There are four ways a substance gets across a membrane, and exam questions almost always come down to telling them apart. Two questions sort it every time: which way is it going relative to the concentration gradient, and does it need a protein. Answer those and the name falls out.
📘 What you need to know
Simple diffusion — the net movement of a substance from where it is more concentrated to where it is less concentrated, caused by the random motion of its particles. No protein, no energy.
Osmosis — the net movement of water across a partially permeable membrane, from a dilute solution to a more concentrated one. Water moves from higher water potential to lower water potential.
Facilitated diffusion — down the gradient, but through a channel or carrier protein. Still passive.
Active transport — against the gradient, using a carrier protein (pump) and energy from ATP.
Random motion comes from the kinetic energy of the particles. Given long enough, diffusion reaches equilibrium.
Rate of diffusion depends on the steepness of the gradient, temperature, surface area and the properties of the particle.
Facilitated diffusion and active transport are what make membranes selectively permeable.
Simple diffusion
Particles are always moving, in random directions, because they have kinetic energy. If there are more of them on one side of a membrane than the other, then simply by chance more will wander across one way than the other. That imbalance is the net movement, and we call it diffusion.
Nothing is pushing the particles. Nothing is spending energy. The particles are just bouncing around, and there happen to be more of them starting on one side.
The direction, every time
higher concentration → lower concentration (down the gradient)
Oxygen diffuses into cells from the capillaries. Respiration keeps using it up, so its concentration inside stays low and the gradient is maintained.
Carbon dioxide diffuses out. Respiration keeps making it, so its concentration inside stays high.
Why the gradient never runs out. Diffusion would stop at equilibrium — but respiration keeps consuming oxygen and producing carbon dioxide, so the cell continually restores the difference. A gradient that is being actively maintained is why gas exchange never simply stops.
What changes the rate
Steepness of the concentration gradient — a bigger difference across the membrane means a faster rate.
Temperature — more kinetic energy means faster random movement, so a faster rate.
Surface area — more membrane to cross means a faster rate.
Properties of the particle — large molecules are slower; uncharged and non-polar molecules are faster because they dissolve in the tails. Small polar molecules such as urea manage it, but only at low rates.
Four routes, side by side
Look at the arrow direction before anything else. Down the gradient means passive; up the gradient means active, and something must be paying for it.
Osmosis
Osmosis is diffusion, but we give it its own name because it is about water. Water moves across a partially permeable membrane from a dilute solution to a more concentrated one.
That sounds backwards until you think about the water rather than the solute. A dilute solution has more water molecules per unit volume; a concentrated one has fewer. So the water is doing what everything else does — moving from where there is more of it to where there is less.
The version to write in exams
water moves from higher water potential → lower water potential
Water potential is used instead of “water concentration” to avoid confusion between the water and the solute. Pure water has the highest water potential; adding solute lowers it.
Water is polar, so strictly it should struggle to cross a bilayer. It gets away with it because it is very small, and because many membranes contain channel proteins called aquaporins that let water through far more freely. Water is genuinely the odd one out here.
The solute cannot move, so the water has to. That is the whole trick of a partially permeable membrane: it decides which of the two is allowed to even out.
Facilitated diffusion
Large molecules, polar molecules and ions cannot get through the hydrophobic core. They still need to get in and out, so they go through transport proteins instead. That is facilitated diffusion — diffusion with help.
It is still passive. No ATP is used, because the substance is still moving down its gradient.
The proteins are highly specific, so only one type of molecule or ion goes through each one.
The direction depends purely on the relative concentrations on each side. Reverse the gradient and the traffic reverses too.
Channel proteins are pores, some of them gated. Carrier proteins bind the substance and switch shape.
If a graph plateaus, something is limiting it. Here that something is the number of transport proteins in the membrane — exactly the same reasoning as an enzyme running at maximum rate.
Active transport
Sometimes a cell needs a substance where there is already plenty of it, or needs to hold something out that keeps drifting in. Diffusion will not do that, because diffusion only ever runs downhill. Going uphill costs energy.
Movement is from lower to higher concentration — against the gradient.
It uses carrier proteins, often called pumps, such as the sodium–potassium pump.
Energy comes from ATP, made in respiration. The ATP is hydrolysed to release that energy.
The energy is used to make the carrier protein change shape, carrying the substance across as it does.
🤔 Facilitated diffusion and active transport both use carrier proteins — so what is the difference?
Only the direction and the bill. In facilitated diffusion the carrier is nudged along by the gradient itself, so nothing has to be paid. In active transport the carrier is forced to work against the gradient, and ATP pays for the shape change. This is why anything that stops respiration — no oxygen, or a metabolic poison — stops active transport but leaves diffusion running.
Feature
Simple diffusion
Osmosis
Facilitated diffusion
Active transport
What moves
Small non-polar molecules
Water
Ions, polar and large molecules
Ions and molecules
Direction
Down the gradient
High to low water potential
Down the gradient
Against the gradient
Protein needed?
No
No, but aquaporins help
Yes, channel or carrier
Yes, carrier (pump)
ATP needed?
No
No
No
Yes
Passive or active
Passive
Passive
Passive
Active
Selective permeability
Put the pieces together and you get a membrane that chooses. Facilitated diffusion and active transport let the cell decide which substances cross, because the cell decides which transport proteins are in its membrane.
Simple diffusion gives far less control. A small non-polar molecule crosses whether the cell likes it or not, so there is no selectivity there at all. Where simple diffusion is selective is by exclusion: large and polar substances simply cannot get through the bilayer without a protein.
🧩 How to name the transport type in three questions
Is it water crossing a partially permeable membrane? If yes, it is osmosis. Stop here.
Which way is it going? Against the gradient means active transport, and ATP must be involved.
Down the gradient — does it use a protein? Yes gives facilitated diffusion; no gives simple diffusion.
Worked examples
WORKED EXAMPLE
Root hair cells take up nitrate ions from soil water, even though the concentration of nitrate inside the cell is already higher than outside. Name the process and explain how it works. [3]
Step 1: check the direction
Low outside to high inside means movement against the concentration gradient.
Step 2: name it
Active transport, using carrier proteins (pumps) in the membrane.
Step 3: explain the energy
ATP from respiration is hydrolysed, and the energy released changes the shape of the carrier protein, moving the ion across.
Active transport, powered by ATPThe phrase “already higher inside” is the examiner telling you it is active transport.
WORKED EXAMPLE
Explain why the rate of facilitated diffusion reaches a plateau as the concentration gradient increases, while simple diffusion does not. [3]
Point 1
Facilitated diffusion depends on transport proteins, and there is a fixed number of them in the membrane.
Point 2
Once every protein is occupied, increasing the gradient cannot increase the rate any further, so the graph levels off.
Point 3
Simple diffusion does not use proteins, so nothing saturates and the rate keeps rising with the gradient.
Proteins are the limiting factor for one line and not the other“Saturated” is a good word here, as long as you explain what it means.
WORKED EXAMPLE
A plant cell is placed in a concentrated sugar solution. Predict the direction of water movement and explain your answer. [3]
Step 1: compare water potential
The sugar solution is concentrated, so it has a lower water potential than the cell contents.
Step 2: apply the rule
Water moves from higher to lower water potential, across the partially permeable membrane.
Step 3: state the direction
So water leaves the cell by osmosis.
Water moves out of the cellAlways name both sides’ water potential before stating a direction.
WORKED EXAMPLE
Cells are treated with a chemical that stops respiration. Explain the effect on the uptake of oxygen and on the uptake of potassium ions by pumps. [3]
Point 1
No respiration means no ATP is produced.
Point 2
Active transport of potassium ions stops, because the pumps cannot change shape without ATP.
Point 3
Oxygen uptake is by simple diffusion, which needs no ATP, so it continues — though the gradient will fade once respiration is no longer using the oxygen up.
Pumps stop; diffusion carries onThat last clause about the gradient fading is the sort of detail that lifts a good answer.
💡 Exam tip
Always write net movement. Particles move both ways; diffusion is about the overall imbalance.
Say down or against the concentration gradient explicitly. It is usually a mark on its own.
For osmosis, use water potential and the phrase partially permeable membrane.
Only active transport uses ATP. Facilitated diffusion uses proteins but no energy.
If a graph plateaus, name the limiting factor: all the transport proteins are occupied.
For rate questions, have the four factors ready: gradient, temperature, surface area, and the nature of the particle.
⚠ Common mix-up
Thinking facilitated diffusion needs energy because it uses proteins. It does not. Proteins and ATP are separate questions.
Saying osmosis is water moving from low to high concentration. Say it in terms of the solution: dilute to concentrated, or high to low water potential.
Describing diffusion as particles “wanting” to spread out. It is random movement plus an imbalance in numbers.
Forgetting that diffusion is still two-way. Particles cross both ways; the net flow is one way.
Using “active transport” for anything that involves a protein. Check the gradient direction first.
Saying equilibrium means movement has stopped. Particles still move; there is simply no net change.
Confusing water potential with solute concentration. High solute means low water potential.
Up next: Glycolipids & Glycoproteins — the carbohydrate chains on the outer surface, and how a cell uses them to say who it is.
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