Three ways of getting across a membrane without spending any energy: straight through the lipids, through a protein, or — if you are water — a bit of both. All three are the same idea underneath, so learn the idea once and the three names look after themselves.
📚 What you need to know
Simple diffusion is the net movement of particles from a region of higher concentration to a region of lower concentration, caused by their random motion.
Particles move down a concentration gradient and eventually reach equilibrium, where they are evenly spread.
The rate of diffusion depends on the steepness of the gradient, temperature, surface area and the size, charge and polarity of the particles.
Osmosis is the diffusion of water across a partially permeable membrane, from a dilute solution to a more concentrated one.
Osmosis can also be described as water moving from higher water potential to lower water potential.
Aquaporins are channel proteins that let water cross far more quickly than it could alone.
Facilitated diffusion uses channel or carrier proteins to move large, polar or charged particles down their gradient.
All three are passive — they need no ATP, because the gradient supplies the energy.
Simple diffusion
Learn this definition
the net movement, as a result of random motion, of molecules or ions from a region of higher concentration to a region of lower concentration
Two words in that definition do all the work.
Random. Particles are not steered anywhere. Each one is moving in a random direction because it has kinetic energy. Nothing pushes them from the crowded side to the empty side.
Net. Particles cross the membrane in both directions the whole time. It is simply that when one side is crowded, more particles happen to set off from that side than from the other. The overall result — the net movement — is down the gradient. When the two sides are equal, particles keep crossing but there is no net movement. That is equilibrium, and it is not the same thing as everything stopping.
The classic examples are the respiratory gases:
Oxygen diffuses into cells from the surrounding capillaries. Respiration keeps using it up, so its concentration inside stays low and the gradient is maintained.
Carbon dioxide diffuses out of cells into the capillaries. Respiration keeps producing it, so its concentration inside stays high.
Notice that in both cases it is respiration that maintains the gradient. Diffusion itself is passive, but the cell is quietly spending energy elsewhere to keep the gradient steep. That distinction wins marks in six-mark questions.
The only difference between the three routes is what the particle travels through. The direction is set by the gradient in every case.
What changes the rate of diffusion
Factor
Effect on rate
Why
Steepness of the gradient
Steeper gradient, faster diffusion
A bigger difference in concentration means a bigger imbalance in random crossings
Temperature
Higher temperature, faster diffusion
Particles have more kinetic energy, so they move about faster
Surface area
Larger area, faster diffusion
More membrane means more places to cross at once
Size of particle
Larger particles diffuse more slowly
They need more energy to move and squeeze through
Charge
Charged particles are effectively blocked
The hydrophobic core repels them
Polarity
Non-polar particles diffuse fastest
They dissolve in the non-polar core; small polar ones cross only slowly
Membrane thickness matters too: the thinner the barrier, the faster the diffusion. That is why alveoli and capillary walls are one cell thick.
Osmosis
Learn this definition
the diffusion of water molecules from a dilute solution to a solution with a higher solute concentration, across a partially permeable membrane
Osmosis is just diffusion with the spotlight on water. A dilute solution has few solute particles and therefore lots of free water molecules; a concentrated solution has many solute particles and fewer free water molecules. Water therefore has its own concentration gradient, and it moves down it — from the dilute side to the concentrated side.
The membrane has to be partially permeable: it lets water through but not the solute. If the solute could cross too, the solute would simply diffuse until both sides matched and nothing dramatic would happen to the water.
Water is going both ways all the time. There is simply more of it setting off from the dilute side, so the net flow is to the right.
Water potential
There is a second way of saying exactly the same thing, and examiners like it because it removes an ambiguity. If you talk about a “concentrated” solution, it is easy to muddle whether you mean concentrated in solute or in water. Water potential avoids that: it describes the tendency of water to move.
A dilute solution has a higher water potential.
A concentrated solution has a lower water potential.
Water moves from higher water potential to lower water potential, through a partially permeable membrane.
Water and the bilayer. Water is polar, so in theory it should be blocked — but it is so small that a little of it slips between the phospholipids anyway. Cells that need to move a lot of water quickly, such as those in the kidney, also have aquaporins: channel proteins built specifically to let water through fast.
Facilitated diffusion
Large molecules, polar molecules and ions cannot get through the hydrophobic core on their own. They cross using transport proteins instead, and that is facilitated diffusion.
It is still passive. No ATP is used, because the particles still move down their concentration gradient.
The direction depends only on the relative concentrations on each side, not on the protein.
Every transport protein is specific, so the cell decides what is allowed a route in by deciding which proteins to make.
Channel proteins provide a pore, used mostly by ions. Carrier proteins change shape, and handle larger molecules such as glucose.
The word “facilitated” just means “helped”. Nothing is being pushed — the protein simply provides a route through the part of the membrane that would otherwise be impassable. Students often assume a protein means energy is being spent. It does not.
Putting the three together
Question
Simple diffusion
Facilitated diffusion
Osmosis
What moves?
Small non-polar particles
Large, polar or charged particles
Water
Protein needed?
No
Yes — channel or carrier
Not essential, but aquaporins speed it up
Direction
Down the gradient
Down the gradient
Down the water potential gradient
ATP used?
No
No
No
Example
Oxygen entering a cell
Glucose entering a cell
Water entering a root hair cell
🧠
Sorting out which is which
Ask two questions in order. Is it water? If yes, it is osmosis. Did it need a protein? If yes, it is facilitated diffusion; if no, it is simple diffusion. Two questions, three answers.
Worked examples
WE 1
Why the gradient never runs out
Explain how a cell maintains a concentration gradient for oxygen across its plasma membrane. (3 marks)
Point 1: what happens insideAerobic respiration in the mitochondria continually uses up oxygen.
Point 2: the effect on concentration
This keeps the concentration of oxygen inside the cell lower than outside it.
Point 3: the consequence
A gradient is therefore maintained, so oxygen keeps diffusing in and equilibrium is never reached.
the cell uses oxygen up, so the gradient is constantly renewed“maintain” questions almost always want a process that keeps consuming or producing something
WE 2
Reading a rate graph
A student measures the rate of uptake of two substances as their external concentration rises. Substance A rises in a straight line. Substance B rises and then levels off. Suggest which substance enters by facilitated diffusion, and explain your answer. (3 marks)
Step 1: identify the shape
Substance B levels off, so something is limiting the rate.
Step 2: name the limit
Facilitated diffusion needs transport proteins, and there is a fixed number of them. Once they are all working flat out, the rate cannot rise further.
Step 3: compare with A
Substance A has no such ceiling, so it must be crossing directly through the phospholipids by simple diffusion.
B enters by facilitated diffusion — the transport proteins become saturateda curve that plateaus is nearly always a clue that a protein is the limiting factor
WE 3
Predicting the direction of osmosis
A plant cell with a dilute cytoplasm is placed in a concentrated sucrose solution. State the direction of net water movement and explain your answer using water potential. (3 marks)
Point 1: compare the two
The cytoplasm is dilute, so it has a higher water potential; the sucrose solution is concentrated, so it has a lower water potential.
Point 2: apply the rule
Water moves from higher to lower water potential, across the partially permeable plasma membrane.
Point 3: state the direction
Water therefore leaves the cell by osmosis.
water moves out of the cellsay “partially permeable” somewhere — it is often a mark on its own
💡 Exam tips
Always write net movement. Particles move both ways; the net direction is what the gradient decides.
Use the phrase down a concentration gradient rather than “from high to low” if you can — both score, but the first sounds like a biologist.
For osmosis, name the membrane as partially permeable, not “semi-permeable”.
If a question mentions water potential, answer in water potential. If it mentions concentration, answer in concentration. Match the wording you are given.
Say that facilitated diffusion is passive explicitly. Many students lose the mark by leaving it implied.
Equilibrium means no net movement, not “no movement”.
⚠ Common mistakes
Saying particles “want” to spread out. They move randomly; the spreading is a statistical result, not a preference.
Thinking facilitated diffusion needs ATP. A protein is involved, but no energy is spent.
Getting osmosis backwards. Water moves towards the more concentrated solution, because that is where water is scarcer.
Saying diffusion stops at equilibrium. Movement continues; only the net movement is zero.
Forgetting temperature and surface area. Gradient steepness is not the only factor.
Describing water as unable to cross the bilayer. It crosses slowly on its own; aquaporins just make it much faster.
Up next: Glycolipids & Glycoproteins — the sugary coating on the outside of every cell, and how your immune system uses it to tell you apart from an invader.
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