S2 Movement of Substances

Part A · Movement of Substances and Diffusion

1What "Movement of Substances" Means

Every living cell has to take in raw materials and push out waste. The substances involved are small enough to matter at the scale of a single cell: gases, water, mineral ions, sugars and amino acids. Movement of substances is simply the study of how those particles get from one place to another — across a cell membrane, along a transport system, or through the fluid around a cell.

Movement of substances: the transfer of particles into, out of, or within an organism, so that the substances a cell needs arrive and the substances it has produced can be removed.

How an organism manages this depends on how many cells it has.

Organism type	How substances are exchanged
Unicellular (e.g. Amoeba, Euglena)	One cell only. Every part of the organism sits close to the surrounding water, so gases, water and food particles cross the cell membrane directly. No transport system is required.
Multicellular (e.g. a mammal, a flowering plant)	Millions of cells, most of them deep inside the body and far from the outside world. Exchange at the body surface is not enough, so multicellular organisms have specialised exchange surfaces (lungs or gills, leaves) backed up by a transport system (blood, xylem and phloem) to carry substances the last stretch to each cell.

Why the size of an organism matters: as a body gets bigger, its volume grows far faster than its surface area, so the surface area to volume ratio falls. The surface can no longer serve the whole body by direct exchange, and a transport system becomes necessary.

The route into a cell is guarded by the cell membrane, which is partially permeable: some particles pass through it easily, others are held back. The membrane is also the site of almost all the movement described in this chapter.

2Diffusion

Diffusion is the most basic form of movement, and the only one that needs no machinery at all.

Diffusion: the net movement of particles down a concentration gradient — from a region where they are more concentrated to a region where they are less concentrated — with no energy supplied by the cell.

Unpack the definition, term by term:

Term	What it tells us
Net movement	Individual particles travel in every direction at once. "Net" refers to the overall result: more particles end up moving from the crowded region to the sparse one than the other way round, so the crowd spreads out.
Concentration gradient	The difference in concentration between two regions. A gradient is what drives diffusion; remove the difference and the movement stops.
No energy supplied	Diffusion is passive. The cell spends no energy on it. The particles already carry kinetic energy of their own — that is all that is needed.

At equilibrium the concentration is the same throughout, so there is no net movement. This does not mean the particles have stopped: they are still moving at random in all directions, just with equal numbers crossing in both directions.

No membrane is needed for diffusion. It happens just as readily in a beaker of still water as it does across a cell membrane — the membrane changes the route, not the process.

The particle view

Picture a drop of purple dye placed at the bottom of a beaker of water. The dye particles are packed closely together at the start, so the region around the drop is highly concentrated and the rest of the beaker is almost clear.

Because particles in a fluid are in constant, random motion, dye particles begin to wander away from the crowded region. In any short moment, particles near the drop are more likely to leave it than to return, simply because there are more of them there to move. Slightly more particles therefore travel outwards than inwards. Over time the dye spreads upwards and outwards until the purple colour is even from top to bottom — equilibrium has been reached, and the visible spreading stops even though the particles have not.

The same view explains exchange in living things. A cell that has just used oxygen in respiration has a lower oxygen concentration inside than in the fluid outside it, so oxygen particles diffuse inwards. A cell that has just released carbon dioxide is more concentrated in carbon dioxide than its surroundings, so carbon dioxide diffuses outwards. In both cases the net movement is down the gradient and costs the cell nothing.

3What Makes Diffusion Faster or Slower

Five factors control how quickly a substance spreads from one region to another. Each one can be traced back to the particle view above.

Factor	Change	Effect on rate of diffusion	Reason
Concentration gradient	Steeper gradient	Faster	A bigger difference in concentration means a bigger excess of particles on the crowded side, so more of them travel the "right" way each second.
Temperature	Higher temperature	Faster	Heating gives particles more kinetic energy. They move more quickly and collide more often, so they spread out sooner.
Particle mass	Lighter particles	Faster	At the same temperature, lighter particles move at higher speeds than heavier ones, so they cover the distance sooner. A gas made of small light molecules therefore diffuses more quickly than a heavier one.
Distance to travel	Shorter distance	Faster	Particles have to travel less far, and a thinner barrier means fewer obstructions on the way. Exchange surfaces are built thin for exactly this reason.
Surface area	Larger surface area	Faster	More particles can cross at the same instant, so the total amount moved per second goes up even though no individual particle travels any faster.

Do not confuse the two "area" ideas. The diffusion factor table lists surface area; the point about body size earlier was about the surface area to volume ratio. A large surface area helps; a large surface area spread over an even larger volume does not.

Rate of diffusion can be described as the amount of substance crossing a given area per unit time. Changing any one factor above changes that rate — and in an exam you must name both the factor and its reason, not just the direction of change.

4Worked Reasoning: A Crystal at Two Temperatures

Two identical glass tanks are filled with 500 cm³ of still water. Tank P is kept at 20 °C; tank Q is kept at 45 °C. A single crystal of purple potassium manganate(VII) is dropped into each tank at the same moment, and the time taken for the purple colour to reach the far end of each tank is recorded.

Observation: the colour reaches the far end of tank Q well before it reaches the far end of tank P. The crystal in both tanks is the same size, and the tanks and the volume of water are identical.

Reasoning step 1 — identify what is diffusing and what the gradient is. The purple colour comes from potassium manganate(VII) particles. Where the crystal sits the concentration is very high; everywhere else it is close to zero. That steep difference in concentration is the gradient driving the spreading.

Reasoning step 2 — identify what is held constant. The gradient at the start, the distance the particles must travel, the surface area of liquid exposed, and the mass of the particles are all the same in both tanks. Only one factor has been changed: temperature. This is a fair test because only one variable differs.

Reasoning step 3 — explain the difference. Water and dissolved particles in tank Q have more kinetic energy than in tank P. They move at higher speeds, so each particle wanders away from the crystal more quickly, and the purple region reaches the far end sooner. The gradient itself is identical; only the speed at which the particles can travel down it has changed.

Reasoning step 4 — predict the end point. If both tanks were left undisturbed for long enough, both would end up the same even shade of purple. Once the concentration is the same throughout, there is no net movement in either tank — though the particles keep moving at random. The hot tank simply gets there first; it does not end up more concentrated.

When you are asked to explain a difference in rate, do not answer "because it is faster". Work through the three things an examiner looks for: what the gradient is, which factor has changed, and why that factor changes how quickly particles move down the gradient.

Part B · Osmosis and Water Potential

5What Osmosis Actually Means

Osmosis is the movement of water molecules, down a water potential gradient, through a partially permeable membrane, from the side where water potential is higher to the side where it is lower, until equilibrium is reached. It is a passive process — the cell does not have to supply any energy for it to happen.

Do not confuse it with diffusion. Diffusion moves any particles and needs no membrane at all; osmosis moves water only, and it cannot happen without a partially permeable membrane in the way. With no membrane between two regions, water still spreads out — but call that diffusion, not osmosis.

Water potential in your own words

Water potential is a way of scoring how keen water molecules are to move away from where they are. Pure water scores highest. Dissolving something replaces some water molecules with solute particles, so there is less free water and the score drops.

Key Point: The more concentrated the solute, the lower the water potential. Water always moves down a water potential gradient — from a dilute solution towards a concentrated one, or from a dilute solution into a cell with concentrated cell sap.

So the ranking of water potential for the solutions we use in class runs:

Solution	Water potential	Why
Distilled water	Highest	No solute at all — all the molecules are free water
0.1 mol/dm³ sodium chloride	High	Very little solute dissolved, so few water molecules are occupied
0.4 mol/dm³ sodium chloride	Lower	More solute dissolved, so far fewer free water molecules
1.0 mol/dm³ sodium chloride	Lowest	Most concentrated — least free water

The ranking of water potential is the exact opposite of the ranking of concentration.

6Partially Permeable or Freely Permeable?

These labels describe what a membrane lets through, and they are not interchangeable.

Membrane type	What it allows through	Can osmosis happen?
Freely permeable	Every particle, whatever its size or charge — solute and water both pass straight through	No. If the membrane holds nothing back, the two sides simply mix, so there is no membrane for osmosis to occur across
Partially permeable (also called semi-permeable)	Small molecules such as water, and sometimes small ions, pass through; large solute particles such as dissolved protein are held back	Yes. Water crosses while the solute cannot, so a water potential gradient is maintained across the membrane

A living cell membrane is partially permeable, and so is Visking tubing — which is why the tubing is used as a model. It has no phospholipid bilayer; its pores are simply too small to let large dissolved particles through, but it behaves in the same useful way.

Exam tip: If a question says the membrane is freely permeable, osmosis is impossible. Do not write a water-potential answer for that set-up; state that diffusion alone occurs and both sides eventually reach the same concentration.

7Plant Cells in Concentrated and Dilute Solutions

A plant cell has a strong cell wall sitting outside its partially permeable membrane. The wall is freely permeable to water and to solutes, so it never blocks osmosis — but it does mean the cell cannot expand forever, because the wall pushes back.

Solution around the cell	Water potential compared with the cell sap	Net water movement	Result
Dilute — e.g. distilled water	Higher outside than inside	Water moves IN, down the gradient	The vacuole swells and pushes the membrane hard against the cell wall. The cell is turgid; the wall stops it bursting.
Same concentration as the cell sap	Equal on both sides	No net movement — water still crosses in both directions, at equal rates	No change; the cell stays the same size
Concentrated — e.g. strong sodium chloride solution	Lower outside than inside	Water moves OUT, down the gradient	The membrane pulls away from the cell wall as the vacuole shrinks. The cell is plasmolysed.

Turgor pressure is the push of the swollen cell contents against the cell wall. It keeps soft young stems upright and petals open; when it is lost the cells become flaccid and the plant droops — that is wilting.

Key Point: Plasmolysis is reversible while the cell is alive: move it into a solution of higher water potential and water enters, the vacuole refills and the membrane presses back onto the wall. Left too long, the cell is permanently damaged.

Exam tip: Name the direction of water movement before the end state. The chain is: compare water potentials → state net direction → state turgid or plasmolysed.

8Animal Cells in Concentrated and Dilute Solutions

An animal cell has no cell wall, so nothing pushes back when water floods in — one difference, a completely different outcome.

Solution around the cell	Water potential compared with the cytoplasm	Net water movement	Result
Dilute — e.g. distilled water	Higher outside than inside	Water moves IN	The cell swells until the cell membrane tears and the contents escape. This bursting is called haemolysis in red blood cells, and it cannot be undone.
Same concentration as the cytoplasm	Equal on both sides	No net movement	The cell keeps its normal shape — the only condition in which an animal cell stays healthy for long
Concentrated — e.g. strong sodium chloride solution	Lower outside than inside	Water moves OUT	The cell loses water, shrinks and its membrane becomes wrinkled: the cell is crenated. This is reversible if the cell is returned to a suitable solution in time.

Exam tip: Contrast the two cell types in one sentence: a plant cell becomes turgid because the wall resists expansion, while an animal cell in the same dilute solution swells and bursts because it has no wall.

9Setting Up and Reading a Classic Osmosis Experiment

We can measure osmosis directly by sealing a salt solution inside Visking tubing and weighing what happens to it.

Setting it up

Cut four equal lengths of Visking tubing. Soak them in distilled water for a minute so the tubing softens and will not split.
Into the first three tubes, pipette 10 cm³ of sodium chloride solution at 0.1, 0.4 and 1.0 mol/dm³. Into the fourth, pipette 10 cm³ of distilled water to act as a control.
Knot each tube tightly and check for leaks. Blot the outside of every tube dry, then record its starting mass on a balance.
Stand all four tubes in separate beakers of distilled water and leave them for 30 minutes.
Remove each tube, blot it dry again in the same way and record the final mass.

Key Point: Every tube starts with the same volume of liquid, is blotted the same way and is left for the same time. That is what makes the comparison fair — the only thing you deliberately change is the concentration of sodium chloride inside the tubing.

Reading the results

Inside every tube the sodium chloride solution has a lower water potential than the distilled water outside, so water moves in through the Visking tubing. The more concentrated the salt solution, the lower its water potential, and the more water it takes up.

Contents of tubing	Water potential inside, compared with the water outside	Net water movement	Change in mass after 30 minutes
Distilled water (control)	Equal — no gradient	None	No change
0.1 mol/dm³ sodium chloride	Slightly lower	Water moves in slowly	Small increase
0.4 mol/dm³ sodium chloride	Lower	Water moves in faster	Larger increase
1.0 mol/dm³ sodium chloride	Lowest	Water moves in fastest	Largest increase

Plot change in mass against concentration and the line slopes upwards: the strongest solution gains the most mass. To improve the method, blot every tube the same way before each weighing so surface water is not mistaken for absorbed water, and run several tubes at each concentration so one unusual tube does not distort the result.

Exam tip: A mass increase is evidence, not explanation. Show the chain: the solution inside has the lower water potential → water moves in by osmosis through the partially permeable membrane → the tubing gains mass.

Part C · Active Transport & Comparing the Three Processes

10Active Transport — Paying to Move Uphill

Active transport: the net movement of a substance from a region of lower concentration to a region of higher concentration — that is, against a concentration gradient — using energy released by respiration and carrier proteins in the cell membrane.

Everything in Part A and Part B is passive. Particles drift down a concentration gradient or water drifts down a water potential gradient because random motion happens to carry more of them one way than the other. Nothing has to be spent, and the process stops on its own once the two sides are level.

Active transport reverses that. The cell wants a substance that is already more concentrated inside than outside, so random motion would only carry it back out. To keep pulling it in, the cell has to do work — the same way you can roll a ball downhill for free but must push to roll it uphill. The energy comes from respiration, so any cell doing a lot of active transport needs a good oxygen supply and plenty of mitochondria.

Two things carry a substance across a membrane — and only one of them charges you.

Channel proteins form a water-filled pore. Substances slip through down the gradient. No energy is used, so this is still facilitated diffusion.
Carrier proteins bind the substance, then use energy from respiration to change shape and release it on the far side. This shape change is what makes the movement go up the gradient.

So do not decide the process from the word "protein". A protein in the membrane may be doing diffusion or active transport. Decide from the direction: down the gradient is passive, against the gradient must be active transport.

Where you meet it

Root hair cells taking in mineral ions. Soil water is usually far more dilute than the sap inside a root cell, so mineral ions such as nitrate and potassium are more concentrated inside the cell than in the soil. Diffusion would push them out. The root hair cell therefore uses active transport to keep absorbing them, and the long, narrow hair gives the cell a large surface area over which to do it. A plant that cannot get these ions — because the soil solution is too dilute or the roots are short of oxygen, as in waterlogged soil — cannot build the proteins and chlorophyll it needs, which is why it yellows and grows slowly.

The gut absorbing glucose. Early on, glucose in the small intestine is plentiful and moves into the lining cells by diffusion. By the time most of it has been taken up, the remaining glucose in the intestine is more dilute than the glucose already inside the lining cells. Absorption must carry on, so the cells switch to active transport to draw in the last of it. This is one reason the lining cells of the small intestine are packed with mitochondria and are folded into tiny projections that multiply the surface available for uptake.

Use a respiratory inhibitor as a test. If you treat cells with a substance that stops respiration — or simply cut off their oxygen supply — active transport stops, because the energy supply has gone. Diffusion and osmosis carry on unaffected, because neither ever needed energy in the first place. This pair of observations, on the same cells, is the cleanest way to show that a movement is active. When an exam question says "even though the concentration outside is lower" or "against the concentration gradient", the answer is active transport, and you should say where the energy comes from.

11The Three Processes — One Decision Table

When a question describes a movement of substances, do not guess from the substance. Work down the questions in the left-hand column in order: find out which way it goes, then whether anything was spent, then what in the membrane did the work. The answer falls out of the first row you can settle.

Ask yourself…	Diffusion	Osmosis	Active transport
Which way does the net movement go?	From higher to lower concentration, down the concentration gradient	From higher to lower water potential, down the water potential gradient	From lower to higher concentration, against the concentration gradient
Does it stop by itself?	Yes — at equilibrium, with no net movement	Yes — at equilibrium, with no net movement	No — it continues as long as the cell needs the substance and respiration supplies energy
Is energy used?	No — passive	No — passive	Yes — energy from respiration
What in the membrane does the work?	Nothing is needed; the substance crosses the bilayer directly, or through a channel protein, depending on the particle	Nothing is needed beyond the partially permeable membrane itself, which holds back the solute	A carrier protein that binds the substance and changes shape using energy
What actually moves?	Any particle small enough to cross — gases, simple sugars, ions	Water molecules only	Dissolved substances the cell needs — mineral ions, glucose, amino acids
An example you can name	Oxygen entering a respiring muscle cell; carbon dioxide leaving a leaf through a stoma by day	Water drawn into a root hair cell from damp soil; a limp stem stiffening again after watering	Nitrate ions entering a root hair cell from dilute soil water; the gut taking in the last of the glucose from digested food
Effect of stopping respiration	Unaffected	Unaffected	Stops

The one-line summary: diffusion and osmosis are the cell letting things happen; active transport is the cell making something happen. All three move substances across the same membrane, but only the third one costs the cell anything, and only the third one can run against the gradient.

Part D · Investigations, Exam Technique & Closing Practice

12Writing Up an Investigation

Practical questions on diffusion or osmosis are marked on the same three things: what you changed, what you measured, and what you held constant.

What to name	How to write it
Independent variable	The concentration of the sugar or salt solution the tissue is placed in — what you change.
Dependent variable	The mass or length of the tissue before and after — what you measure.
Controlled variables	Temperature, volume of solution, time in the solution, and size of the tissue.
Why control matters	Each one also affects the rate of water movement, so if two change at once you cannot tell which caused the result.

Reasons for the usual controls:

Temperature — warmer water molecules have more kinetic energy and diffuse faster, so a warm bench would enlarge the mass change for an unrelated reason.
Time in the solution — water keeps moving until equilibrium, so longer always means a bigger change.
Volume of solution — a small volume would have its own water potential altered by the tissue.
Same tissue, same size — cut every cylinder from one potato with the same cork borer, because cell sap concentration differs between potatoes and a larger piece has more cells exchanging water.

Blot each piece dry before weighing. Surface water would be counted as mass gained by the tissue — a systematic error, not a random one.

Express results as a percentage change in mass: (final mass − initial mass) ÷ initial mass × 100. This turns every cylinder into the same starting point, so a piece that began slightly heavier does not tilt the result. State the direction as well: positive means water moved in, negative means water moved out.

On the graph, put solution concentration on the x-axis and percentage change in mass on the y-axis, draw one best-fit line rather than joining the dots, and label both axes with units.

The line slopes downwards from left to right, because a more concentrated sugar solution has a lower water potential, so more water leaves the cells. It crosses zero at one concentration only: that is where the solution has the same water potential as the cell sap, so there is no net movement of water and the mass does not change. That crossing point gives the water potential of the cell sap. A steep line means a small change in concentration produces a large change in water movement.

Read the crossing point from your own line, not from a data point. If it falls between two plotted concentrations, answer with a value between them (for example “about 0.43 mol/dm³”) instead of snapping to the nearest label.

13Mistakes That Cost Marks

Mistake	Write instead
Using “diffusion” and “osmosis” interchangeably	Name the process, then the substance. Water crossing a partially permeable membrane is osmosis; a gas or solute spreading out is diffusion.
Saying water moves “because of concentration” without naming the gradient	From a region of higher water potential to a region of lower water potential, down a water potential gradient.
Forgetting that active transport needs energy	Against the concentration gradient, using energy from respiration — for example mineral salts absorbed when the soil solution is more dilute than the cell.
Giving the result of water movement without its direction	Direction first — water moves out of the cell — then the result — so the cell becomes plasmolysed.
Treating “water concentration” and “water potential” as rising together	They move in opposite directions: a more concentrated solution has a lower water potential. Say solute concentration for the sugar or salt.
Saying osmosis needs energy, or that diffusion cannot cross a membrane	Both diffusion and osmosis are passive: write without the use of energy. Only active transport requires energy.
Claiming a plant cell bursts in pure water	The cell becomes turgid and the cell wall prevents it from bursting. Bursting happens to an animal cell, which has no wall.
Saying movement stops at equilibrium	There is no net movement, but the molecules continue to move randomly in all directions.

Two habits pick up easy marks. Underline the command word: state needs one short sentence, describe needs what happens next, and explain needs a cause and its consequence joined by “because”. And name the barrier when one is involved — “water moves in” is weaker than “water moves in, across the partially permeable cell membrane, from the solution to the cell.”

14Closing Practice — Osmosis in Potato Tissue (10 marks)

A student cut six cylinders from the same potato with the same cork borer. Each was blotted dry, weighed, and placed in a labelled test tube containing 20 cm³ of sucrose solution. All six tubes were kept at the same temperature for 45 minutes, then the cylinders were blotted dry and reweighed.

Concentration of sucrose / mol dm−³	Initial mass / g	Final mass / g	Percentage change in mass
0.0	2.00	2.37	+18.5
0.2	2.00	2.19	+9.5
0.4	2.00	2.02	+1.0
0.6	2.00	1.87	−6.5
0.8	2.00	1.72	−14.0

State the independent variable and the dependent variable, and give one variable that must be controlled, explaining why. (3 marks)
Explain why the student blotted each cylinder dry before weighing it. (2 marks)
The student plotted percentage change in mass against concentration. Explain how the graph would be used to find the concentration of sucrose with the same water potential as the potato cell sap, and give that value. (3 marks)
At 0.0 mol/dm³ the cylinders gained mass but did not burst. Explain why. (2 marks)

Model answers

The independent variable is the concentration of the sucrose solution the cylinders were placed in. The dependent variable is the mass of each cylinder, measured before and after the 45 minutes. One controlled variable is temperature, because a higher temperature gives the water molecules more kinetic energy so they diffuse faster, and the cylinders would then show a larger change in mass for a reason other than the concentration. (1 mark each for: independent variable; dependent variable; a named controlled variable with a reason.)
Water left on the outside of a cylinder would be weighed with the tissue, so the final mass would be too high and the percentage change would look larger than the water movement really was, making the result inaccurate in the same direction every time. (1 mark for surface water counted as mass gained; 1 mark for the recorded change being inaccurate — accept “systematic error”.)
The graph is a straight line sloping downwards from left to right, because a more concentrated sucrose solution has a lower water potential, so more water leaves the cells. I would read the value on the x-axis where the line crosses zero, because there is no net movement of water there, meaning the solution and the cell sap have the same water potential. The crossing lies between the 0.4 mol/dm³ point (+1.0%) and the 0.6 mol/dm³ point (−6.5%), so the value is about 0.43 mol/dm³. (1 mark for the intercept at zero change; 1 mark for explaining that no net movement means equal water potential; 1 mark for a value between 0.4 and 0.6.)
At 0.0 mol/dm³ the solution is pure water, which has a higher water potential than the cell sap, so water moves into the cells by osmosis and the cells become turgid. They do not burst because each plant cell has a rigid cell wall that resists the pressure as the cell swells and stops it expanding any further. (1 mark for water moving in from a higher water potential; 1 mark for the cell wall preventing bursting.)

Every model answer names the process and the direction before the result, and the numerical value is read from the crossing point rather than a listed row. Do the same and the marks follow.