Moving Materials: Diffusion & Osmosis
Learn how a selectively permeable membrane lets materials cross by diffusion and osmosis, predict which way water moves in salty or pure water, and see why it needs no energy.
What you'll do in this lesson
A voice-first session with the Crimsora tutor on Moving Materials: Diffusion & Osmosis, then targeted practice and FRQs — with the tutor adapting to where you get stuck.
What this lesson covers
Every cell is wrapped in a thin border called the cell membrane, and that border is doing a job every second: deciding what gets in and what stays out. Oxygen slips in, carbon dioxide slips out, water flows both directions — and the cell never spends a bit of energy making it happen.
This lesson explains the two movements behind that traffic: diffusion, the spreading out of particles from crowded areas to less crowded areas, and osmosis, the diffusion of water across a membrane. By the end you should be able to look at a cell sitting in salty water or in pure water and say confidently which way water will move, what will happen to the cell, and why the cell's energy supply is not involved at all. These ideas explain wilting plants, wrinkled fingertips in the bath, and why a saltwater fish cannot survive in a freshwater lake.
This lesson explains the two movements behind that traffic: diffusion, the spreading out of particles from crowded areas to less crowded areas, and osmosis, the diffusion of water across a membrane. By the end you should be able to look at a cell sitting in salty water or in pure water and say confidently which way water will move, what will happen to the cell, and why the cell's energy supply is not involved at all. These ideas explain wilting plants, wrinkled fingertips in the bath, and why a saltwater fish cannot survive in a freshwater lake.
The Selectively Permeable Membrane
The cell membrane is not a solid wall and it is not an open doorway. It is selectively permeable, meaning it lets some substances cross while blocking others. A good mental picture is a window screen: air and light pass through easily, but mosquitoes and leaves do not.
What decides who passes? Mostly size and chemistry. Small particles such as oxygen, carbon dioxide, and water molecules slip between the molecules of the membrane on their own. Large particles such as starch molecules, proteins, and most sugars are too big to squeeze through without help. Charged particles like sodium ions also have trouble crossing on their own, even though they are small.
This selectivity is what makes a cell a cell. If the membrane blocked everything, the cell would starve. If it let everything through, the cytoplasm would simply mix with the surroundings and the cell would stop being a separate living thing. The membrane keeps the inside chemically different from the outside while still allowing an exchange of supplies and wastes.
A common misconception is that the membrane "chooses" or "decides" like a guard with a list. It does not think. The selectivity comes entirely from the physical structure of the membrane — a double layer of fatty molecules with protein channels embedded in it. Some substances physically fit through; others physically do not. Whenever you explain permeability, describe the structure, not a decision.
Remember that plant cells have a cell wall outside the membrane, but the wall is fully permeable. It gives support and shape; the membrane is still the part that controls what enters the cytoplasm.
What decides who passes? Mostly size and chemistry. Small particles such as oxygen, carbon dioxide, and water molecules slip between the molecules of the membrane on their own. Large particles such as starch molecules, proteins, and most sugars are too big to squeeze through without help. Charged particles like sodium ions also have trouble crossing on their own, even though they are small.
This selectivity is what makes a cell a cell. If the membrane blocked everything, the cell would starve. If it let everything through, the cytoplasm would simply mix with the surroundings and the cell would stop being a separate living thing. The membrane keeps the inside chemically different from the outside while still allowing an exchange of supplies and wastes.
A common misconception is that the membrane "chooses" or "decides" like a guard with a list. It does not think. The selectivity comes entirely from the physical structure of the membrane — a double layer of fatty molecules with protein channels embedded in it. Some substances physically fit through; others physically do not. Whenever you explain permeability, describe the structure, not a decision.
Remember that plant cells have a cell wall outside the membrane, but the wall is fully permeable. It gives support and shape; the membrane is still the part that controls what enters the cytoplasm.
Diffusion: Spreading Out From Crowded to Less Crowded
All particles are in constant random motion. They jiggle, bump into each other, and bounce off in new directions. That motion never stops as long as the substance is above absolute zero, and it speeds up as temperature rises.
Because the motion is random, particles tend to spread out. If one region is crowded with a substance and a nearby region has very little of it, random bumping sends more particles out of the crowded region than back into it. Over time the substance ends up evenly spread. That net movement from higher concentration to lower concentration is diffusion.
The difference in concentration between two areas is the concentration gradient. Particles diffuse "down" the gradient, from high to low. Once the concentrations are equal, the particles keep moving, but equal numbers go each way, so there is no more net change. Scientists call that state equilibrium — motion continues, but the overall amounts stop shifting.
Diffusion across a cell membrane is how your cells get oxygen. Blood delivers oxygen-rich fluid around a muscle cell. Inside the cell, oxygen is constantly being used up, so its concentration stays low. Oxygen therefore diffuses inward. Meanwhile the cell produces carbon dioxide, so carbon dioxide is more concentrated inside and diffuses outward. No pump, no effort — just gradients.
Where students often go wrong: writing that particles "want" to spread out or that diffusion happens "to help the cell." Diffusion happens whether it helps or not. A poison will diffuse into a cell just as readily as oxygen if the gradient points inward. Also, diffusion does not stop at equilibrium — the individual particles never stop moving.
Because the motion is random, particles tend to spread out. If one region is crowded with a substance and a nearby region has very little of it, random bumping sends more particles out of the crowded region than back into it. Over time the substance ends up evenly spread. That net movement from higher concentration to lower concentration is diffusion.
The difference in concentration between two areas is the concentration gradient. Particles diffuse "down" the gradient, from high to low. Once the concentrations are equal, the particles keep moving, but equal numbers go each way, so there is no more net change. Scientists call that state equilibrium — motion continues, but the overall amounts stop shifting.
Diffusion across a cell membrane is how your cells get oxygen. Blood delivers oxygen-rich fluid around a muscle cell. Inside the cell, oxygen is constantly being used up, so its concentration stays low. Oxygen therefore diffuses inward. Meanwhile the cell produces carbon dioxide, so carbon dioxide is more concentrated inside and diffuses outward. No pump, no effort — just gradients.
Where students often go wrong: writing that particles "want" to spread out or that diffusion happens "to help the cell." Diffusion happens whether it helps or not. A poison will diffuse into a cell just as readily as oxygen if the gradient points inward. Also, diffusion does not stop at equilibrium — the individual particles never stop moving.
Osmosis and Predicting Which Way Water Moves
Osmosis is diffusion of water across a selectively permeable membrane. Water follows the same rule as everything else: it moves from where water is more concentrated to where water is less concentrated.
The tricky part is that we usually describe solutions by how much salt or sugar is dissolved, not by how much water. Keep this translation in mind: more dissolved particles means less water; fewer dissolved particles means more water. So water moves toward the saltier side.
A red blood cell dropped into pure distilled water takes in water until it bursts. The same cell in concentrated salt water shrivels into a spiky ball. This is exactly why a wilted plant perks up after watering: the soil water is purer than the cell sap, so water enters, the vacuoles fill, and the cells press outward against their walls.
The most common error is reversing the direction — saying water moves toward the pure water side because "it wants to join the water." Slow down and ask which side has fewer water molecules per unit of volume. That crowded, salty side is where water is headed.
The tricky part is that we usually describe solutions by how much salt or sugar is dissolved, not by how much water. Keep this translation in mind: more dissolved particles means less water; fewer dissolved particles means more water. So water moves toward the saltier side.
| Outside the cell | Water moves | Effect on an animal cell | Effect on a plant cell |
|---|---|---|---|
| Saltier than the cell (hypertonic) | Out of the cell | Shrinks, shrivels | Wilts; membrane pulls away from the wall |
| Purer than the cell (hypotonic) | Into the cell | Swells, may burst | Becomes firm and crisp; wall prevents bursting |
| Same as the cell (isotonic) | Equal both ways | Stays the same size | Slightly limp but stable |
The most common error is reversing the direction — saying water moves toward the pure water side because "it wants to join the water." Slow down and ask which side has fewer water molecules per unit of volume. That crowded, salty side is where water is headed.
Why Passive Transport Costs No Energy
Diffusion and osmosis are both forms of passive transport: the cell spends none of its own energy to make them happen.
The energy is already there, in the random motion of the particles themselves. That motion comes from thermal energy in the environment, not from the cell's food supply. Think of a ball on a hill. It rolls downhill on its own because it is already high up; you only need to spend energy to push it uphill. Particles moving down a concentration gradient are rolling downhill. The gradient is the hill.
The contrast is active transport, which moves substances the other way — from low concentration to high concentration, up the gradient. That is the uphill push, and it requires protein pumps in the membrane plus energy supplied by the mitochondria. A root hair cell pulling in scarce minerals from soil that already has less mineral than the cell does is using active transport.
Students sometimes assume that because a process is important, it must be expensive. Getting oxygen is essential, yet it is free. Others assume passive means nothing is moving. Passive means the cell is not paying, not that nothing happens. A dead cell whose mitochondria have stopped will still let water in or out by osmosis — which is exactly why meat left in brine gets salty and why cut potatoes shrink in salt water.
The energy is already there, in the random motion of the particles themselves. That motion comes from thermal energy in the environment, not from the cell's food supply. Think of a ball on a hill. It rolls downhill on its own because it is already high up; you only need to spend energy to push it uphill. Particles moving down a concentration gradient are rolling downhill. The gradient is the hill.
The contrast is active transport, which moves substances the other way — from low concentration to high concentration, up the gradient. That is the uphill push, and it requires protein pumps in the membrane plus energy supplied by the mitochondria. A root hair cell pulling in scarce minerals from soil that already has less mineral than the cell does is using active transport.
| Feature | Passive transport | Active transport |
|---|---|---|
| Direction | High to low concentration | Low to high concentration |
| Cell energy used | None | Yes, from the mitochondria |
| Examples | Diffusion, osmosis | Mineral uptake by root cells |
Key terms
- Selectively permeable.
- A property of the cell membrane that lets some substances cross while blocking others, based on their size and chemistry.
- Diffusion.
- The net movement of particles from an area of higher concentration to an area of lower concentration, caused by their random motion.
- Osmosis.
- The diffusion of water across a selectively permeable membrane, from the side with more water to the side with less water.
- Concentration gradient.
- The difference in the amount of a substance between two areas; particles diffuse down the gradient from high to low.
- Equilibrium.
- The state reached when concentrations are equal on both sides; particles keep moving, but there is no further net change.
- Passive transport.
- Movement of materials across the membrane that uses no energy from the cell, including diffusion and osmosis.
- Active transport.
- Movement of materials up a concentration gradient, from low to high, using protein pumps and energy from the cell.
- Hypertonic solution.
- A surrounding solution with more dissolved particles (and therefore less water) than the inside of the cell, causing the cell to lose water.
Worked example
A student cuts two identical potato cubes, each with a starting mass of 8.0 grams. Cube A is placed in a beaker of distilled (pure) water. Cube B is placed in a beaker of 15 percent salt solution. After one hour, the student blots each cube dry and re-measures the mass. Predict which cube gained mass and which lost mass, explain the movement of water in each beaker, and state whether the potato cells used energy.
Step 1: Identify what is inside the potato cells. Potato cytoplasm contains water plus dissolved sugars, salts, and other solutes. It is not pure water.
Step 2: Compare each solution to the inside of the cells. Distilled water has zero dissolved particles, so it has a higher concentration of water than the potato cytoplasm does. The 15 percent salt solution has far more dissolved particles than the cytoplasm, so it has a lower concentration of water.
Step 3: Apply the rule that water moves from higher water concentration to lower water concentration, which is the same as saying water moves toward the saltier side.
Cube A: water moves from the beaker into the cells. The cells swell, press against their cell walls, and the cube becomes firm. Its mass increases above 8.0 grams — a value near 8.6 grams would be reasonable.
Cube B: water moves out of the cells into the salty beaker. The cells lose water, the cube becomes limp and flexible, and its mass drops below 8.0 grams — perhaps near 7.2 grams.
Step 4: Compute the change to report the result numerically. Using the sample data, and .
Step 5: Address the energy question. Both changes happened by osmosis, a form of passive transport driven by the random motion of water molecules. The potato cells spent none of their own stored energy. The mass change would still occur in cells whose mitochondria had stopped working.
Step 2: Compare each solution to the inside of the cells. Distilled water has zero dissolved particles, so it has a higher concentration of water than the potato cytoplasm does. The 15 percent salt solution has far more dissolved particles than the cytoplasm, so it has a lower concentration of water.
Step 3: Apply the rule that water moves from higher water concentration to lower water concentration, which is the same as saying water moves toward the saltier side.
Cube A: water moves from the beaker into the cells. The cells swell, press against their cell walls, and the cube becomes firm. Its mass increases above 8.0 grams — a value near 8.6 grams would be reasonable.
Cube B: water moves out of the cells into the salty beaker. The cells lose water, the cube becomes limp and flexible, and its mass drops below 8.0 grams — perhaps near 7.2 grams.
Step 4: Compute the change to report the result numerically. Using the sample data, and .
Step 5: Address the energy question. Both changes happened by osmosis, a form of passive transport driven by the random motion of water molecules. The potato cells spent none of their own stored energy. The mass change would still occur in cells whose mitochondria had stopped working.
Practice questions
A red blood cell is placed in a beaker of concentrated salt water. What will happen and why?
- Water moves into the cell and it swells, because water is attracted to the cell membrane.
- Water moves out of the cell and it shrivels, because the water concentration is lower outside the cell.
- Salt moves into the cell and it swells, because salt diffuses down its gradient.
- Nothing happens, because the membrane is selectively permeable and blocks water.
Answer: Water moves out of the cell and it shrivels, because the water concentration is lower outside the cell.
Concentrated salt water contains many dissolved particles, so it holds relatively less water than the cytoplasm does. Water diffuses from the side with more water (inside the cell) to the side with less water (the beaker), and the cell shrivels. The choice about salt entering is a common wrong answer — some salt ions may cross slowly, but the dramatic shriveling is caused by water leaving. The last choice misreads selective permeability: water is one of the small molecules that crosses the membrane easily.
Explain why oxygen keeps diffusing into a working muscle cell hour after hour, and why the cell does not have to spend any energy to bring the oxygen in.
Answer: The muscle cell constantly uses oxygen during respiration, so oxygen concentration inside stays low. Blood keeps oxygen concentration outside high. That maintained concentration gradient means random particle motion carries more oxygen molecules in than out, so oxygen diffuses inward continuously. No cell energy is needed because the movement is down the gradient, powered by the thermal motion the particles already have.
A complete answer has two parts. First, the gradient never disappears because the cell is a constant consumer — this is why diffusion does not just reach equilibrium and stop. Second, the energy point: passive transport uses the existing random motion of molecules, not energy from the mitochondria. Cells only pay energy when moving a substance from low concentration to high concentration, which is not what is happening here.
A gardener sprinkles far too much fertilizer on a flowerbed and waters it. By the next morning the plants are badly wilted even though the soil is wet. Explain what happened at the cell level.
Answer: The excess fertilizer dissolved and made the soil water very concentrated with dissolved particles, so the soil water had a lower water concentration than the root cell cytoplasm. Water therefore moved by osmosis out of the root cells into the soil instead of into the plant. The cells lost water, their vacuoles shrank, they stopped pressing against their cell walls, and the plant wilted.
This question checks whether you can apply the direction rule when the salty side is outside the plant rather than in a beaker. Students often answer that the plant needed more water, but the soil is already wet — the problem is the direction of osmosis, not the amount of liquid present. Wilting happens whenever plant cells lose enough water that they no longer push outward on their cell walls.
FAQ
- What is the difference between diffusion and osmosis?
- Osmosis is a specific type of diffusion. Diffusion is the general spreading of any particle from high concentration to low concentration. Osmosis refers only to water moving across a selectively permeable membrane. Every case of osmosis is diffusion, but diffusion of oxygen or perfume in air is not osmosis.
- Which way does water move if a cell is in salty water?
- Water moves out of the cell. Salty water contains lots of dissolved particles and therefore relatively less water than the cytoplasm does. Water always diffuses toward the side with less water, which is the saltier side, so the cell loses water and shrinks or wilts.
- Why do plant cells not burst in pure water when animal cells do?
- Plant cells have a rigid cell wall outside the membrane. As water enters and the cell swells, the wall pushes back and stops the cell from expanding any further, leaving the cell firm and crisp. Animal cells have only a flexible membrane, so nothing stops the swelling and the cell can rupture.
- Does diffusion stop once concentrations are equal?
- The particles never stop moving. What stops is the net change. At equilibrium, just as many particles cross one way as the other, so the concentrations stay the same even though random motion continues in both directions.
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