Passive Transport
Cells move many substances across their membranes without spending any energy at all, they simply let physics do the work. Particles spread from where they are crowded to where they are sparse. This lesson explains the three passive processes, simple diffusion, osmosis and facilitated diffusion, and you will demonstrate two of them on the bench.
Get oriented
Make a prediction, read the goals and the key terms, then clear the trap before you start.
Before You Read
warm-upSpray perfume in one corner of a room and minutes later you can smell it across the room.
Before reading: explain in your own words why the smell spreads, and which way the perfume molecules move, from where there are many to where there are few, or the reverse?
Learning Intentions
goalsKnow
- The definitions of diffusion, osmosis and facilitated diffusion
- That all three are passive (no energy from the cell)
- How to demonstrate diffusion and osmosis practically
Understand
- Why particles move down a concentration gradient
- How osmosis differs from diffusion (water, partially permeable membrane)
- Why some molecules need a protein channel (facilitated diffusion)
Can Do
- Explain each of the three passive processes
- Design and run a diffusion or osmosis demonstration
- Classify a described example as diffusion, osmosis or facilitated diffusion
Scan these before reading
vocabMisconceptions To Fix
watch outBuild the first idea
The core of the dot point, then two checks you can answer straight from the card.
Core Content
Simple Diffusion and Osmosis
+5 XPTwo passive processes driven by gradients
Particles in liquids and gases are always moving randomly, and this random motion alone spreads them out evenly over time.
Simple diffusion is the net movement of particles from a region of high concentration to a region of low concentration, down the concentration gradient, until evenly spread. Small non-polar molecules such as O₂ and CO₂ diffuse straight through the bilayer. Osmosis is a special case for water: the net movement of water molecules across a partially permeable membrane from a region of higher water concentration (dilute solution) to lower water concentration (concentrated solution). Both are passive, they need no energy from the cell.
Pause, copy the highlighted definitions of diffusion and osmosis into your book.
Net movement of water across a partially permeable membrane, from high to low water concentration, is called _____.
Two truths, one lie. Three of these statements about diffusion and osmosis are correct. Which one is the lie?
Take it further
The next move, with the diagram to work from and two more checks.
Facilitated Diffusion
+5 XPWhen a molecule needs a protein doorway, but still no energy
We just saw O₂ and water cross freely. That raises a question: how do ions and glucose, which the hydrophobic core blocks, get across passively? This card answers it → through membrane proteins, by facilitated diffusion.
Charged or large polar molecules cannot cross the lipid core, so they pass through specific membrane proteins while still moving down their gradient.
Text description
- The membrane (phospholipid bilayer) separates a region of higher concentration (top) from lower concentration (bottom). Every arrow points down the gradient.
- Simple diffusion: small non-polar molecules such as O₂ and CO₂ pass straight through the bilayer.
- Facilitated diffusion through a channel protein: ions pass through a water-filled pore.
- Facilitated diffusion through a carrier protein: glucose binds, the protein changes shape and releases it on the other side.
- All three are passive: down the concentration gradient, with no ATP.
Facilitated diffusion is the passive movement of ions or large polar molecules (e.g. glucose) down their concentration gradient, through a specific channel or carrier protein in the membrane. Because it follows the gradient, it needs no energy from the cell, the protein simply provides a route through the hydrophobic core. The rate depends on how many transport proteins are available.
Pause, copy the highlighted definition, noting facilitated diffusion is still passive.
Which statement about facilitated diffusion is correct?
Fill the gaps. Facilitated diffusion moves ions and large polar molecules their concentration gradient, through a channel or protein. Because it follows the gradient it needs energy from the cell.
Work it yourself
Put the idea on paper in your own book before you read on.
Activity 1
Design the Practical
Pattern B, Plan an investigation
Outline a method to demonstrate diffusion (e.g. potassium permanganate crystal in water, or agar cubes with dye) AND osmosis (e.g. Visking/dialysis tubing filled with concentrated sugar solution placed in water, or potato cylinders in different sugar concentrations). For each, state what you would measure and the expected result.
Finish the picture
The last piece of the dot point, then a true or false check across all three cards.
Demonstrating Diffusion and Osmosis
+5 XPThe bench evidence for both processes
We just explained the three processes. That raises a question: how do we actually SEE them happen? This card answers it → two classic demonstrations, one for each.
Both diffusion and osmosis can be shown directly, and the observed change is the evidence that the process occurred.
Diffusion demo: a crystal of potassium permanganate dropped into still water spreads colour outward over time with no stirring, visible evidence of particles moving down a concentration gradient. Osmosis demo: Visking (dialysis) tubing filled with concentrated sugar solution and placed in water gains mass as water moves in by osmosis; potato cylinders in concentrated sugar solution lose mass. Measuring mass change before and after is the quantitative evidence for osmosis. Controls (e.g. tubing in water vs water in water) and repeats improve reliability.
Add the diffusion (colour spread) and osmosis (mass change) evidence to your notes before the check.
Diffusion, osmosis and facilitated diffusion are all passive (no energy from the cell).
Visking tubing of concentrated sugar solution placed in water gains mass as water enters by osmosis.
Osmosis is the diffusion of any solute across a membrane.
See where it goes
Where this shows up outside the classroom, and why the dot point is worth knowing.
Why a salty meal makes you thirsty
Eat a packet of salty chips and you soon feel thirsty, that is osmosis at work in your own body. The extra salt raises the solute concentration of your blood, so water moves out of your cells by osmosis, leaving them slightly dehydrated. Your brain detects this and triggers thirst, prompting you to drink and restore balance. The same partially permeable membranes you study on the bench are regulating water in every cell you own.
Explain the processes of simple diffusion, osmosis and facilitated diffusion.
Explain asks for the cause, not just the definition. For all three processes the cause is the same — random particle motion plus a concentration gradient — so the marks are for showing how one cause produces three different-looking processes.
6 marks, three processes. Give each a definition, the route taken across the membrane, and the reason no energy is required. State the shared cause once, at the start.
Particles in a liquid or gas are in constant random motion. Where a substance is more concentrated in one region than another, this random motion alone produces a net movement from the region of higher concentration to that of lower concentration, down the concentration gradient. Because the motion is thermal and already occurring, none of the three processes requires the cell to expend energy, which is why all three are described as passive.
Simple diffusion is the net movement of particles down their concentration gradient directly through the phospholipid bilayer, continuing until the particles are evenly distributed and no net gradient remains. Only substances that can dissolve in the hydrophobic core take this route, that is small non-polar molecules such as oxygen and carbon dioxide, and small lipid-soluble molecules.
Osmosis is the special case of diffusion applied to water: the net movement of water molecules across a partially permeable membrane, from a region of higher water concentration, meaning a more dilute solution, to a region of lower water concentration, meaning a more concentrated one. The membrane is permeable to water but not to the dissolved solute, so the solute cannot move to even out the difference and the water moves instead.
Facilitated diffusion is the net movement of ions or large polar molecules, such as glucose, down their concentration gradient through a specific channel or carrier protein embedded in the membrane. It is needed because the hydrophobic core repels charged and polar substances, so the protein provides a hydrophilic route through it; the movement still follows the gradient, so no ATP is required. Its rate depends on the number of available transport proteins as well as on the steepness of the gradient, so it saturates once every protein is occupied.
In all three processes the direction of movement is determined by the concentration gradient and the energy by random thermal motion; what differs is only the route — through the bilayer, through a partially permeable membrane in the case of water, or through a transport protein — and the route is set by the properties of the substance being moved.
Particles in a liquid or gas are in constant random motion, and where a substance is more concentrated in one region than another that motion alone produces a net movement down the concentration gradient; because this thermal motion is already occurring, none of the three processes requires the cell to expend energy, which is why all are passive. Simple diffusion is the net movement of particles down their gradient directly through the phospholipid bilayer until they are evenly distributed, and it is available only to substances that dissolve in the hydrophobic core, namely small non-polar molecules such as oxygen and carbon dioxide and small lipid-soluble molecules. Osmosis is the special case for water: the net movement of water molecules across a partially permeable membrane from a more dilute solution, of higher water concentration, to a more concentrated one, of lower water concentration; it is a distinct process because the membrane blocks the dissolved solute, so the solute cannot move to even out the difference and the water moves instead. Facilitated diffusion is the net movement of ions or large polar molecules such as glucose down their gradient through a specific channel or carrier protein, required because the hydrophobic core repels charged and polar substances while the protein offers a hydrophilic route through it; it needs no ATP because the movement still follows the gradient, and its rate depends on the number of transport proteins as well as the gradient, so it saturates once all are occupied. In every case the gradient sets the direction and random thermal motion supplies the energy; only the route differs, and the route is determined by the properties of the substance being moved.
Compare simple diffusion and facilitated diffusion.
Compare requires similarities and differences on shared criteria. Two paragraphs, one per process, is a Describe answer no matter how accurate it is.
5 marks. One sentence of similarities, then three criteria, each stated for both processes in the same sentence.
Both are passive processes: both move substances down a concentration gradient, from higher to lower concentration, both are driven by the random thermal motion of particles rather than by energy from the cell, so neither requires ATP, and both continue until the gradient is abolished and no net movement remains.
Simple diffusion takes substances directly through the phospholipid bilayer, dissolving in the hydrophobic core and emerging on the other side, whereas facilitated diffusion takes them through a channel or carrier protein that provides a hydrophilic path across, so the substance never contacts the hydrophobic core.
Simple diffusion is restricted to small non-polar and lipid-soluble molecules such as oxygen and carbon dioxide, which can dissolve in the lipid core, whereas facilitated diffusion carries ions and large polar molecules such as glucose, which are repelled by that core and could not otherwise cross.
Simple diffusion is non-specific and its rate increases in direct proportion to the steepness of the concentration gradient, with no upper limit imposed by the membrane, whereas facilitated diffusion is specific, since each carrier or channel accepts only particular substances, and its rate also depends on the number of transport proteins present, so it plateaus at a maximum once every protein is occupied even if the gradient is steepened further.
Simple diffusion and facilitated diffusion are both passive: both move substances down a concentration gradient from higher to lower concentration, both are driven by the random thermal motion of particles rather than by cellular energy so neither requires ATP, and both cease when the gradient is abolished. They differ in three respects. Simple diffusion carries substances directly through the phospholipid bilayer, the substance dissolving in the hydrophobic core, whereas facilitated diffusion carries them through a channel or carrier protein that provides a hydrophilic route so the substance never contacts that core. Consequently simple diffusion is restricted to small non-polar or lipid-soluble molecules such as oxygen and carbon dioxide, while facilitated diffusion handles ions and large polar molecules such as glucose, which the hydrophobic core repels. Finally, simple diffusion is non-specific and its rate rises in direct proportion to the steepness of the gradient with no membrane-imposed ceiling, whereas facilitated diffusion is specific, each protein accepting only particular substances, and its rate depends on the number of transport proteins as well as the gradient, so it plateaus at a maximum once all the proteins are occupied.
Practise independently
Answer in your own words first. The model answers are for checking, not for copying.
Multiple Choice
+5 XPA fresh set drawn from this lesson's question bank, feedback shown immediately. +5 XP per correct · +25 XP all correct
Pick your answer, then rate your confidence, that tells the system what to drill next.
Short Answer, 12 marks
+5 XPUnderstandBand 5(6 marks) 1. Explain the processes of simple diffusion, osmosis and facilitated diffusion. Then outline how you could demonstrate osmosis using Visking tubing, stating what you would measure.
3 marks: correct explanation of each of the three processes (gradient, passive, water vs solute, role of protein) · 3 marks: a valid osmosis demonstration with a stated measurement and expected result
ApplyBand 4(3 marks) 2. Visking tubing filled with concentrated sugar solution is tied off, dried, weighed and placed in a beaker of distilled water. Predict the change in mass after 30 minutes and explain it using the process of osmosis.
1 mark: predicts an increase in mass · 1 mark: identifies the direction of net water movement and why · 1 mark: names the partially permeable membrane as the reason the sugar stays in
EvaluateBand 5–6(3 marks) 3. In the Visking tubing investigation above, describe one control and one repeat measure you would include, and explain what each contributes to the quality of the result.
1 mark: a valid control described · 1 mark: explains what the control rules out · 1 mark: repeats described and linked to reliability
Show all answers
Multiple choice
MC answers and full explanations are shown inline as you complete each question. Use the retry button to attempt a fresh set from the lesson bank.
Short Answer Model Responses
Q1 (6 marks): Simple diffusion is the net movement of particles from a region of high concentration to a region of low concentration (down the concentration gradient) until evenly spread; small non-polar molecules like O₂ pass straight through the bilayer, and no energy is required. Osmosis is the net movement of water molecules across a partially permeable membrane from a region of higher water concentration to lower water concentration. Facilitated diffusion is the passive movement of ions or large polar molecules down their concentration gradient through a specific channel or carrier protein; it needs no energy because it follows the gradient. Demonstration of osmosis: fill a length of Visking (dialysis) tubing with concentrated sugar solution, tie the ends, record its starting mass, then submerge it in distilled water; after a set time, remove, dry and re-weigh it. The tubing gains mass because water moves into it by osmosis. Including a control and repeats improves reliability.
Q2 (3 marks): The mass of the tubing will increase [1]. The distilled water outside has a higher water concentration than the concentrated sugar solution inside, so there is a net movement of water into the tubing by osmosis [1]. The Visking tubing is partially permeable: water molecules are small enough to cross but sugar molecules are not, so the sugar stays inside and the gradient is maintained, and the gain in water shows as a gain in mass [1].
Q3 (3 marks): A control would be a second piece of tubing filled with distilled water rather than sugar solution and placed in the same beaker [1]. If that tubing shows no mass change, any change in the experimental tubing can be attributed to the concentration difference rather than to the tubing itself, water clinging to the outside, or evaporation, which makes the result valid [1]. Setting up several identical tubes and taking a mean, rather than relying on one, improves reliability, because a single anomalous reading (a leak, a poorly dried tube) no longer decides the result [1].
Retrieve what stuck
Work the spaced-review set first. It mixes this lesson with earlier ones on purpose.
Check what actually stuck
A short retrieval set drawn from this lesson and the ones before it, so the earlier work keeps coming back rather than fading.
Lock it in
bossYou can now explain all three passive processes and demonstrate diffusion and osmosis. Next lesson applies osmosis to real cells, what happens to a cell placed in a hypertonic, hypotonic or isotonic solution.
↩ Back to focus-area hubCompare it with where you started
Read your opening prediction again, then mark the lesson complete.
How did your thinking change?
At the start you explained why perfume spreads across a room. It is diffusion: random particle motion gives a net movement from high concentration (where you sprayed) to low concentration (across the room), needing no energy. If you thought facilitated diffusion requires energy, revisit the misconception box, it is still passive.