Why Salted Cucumber Slices Go Limp While Raisins Soaked in Water Swell Up
Tell diffusion, osmosis and imbibition apart, understand semi-permeable and selectively permeable membranes, predict what hypotonic, hypertonic and isotonic solutions do to a plant cell, and explain turgidity, plasmolysis and deplasmolysis with the thistle funnel and potato osmoscope experiments.
Why do some foods shrink in salt but swell in water?
Sprinkle salt on sliced cucumber for a salad and within minutes the slices go soft and a pool of water collects on the plate. Soak a handful of raisins in plain water overnight and they swell up plump and round. Both changes are caused by the same process moving water in opposite directions: osmosis.
Water moves across the membranes of living cells according to how concentrated the solutions on either side are.
- Salt on cucumber makes the liquid outside the cells more concentrated than the cell sap, so water leaves the cells and they go limp
- Plain water around raisins is less concentrated than the sugary contents of the cells, so water enters and they swell
Plants depend on exactly the same movement. A root absorbs water from the soil because the sap inside its cells is more concentrated than the soil water. When too much fertiliser is added and the soil water becomes more concentrated than the cell sap, the direction reverses and the plant wilts — the gardener's complaint that fertiliser has burnt the plant.
This part builds the ideas behind water absorption by roots:
- Diffusion, osmosis and imbibition — three ways substances move, with a biological example of each
- Semi-permeable and selectively permeable membranes, and endosmosis and exosmosis
- Hypotonic, hypertonic and isotonic solutions, and what each does to a plant cell
- Turgidity, flaccidity, plasmolysis and deplasmolysis, and two experiments that demonstrate osmosis
A simple rule to keep in mind throughout. Water always moves towards the more concentrated solution — from where water is more plentiful to where it is less. Every prediction in this chapter follows from that one rule and from knowing which membrane the water has to cross.
Part 2 then uses these ideas to explain how root hairs take in water and minerals and how water rises to the top of a tall tree.
This page covers the first part of the ICSE Class 10 Biology chapter on absorption by roots: diffusion, osmosis and imbibition, membranes, types of solution, and plasmolysis with experiments.
Water moves across the membranes of living cells according to how concentrated the solutions on either side are.
- Salt on cucumber makes the liquid outside the cells more concentrated than the cell sap, so water leaves the cells and they go limp
- Plain water around raisins is less concentrated than the sugary contents of the cells, so water enters and they swell
Plants depend on exactly the same movement. A root absorbs water from the soil because the sap inside its cells is more concentrated than the soil water. When too much fertiliser is added and the soil water becomes more concentrated than the cell sap, the direction reverses and the plant wilts — the gardener's complaint that fertiliser has burnt the plant.
This part builds the ideas behind water absorption by roots:
- Diffusion, osmosis and imbibition — three ways substances move, with a biological example of each
- Semi-permeable and selectively permeable membranes, and endosmosis and exosmosis
- Hypotonic, hypertonic and isotonic solutions, and what each does to a plant cell
- Turgidity, flaccidity, plasmolysis and deplasmolysis, and two experiments that demonstrate osmosis
A simple rule to keep in mind throughout. Water always moves towards the more concentrated solution — from where water is more plentiful to where it is less. Every prediction in this chapter follows from that one rule and from knowing which membrane the water has to cross.
Part 2 then uses these ideas to explain how root hairs take in water and minerals and how water rises to the top of a tall tree.
This page covers the first part of the ICSE Class 10 Biology chapter on absorption by roots: diffusion, osmosis and imbibition, membranes, types of solution, and plasmolysis with experiments.
What are diffusion, osmosis and imbibition, and how do they differ?
Diffusion is the spreading of molecules from a region of higher to lower concentration; osmosis is the movement of water molecules through a semi-permeable membrane from a dilute to a more concentrated solution; imbibition is the absorption of water by solid substances such as dry seeds and wood, which then swell.
1. Diffusion. The movement of molecules of a substance — gas, liquid or dissolved solid — from a region of their higher concentration to a region of their lower concentration, until they are evenly spread.
- No membrane is needed, and no energy is used
- Biological example: exchange of gases in leaves — carbon dioxide diffuses into the leaf through the stomata, and oxygen diffuses out during photosynthesis
- Everyday example: the smell of a lit incense stick spreading through a room
2. Osmosis. The 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 semi-permeable membrane.
- Only water moves, and a membrane is essential
- Biological example: absorption of water by root hairs from the soil
- Another example: water entering or leaving guard cells, which opens and closes the stomata
3. Imbibition. The absorption of water by solid substances, such as dry seeds, wood and dried fruit, causing them to swell, without forming a solution.
- Substances that imbibe are hydrophilic — they attract water — such as cellulose, starch and proteins
- Biological example: dry seeds soaking up water before germination, swelling and bursting the seed coat
- The swelling can exert considerable pressure, called imbibition pressure
The three compared:
- What moves: diffusion — any molecules; osmosis — water only; imbibition — water into a solid
- Membrane: diffusion — not needed; osmosis — semi-permeable membrane needed; imbibition — not needed
- Result: diffusion — even spreading; osmosis — water shifts between solutions; imbibition — swelling of the solid
Worked example — name the process.
- Chana soaked overnight grows much larger — imbibition by the dry seed
- The smell of cooking reaches the next room — diffusion
- A root hair takes in water from moist soil — osmosis
- Oxygen passes from the air sacs of the lungs into the blood — diffusion
An everyday example. Wooden doors and windows that close easily in winter often jam during the monsoon. The dry wood imbibes moisture from the humid air and swells — imbibition happening in your own home.
The boundary case — osmosis is a special case of diffusion. Osmosis is the diffusion of water, but across a semi-permeable membrane that stops the dissolved substance from following. Without the membrane, the solute would simply diffuse too, and the solutions would mix instead of water moving one way.
1. Diffusion. The movement of molecules of a substance — gas, liquid or dissolved solid — from a region of their higher concentration to a region of their lower concentration, until they are evenly spread.
- No membrane is needed, and no energy is used
- Biological example: exchange of gases in leaves — carbon dioxide diffuses into the leaf through the stomata, and oxygen diffuses out during photosynthesis
- Everyday example: the smell of a lit incense stick spreading through a room
2. Osmosis. The 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 semi-permeable membrane.
- Only water moves, and a membrane is essential
- Biological example: absorption of water by root hairs from the soil
- Another example: water entering or leaving guard cells, which opens and closes the stomata
3. Imbibition. The absorption of water by solid substances, such as dry seeds, wood and dried fruit, causing them to swell, without forming a solution.
- Substances that imbibe are hydrophilic — they attract water — such as cellulose, starch and proteins
- Biological example: dry seeds soaking up water before germination, swelling and bursting the seed coat
- The swelling can exert considerable pressure, called imbibition pressure
The three compared:
- What moves: diffusion — any molecules; osmosis — water only; imbibition — water into a solid
- Membrane: diffusion — not needed; osmosis — semi-permeable membrane needed; imbibition — not needed
- Result: diffusion — even spreading; osmosis — water shifts between solutions; imbibition — swelling of the solid
Worked example — name the process.
- Chana soaked overnight grows much larger — imbibition by the dry seed
- The smell of cooking reaches the next room — diffusion
- A root hair takes in water from moist soil — osmosis
- Oxygen passes from the air sacs of the lungs into the blood — diffusion
An everyday example. Wooden doors and windows that close easily in winter often jam during the monsoon. The dry wood imbibes moisture from the humid air and swells — imbibition happening in your own home.
The boundary case — osmosis is a special case of diffusion. Osmosis is the diffusion of water, but across a semi-permeable membrane that stops the dissolved substance from following. Without the membrane, the solute would simply diffuse too, and the solutions would mix instead of water moving one way.
How does a semi-permeable membrane differ from a selectively permeable one, and what are endosmosis and exosmosis?
A semi-permeable membrane lets only water pass, while a selectively permeable membrane — such as a living cell membrane — lets water and certain chosen substances pass; endosmosis is water entering a cell and exosmosis is water leaving it.
1. Semi-permeable membrane. A membrane that allows only solvent molecules — water — to pass through, and not the dissolved solute.
- Examples: parchment paper, cellophane and the membrane lining an eggshell
- These are non-living, and are used in experiments to demonstrate osmosis
2. Selectively permeable membrane. A membrane that allows water and some selected solutes to pass, but stops others.
- Example: the cell membrane, or plasma membrane, of a living cell, and the membrane around the vacuole
- It is living and can control which substances enter and leave
3. Freely permeable. For contrast, the cell wall of a plant cell is freely permeable — it lets water and dissolved substances pass easily. It is the cell membrane, not the wall, that controls osmosis.
4. Endosmosis. The entry of water into a cell when it is placed in a solution less concentrated than its own contents.
- The cell swells
- Example: raisins placed in water swell up
5. Exosmosis. The exit of water from a cell when it is placed in a solution more concentrated than its own contents.
- The cell shrinks
- Example: grapes placed in strong sugar syrup shrivel, and salted vegetables release water
Worked example — predict the direction. A cell whose sap is a sugar solution is placed in each of three solutions. Which way does water move?
- ** sugar solution — outside is more dilute, so water enters: endosmosis
- sugar solution — equal concentration, so no net movement
- sugar solution — outside is more concentrated, so water leaves: exosmosis
The water always moves towards the more concentrated side.
An everyday example. Families preparing mango or lemon pickle often salt the pieces and leave them in the sun. Water drawn out of the fruit by exosmosis collects around the pieces, and the fruit shrinks and softens before the spices are added.
The boundary case — why the difference between the two membranes matters. A semi-permeable membrane in an experiment passes water only, giving a clean demonstration of osmosis. A living cell's selectively permeable membrane also lets through certain dissolved minerals, and can move some of them using energy** — which is how roots take in minerals, the subject of Part 2.
1. Semi-permeable membrane. A membrane that allows only solvent molecules — water — to pass through, and not the dissolved solute.
- Examples: parchment paper, cellophane and the membrane lining an eggshell
- These are non-living, and are used in experiments to demonstrate osmosis
2. Selectively permeable membrane. A membrane that allows water and some selected solutes to pass, but stops others.
- Example: the cell membrane, or plasma membrane, of a living cell, and the membrane around the vacuole
- It is living and can control which substances enter and leave
3. Freely permeable. For contrast, the cell wall of a plant cell is freely permeable — it lets water and dissolved substances pass easily. It is the cell membrane, not the wall, that controls osmosis.
4. Endosmosis. The entry of water into a cell when it is placed in a solution less concentrated than its own contents.
- The cell swells
- Example: raisins placed in water swell up
5. Exosmosis. The exit of water from a cell when it is placed in a solution more concentrated than its own contents.
- The cell shrinks
- Example: grapes placed in strong sugar syrup shrivel, and salted vegetables release water
Worked example — predict the direction. A cell whose sap is a sugar solution is placed in each of three solutions. Which way does water move?
- ** sugar solution — outside is more dilute, so water enters: endosmosis
- sugar solution — equal concentration, so no net movement
- sugar solution — outside is more concentrated, so water leaves: exosmosis
The water always moves towards the more concentrated side.
An everyday example. Families preparing mango or lemon pickle often salt the pieces and leave them in the sun. Water drawn out of the fruit by exosmosis collects around the pieces, and the fruit shrinks and softens before the spices are added.
The boundary case — why the difference between the two membranes matters. A semi-permeable membrane in an experiment passes water only, giving a clean demonstration of osmosis. A living cell's selectively permeable membrane also lets through certain dissolved minerals, and can move some of them using energy** — which is how roots take in minerals, the subject of Part 2.
What happens to a plant cell in hypotonic, hypertonic and isotonic solutions?
In a hypotonic solution a plant cell gains water and becomes turgid; in a hypertonic solution it loses water, becomes flaccid and may plasmolyse; in an isotonic solution there is no net movement of water and the cell is unchanged.
1. Hypotonic solution. The outside solution is less concentrated than the cell sap — it has more water.
- Water enters the cell by endosmosis
- The vacuole swells and presses the cytoplasm against the cell wall
- The cell becomes turgid, but does not burst, because the strong cell wall resists further swelling
2. Hypertonic solution. The outside solution is more concentrated than the cell sap — it has less water.
- Water leaves the cell by exosmosis
- The vacuole shrinks and the cell becomes flaccid
- If enough water is lost, the protoplast pulls away from the cell wall — plasmolysis
3. Isotonic solution. The outside solution has the same concentration as the cell sap.
- Water enters and leaves at the same rate, so there is no net movement
- The cell remains unchanged
These terms are always comparisons. A solution is hypotonic or hypertonic relative to a particular cell — the same solution can be hypotonic to one cell and hypertonic to another.
Worked example — predicting the effect. The cell sap of some onion cells is equivalent to a molar sugar solution. Predict what happens in , and molar sugar solutions.
- ** molar — less concentrated than the sap: hypotonic — endosmosis — cells become turgid
- molar — equal: isotonic — no change
- molar — more concentrated: hypertonic — exosmosis — cells become flaccid and plasmolyse
Plant cells versus animal cells in water. A plant cell placed in pure water becomes turgid but its cell wall stops it bursting. An animal cell such as a red blood cell has no cell wall, so in pure water it swells until it bursts. That is why the salt solution used in hospital drips is made isotonic with blood** — normal saline contains about sodium chloride — so that blood cells neither swell nor shrink.
An everyday example. Adding too much fertiliser to a potted plant makes the soil water hypertonic to the root cells. Water leaves the roots by exosmosis instead of entering, and the plant wilts — even though the soil is moist.
The boundary case. Salt spread on weeds in some paths kills them for the same reason: the soil solution becomes so hypertonic that the plant cells lose water continuously and cannot recover. The chemistry that keeps a plant alive in normal soil kills it when the concentrations are reversed.
1. Hypotonic solution. The outside solution is less concentrated than the cell sap — it has more water.
- Water enters the cell by endosmosis
- The vacuole swells and presses the cytoplasm against the cell wall
- The cell becomes turgid, but does not burst, because the strong cell wall resists further swelling
2. Hypertonic solution. The outside solution is more concentrated than the cell sap — it has less water.
- Water leaves the cell by exosmosis
- The vacuole shrinks and the cell becomes flaccid
- If enough water is lost, the protoplast pulls away from the cell wall — plasmolysis
3. Isotonic solution. The outside solution has the same concentration as the cell sap.
- Water enters and leaves at the same rate, so there is no net movement
- The cell remains unchanged
These terms are always comparisons. A solution is hypotonic or hypertonic relative to a particular cell — the same solution can be hypotonic to one cell and hypertonic to another.
Worked example — predicting the effect. The cell sap of some onion cells is equivalent to a molar sugar solution. Predict what happens in , and molar sugar solutions.
- ** molar — less concentrated than the sap: hypotonic — endosmosis — cells become turgid
- molar — equal: isotonic — no change
- molar — more concentrated: hypertonic — exosmosis — cells become flaccid and plasmolyse
Plant cells versus animal cells in water. A plant cell placed in pure water becomes turgid but its cell wall stops it bursting. An animal cell such as a red blood cell has no cell wall, so in pure water it swells until it bursts. That is why the salt solution used in hospital drips is made isotonic with blood** — normal saline contains about sodium chloride — so that blood cells neither swell nor shrink.
An everyday example. Adding too much fertiliser to a potted plant makes the soil water hypertonic to the root cells. Water leaves the roots by exosmosis instead of entering, and the plant wilts — even though the soil is moist.
The boundary case. Salt spread on weeds in some paths kills them for the same reason: the soil solution becomes so hypertonic that the plant cells lose water continuously and cannot recover. The chemistry that keeps a plant alive in normal soil kills it when the concentrations are reversed.
What are turgidity, flaccidity, plasmolysis and deplasmolysis, and how is osmosis demonstrated?
A turgid cell is full of water and firm, a flaccid cell has lost water and is soft, plasmolysis is the shrinking of the protoplast away from the cell wall in a hypertonic solution, and deplasmolysis is its recovery in water; osmosis is demonstrated with a thistle funnel or a potato osmoscope.
1. Turgidity. The state of a cell that has absorbed as much water as it can, so that its cytoplasm is pressed firmly against the cell wall. The pressure of the contents on the wall is turgor pressure.
- Importance: keeps leaves and soft stems firm and upright, and opens the stomata when guard cells are turgid
2. Flaccidity. The state of a cell that has lost water, so that it is soft and no longer pressed against its wall.
- A plant whose cells are flaccid wilts
3. Plasmolysis. The shrinkage of the protoplast — the cell membrane with its cytoplasm and vacuole — away from the cell wall, caused by loss of water by exosmosis in a hypertonic solution.
- The space between the wall and the shrunken protoplast fills with the outside solution, because the cell wall is freely permeable
4. Deplasmolysis. The return of a plasmolysed cell to its normal state when it is placed in water or a hypotonic solution, as water re-enters by endosmosis.
- Deplasmolysis is possible only if plasmolysis has not lasted too long; prolonged plasmolysis kills the cell
Experiment 1 — the thistle funnel.
- Setup: tie a semi-permeable membrane, such as an egg membrane or parchment paper, tightly over the mouth of a thistle funnel. Fill the funnel with concentrated sugar solution and invert it in a beaker of water. Mark the level of the solution in the stem
- Observation: after some hours, the level in the stem rises
- Inference: water has passed through the membrane into the sugar solution — osmosis
- Control: a funnel filled with water instead of sugar solution shows no rise
Experiment 2 — the potato osmoscope.
- Setup: peel a large potato, cut its base flat, and hollow out a cavity in the top. Pour concentrated sugar solution into the cavity and mark its level. Stand the potato in a dish of water
- Observation: after some hours, the level in the cavity rises
- Inference: water from the dish has passed through the living potato cells into the sugar solution — osmosis, with the cells acting as a membrane
- Control: a boiled potato set up the same way shows no rise, because boiling kills the cells and destroys their membranes
Worked example — reversing the osmoscope. The cavity is filled with water and the potato is placed in concentrated sugar solution. What happens to the level in the cavity?
The level falls, because water now moves out of the cavity, through the potato cells, into the more concentrated solution outside.
An everyday example. Leafy vegetables such as spinach droop in a hot market by afternoon as their cells turn flaccid. Sprinkling them with water restores their firmness — the cells regain turgidity by endosmosis.
The boundary case. A plant can wilt without any of its cells plasmolysing. Wilting only needs the cells to become flaccid; plasmolysis is the more extreme stage in which the protoplast actually separates from the wall.
1. Turgidity. The state of a cell that has absorbed as much water as it can, so that its cytoplasm is pressed firmly against the cell wall. The pressure of the contents on the wall is turgor pressure.
- Importance: keeps leaves and soft stems firm and upright, and opens the stomata when guard cells are turgid
2. Flaccidity. The state of a cell that has lost water, so that it is soft and no longer pressed against its wall.
- A plant whose cells are flaccid wilts
3. Plasmolysis. The shrinkage of the protoplast — the cell membrane with its cytoplasm and vacuole — away from the cell wall, caused by loss of water by exosmosis in a hypertonic solution.
- The space between the wall and the shrunken protoplast fills with the outside solution, because the cell wall is freely permeable
4. Deplasmolysis. The return of a plasmolysed cell to its normal state when it is placed in water or a hypotonic solution, as water re-enters by endosmosis.
- Deplasmolysis is possible only if plasmolysis has not lasted too long; prolonged plasmolysis kills the cell
Experiment 1 — the thistle funnel.
- Setup: tie a semi-permeable membrane, such as an egg membrane or parchment paper, tightly over the mouth of a thistle funnel. Fill the funnel with concentrated sugar solution and invert it in a beaker of water. Mark the level of the solution in the stem
- Observation: after some hours, the level in the stem rises
- Inference: water has passed through the membrane into the sugar solution — osmosis
- Control: a funnel filled with water instead of sugar solution shows no rise
Experiment 2 — the potato osmoscope.
- Setup: peel a large potato, cut its base flat, and hollow out a cavity in the top. Pour concentrated sugar solution into the cavity and mark its level. Stand the potato in a dish of water
- Observation: after some hours, the level in the cavity rises
- Inference: water from the dish has passed through the living potato cells into the sugar solution — osmosis, with the cells acting as a membrane
- Control: a boiled potato set up the same way shows no rise, because boiling kills the cells and destroys their membranes
Worked example — reversing the osmoscope. The cavity is filled with water and the potato is placed in concentrated sugar solution. What happens to the level in the cavity?
The level falls, because water now moves out of the cavity, through the potato cells, into the more concentrated solution outside.
An everyday example. Leafy vegetables such as spinach droop in a hot market by afternoon as their cells turn flaccid. Sprinkling them with water restores their firmness — the cells regain turgidity by endosmosis.
The boundary case. A plant can wilt without any of its cells plasmolysing. Wilting only needs the cells to become flaccid; plasmolysis is the more extreme stage in which the protoplast actually separates from the wall.
Exam tip
What earns full marks on osmosis and plasmolysis questions?
Define each process with its key condition — membrane, direction and type of solution — and describe experiments in the order setup, observation, inference and control.
- Define osmosis fully: water molecules, from dilute to concentrated solution, through a semi-permeable membrane
- Give a biological example for each of diffusion, osmosis and imbibition
- Name examples of semi-permeable membranes — parchment paper, cellophane, egg membrane — and say the cell membrane is selectively permeable
- State that the cell wall is freely permeable
- Define endosmosis and exosmosis by the direction of water movement
- Compare the solution with the cell sap when using hypotonic, hypertonic or isotonic
- Explain why a plant cell does not burst in water — the cell wall
- Describe plasmolysis as the protoplast shrinking away from the wall, and deplasmolysis as its recovery
- For the osmoscope, mention the boiled potato control and why it shows no change
- For the thistle funnel, name the membrane and the solutions inside and outside
The misconception to name. In osmosis, the solute does not move. Water moves towards the more concentrated solution, not sugar towards the water. Writing that sugar passes out through the membrane contradicts the definition of a semi-permeable membrane.
A second trap. Mixing up hypotonic and hypertonic. Hyper- means more concentrated than the cell, so water leaves and the cell shrinks. Linking the prefix to more solute outside keeps the direction of water movement correct.
- Define osmosis fully: water molecules, from dilute to concentrated solution, through a semi-permeable membrane
- Give a biological example for each of diffusion, osmosis and imbibition
- Name examples of semi-permeable membranes — parchment paper, cellophane, egg membrane — and say the cell membrane is selectively permeable
- State that the cell wall is freely permeable
- Define endosmosis and exosmosis by the direction of water movement
- Compare the solution with the cell sap when using hypotonic, hypertonic or isotonic
- Explain why a plant cell does not burst in water — the cell wall
- Describe plasmolysis as the protoplast shrinking away from the wall, and deplasmolysis as its recovery
- For the osmoscope, mention the boiled potato control and why it shows no change
- For the thistle funnel, name the membrane and the solutions inside and outside
The misconception to name. In osmosis, the solute does not move. Water moves towards the more concentrated solution, not sugar towards the water. Writing that sugar passes out through the membrane contradicts the definition of a semi-permeable membrane.
A second trap. Mixing up hypotonic and hypertonic. Hyper- means more concentrated than the cell, so water leaves and the cell shrinks. Linking the prefix to more solute outside keeps the direction of water movement correct.
Did you know
Why do pickles and murabba keep for months without spoiling?
A jar of mango pickle made with plenty of salt, or a jar of sweet murabba soaked in thick sugar syrup, can sit in a kitchen cupboard for months. Fresh fruit left on the same shelf would rot within days. The difference is osmosis, working against the microbes that cause spoilage.
Bacteria and moulds are living cells, surrounded by membranes just like plant cells. To grow and multiply, they need water.
- The heavily salted liquid in a pickle, or the thick sugar syrup of a murabba, is strongly hypertonic compared with the contents of any microbe that lands in it
- Water moves out of the microbe by exosmosis, towards the more concentrated liquid outside
- The microbe loses water, its protoplast shrinks — it is effectively plasmolysed — and it cannot grow
So salt and sugar preserve food not by poisoning microbes, but by drawing water out of them.
The same principle explains several kitchen practices.
- Jams and jellies keep well because of their very high sugar content
- Salted, sun-dried fish and dried fruits resist spoilage because their surfaces are too concentrated for microbes to take up water
- Traditional pickling often begins by salting the fruit pieces and leaving them in the sun, drawing water out of the fruit itself before the oil and spices are added
And it explains why these foods can still spoil. If a wet spoon adds water to the jar, or the pickle has too little salt, the liquid near the surface becomes less concentrated. Microbes there can then take up water and grow — which is why a layer of oil on top and a dry spoon are part of the traditional pickle-making rules in many homes.
One process, then, links a wilting plant in over-fertilised soil, a salted cucumber on a salad plate and a jar of pickle that lasts a whole season — in each case, water moving out of living cells towards a more concentrated solution.
Bacteria and moulds are living cells, surrounded by membranes just like plant cells. To grow and multiply, they need water.
- The heavily salted liquid in a pickle, or the thick sugar syrup of a murabba, is strongly hypertonic compared with the contents of any microbe that lands in it
- Water moves out of the microbe by exosmosis, towards the more concentrated liquid outside
- The microbe loses water, its protoplast shrinks — it is effectively plasmolysed — and it cannot grow
So salt and sugar preserve food not by poisoning microbes, but by drawing water out of them.
The same principle explains several kitchen practices.
- Jams and jellies keep well because of their very high sugar content
- Salted, sun-dried fish and dried fruits resist spoilage because their surfaces are too concentrated for microbes to take up water
- Traditional pickling often begins by salting the fruit pieces and leaving them in the sun, drawing water out of the fruit itself before the oil and spices are added
And it explains why these foods can still spoil. If a wet spoon adds water to the jar, or the pickle has too little salt, the liquid near the surface becomes less concentrated. Microbes there can then take up water and grow — which is why a layer of oil on top and a dry spoon are part of the traditional pickle-making rules in many homes.
One process, then, links a wilting plant in over-fertilised soil, a salted cucumber on a salad plate and a jar of pickle that lasts a whole season — in each case, water moving out of living cells towards a more concentrated solution.
Exam relevance
How does osmosis carry into NEET Biology and Class 12 Chemistry?
This is foundation work for Class 11 Cell: The Unit of Life in NEET Biology, and for Class 12 Solutions in Chemistry, which is examined in both NEET and JEE Main. The treatment of water movement in plants in NEET depends on the current official syllabus, so check how far plant transport is covered; the membrane and osmosis ideas below are used regardless.
Where membranes lead. Class 11 Cell: The Unit of Life describes the plasma membrane as selectively permeable and explains passive transport — diffusion and osmosis — and active transport, which uses energy. Statement-based questions on which substances cross membranes, and how, build directly on the definitions in this lesson.
Where osmosis leads in Chemistry. Class 12 Solutions treats osmosis quantitatively. Osmotic pressure — the pressure needed to stop osmosis — is calculated from the concentration of the solution and the temperature, and solutions are classified as isotonic, hypotonic and hypertonic exactly as here. Numericals on osmotic pressure, and questions on why blood cells swell or shrink in different solutions, appear in both NEET and JEE Main.
Where plasmolysis leads. The same chemistry chapter explains reverse osmosis, used to purify drinking water and desalinate seawater, and uses plasmolysis and the swelling of raisins as examples of osmosis in living cells. The idea that water moves towards the more concentrated solution is the qualitative core of every such question.
Where the experiments lead. Understanding the control in the potato osmoscope — a boiled potato with destroyed membranes — is the kind of reasoning tested in experimental-design questions.
Question types to expect. At this level: definitions, differences, predictions for types of solution, and experiments. In competitive papers: membrane transport statements, osmotic pressure numericals, isotonic solutions such as normal saline, and reverse osmosis applications.
The single trap that costs marks. Reversing the direction of water movement. Water moves from the hypotonic to the hypertonic side — from lower to higher solute concentration — and options that describe solute moving instead are common distractors.
A second trap. Treating the cell wall as the membrane that controls osmosis. The cell wall is freely permeable; the cell membrane is selectively permeable, and questions on plasmolysis depend on that difference.
Board versus competitive emphasis. The ICSE paper marks precise definitions, labelled experiments and predictions; competitive papers mark statements, calculations and applications. The transferable habit is always identifying which side is more concentrated before deciding where water goes.
Where membranes lead. Class 11 Cell: The Unit of Life describes the plasma membrane as selectively permeable and explains passive transport — diffusion and osmosis — and active transport, which uses energy. Statement-based questions on which substances cross membranes, and how, build directly on the definitions in this lesson.
Where osmosis leads in Chemistry. Class 12 Solutions treats osmosis quantitatively. Osmotic pressure — the pressure needed to stop osmosis — is calculated from the concentration of the solution and the temperature, and solutions are classified as isotonic, hypotonic and hypertonic exactly as here. Numericals on osmotic pressure, and questions on why blood cells swell or shrink in different solutions, appear in both NEET and JEE Main.
Where plasmolysis leads. The same chemistry chapter explains reverse osmosis, used to purify drinking water and desalinate seawater, and uses plasmolysis and the swelling of raisins as examples of osmosis in living cells. The idea that water moves towards the more concentrated solution is the qualitative core of every such question.
Where the experiments lead. Understanding the control in the potato osmoscope — a boiled potato with destroyed membranes — is the kind of reasoning tested in experimental-design questions.
Question types to expect. At this level: definitions, differences, predictions for types of solution, and experiments. In competitive papers: membrane transport statements, osmotic pressure numericals, isotonic solutions such as normal saline, and reverse osmosis applications.
The single trap that costs marks. Reversing the direction of water movement. Water moves from the hypotonic to the hypertonic side — from lower to higher solute concentration — and options that describe solute moving instead are common distractors.
A second trap. Treating the cell wall as the membrane that controls osmosis. The cell wall is freely permeable; the cell membrane is selectively permeable, and questions on plasmolysis depend on that difference.
Board versus competitive emphasis. The ICSE paper marks precise definitions, labelled experiments and predictions; competitive papers mark statements, calculations and applications. The transferable habit is always identifying which side is more concentrated before deciding where water goes.
Key takeaways
What must you be able to do from this part?
Three processes, three kinds of membrane, three kinds of solution, four cell states and two experiments.
- Diffusion: molecules move from higher to lower concentration, no membrane — gas exchange in leaves
- Osmosis: water moves from dilute to concentrated solution through a semi-permeable membrane — water uptake by root hairs
- Imbibition: absorption of water by solids such as dry seeds and wood, causing swelling
- Osmosis is diffusion of water across a membrane that stops the solute
- Semi-permeable membrane: passes only water — parchment paper, cellophane, egg membrane
- Selectively permeable membrane: passes water and chosen solutes — the living cell membrane
- Cell wall: freely permeable
- Endosmosis: water enters the cell — raisins swell in water; exosmosis: water leaves — grapes shrivel in syrup
- Hypotonic: outside less concentrated — cell gains water and becomes turgid
- Hypertonic: outside more concentrated — cell loses water, becomes flaccid and plasmolyses
- Isotonic: equal concentration — no net change
- Plant cells do not burst in water because of the cell wall; red blood cells do
- Turgidity: cell full of water, pressed against the wall; flaccidity: soft cell that has lost water
- Plasmolysis: protoplast shrinks from the wall in a hypertonic solution; deplasmolysis: recovery in water
- Thistle funnel: sugar solution behind a membrane, level rises in water; control with water shows no rise
- Potato osmoscope: sugar solution in the cavity rises; boiled potato control shows no rise
The sharpest self-test is three beakers and one onion. Imagine onion peel placed in pure water, in a solution matching its cell sap and in strong salt solution, and describe what a microscope would show in each — then say what would happen if the salted peel were moved back into water.
- Diffusion: molecules move from higher to lower concentration, no membrane — gas exchange in leaves
- Osmosis: water moves from dilute to concentrated solution through a semi-permeable membrane — water uptake by root hairs
- Imbibition: absorption of water by solids such as dry seeds and wood, causing swelling
- Osmosis is diffusion of water across a membrane that stops the solute
- Semi-permeable membrane: passes only water — parchment paper, cellophane, egg membrane
- Selectively permeable membrane: passes water and chosen solutes — the living cell membrane
- Cell wall: freely permeable
- Endosmosis: water enters the cell — raisins swell in water; exosmosis: water leaves — grapes shrivel in syrup
- Hypotonic: outside less concentrated — cell gains water and becomes turgid
- Hypertonic: outside more concentrated — cell loses water, becomes flaccid and plasmolyses
- Isotonic: equal concentration — no net change
- Plant cells do not burst in water because of the cell wall; red blood cells do
- Turgidity: cell full of water, pressed against the wall; flaccidity: soft cell that has lost water
- Plasmolysis: protoplast shrinks from the wall in a hypertonic solution; deplasmolysis: recovery in water
- Thistle funnel: sugar solution behind a membrane, level rises in water; control with water shows no rise
- Potato osmoscope: sugar solution in the cavity rises; boiled potato control shows no rise
The sharpest self-test is three beakers and one onion. Imagine onion peel placed in pure water, in a solution matching its cell sap and in strong salt solution, and describe what a microscope would show in each — then say what would happen if the salted peel were moved back into water.