What you need to know
- 2.5(i–ii)Passive transport and the roles of channel and carrier proteinA membrane protein that binds a specific molecule and changes shape to move it across the membrane, in facilitated diffusion or active transport..
By the end, you should be able to…
- Define diffusionThe net movement of molecules from a region of higher concentration to a region of lower concentration. It is passive. as net movement caused by random molecular motion.
- Compare simple and facilitated diffusionPassive movement of molecules down their concentration gradient through channel or carrier proteins..
- Interpret the effect of a concentration gradientThe difference in concentration of a substance between two regions. on transport.
Build your understanding
Random motion, directional net movement
Particles move randomly because of their kinetic energy. Where a concentration difference exists, more particles tend to leave the more concentrated region than move back, producing net movement down the gradient. Individual particles can move in either direction.
At dynamic equilibriumA state that is kept roughly constant by continual small corrections, so it fluctuates around a set point., particles still move, but equal opposing movements give no net change. diffusionThe net movement of molecules from a region of higher concentration to a region of lower concentration. It is passive. does not stop because the particles stop moving. Passive movement does not require ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi. hydrolysisA reaction that breaks a covalent bond using a molecule of water. by the transport mechanism.
Two passive routes
Simple diffusion through a membrane occurs directly through the phospholipid bilayerTwo layers of phospholipids with the hydrophobic tails inside and the hydrophilic heads facing the water on each side., especially for small non-polarHaving no separated charges, so the molecule does not attract water and does not dissolve well in it. molecules. facilitated diffusionPassive movement of molecules down their concentration gradient through channel or carrier proteins. uses membrane proteins to move suitable particles down their gradient. It is still passive even though a protein is involved.
Two passive routes (continued)
channel proteinA membrane protein with a water-filled pore that lets specific ions or polar molecules diffuse through. form selective hydrophilic pathways, often for particular ions. carrier proteinA membrane protein that binds a specific molecule and changes shape to move it across the membrane, in facilitated diffusion or active transport. bind a substance and change conformation to move it across. Both provide alternatives to the hydrophobic membrane interior; a carrier is not a permanently open pore. For ions, electrical differences also influence movement, so concentration alone may not predict direction in every setting.
Rates and limits
A steeper gradient usually increases net diffusion rate if other factors are unchanged. More exchange area and a shorter path also favour faster diffusion. Temperature affects molecular motion, but in biological membranes large temperature changes may also alter structure.
Carrier-mediated transport has a finite capacity. At high substrateThe molecule that binds to an enzyme’s active site and is changed in the reaction. concentrations, available carriers can become occupied and cycling rate can limit further increases. A plateau is consistent with a limited transport mechanism, but is not enough by itself to prove whether ATP is used.
Follow the process
Particles have kinetic energy and move at random → a concentration difference means more leave the concentrated side than return → net movement is down the gradient → at dynamic equilibrium the two directions balance and net movement is zero. The particles have not stopped.
If the substance cannot cross the phospholipid bilayer, the route branches. A channel protein is a selective pore, often for an ion. A carrier protein binds the substance and changes shape. Both are facilitated diffusion while movement is down the gradient and the transporter does not use ATP. State the direction and the gradient in the answer.
NOT: facilitated diffusion is not movement against the gradient.
- Both routes are passive: down the gradient, no ATP
- A carrier can do facilitated diffusion or, separately, active transport
- Net movement stops at equilibrium; molecules still move
- Charge and lipid solubility matter, not size alone
Two traps this diagram blocks
- Do not say facilitated diffusion uses ATP because a protein is involved.
- Do not describe equilibrium as the point when all movement stops.
From the diagram alone
Question
Compare simple diffusion and facilitated diffusion of a substance across a cell-surface membrane. [3]
Show a model answer
Both are passive net movements down a concentration gradient and neither uses ATP. Simple diffusion of a small non-polar molecule is through the phospholipid bilayer. Facilitated diffusion instead uses a channel or carrier protein.
| Route | Pathway | ATP used directly? |
|---|---|---|
| Simple diffusion | Phospholipid bilayer | No |
| Facilitated diffusion | Channel or carrier protein | No |
Make your reasoning visible
Build the explanation
X happens because Y, leading to Z. X: net movement of the solute is into the cell. Y: its concentration is higher outside, and a carrier lets it cross down that gradient. Z: the concentrations become more alike until net movement stops. Incomplete: “It diffuses because of a protein, so it is active.” Improved: “The carrier allows facilitated diffusionPassive movement of molecules down their concentration gradient through channel or carrier proteins. down the concentration gradientThe difference in concentration of a substance between two regions., without ATPAdenosine triphosphate: the molecule that releases energy for cell processes when it is hydrolysed to ADP and Pi..” The improvement separates the protein from the energy source.
Say “net movement”
Particles move in both directions. The gradient determines the overall balance of movement, not the direction of every molecule.
Key explanations
- Oxygen and carbon dioxide are examples that cross in this way.
- At equilibrium, molecules continue to move but there is no net movement.
Avoid this misconception
The claim: Facilitated diffusion moves substances against their gradient.
The correction: It enables passive movement down a gradient using a protein. Movement against the relevant gradient requires an energy source.
Why molecules spread out, even though each one moves at random
Molecules are always moving in random directions. When there are more of them on one side of a membrane, more of them happen to cross from that side, so overall they spread out. The lab below lets you watch this happen, count the crossings and measure how fast it goes.
What the model shows
A piece of cell membrane with the outside of the cell above and the cytoplasm below. Oxygen can slip through the lipid part of the membrane. Glucose needs carrier proteins. In the osmosis mode only water can cross, through channels called aquaporins.
What to look for
The graph lines moving together, and the arrow showing net movement. When the lines meet, look at the crossings counter: are molecules still crossing? Then compare how fast oxygen and glucose cross when you start with more of them.
What it tells us
Diffusion is net movement down a concentration gradient, and it does not stop at equilibrium: the crossings just balance. Carrier proteins can only work so fast, so facilitated diffusion levels off. Water moves by osmosis towards the side with more solute, which has the lower water potential.
Be careful what you claim
Real cells have far more molecules, so their curves are smoother. The two halves here cannot swell, so in osmosis the side that gains water gets crowded and the flow levels off; the ψ bars show only the solute effect, not that push-back, which in real cells is pressure potential (see the next lesson). The water potentials are made-up model values to show the direction, not real measurements.
Try it in the model: just watch at first: let the oxygen spread until the two graph lines meet, then check whether molecules are still crossing.
Watch diffusion and osmosis
Particles crossing a membrane between the outside of a cell and its cytoplasm. Compare simple diffusion, facilitated diffusion through carriers, and osmosis through aquaporins. Particle numbers are small and illustrative.
What you see
- A box split by a membrane: outside the cell at the top, inside at the bottom. Both halves have the same volume, so the number of particles stands for the concentration.
- Oxygen (red) crossing the lipid core, glucose (amber) using carrier proteins (green), or water (blue) using aquaporins while sucrose (purple) stays put.
- A graph of the amount on each side over time, a net-movement arrow, crossings each way, and water-potential bars in the osmosis mode.
How to use it
- Choose a process, set how many particles start on each side, and let it run. Changing a setting starts a new run.
- Each run records its starting rate after 5 seconds. The rate graph compares your runs with the average curves.
- Temperature changes how fast the particles move.
What it shows
- Net movement down a concentration gradient, and dynamic equilibrium.
- Simple diffusion rises in proportion to the gradient; facilitated diffusion levels off when the carriers are busy.
- Osmosis: water moves towards the side with the lower (more negative) water potential.
Limits of the model
- Particle numbers are small and the box is two-dimensional, so single runs are noisy. The rate curves are averages tuned to the particle model.
- The two halves keep the same volume. A real cell would swell or shrink (see the 2A 3 cell lab), and pressure is not modelled here.
- Water potentials are illustrative model values in kPa, worked out from the solute fraction on each side.
Guided tour · step 1 of 7
- In Set up, just below these steps, keep Process on Simple diffusion (options: Simple diffusion, Facilitated, Osmosis).
- Oxygen starts outside the cell. Watch the graph in the simulator until the two lines meet (within a few molecules).
Every molecule moves at random, but while there are more outside, more cross inwards than outwards: net movement down the concentration gradient.
Set up
Changing a setting starts a new run; each run records its starting rate after 5 s.
Readings
Time 0 s. Both halves have the same volume, so a count stands for a concentration.
Rate against gradient
Lines: average model rates (red simple, green facilitated with 3 carriers). Dots: your runs. Single runs are noisy.
Questions
Check your understanding
Try each question before opening the suggested answer.
Practice question
Retrieval and explanation · suggested answer
A molecule crosses through a carrier from high to low concentration without ATP use. Name the process.
Show suggested answer
Facilitated diffusion: it is passive movement down the concentration gradient through a carrier protein.
Practice question
Retrieval and explanation · suggested answer
What happens to molecular movement at diffusion equilibrium?
Show suggested answer
Random motion continues in both directions, but the opposing rates are equal, so there is no net movement.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A transport-rate curve plateaus at high substrate concentration. Can you conclude that the transport is active?
Show a hint
Ask what a plateau tells you and what it does not.
Explore the reasoning
No. A finite number of carriers can limit both facilitated diffusion and active transport. Evidence about direction relative to the gradient and dependence on energy is needed to distinguish them.
Recall this lesson
Lesson recall questions
Question 1 of 5: Compare simple diffusion with facilitated diffusion.
Passive, active and bulk transport
Practice question
Retrieval and explanation · suggested answer
Compare simple diffusion with facilitated diffusion.
Lesson: 2A 2: Cell Transport and Diffusion
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
A membrane as part of a treatment system
2A · Cell Transport and DiffusionHaemodialysis passes blood through a filter and uses dialysis fluid to help remove wastes. Selective exchange across a membrane is central to the process. Fresh dialysis fluid helps…Specification 2.5(i)Read & use the scienceClose article
Why this matters
Haemodialysis passes blood through a filter and uses dialysis fluid to help remove wastes. Selective exchange across a membrane is central to the process. Fresh dialysis fluid helps maintain a concentration difference for suitable waste solutes, while the membrane restricts movement of larger components such as blood cells. If both sides approached the same waste concentration, net diffusion would slow even though individual molecules continued moving. Renal technology must coordinate fluid composition and flow rather than rely on a membrane alone. Water removal also involves a controlled pressure difference, so explaining every part of the treatment as osmosis would miss an important mechanism.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.5(i) understand what is meant by passive transport (diffusion, facilitated diffusion), active transport (including the role of ATP as an immediate source of energy), endocytosis and exocytosis
Use the science
Why would continually replacing dialysis fluid help remove a diffusible waste solute more effectively than leaving the same fluid beside the blood?
Explore a possible response
Waste entering unchanged fluid raises its concentration and reduces the concentration difference across the membrane. Replacing it with fluid low in that waste maintains the gradient, sustaining net diffusion out of the blood. Molecules still move both ways; net movement depends on the difference. Pressure-controlled water removal is a separate part of the system.
Explore the source
- NIDDK: haemodialysisNIDDK
Why some medicines work as a patch
2A · Cell Transport and DiffusionA transdermal patch supplies a medicine across the skin over time. Pharmaceutical scientists must match the molecule and formulation to a demanding barrier: the outer skin layer contains…Specification 2.5(i) · 2.2(i)Read & use the scienceClose article
Why this matters
A transdermal patch supplies a medicine across the skin over time. Pharmaceutical scientists must match the molecule and formulation to a demanding barrier: the outer skin layer contains cells surrounded by lipids. Small molecules with suitable lipid solubility can diffuse through more readily than many large or strongly water-soluble molecules. The skin is a tissue, not a single cell membrane, but the principles of gradients and permeability still apply. This explains why changing a tablet into a patch is not a simple packaging decision. Delivery rate, dose and sustained performance must be measured for the particular drug and system.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 1, pp. 15–18. The application extends beyond required recall.
- 2.5(i) understand what is meant by passive transport (diffusion, facilitated diffusion), active transport (including the role of ATP as an immediate source of energy), endocytosis and exocytosis
- 2.2(i) know the structure and properties of cell membranes
Use the science
Why might two drugs with the same concentration in a patch cross the skin at different rates?
Explore a possible response
Their sizes and chemical properties affect partitioning and permeability. Equal concentration does not imply equal diffusion coefficients or equal delivery through a layered tissue.
Explore the source
- Transdermal drug deliveryPrausnitz and Langer, Nature Biotechnology (2008)