OsmosisCambridge IGCSE Biology: Revision notes
Section 1
What is osmosis and how does it differ from diffusion?
Osmosis is the net movement of water molecules from a region of higher water potential (more dilute solution) to a region of lower water potential (more concentrated solution) through a partially permeable membrane.
Key differences from diffusion:
- Osmosis involves water molecules only, whereas diffusion can involve any particles
- Osmosis requires a partially permeable membrane, whereas diffusion does not
- Osmosis is driven by water potential differences, not concentration gradients of solutes
- Osmosis is a passive process — no energy is required, as water molecules move randomly but there is a net movement in one direction
The partially permeable membrane allows water molecules and small dissolved particles to pass through, but blocks larger solute molecules. This selective permeability is essential for osmosis to occur.
Think of a partially permeable membrane like a sieve that lets water through but traps salt grains. Water molecules keep bouncing through in both directions, but more escape from the pure water side into the salt solution side because salt particles 'get in the way' on that side.
Examiners expect you to use the term 'water potential' not 'concentration'. Always state that water moves from higher water potential to lower water potential, and remember that a pure solution has higher water potential than a concentrated solution.
Section 2
How does water move into and out of cells by osmosis?
Water constantly moves across the cell membrane — a partially permeable membrane that surrounds all cells. The direction and extent of this movement depends on the water potential inside the cell compared to outside.
Movement into cells:
- Occurs when the outside solution has higher water potential (is more dilute) than inside the cell
- Water moves in by osmosis, diluting the cell contents
- This is the typical situation for plant cells in normal conditions
Movement out of cells:
- Occurs when the outside solution has lower water potential (is more concentrated) than inside the cell
- Water moves out by osmosis, concentrating the cell contents
- This is why placing cells in concentrated salt or sugar solutions causes water loss
Net movement:
- Even though water moves in both directions constantly, there is a net movement from the region of higher water potential to lower water potential
- The rate of net movement depends on the difference in water potential between the inside and outside of the cell
- At equilibrium, water potential is equal inside and outside, and there is no net movement (though water molecules still move both ways randomly)
Students often think water only moves one way or that osmosis stops immediately. Remember: water molecules move randomly in both directions constantly, but there is a net movement towards lower water potential. Movement continues until water potentials are equal.
Section 3
What are the effects of osmosis on plant cells?
Plant cells respond dramatically to changes in external water potential because they have a rigid cell wall in addition to the flexible cell membrane. Four key states describe these effects:
| State | Definition | Appearance | Water Potential Status |
|---|---|---|---|
| Turgid | Cell is firm and rigid; membrane presses against cell wall | Plant tissue is crisp and firm; leaves stand upright | Water potential inside equals outside; turgor pressure pushes outward |
| Turgor Pressure | The force exerted by water inside the cell pressing outward on the cell wall | Provides support and rigidity to the plant | Created by osmotic entry of water; typical in healthy plant cells |
| Flaccid | Cell has lost turgor pressure; cell membrane no longer presses against cell wall | Plant tissue becomes limp and droops; leaves wilt | Water potential inside is equal to outside; no net water movement |
| Plasmolysed | Cell membrane pulls away from the cell wall due to excessive water loss | Visible gap between cell membrane and cell wall; cytoplasm shrinks | Water has moved out by osmosis; external solution has very low water potential |
Sequence of changes when plant tissue is placed in concentrated solution:
- Water potential outside becomes lower than inside
- Water moves out of the cell by osmosis
- Cell becomes flaccid (turgor pressure lost)
- If water loss continues, cell becomes plasmolysed
- If returned to dilute solution, water re-enters and cell becomes turgid again
Importance for plants:
- Turgor pressure is essential for plant support — it keeps stems rigid and leaves firm
- Without turgor, plants wilt and cannot maintain their shape
- Plants actively absorb water by osmosis to maintain turgidity
Examiners test whether you can distinguish between these states. Use the phrase 'cell membrane pulls away from cell wall' specifically for plasmolysis, and 'water pressure inside decreases' for flaccidity. Always link water movement by osmosis to the observable change.
If a plant cell with water potential of −200 kPa is placed in a solution with water potential of −500 kPa: water moves out by osmosis (from −200 to −500), turgor pressure decreases, and if water loss is severe enough, plasmolysis occurs with the membrane visibly separated from the cell wall.
Section 4
How is osmosis investigated using dialysis tubing?
Dialysis tubing is used as a practical model of a partially permeable membrane to demonstrate osmosis principles. The tubing is made of partially permeable material — similar to the cell membrane in function.
Typical osmosis investigation procedure:
- Fill a piece of dialysis tubing with a concentrated sugar solution
- Tie the ends of the tubing to seal it completely
- Measure the initial mass of the sealed tubing
- Suspend the tubing in a beaker of distilled water (or dilute solution)
- At regular time intervals, remove the tubing, blot dry, and measure its mass
- Record observations of any visible changes (tubing becoming firmer or more rigid)
Expected results:
- The tubing gains mass as water moves in by osmosis
- The tubing may become tighter or firmer as internal pressure increases
- The extent of mass increase depends on the initial solute concentration inside the tubing
- Rate of mass increase slows over time as water potentials approach equilibrium
Why this models osmosis:
- The dialysis tubing is partially permeable (allows water through, blocks large sugar molecules)
- Water molecules move from higher water potential (outside, pure water) to lower water potential (inside, concentrated sugar solution)
- The net result is water moving inward
- The tubing demonstrates that osmosis is a real, measurable phenomenon, not just a theoretical concept
Variables and controls:
- Independent variable: Initial concentration of sugar solution inside tubing
- Dependent variable: Change in mass or time taken to reach equilibrium
- Control: Tubing placed in salt solution of same concentration (to show osmosis is due to water potential difference, not solute type)
When describing this investigation, emphasise that the tubing must be blotted dry before each measurement to remove external water — this ensures you measure only the water inside the tubing due to osmosis. State clearly that water moved in because the sugar solution inside has lower water potential than distilled water outside.
Students often forget to mention 'drying the outside of the tubing' before weighing or assume sugar passes through the membrane. The tubing is partially permeable — water passes through freely, but sugar molecules cannot, which is why water enters to dilute the inside solution.
Section 5
Why is water important as a solvent and what role does osmosis play?
Water is the universal solvent in organisms and has crucial functions in digestion, excretion, and transport. Osmosis is central to maintaining water balance in these systems.
Water as a solvent in digestion:
- Enzymes must work in aqueous solution to break down food molecules
- Soluble products of digestion (glucose, amino acids) dissolve in water for absorption
- Water dissolves nutrients, making them available for cells to use
Water as a solvent in excretion:
- Waste products (urea, excess salts) must dissolve in water to be transported away from cells
- Kidneys concentrate or dilute urine by controlling water reabsorption via osmosis
- Osmosis allows the body to regulate water loss and solute concentration
Water as a solvent in transport:
- Dissolved nutrients, gases, and hormones travel in blood plasma, which is mostly water
- Osmosis ensures water moves into and out of blood capillaries to maintain correct solute concentration
- Lymph transport relies on water to carry dissolved substances
Role of osmosis in water uptake and loss:
- Uptake: Cells absorb water by osmosis when external water potential is higher (e.g. plant root cells absorbing soil water)
- Loss: Cells lose water by osmosis when external water potential is lower (e.g. in dehydration or when placed in concentrated salt solution)
- Regulation: Organisms control water balance by adjusting internal solute concentration, which changes water potential and drives osmosis in the desired direction
- Kidney function: The kidneys use osmosis to recover water from the filtrate — solutes are selectively reabsorbed, lowering water potential in the blood, which draws water back by osmosis
Importance for organisms:
- Without osmotic water movement, cells cannot maintain correct water content
- Dehydration (too little water) or overhydration (too much water) damages cells
- Osmosis allows precise control of cellular water content, which is essential for survival
Examiners expect you to link osmosis to real biological contexts. When explaining water uptake in plants or kidney function, always describe the water potential difference that causes osmosis, then explain why this is important (e.g. 'osmosis allows the kidney to reabsorb water, maintaining blood water potential').
A kidney tubule reabsorbs glucose and ions into the blood. This lowers the water potential of the blood below that of the tubule fluid, causing water to move out by osmosis. This concentrates the urine and prevents dehydration — demonstrating how osmosis is essential for controlling water loss.
Must Know
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Osmosis is the net movement of water molecules from higher water potential (dilute solution) to lower water potential (concentrated solution) through a partially permeable membrane — it is a passive process requiring no energy
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Water moves into cells when external solution has higher water potential; water moves out when external solution has lower water potential; movement continues until water potentials are equal (equilibrium)
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Turgid cells are firm and rigid with water pressure inside pressing outward (turgor pressure); flaccid cells have lost turgor and appear limp; plasmolysed cells have a visible gap between cell membrane and cell wall after excessive water loss
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Turgor pressure provides structural support to plants — without it, plants cannot stand upright. Plasmolysis is reversible: if a plasmolysed cell is returned to a dilute solution, water re-enters by osmosis and the cell becomes turgid again
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Dialysis tubing experiments model osmosis: water enters through the partially permeable membrane into a concentrated sugar solution, increasing the tubing's mass. This demonstrates that osmosis is a real, measurable process driven by water potential differences
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Water is essential as a solvent in digestion (dissolving nutrients), excretion (dissolving wastes), and transport (carrying dissolved substances in blood). Osmosis controls water balance in organisms — the kidneys use osmosis to reabsorb water by changing solute concentration, and plants absorb water from soil by osmosis to maintain turgor. Without osmotic water movement, cells cannot maintain correct water content and organisms cannot survive
That's the notes covered.
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