العلوم
Osmosis: Water Moving Through a Membrane
Osmosis is water crossing a partially permeable membrane towards the more concentrated solution. What the potato-in-sugar-solution practical actually proves.
Osmosis
Osmosis is the net movement of water across a partially permeable membrane, from a dilute solution to a more concentrated one.
- أين يقابله الطلاب
- Grade 9 science, in the unit on movement in and out of cells, and again wherever the potato-and-sugar-solution practical is set at IGCSE level.
الإجابة باختصار
Osmosis is the net movement of water molecules through a partially permeable membrane, from a dilute solution into a more concentrated one. The membrane lets water through but holds back the dissolved substance, so the water moves instead of the solute. No energy is required; the particles are already moving.
مثال
Percentage change in mass = (final − initial) ÷ initial × 100
A potato cylinder weighing 4.0 g is left in distilled water and comes out at 4.4 g, a gain of 10%. An identical cylinder left in concentrated sugar solution comes out at 3.4 g, a loss of 15%. Water entered the first and left the second, so the first solution was more dilute than the potato's cell contents and the second was more concentrated. Percentage change is used rather than grams because the cylinders never start at exactly the same mass.
The water moves because the sugar cannot
A partially permeable membrane has gaps that small water molecules pass through and larger dissolved molecules do not. Put a concentrated solution on one side and a dilute one on the other, and the difference has only one way to even out. The solute is stuck, so the water travels.
The word net is doing real work in the definition. Water molecules cross the membrane in both directions the whole time, because they are moving randomly and the membrane does not know which way is which. On the dilute side there are more free water molecules available to cross, so more go one way than the other. When the two sides reach the same concentration, crossings continue but the traffic balances and the level stops changing.
Nothing is spending energy on any of this. Osmosis is powered by the random motion the particles already have, which is why it happens just as well in a dead potato as a living one.
What the potato practical proves
Cylinders of potato are cut, weighed, and left in sugar solutions of different concentrations for a fixed time, then blotted dry and weighed again. Mass gained means water moved in, so the solution outside was more dilute than the contents of the cells. Mass lost means the reverse.
The result the experiment is really after is the concentration at which the mass does not change. At that point water is entering and leaving at the same rate, which means the solution outside matches the concentration inside the potato cells. Plotting percentage change against concentration and reading where the line crosses zero gives an estimate of that internal concentration, and that is the point of the whole exercise.
Two procedural details decide whether the numbers mean anything: blot the cylinders before weighing so surface liquid is not weighed as gain, and use percentage change rather than raw grams so cylinders of slightly different starting size can be compared.
Plant cells survive what animal cells do not
Put a plant cell in a dilute solution and water enters by osmosis. The vacuole swells and pushes the cell contents against the cell wall, which is strong enough to resist, and the cell becomes firm — turgid. That pressure is what holds a non-woody plant upright, and it is why an unwatered plant wilts.
Lose water instead and the cell goes limp, or flaccid; keep going and the membrane pulls away from the wall altogether, which is called plasmolysis. An animal cell has no wall to push back. In pure water it swells until it bursts, and in a concentrated solution it shrivels. That is why fluids given to patients are matched to the concentration of blood plasma rather than being pure water.
أسئلة شائعة
What is the difference between osmosis and diffusion?
Diffusion is any particle spreading from a crowded region to a less crowded one, with or without a membrane. Osmosis is specifically water, and specifically across a partially permeable membrane. When a question involves water and a membrane, write osmosis; that is the word the mark scheme is looking for, and diffusion will usually not be credited.
Does osmosis require energy?
No. It is a passive process driven by the random movement particles already have, so it needs no energy from respiration and continues in cells that are no longer alive. This is one of the clearest ways to separate it from active transport, which moves substances the other way along a gradient and stops when respiration stops.
Why does a potato cylinder gain mass in pure water?
Because the cell contents are a fairly concentrated solution of sugars and salts, while distilled water contains almost nothing. Water therefore moves into the cells through their partially permeable membranes, the vacuoles swell, and the cylinder gets heavier and firmer. The cell walls stop the cells bursting, which is why the potato holds its shape.
What does partially permeable mean?
That the membrane lets some substances through and blocks others, sorting mainly by size. Water molecules are small enough to pass; sugar and salt particles in solution largely are not. Older textbooks call it semi-permeable, and it means the same thing, but examiners generally prefer partially permeable.
Why calculate percentage change in mass rather than the difference?
Because no two potato cylinders start at exactly the same mass, and a 0.4 g gain means something different for a 4 g cylinder than for a 6 g one. Converting to a percentage puts every result on the same scale so the concentrations can be compared fairly, and it is what allows several groups' results to be pooled.
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