane lets some substances cross freely and blocks others. The phospholipid bilayer has a nonpolar interior, so the rules follow from chemistry:
When concentrations become equal on both sides, the system reaches dynamic equilibrium. This is the single most common misconception in the unit: equilibrium does not mean the molecules stop. They keep crossing in both directions, but at equal rates, so the net movement is zero. A related error is thinking diffusion "stops when the cell has enough." Simple diffusion has no sensor and no off switch — it responds only to the gradient, which is why cells need proteins and ATP to control anything precisely.
A selectively permeable membrane lets some substances cross freely and blocks others. The phospholipid bilayer has a nonpolar interior, so the rules follow from chemistry:
| Substance | Crosses the bilayer directly? | Why |
|---|---|---|
| , , small nonpolar molecules | Yes, easily | Nonpolar, dissolve in the lipid tails |
| Water | Slowly on its own; fast through aquaporins | Small but polar |
| Glucose, amino acids | No | Polar and too large |
| , , , | No | Charged; repelled by the nonpolar core |
Three Flavors of Passive Transport
Passive transport is any movement down a concentration gradient that costs the cell no ATP. The energy is already stored in the gradient itself.
Simple diffusion is direct passage through the bilayer, no protein involved. Oxygen diffusing into a muscle cell and carbon dioxide diffusing out are the classic examples.
Facilitated diffusion uses a channel protein (a hydrophilic tunnel, often gated) or a carrier protein (binds the solute and changes shape) to move substances that cannot cross the lipid core. Glucose entering a red blood cell through GLUT transporters and ions moving through ion channels are facilitated diffusion. Note the key point students often miss: a protein is involved, but no ATP is. The substance still moves high to low. Because there is a limited number of transport proteins, facilitated diffusion saturates — the rate levels off at high concentrations, while simple diffusion keeps speeding up.
Osmosis is the diffusion of water across a selectively permeable membrane, from where water is more concentrated (fewer dissolved solutes) toward where water is less concentrated (more solutes). Aquaporins are channel proteins that greatly speed it up, which makes osmosis a special case of facilitated diffusion for water.
The sentence to internalize: water follows solute. When solute cannot cross the membrane, the water moves instead. Students frequently write that "solute moves to balance the concentrations," but in most cell problems the membrane blocks the solute — so the only thing that can equalize concentrations is water shifting, which changes cell volume.
Simple diffusion is direct passage through the bilayer, no protein involved. Oxygen diffusing into a muscle cell and carbon dioxide diffusing out are the classic examples.
Facilitated diffusion uses a channel protein (a hydrophilic tunnel, often gated) or a carrier protein (binds the solute and changes shape) to move substances that cannot cross the lipid core. Glucose entering a red blood cell through GLUT transporters and ions moving through ion channels are facilitated diffusion. Note the key point students often miss: a protein is involved, but no ATP is. The substance still moves high to low. Because there is a limited number of transport proteins, facilitated diffusion saturates — the rate levels off at high concentrations, while simple diffusion keeps speeding up.
Osmosis is the diffusion of water across a selectively permeable membrane, from where water is more concentrated (fewer dissolved solutes) toward where water is less concentrated (more solutes). Aquaporins are channel proteins that greatly speed it up, which makes osmosis a special case of facilitated diffusion for water.
The sentence to internalize: water follows solute. When solute cannot cross the membrane, the water moves instead. Students frequently write that "solute moves to balance the concentrations," but in most cell problems the membrane blocks the solute — so the only thing that can equalize concentrations is water shifting, which changes cell volume.
Active Transport: Paying ATP to Go Uphill
Active transport moves a substance against its concentration gradient, from low to high, and therefore requires energy — usually ATP hydrolysis. It always uses a carrier protein, often called a pump.
The textbook example is the sodium-potassium pump in animal cells. It exports three out and imports two in per ATP spent, keeping sodium high outside and potassium high inside. Both ions are moving uphill, and both would leak back down if the pump ever stopped. This pump maintains the electrical gradient that nerve and muscle cells depend on, and it explains why cells that fire signals burn so much energy. Root hair cells use similar pumps to pull minerals out of soil where the mineral concentration is far lower than inside the cell.
Bulk transport handles material too large for any protein. In endocytosis the membrane folds inward to engulf material into a vesicle (phagocytosis for large particles, pinocytosis for fluid); in exocytosis a vesicle fuses with the membrane to release contents. Both require ATP.
A reliable test for checking your own answers: if the substance ended up more concentrated on the side it moved to, ATP was spent. If a poison blocks ATP production, active transport halts while simple diffusion continues unaffected.
The textbook example is the sodium-potassium pump in animal cells. It exports three out and imports two in per ATP spent, keeping sodium high outside and potassium high inside. Both ions are moving uphill, and both would leak back down if the pump ever stopped. This pump maintains the electrical gradient that nerve and muscle cells depend on, and it explains why cells that fire signals burn so much energy. Root hair cells use similar pumps to pull minerals out of soil where the mineral concentration is far lower than inside the cell.
| Feature | Simple diffusion | Facilitated diffusion | Active transport |
|---|---|---|---|
| Direction | Down gradient | Down gradient | Against gradient |
| Protein needed | No | Yes | Yes (pump) |
| ATP needed | No | No | Yes |
| Examples | , | Glucose, ions, water via aquaporins | pump, mineral uptake |
A reliable test for checking your own answers: if the substance ended up more concentrated on the side it moved to, ATP was spent. If a poison blocks ATP production, active transport halts while simple diffusion continues unaffected.
Tonicity: Predicting Water Movement and Cell Fate
Tonicity compares the solute concentration of a solution to the solute concentration inside a cell. The prefix always describes the solution outside, and this is where most mistakes happen — students say "the cell is hypertonic" when they mean the surrounding fluid is.
Work it in this order: (1) compare solute concentrations, (2) remember water moves toward higher solute, (3) state whether the cell gains or loses water, (4) describe the volume change.
Plant cells behave differently because a rigid cell wall resists expansion. In a hypotonic solution a plant cell takes up water until turgor pressure builds and the cell becomes firm — the wall prevents lysis, which is why watered plants stand upright. In a hypertonic solution the cell loses water, the vacuole shrinks, and the membrane pulls away from the wall in plasmolysis; the whole plant wilts. Salting a slug, or salting eggplant to draw out moisture, is hypertonic conditions at work.
One more trap: "hypotonic" and "hypertonic" are relative terms. A salt solution is hypertonic to a cell with salt but hypotonic to a cell with salt. Always name what you are comparing to what.
| Outside solution | Solute comparison | Net water movement | Animal cell result |
|---|---|---|---|
| Hypotonic | Less solute outside than inside | Water enters cell | Swells; may lyse (burst) |
| Hypertonic | More solute outside than inside | Water leaves cell | Shrinks, crenates |
| Isotonic | Equal solute | No net movement | No volume change |
Plant cells behave differently because a rigid cell wall resists expansion. In a hypotonic solution a plant cell takes up water until turgor pressure builds and the cell becomes firm — the wall prevents lysis, which is why watered plants stand upright. In a hypertonic solution the cell loses water, the vacuole shrinks, and the membrane pulls away from the wall in plasmolysis; the whole plant wilts. Salting a slug, or salting eggplant to draw out moisture, is hypertonic conditions at work.
One more trap: "hypotonic" and "hypertonic" are relative terms. A salt solution is hypertonic to a cell with salt but hypotonic to a cell with salt. Always name what you are comparing to what.
Reading Transport Problems Without Getting Tricked
Most transport questions on classwork give you numbers and a membrane description, then ask for direction of movement. Three habits keep you from misreading them.
First, check what the membrane is permeable to. If the problem says the membrane is permeable to water but not to sucrose, sucrose is frozen in place; only water can respond. If it says the membrane is permeable to both, then both diffuse down their own gradients independently — sucrose one way, water the other — until each is balanced.
Second, convert to solute-versus-water language deliberately. A solution that is solute is water. Higher solute automatically means lower water concentration. Many wrong answers come from a student correctly identifying the hypertonic side and then sending water the wrong way; writing both percentages down prevents it.
Third, watch for the word "against" or a described energy source. If a diagram shows a substance accumulating inside a cell where it is already concentrated, that is active transport, even if the diagram never mentions ATP. Conversely, a protein in the picture does not automatically mean active transport — channels and carriers do plenty of passive work.
Finally, when a question asks you to predict a long-term outcome, ask whether equilibrium is reachable. A red blood cell in distilled water cannot reach equilibrium by osmosis alone, because water keeps entering as long as any solute remains inside; the cell bursts first. That is why patients receive isotonic saline rather than pure water intravenously.
First, check what the membrane is permeable to. If the problem says the membrane is permeable to water but not to sucrose, sucrose is frozen in place; only water can respond. If it says the membrane is permeable to both, then both diffuse down their own gradients independently — sucrose one way, water the other — until each is balanced.
Second, convert to solute-versus-water language deliberately. A solution that is solute is water. Higher solute automatically means lower water concentration. Many wrong answers come from a student correctly identifying the hypertonic side and then sending water the wrong way; writing both percentages down prevents it.
Third, watch for the word "against" or a described energy source. If a diagram shows a substance accumulating inside a cell where it is already concentrated, that is active transport, even if the diagram never mentions ATP. Conversely, a protein in the picture does not automatically mean active transport — channels and carriers do plenty of passive work.
Finally, when a question asks you to predict a long-term outcome, ask whether equilibrium is reachable. A red blood cell in distilled water cannot reach equilibrium by osmosis alone, because water keeps entering as long as any solute remains inside; the cell bursts first. That is why patients receive isotonic saline rather than pure water intravenously.
Key terms
- Selectively permeable.
- A property of the cell membrane by which some substances pass freely while others are blocked or require transport proteins.
- Concentration gradient.
- A difference in the concentration of a substance between two regions; movement from high to low concentration is movement down the gradient.
- Simple diffusion.
- Net movement of a small nonpolar substance directly through the phospholipid bilayer, down its gradient, with no protein and no ATP.
- Facilitated diffusion.
- Passive movement down a gradient through a channel or carrier protein; needed by polar and charged substances, and it saturates when all proteins are in use.
- Osmosis.
- Diffusion of water across a selectively permeable membrane from the region of lower solute concentration to the region of higher solute concentration.
- Active transport.
- Protein-mediated movement of a substance against its concentration gradient, powered by ATP; the sodium-potassium pump is the standard example.
- Tonicity.
- A comparison of the solute concentration of the surrounding solution to that inside the cell, described as hypotonic, hypertonic, or isotonic.
- Plasmolysis.
- Shrinking of a plant cell's cytoplasm and separation of the membrane from the cell wall after water loss in a hypertonic solution.
Worked example
A plant cell has an internal solute concentration of . It is placed in a beaker of solution that is solute. The membrane is permeable to water but not to the solute. (a) Is the beaker solution hypotonic, hypertonic, or isotonic to the cell? (b) Which way does water move, and by what process? (c) Describe the cell after 30 minutes. (d) The cell also pulls in potassium ions even though potassium is far more concentrated inside the cell than in the beaker. Name that process and state what it requires.
(a) Compare solute concentrations: outside is , inside is . More solute is outside, so the beaker solution is hypertonic to the cell. Remember the prefix describes the solution, not the cell.
(b) Translate solute into water: the beaker is water and the cell is water. Water diffuses from higher water concentration to lower, so water leaves the cell and enters the beaker. Because water is crossing a selectively permeable membrane, the process is osmosis, and it is passive — no ATP required. The solute itself cannot move, so water is the only thing that can respond to the gradient.
(c) Losing water, the central vacuole shrinks and the cytoplasm pulls away from the rigid cell wall. This is plasmolysis, and the cell loses turgor pressure. The cell wall itself keeps its shape, so the cell does not collapse into a ball the way an animal cell crenates; in a whole plant, this appears as wilting.
(d) Potassium is moving from low concentration outside to high concentration inside, which is against its gradient. That is active transport, and it requires a carrier protein acting as a pump plus energy from ATP hydrolysis. If a metabolic poison stopped ATP production, potassium uptake would stop while osmosis in part (b) would continue.
(b) Translate solute into water: the beaker is water and the cell is water. Water diffuses from higher water concentration to lower, so water leaves the cell and enters the beaker. Because water is crossing a selectively permeable membrane, the process is osmosis, and it is passive — no ATP required. The solute itself cannot move, so water is the only thing that can respond to the gradient.
(c) Losing water, the central vacuole shrinks and the cytoplasm pulls away from the rigid cell wall. This is plasmolysis, and the cell loses turgor pressure. The cell wall itself keeps its shape, so the cell does not collapse into a ball the way an animal cell crenates; in a whole plant, this appears as wilting.
(d) Potassium is moving from low concentration outside to high concentration inside, which is against its gradient. That is active transport, and it requires a carrier protein acting as a pump plus energy from ATP hydrolysis. If a metabolic poison stopped ATP production, potassium uptake would stop while osmosis in part (b) would continue.
Practice questions
A red blood cell with an internal solute concentration of is placed in a salt solution. What is the most likely outcome?
- Water leaves the cell and it crenates, because the solution is hypertonic
- Water enters the cell and it may lyse, because the solution is hypotonic
- No net water movement occurs, because the solution is isotonic
- Salt moves into the cell by active transport until concentrations are equal
Answer: Water enters the cell and it may lyse, because the solution is hypotonic
The outside solution has less solute () than the cell (), so it is hypotonic to the cell and contains more water per unit volume. Water moves toward the higher solute concentration, meaning into the cell. An animal cell has no cell wall to resist the incoming water, so it swells and can burst (lyse). The salt-moving choice is wrong because salt would be moving from low to high concentration only if the cell spent ATP, and nothing in the problem indicates a pump; also, osmosis of water is the fast, dominant response here.
A cell is surrounded by a solution containing glucose at a lower concentration than inside the cell, yet the cell continues to accumulate glucose. Identify the transport process, explain how you know, and predict what would happen if a drug blocked ATP synthesis in this cell.
Answer: This is active transport. Glucose is moving from low concentration outside to high concentration inside, which is against the concentration gradient, and only ATP-powered pumps can do that. If ATP synthesis were blocked, glucose accumulation would stop, and glucose would then leak out by facilitated diffusion down its gradient until concentrations equalized.
The decisive clue is direction relative to the gradient, not the presence of a protein. Both facilitated diffusion and active transport use proteins, so a protein in a diagram proves nothing by itself; movement uphill proves ATP involvement. The second half of the answer connects to a real experimental design: blocking ATP is how biologists distinguish active from passive transport, because passive processes continue in a poisoned cell while pumps shut down and gradients slowly collapse.
Explain why a wilted stalk of celery becomes crisp again after sitting in fresh water, using the terms hypotonic, osmosis, and turgor pressure.
Answer: Fresh water is hypotonic to the celery cells, so water moves into the cells by osmosis. The cells fill and press outward against their rigid cell walls, generating turgor pressure that stiffens the tissue and makes the celery crisp.
Fresh water has almost no solute, so it has a higher water concentration than the cell interior; water therefore diffuses inward. The cell wall is the reason the outcome differs from an animal cell in the same beaker: instead of lysing, the plant cell reaches a firm, turgid state where wall pressure balances further water entry. Wilting is the reverse situation — water loss to a drier or saltier environment reduces turgor pressure and the tissue goes limp.
FAQ
- Is osmosis a type of diffusion?
- Yes. Osmosis is the diffusion of water specifically, across a selectively permeable membrane. It is passive and requires no ATP. Because most water crossing a real membrane travels through aquaporin channel proteins, osmosis is usually classified as a form of facilitated diffusion for water.
- Does facilitated diffusion use ATP?
- No. Facilitated diffusion uses a channel or carrier protein but moves substances down their concentration gradient, so the energy comes from the gradient itself. Seeing a protein in a diagram does not mean ATP is involved — only movement against the gradient requires ATP.
- Does hypotonic describe the cell or the solution?
- By convention it describes the solution surrounding the cell. "The solution is hypotonic to the cell" means the solution has less solute than the cell, so water will move into the cell. Always name both sides of the comparison, because the same solution can be hypotonic to one cell and hypertonic to another.
- Why does a plant cell not burst in pure water when an animal cell does?
- The plant cell has a rigid cellulose cell wall. As water enters, the cell presses against the wall and turgor pressure builds until it opposes further water entry, leaving the cell firm rather than ruptured. An animal cell has only a flexible membrane, so nothing stops the swelling and it can lyse.
Learn this with a teacher, not a page
The Crimsora tutor teaches Passive Transport, Active Transport & Tonicity live — explaining on a whiteboard, asking you questions, and adapting to where you get stuck.