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A short class with Sarah and her students. The other ways are under More.
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We have studied diffusion. Osmosis is a special case of it. What do you think makes it special?
It's only about water?
Yes: the diffusion of water through a partially permeable membrane. What does 'partially permeable' mean?
Some molecules can get through, like water, but bigger ones like sugar can't.
Exactly. So if there is sugar solution on one side and pure water on the other, which way will water move overall?
Towards the sugar solution, because there is less free water there.
Right: water moves towards the side with more solute, and it needs no energy to do so.
This bag of Visking tubing contains concentrated sucrose solution and is joined to a thin glass tube. I'm standing it in water. What will happen?
Water will move into the bag by osmosis.
And what will we see in the glass tube?
The liquid level will rise, because the bag is gaining water.
Why doesn't the sucrose just move out into the water instead?
Its molecules are too big to pass through the tubing.
Good. The tubing is our model of a cell membrane.
We will put potato cylinders into sucrose solutions of different concentrations. What is our independent variable?
The concentration of the sucrose solution.
And why measure percentage change in mass, not just the change in grams?
Because the cylinders don't all start with exactly the same mass.
Exactly. Our sample results go from plus 4.0% at 0.2 to minus 5.2% at 0.4. What does that tell us?
Somewhere in between, the potato neither gains nor loses water.
Yes, at about 0.29 mol/dm³. There, the solution matches the water concentration inside the potato cells.
The diffusion of water through a partially permeable membrane, from a dilute to a more concentrated solution.
Osmosis, like all diffusion, needs no energy from the cell.
Water crosses both ways, but more crosses from the side with more free water molecules.
An artificial membrane shows water entering a concentrated sucrose solution.
Percentage change in mass against concentration reveals the point of no net water movement.
A Visking tubing bag of sucrose solution standing in water gains water, so the liquid rises up the glass tube attached to it.
A potato cylinder left in pure water gains mass, while one left in concentrated sucrose solution loses mass.
A wilted stick of celery placed in a glass of water becomes firm again as its cells take up water by osmosis.
Water will go where the solute is more, crossing the membrane like guests through a door.
A crowd of water molecules gathers on both sides of a gate. On one side, sugar molecules hold on to many of them, so more of the free molecules on the other side wander through the gate, until the crowds are balanced.
Dilute to Denser: water moves from the dilute solution to the denser, more concentrated one.
W.A.S.P. - Water Always Seeks Particles: water moves towards the side with more solute particles.
Term
Define osmosis.
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Partially permeable membrane
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Which way does water move by osmosis?
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Why use percentage change in mass in the potato investigation?
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What does the point where the graph crosses zero show?
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The net movement of water molecules from a dilute solution to a more concentrated solution through a partially permeable membrane.
A membrane that lets some molecules, such as water, pass through but not others, such as sucrose; also called semi-permeable.
A substance dissolved in a solvent, such as sucrose in water.
The liquid in which a solute dissolves; in living things, usually water.
A solution with little solute and therefore a high concentration of water.
A solution with a lot of solute and therefore a lower concentration of water.
An artificial partially permeable membrane used to model a cell membrane.
The change in mass divided by the starting mass, multiplied by 100.
Osmosis is the net movement of water molecules from a dilute solution, which has more free water, to a more concentrated solution, which has less, through a partially (semi-) permeable membrane; it is a special case of diffusion and needs no energy.
This lesson explains osmosis as the diffusion of water across a semi-permeable membrane. It shows, at the level of particles, why more water molecules cross from the dilute side than from the concentrated side, models the process with Visking tubing, and then works through a full potato-cylinder investigation in the four stages of scientific inquiry: formulating a question, planning, recording and presenting data, and interpreting results, with sample data and calculations of percentage change in mass.
Water is the substance that moves in and out of cells most of all, and it does so by a special kind of diffusion.
Osmosis is the net movement of water molecules from a region of higher water concentration (a dilute solution) to a region of lower water concentration (a concentrated solution) through a partially permeable membrane. It is diffusion, but of water only, and like all diffusion it is passive: the cell uses no energy.
A partially permeable (semi-permeable) membrane lets small molecules such as water pass through but stops larger solute molecules such as sucrose. The cell membrane behaves in this way, and many cells also have special channel proteins that let water cross quickly.
Water molecules move about randomly and cross the membrane in both directions. In a concentrated solution, many water molecules are attracted to the solute particles and cluster around them, so fewer are free to move through the membrane. On the dilute side, more water molecules are free. So, in any second, more water molecules cross from the dilute side to the concentrated side than cross back. The result is a net flow of water towards the more concentrated solution, which continues until the two sides have the same water concentration, or until something such as a plant cell wall pushes back.
A quick way to remember the direction: water moves towards the side with more solute.
Visking tubing is an artificial partially permeable membrane. If a bag of it is filled with concentrated sucrose solution, attached to a narrow glass tube and stood in a beaker of water, the liquid slowly rises up the tube. Water is entering the bag by osmosis; the sucrose molecules are too large to leave. If the bag held water and the beaker held sucrose solution, the bag would lose water instead.
1. Formulating a scientific question. How does the concentration of a sucrose solution affect the percentage change in mass of potato cylinders? Hypothesis: in dilute solutions the cylinders will gain mass, in concentrated solutions they will lose mass, and at one concentration, where the solution matches the water concentration of the potato cells, there will be no change.
2. Planning the investigation.
3. Recording and presenting data. Sample results for one cylinder at each concentration:
Percentage change = (final mass − starting mass) ÷ starting mass × 100. For 0.2 mol/dm³: 0.10 ÷ 2.48 × 100 = +4.0%. Percentages are used because the cylinders start with slightly different masses. The results are then plotted on a graph, with concentration on the x-axis and percentage change on the y-axis.
4. Interpreting the results. Cylinders in water and in 0.2 mol/dm³ sucrose gained mass: the solution was more dilute than the cell contents, so water entered the cells by osmosis. In 0.4 mol/dm³ and above they lost mass, because water left the cells. The line crosses zero between 0.2 and 0.4 mol/dm³. Assuming a straight line between those two points, the crossing is at 0.2 + 0.2 × (4.0 ÷ 9.2), which is about 0.29 mol/dm³. This estimates the concentration at which the solution and the potato cells have the same water concentration, so there is no net osmosis.