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Osmosis is the diffusion of water from a dilute to a concentrated solution through a partially permeable membrane. Free notes and 13 questions answered.
Understanding Osmosis is part of Transport in cells in GCSE AQA Higher Biology.
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Osmosis is the diffusion of water from a dilute to a concentrated solution through a partially permeable membrane. Free notes and 13 questions answered.
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Use this lesson to stay inside Transport in cells while connecting the detail back to the wider Cell biology topic.
Read the core explanations from Understanding Osmosis before testing yourself from memory.
Picture two glasses of water separated by a special membrane. One glass has pure water, the other has sugar dissolved in it. Leave them overnight and something fascinating happens - water moves from the pure side to the sugary side, even though nothing is pushing it.
This movement has a name: osmosis. It's happening in your body right now - in every cell, keeping you alive.
Trap: Many students think osmosis needs energy or a pump. Actually, it's completely passive - water moves down its concentration gradient naturally, just like diffusion.
Osmosis can't happen through any old barrier. It needs a partially permeable membrane - think of it as a molecular bouncer that's very picky about who gets through.
Cell membranes are partially permeable. Water molecules are tiny and can pass through, but many solute particles cannot. Glucose and proteins are too large to pass through easily, and ions are also restricted by the membrane's selective properties.
This creates the perfect setup for osmosis. Water can move to balance things out, but the dissolved substances can't follow.
Here's the key insight: water doesn't actually 'want' to move anywhere. It's all about concentration gradients and random molecular motion.
In pure water, water molecules are bouncing around randomly. In sugar water, some of those bouncing spaces are taken up by sugar molecules. So there are fewer water molecules per unit volume - it's more dilute in terms of water.
Water molecules move randomly in all directions, but there's a net movement from where water is more concentrated (pure water) to where it's less concentrated (sugar water).
Trap: Students often think 'concentrated solution' means lots of water. Actually, a concentrated sugar solution has less water per unit volume because the sugar takes up space.
Now for the real-world application. Every cell in your body is surrounded by a partially permeable membrane, making osmosis crucial for life.
Plant cells have an advantage - their cell wall prevents them from bursting when water enters by osmosis. Animal cells aren't so lucky and can burst if they take in too much water.
Worked example: Predicting Osmosis in Cells
A plant cell with 5% sugar solution is placed in pure water. Predict what happens.
Answer: Water enters the cell by osmosis, making it turgid and firm
Osmosis questions are exam favourites because they test both knowledge and application. Here's how to nail them every time.
Common exam mistake: Writing 'water moves to balance concentrations'. This is vague. Instead write: 'water moves from dilute solution to concentrated solution by osmosis through the partially permeable membrane'.
Master this process and you'll find osmosis questions become straightforward mark-winners. The key is understanding the direction of water movement and being able to explain it clearly using the correct scientific terminology.
In plant tissues, osmosis supports structure by maintaining turgor pressure in cells. In animal cells, there is no cell wall, so water movement can cause cells to burst in very dilute conditions or shrink in concentrated conditions. In exam answers, always state direction as water moving from dilute to concentrated solution through a partially permeable membrane.
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Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.
Osmosis is the diffusion of water through a partially permeable membrane from a dilute solution to a concentrated solution. It is completely passive: water moves down its concentration gradient naturally, just like diffusion, without needing energy or a pump.
Osmosis is the diffusion of water through a partially permeable membrane. Like simple diffusion, it is completely passive and goes down a concentration gradient, but in osmosis water can move to balance things out while the dissolved substances can't follow.
A partially permeable membrane lets tiny water molecules pass through, but many solute particles cannot. Cell membranes are partially permeable: glucose and proteins are too large to pass through easily, and ions are also restricted by the membrane's selective properties.
In plant cells, the cell wall prevents bursting when water enters by osmosis, and osmosis maintains turgor pressure that supports plant structure. Animal cells have no cell wall, so water movement can make them burst in very dilute conditions or shrink in concentrated conditions.
A plant cell in pure water takes in water by osmosis, because the cell contents are more concentrated than the surrounding water. The cell swells and becomes turgid, but the cell wall prevents it from bursting.
Animal cells in a concentrated solution shrink, because water moves out of the cell by osmosis into the more concentrated solution. Animal cells have no cell wall, so in very dilute conditions they can instead burst.
Turgid means a plant cell is firm because water has entered it by osmosis. The cell wall prevents the cell from bursting, and turgid cells provide structural support, which is how plants stay upright without bones.
Osmosis does not need energy or a pump; it is completely passive. Water moves down its concentration gradient naturally, from a dilute solution to a concentrated solution through a partially permeable membrane, just like diffusion.
Animal cells can burst in pure water because they have no cell wall, so they keep taking in water by osmosis. Plant cells also swell as water enters, but their cell wall prevents them from bursting, so they become turgid instead.
A concentrated solution has less water per unit volume, not more, because the dissolved substance, such as sugar, takes up space. So in osmosis water moves from a dilute solution, where water is more concentrated, to a concentrated solution.
Sugar cannot easily cross a partially permeable membrane because glucose molecules are too large, while water molecules are tiny and can pass through. Water can move to balance things out, but the dissolved substances cannot follow, which creates the setup for osmosis.
Osmosis should be explained as water moving from a dilute solution to a concentrated solution through a partially permeable membrane. Writing that 'water moves to balance concentrations' is too vague to earn credit.
Water moves from a dilute to a concentrated solution because a concentrated solution has fewer water molecules per unit volume. Water molecules move randomly in all directions, but there is a net movement from where water is more concentrated to where it is less concentrated.