Cambridge O Level Biology · Syllabus 5090 · Movement Into and Out of Cells
Osmosis
What is Osmosis?
Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane. Only water molecules move; the membrane holds back larger solute particles. Like diffusion it is driven by the random movement of the water molecules themselves and requires no energy from the cell, and at equilibrium water continues to cross in both directions with no net movement.
This definition is part of the Movement Into and Out of Cells chapter in Cambridge O Level Biology.
Osmosis in context
Every cell is surrounded by a membrane, and everything a cell needs must cross it. Three processes do the crossing. Diffusion is the net movement of molecules or ions from higher to lower concentration, down a concentration gradient, as a result of random movement. Osmosis is the same random movement applied to water molecules only, from higher to lower water potential, through a partially permeable membrane. Neither of those two costs the cell anything — the energy comes from the kinetic energy the particles already have. Active transport is the exception: it drags molecules or ions against a concentration gradient, from lower to higher concentration, through a cell membrane, using energy released during respiration. Read every question in this topic by asking three things in order: which particle is moving, which way relative to the gradient, and who is paying for it?
Water acts as a solvent in living organisms. Substances that dissolve in it can be transported in a liquid, can react in solution, and can be excreted dissolved in a liquid. Digested food must dissolve before it can be absorbed; blood plasma carries dissolved glucose, amino acids, ions, hormones, carbon dioxide and urea; urine removes dissolved urea and excess ions; and plant xylem carries water containing dissolved mineral ions. Every process in the rest of this chapter — diffusion, osmosis and active transport — assumes that the substance being moved is already in solution.
Molecules and ions possess kinetic energy and are in constant, random motion. Each particle travels until it collides, then moves off in a new direction; no particle knows where the concentration is lower and none is aiming for it. But where particles are crowded, more of them happen to wander out of that region than happen to wander in, so the population as a whole shows a net movement from higher to lower concentration. Because the driving energy is the particles’ own random motion, the cell supplies no energy for diffusion or for osmosis.
Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane. Water potential describes the tendency of water molecules to move out of a region. Pure water has the highest water potential of all; adding solute lowers it, so a dilute solution has a higher water potential than a concentrated one. Water therefore moves from dilute towards concentrated — but you must say it in water-potential language, because that is the language the definition is marked in.
Turgor pressure is the outward pressure of the cell contents against the cell wall. Water entering by osmosis expands the vacuole, which presses the cytoplasm and membrane against the rigid cellulose wall; the wall pushes back and the pressure rises until the cell is turgid. Lose that water and turgor pressure falls: the cell becomes flaccid, and with severe loss the cell membrane and cell contents pull away from the cell wall — the cell is plasmolysed. Because non-woody plants are held up by the turgor of their cells rather than by wood, losing turgor across a whole tissue makes the plant wilt.
Active transport is the movement of molecules or ions through a cell membrane from a region of lower concentration to a region of higher concentration, against a concentration gradient, using energy released during respiration. Diffusion and osmosis can only take a substance downhill. When a cell needs something that is already more concentrated inside than outside — nitrate ions in a root hair, glucose in the last stretch of the small intestine, sodium ions in a kidney tubule — the only way to get it is to spend energy moving it uphill. That energy is released by respiration; the mitochondria transfer it, they do not manufacture it.
Common mistakes with Osmosis
- 1. “Osmosis is the movement of water from a high concentration to a low concentration.” Why wrong“High concentration” of what? Of water, or of solute? The two point in opposite directions, so the sentence is ambiguous, and a definition mark cannot be given for an ambiguous statement. CorrectWater moves from higher water potential to lower water potential, through a partially permeable membrane. Adding solute lowers water potential, so water moves from dilute towards concentrated. Say this“Osmosis is the net movement of water molecules from a region of higher water potential to a region of lower water potential through a partially permeable membrane.” CheckState which has the higher water potential: pure water, or a 0.5 \(\mathrm{mol/dm^3}\) sucrose solution. (Pure water.)
- 2. “Osmosis is just water diffusing, so it does not need a membrane.” Why wrongWithout a partially permeable barrier the solute diffuses as freely as the water and nothing accumulates on either side. The membrane is what makes the water movement one-sided, and it is part of the definition. CorrectOsmosis is a special case of diffusion in which the particle is water and the barrier is a partially permeable membrane. Both conditions are required. Say this“Osmosis requires a partially permeable membrane, which allows water molecules through but not the solute.” CheckTwo sucrose solutions of different concentration are mixed in an open beaker. Is that osmosis? (No — there is no partially permeable membrane; the sucrose and the water simply diffuse.)
- 3. “Diffusion needs energy from the cell.” Why wrongDiffusion happens perfectly well in a beaker with no cells in it, and continues in dead tissue. Nothing is spending energy on it. CorrectThe energy is the kinetic energy the particles already have. Their random movement produces the net movement without any contribution from the cell. Say this“Diffusion results from the random movement of particles and requires no energy from the cell.” CheckCyanide stops respiration. Which of diffusion, osmosis and active transport does it stop? (Only active transport.)
- 13. “The answer is 10%, so the sign does not matter.” Why wrongThe sign is the biology. A negative value means water left the cells; a positive value means water entered. Dropping it discards the direction of osmosis. CorrectKeep the sign, and then say what it means: “\(-10.0\%\), so the cylinder lost water to a solution of lower water potential.” Say this“The negative sign shows a loss of mass, so water moved out of the cells by osmosis.” CheckA cylinder changes from \(5.00\ \text{g}\) to \(5.35\ \text{g}\). Give the percentage change with its sign and its meaning. (\(+7.0\%\); water entered the cells.)
- 14. “The agar block is a model cell, so the acid enters the cell.” Why wrongAgar has no membrane, no cytoplasm and no respiration. It models the geometry of diffusion and nothing else. CorrectThe agar block models how surface-area-to-volume ratio and diffusion distance affect the time taken for a substance to reach the centre of an object. Say this“The agar block is a model of diffusion into an object; it does not model a membrane or any transport requiring energy.” CheckName one thing this model cannot demonstrate. (Active transport, selective permeability, or osmosis.)
Questions students ask about Osmosis
What is the difference between diffusion and osmosis?
Diffusion is the net movement of molecules or ions from a region of higher concentration to a region of lower concentration, down a concentration gradient, as a result of random movement. Osmosis is the net movement of water molecules only, from a region of higher water potential to a region of lower water potential, through a partially permeable membrane. Both are driven by the random movement of the particles themselves and need no energy from the cell; the restriction to water and the need for a partially permeable membrane are what make osmosis a special case.
Does diffusion need energy from the cell?
No. Diffusion happens in a beaker with no cells in it and continues in dead tissue. The energy comes from the kinetic energy that molecules and ions already possess, which makes them move randomly; where particles are crowded, more of them wander outwards than inwards, giving net movement down the concentration gradient. Only active transport uses energy released during respiration, which is why a respiratory inhibitor reduces mineral-ion uptake but does not stop diffusion or osmosis.
Why does a plant cell not burst when it takes in water?
Because it is enclosed by a rigid cellulose cell wall. As water enters by osmosis the vacuole expands and the cell contents press outwards on the wall, producing turgor pressure. The wall resists, so the pressure inside rises instead of the volume increasing without limit, and the cell becomes turgid. An animal cell has no cell wall, so in a solution of higher water potential it swells and may burst.
What must a definition of osmosis include to score the marks?
Three things: water molecules, water potential (from higher to lower) and a partially permeable membrane — plus the word "net", because particles never stop moving. Drop any one of them and the definition scores nothing. Writing "water moves from a high concentration to a low concentration" is ambiguous, because a high concentration of water and a high concentration of solute point in opposite directions, so an examiner cannot award a definition mark for it.