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 from respiration, and only active transport goes against the concentration gradient.” Write that sentence out once, from memory, before continuing.
• Osmosis is the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane. Four elements, and a complete answer needs all four: net, water molecules, higher to lower water potential, partially permeable membrane.
• Plants are supported by the pressure of water inside the cells pressing outwards on the cell wall. Learn that sentence as it stands. A non-woody plant has no wood to hold it up — what holds it up is water pressure inside millions of cells, each pressing on its own wall.
• Protein carriers move molecules or ions across a membrane during active transport. That is the whole statement. It begins with state, so one clear sentence is a complete answer.
• 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 from respiration. If you can write that sentence and then defend every clause in it, the chapter is done.

• Describe diffusion as the net movement of particles from a region of their higher concentration to a region of their lower concentration — down a concentration gradient — as a result of their random movement. (3.1.1)
• State that the energy for diffusion comes from the kinetic energy of the random movement of molecules and ions. (3.1.2)
• State that some substances move into and out of cells by diffusion through the cell membrane. (3.1.3)
• Describe the importance of the diffusion of gases and of solutes in living organisms, with named examples. (3.1.4)
• Investigate the factors that influence diffusion, limited to surface area, temperature, concentration gradient and distance — giving the molecular mechanism for each. (3.1.5)
• Describe a controlled agar-block investigation, identify its control variables and state its limitations as a model. (3.1.5, Papers 5 and 6)
• Describe the role of water as a solvent in organisms, with reference to digestion, excretion and transport. (3.2.1)
• State that water diffuses through partially permeable membranes by osmosis. (3.2.2)
• State that water moves into and out of cells by osmosis through the cell membrane. (3.2.3)
• Investigate osmosis using materials such as dialysis tubing, and infer permeability from the evidence rather than assuming it. (3.2.4)
• Investigate and describe the effects on plant tissues of immersing them in solutions of different concentrations, including calculating percentage change in mass and reading the concentration at which the change is zero. (3.2.5)
• State that plants are supported by the pressure of water inside the cells pressing outwards on the cell wall. (3.2.6)
• Describe active transport as the movement of particles through a cell membrane from a region of lower concentration to a region of higher concentration — against a concentration gradient — using energy from respiration. (3.3.1)
• Supplement 3.2.7 Describe osmosis as the net movement of water molecules from a region of higher water potential (dilute solution) to a region of lower water potential (concentrated solution), through a partially permeable membrane.
• Supplement 3.2.8 Explain the effects on plant cells of immersing them in solutions of different concentrations, using the terms turgid, turgor pressure, plasmolysis and flaccid.
• Supplement 3.2.9 Explain the importance of water potential and osmosis in the uptake and loss of water by organisms.
• Supplement 3.3.2 Explain the importance of active transport as a process for the movement of molecules or ions across membranes, including ion uptake by root hairs.
• Supplement 3.3.3 State that protein carriers move molecules or ions across a membrane during active transport.

Why it matters: without active transport, a cell could take up a substance only until its inside matched its outside. Active transport is what lets a cell hold a useful substance at a concentration the outside world cannot supply — and it is the reason a cell that stops respiring soon stops functioning.

• 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 an ambiguous statement cannot be given a mark. Correct (Core)Water diffuses through a partially permeable membrane by osmosis, and the net movement is from the more dilute solution towards the more concentrated one. Say which solution, not which “concentration”. Correct (Supplement)Extended candidates say it in water potential: from higher water potential (dilute solution) to lower water potential (concentrated solution), through a partially permeable membrane. CheckWhich is more concentrated: pure water, or a 0.5 \(\mathrm{mol/dm^3}\) sucrose solution — and which way does water move between them? (The sucrose solution; water moves into it.)
• 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 firm instead. The rising pressure of water inside the cell, pressing outwards on the wall, opposes further net entry of water, and the cell reaches a stable state. (Supplement: that pressure is turgor pressure and the cell is turgid.) Say this“The rigid cellulose cell wall resists the outward pressure, so the cell becomes firm 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. [Supplement] “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. [Supplement] “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 from 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 solution and the cell contents were equally concentrated, so water crossed the membranes equally in both directions and the two movements cancelled. (Supplement: they had the same water potential.) Say this“At this concentration the solution is as concentrated as the cell contents, so there is no net movement of water.” CheckWhat does the zero-change concentration estimate? (The concentration of the solution inside the cells.)
• 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 more concentrated solution.” 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.)

• Core never depends on a Supplement block. You can read this chapter skipping every bordered Supplement block and still have a complete, self-sufficient Core course — a full account of osmosis, a full account of what happens to cells in different solutions, a full account of why a plant stands up, and all three practicals with their calculations. Nothing you need for Core is hidden inside a Supplement box.
• Topic 3 is one of the few topics named outright in the practical specification. The official list of experimental contexts for Papers 5 and 6 names diffusion and osmosis, and lists a partially permeable membrane (for example Visking or dialysis tubing) among the standard apparatus. Whichever practical paper you sit, this topic can be the context for it. The three investigations in this chapter are therefore not optional reading.
• Time discipline. A “state” answer is worth roughly 45 seconds. If you find yourself writing a paragraph for a 2-mark state, you have misread the command word and are spending time you will need for the data question later in the paper.
• Core candidates: this whole section is Supplement, and you can skip it. Nothing in your Core course depends on it. The Core account of what happens to cells in different solutions, and of how water pressure inside cells supports a plant, is complete in the previous section. This section puts precise names to those states and extends the idea from single cells to whole organisms.
• Only the first row is a named requirement here. The other two are included because they show you the same idea in an animal, and because you will meet both again in Topics 7 and 13. Do not memorise them as part of Topic 3; memorise the root hair, and recognise the pattern elsewhere.
• Both routes do this check. Core candidates: this is your complete retrieval check for Topic 3. Extended candidates: do all ten of these first, then go on to the Extended retrieval check, which adds the five Supplement statements.

• Topic 3 is short in the syllabus but it underpins a great deal of what follows, because it supplies the mechanism behind gas exchange, transport in plants, transport in animals, excretion and absorption in the alimentary canal. It reaches you through five papers, depending on your route.
• The sucrose solution outside is more concentrated than the solution inside the cells.
• Water therefore moves out of the cells by osmosis, through the partially permeable cell membrane.
• The cells lose water and shrink, so the tissue becomes softer and shorter/lighter — a potato cylinder in this solution loses mass.
• The water inside the cells no longer presses outwards on the cell walls as strongly, so the tissue is no longer supported and is limp rather than firm.
• “Water leaves because there is more sugar outside” earns at most one mark: it names neither osmosis nor the membrane, and it does not say what the tissue becomes.