Movement Into and Out of Cells

• One sentence that repairs half of these at once: “Diffusion and osmosis are driven by the random movement of particles; only active transport uses energy released during respiration, and only active transport goes against the concentration gradient.” Write that sentence out once, from memory, before continuing.
• The one-sentence summary of the whole chapter: particles move randomly and that alone produces diffusion and osmosis, which cost the cell nothing and always run downhill; active transport is the only process that runs uphill, and the only one the cell has to pay for, with energy released during respiration. If you can write that sentence and then defend every clause in it, the chapter is done.

• Explain that water is a solvent in living organisms and give named examples from digestion, transport and excretion.
• State that molecules and ions possess kinetic energy and move randomly, and that this random movement — not energy from the cell — drives diffusion and osmosis.
• Define diffusion word for word: 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.
• Explain dynamic equilibrium: movement continues in both directions, equal numbers cross each way, and there is no net movement.
• Describe and explain the effect of surface area, temperature, concentration gradient and diffusion distance on the rate of diffusion, giving the molecular mechanism in each case.
• Relate those four factors to real exchange surfaces: alveoli, villi and root hair cells.
• Describe a controlled investigation using agar blocks, identify its control variables and state its limitations as a model.
• Define osmosis word for word: 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.
• Compare the water potential of pure water, a dilute solution and a concentrated solution.
• Predict and explain what happens to an animal cell and to a plant cell in solutions of higher, equal and lower water potential.
• Use the terms turgid, turgor pressure, flaccid and plasmolysis correctly, and describe plasmolysis as the cell membrane and cell contents pulling away from the cell wall.
• Explain how turgor supports non-woody plant tissue, and explain wilting as a consequence of losing it.
• Describe the potato-cylinder investigation in full, calculate percentage change in mass, and interpret the concentration at which the change is zero.
• Describe a dialysis-tubing investigation and infer permeability from the results rather than assuming it.
• Define active transport word for word: 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.
• Explain mineral-ion uptake by root hair cells when the soil solution is more dilute than the cell contents.
• Explain why a respiratory inhibitor reduces ion uptake but does not stop diffusion or osmosis.
• Compare diffusion, osmosis and active transport by particle, direction, gradient, membrane requirement, energy source and biological example.

Why it matters: without active transport, a plant could take up a mineral ion only until its cells matched the soil — which for a nutrient-poor soil would be almost nothing. Active transport is what lets a cell hold a useful substance at a concentration the outside world cannot supply.

Turgor Pressure

Turgor pressure is the outward pressure exerted by the cell contents against the cellulose cell wall of a plant cell as water enters by osmosis and the vacuole expands. A cell in which this pressure is high is described as turgid, and the combined turgor of many cells is what supports the stems and leaves of non-woody plants. When water is lost the pressure falls, the cell becomes flaccid, the tissue loses support and the plant wilts.

Diffusion

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. It requires no energy from the cell: the driving energy is the kinetic energy of the moving particles themselves. Diffusion supplies oxygen to respiring cells, removes carbon dioxide, and moves dissolved food molecules across exchange surfaces such as the alveoli and the villi.

Water as a Solvent

Water dissolves many biological substances, forming solutions in which those substances can be transported around an organism, react with one another, or be removed from the body. Digested food, plasma solutes such as glucose and urea, and mineral ions in xylem sap all travel dissolved in water; substances that do not dissolve in water require other transport arrangements.

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.

Cell Response to Water Potential

A cell placed in a solution of higher water potential gains water by osmosis; one placed in a solution of lower water potential loses water. An animal cell has no cell wall, so it swells and may burst when water enters, and shrinks when water leaves. A plant cell is enclosed by a rigid cellulose cell wall which resists the outward pressure, so it becomes turgid rather than bursting, and becomes flaccid or plasmolysed when water is lost.

Random Molecular Movement

Molecules and ions possess kinetic energy and are in constant motion, colliding and changing direction unpredictably. No individual particle has a preferred direction, but where particles are unevenly distributed this random motion produces a predictable net movement from the crowded region to the less crowded one. It is the source of the energy for diffusion and osmosis, so neither process requires energy from the cell.

Active Transport

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. It is the only one of the three transport processes that requires energy from the cell, and the only one that can move a substance uphill. Root hair cells use it to absorb mineral ions from soil that is more dilute than their own cytoplasm, and contain many mitochondria to release the energy required.

Rate of Diffusion

The rate of diffusion is how quickly the net movement of particles down a concentration gradient occurs. Four factors raise it: a larger surface area, a higher temperature, a steeper concentration gradient and a shorter diffusion distance. Each acts through the random movement of the particles themselves, so none of them involves energy supplied by the cell. Real exchange surfaces such as alveoli, villi and root hair cells are adapted to maximise all four.

• 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.)
• 4. “At equilibrium, the particles stop moving.” Why wrongParticles possess kinetic energy at any temperature above absolute zero. Nothing has removed it. CorrectEquilibrium is dynamic: equal numbers of particles cross in each direction, so there is no net movement, but movement itself continues. Say this“There is no net movement, although the particles continue to move randomly in both directions.” CheckA potato cylinder shows zero percentage change. Has water stopped crossing the membranes? (No — equal amounts cross each way.)
• 5. “A plant cell bursts when water enters it.” Why wrongRigid cellulose walls resist the outward pressure, so the pressure inside rises instead of the volume increasing without limit. CorrectThe cell becomes turgid. Rising turgor pressure opposes further net entry of water, and the cell reaches a stable state. Say this“The cellulose cell wall resists the outward pressure, so the cell becomes turgid rather than bursting.” CheckWhich cell type bursts in distilled water, and why? (An animal cell — it has no cell wall.)
• 6. “The cell wall controls what enters and leaves the cell.” Why wrongThe cellulose wall is fully permeable: water and dissolved substances pass straight through it. It is a structural component, not a gatekeeper. CorrectThe cell membrane is partially permeable and controls entry and exit. The wall provides support and prevents bursting. Say this“The partially permeable cell membrane controls what enters and leaves; the fully permeable cell wall does not.” CheckIn a plasmolysed cell, what fills the gap between the wall and the membrane, and how did it get there? (External solution, straight through the fully permeable wall.)
• 7. “The cell wall becomes plasmolysed.” Why wrongThe wall does not move or change at all. Only the living contents shrink. CorrectIn plasmolysis the cell membrane and the cell contents pull away from the cell wall. Say this“The cell membrane and cell contents have pulled away from the cell wall.” CheckIn a diagram of a plasmolysed cell, which outline is unchanged from the turgid cell? (The cell wall.)
• 8. “Flaccid and plasmolysed are two words for the same thing.” Why wrongThey describe different amounts of water loss, and the diagrams are different. CorrectFlaccid: turgor pressure has fallen, the membrane still lies against the wall. Plasmolysed: so much water has gone that the membrane has separated from the wall. Say this“The cells are flaccid; if more water were lost they would become plasmolysed.” CheckA wilting plant is watered and recovers within an hour. Were its cells flaccid or plasmolysed? (Flaccid — plasmolysis is far more severe and may be irreversible.)
• 9. “Active transport moves substances from high concentration to low concentration.” Why wrongThat is a description of diffusion, and it makes the energy requirement pointless — nothing needs to be paid for when a substance runs downhill. CorrectActive transport moves substances from lower to higher concentration, against the concentration gradient. Say this“Active transport moves ions against the concentration gradient, from a lower to a higher concentration.” CheckWhy can a root hair cell not absorb nitrate ions from dilute soil by diffusion? (The concentration inside is already higher, so diffusion would move them out.)
• 10. “Mitochondria make energy for active transport.” Why wrongEnergy cannot be created. Respiration releases energy that was already stored in nutrient molecules such as glucose. CorrectMitochondria are the site of aerobic respiration, in which energy is released from glucose and made available to the cell. Say this“Active transport uses energy released during respiration, which takes place in the mitochondria.” CheckWhy do cells that carry out a lot of active transport contain many mitochondria? (To release the large amount of energy required.)
• 11. “Zero percentage change in mass means no water moved.” Why wrongIt confuses a net result with the absence of movement, and it contradicts the definition of dynamic equilibrium. CorrectThe tissue and the solution had the same water potential, so water crossed the membranes equally in both directions and the two movements cancelled. Say this“At this concentration the solution has the same water potential as the tissue, so there is no net movement of water.” CheckWhat does the zero-change concentration estimate? (The water potential of the tissue.)
• 12. “Percentage change = (final − initial) ÷ final × 100.” Why wrongDividing by the final value gives a different number and makes results from different cylinders non-comparable. It also produces plausible-looking wrong answers, so it is rarely spotted. CorrectAlways divide by the initial value: \(\dfrac{\text{final} - \text{initial}}{\text{initial}} \times 100\). Say thisNothing — just write the correct formula out before substituting. Check\(4.80\ \text{g} \rightarrow 4.32\ \text{g}\). Correct answer, and the wrong-denominator answer? (\(-10.0\%\); the wrong method gives \(-11.1\%\).)
• 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.)

• Time discipline. A definition is worth roughly 45 seconds. If you find yourself writing a paragraph for a 2-mark “define”, you have misread the command word and are spending time you will need for the data question later in the paper.

• Topic 3 is short in the syllabus but heavily examined, because it supplies the mechanism behind gas exchange, transport in plants, transport in humans, excretion and absorption in the alimentary canal. It appears in three recognisable shapes.
• The concentrated sucrose solution has a lower water potential than the cell contents.
• Water molecules therefore move out of the cell, from higher to lower water potential, by osmosis through the partially permeable cell membrane.
• The vacuole and cytoplasm lose water and shrink, so turgor pressure falls and the cell becomes flaccid.
• If enough water is lost the cell membrane and cell contents pull away from the cell wall: the cell is plasmolysed.
• “Water leaves because the outside is more concentrated” earns at most one mark: it names neither water potential nor osmosis nor the membrane, and it does not say what the cell becomes.