AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/2/2-6)
Unit 2 · Topic 2.6
2.6 Facilitated Diffusion
Facilitated diffusion is passive transport through membrane proteins. Ions like Na⁺ and K⁺ cross through channel proteins, and large polar molecules like glucose cross through carrier proteins, always moving down their gradient with no energy from the cell. Aquaporins are channels that let large amounts of water cross quickly.
Key terms
- facilitated diffusion
- channel protein
- carrier protein
- aquaporin
- ion
What makes it 'facilitated'
Ions and large polar molecules can't get through the hydrophobic core of the bilayer on their own (2.4). Facilitated diffusion solves this by giving them a path through a protein. 'Facilitated' means 'made easier.'
It's still passive transport. Substances move only down their concentration gradient, from high to low, and the cell spends no ATP. The protein just provides the route; the gradient provides the push.
Two kinds of transport proteins
- Channel proteins form a tunnel lined with hydrophilic R groups that lets specific ions or molecules flow through. Many channels are gated, meaning they open and close in response to a signal, such as a change in voltage or the binding of a signal molecule (4.2). Charged ions, including Na⁺ and K⁺, need channel proteins to cross.
- Carrier proteins bind a specific molecule on one side of the membrane, change shape, and release it on the other side. Glucose enters many of your cells this way, through glucose transporter proteins. Carriers are slower than channels because each molecule has to bind and the protein has to change shape.
Ions and charge
Moving ions moves charge. When ions flow through channels, they can change the balance of charge across the membrane, making one side more positive or negative than the other. A membrane with a charge difference across it is said to be polarized. Nerve cells use channels that let Na⁺ and K⁺ rush across in a fraction of a second to send electrical signals.
For an ion, both its concentration gradient and the charge difference across the membrane affect which way it moves. Together, these are its electrochemical gradient (2.8).
Aquaporins: water channels
A little water can cross the bilayer directly, but it's slow. Aquaporins are channel proteins that let large amounts of water pass through quickly, in single file, while blocking ions. Cells that move a lot of water, such as the kidney cells that reabsorb water and many plant root cells, have lots of aquaporins. Water still moves by osmosis, passively, down its water potential gradient (2.7). Aquaporins only speed it up.
The saturation pattern
Because facilitated diffusion depends on a limited number of proteins, its rate behaves differently from simple diffusion. Picture a graph of transport rate (y-axis) against the concentration difference across the membrane (x-axis). For simple diffusion, the line keeps rising steadily. For facilitated diffusion, the curve rises quickly at first, then bends and levels off at a maximum rate. At that plateau, every transport protein is already working as fast as it can, so the proteins are saturated. Adding more transport proteins raises the plateau.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Reading a transport graph
Researchers measure the rate that substance Z enters red blood cells at different outside concentrations (inside concentration stays near zero). The rate rises steeply from 0 to 5 mM, then barely increases between 10 mM and 20 mM. When the cells are treated with a drug that blocks ATP production, the curve doesn't change. How does Z enter the cells?
Show the solutionHide the solution
- Step 1: Z moves from higher concentration outside to near-zero inside, which is down its gradient.
- Step 2: Blocking ATP has no effect, so the cell isn't spending energy to move Z. That rules out active transport.
- Step 3: The rate levels off at high concentration. That's a sign of saturation: a limited number of proteins are all busy. Simple diffusion through the bilayer wouldn't level off like that.
- Step 4: Put it together: passive, down the gradient, protein-limited.
Answer: By facilitated diffusion: it moves down its gradient without ATP, and the plateau shows it depends on a limited number of transport proteins.
- Example 2
Protein means active? (classic trap)
A student says K⁺ leaving a cell through an open K⁺ channel must be active transport, because the K⁺ needs a protein to cross. Evaluate the claim.
Show the solutionHide the solution
- Step 1: Ask what defines active transport: a direct input of energy, often to move a substance against its gradient. Using a protein doesn't by itself make transport active.
- Step 2: Check the direction: K⁺ is more concentrated inside the cell, so leaving the cell is moving down its concentration gradient.
- Step 3: Check energy: an open channel just provides a path. No ATP is used to push the K⁺ through.
- Step 4: Conclude: this is facilitated diffusion, which is passive.
Answer: The claim is wrong. K⁺ flowing out through a channel moves down its gradient with no energy input, so it's facilitated diffusion, a type of passive transport.
Common mistakes
- Calling facilitated diffusion active transport because it uses a protein. It's passive: no direct energy input, and movement only down the gradient.
- Saying water can only cross through aquaporins. Some crosses the bilayer directly; aquaporins greatly speed it up.
- Expecting facilitated diffusion to increase forever with concentration. It levels off when the transport proteins are saturated.
On the exam
- Expect graph-reading questions where you tell simple diffusion (straight line) from protein-mediated transport (leveling off), or test whether transport needs ATP.
- If a question asks how an ion crosses a membrane passively, name a channel protein and say it moves down its gradient.
Connected topics
Videos
Check yourself
4 questions on 2.6 Facilitated Diffusion. Pick an answer to see if you got it, and why.
| Group | Treatment | Average time to burst in hypotonic solution (s) |
|---|---|---|
| 1 | Injected with water | Did not burst within 300 s |
| 2 | Injected with aquaporin mRNA | 35 |
| 3 | Injected with aquaporin mRNA, then treated with a drug that blocks aquaporin channels | Did not burst within 300 s |
Experimental data: frog egg cells, whose membranes normally let little water through, were injected and then placed in a hypotonic solution. Each group had 20 cells.
What is the purpose of Group 1 in this experiment?
Which conclusion is best supported by the data?
Red blood cells take up glucose from the blood through a carrier protein. As the glucose concentration outside the cells is raised, the rate of uptake rises at first and then levels off, even though the concentration keeps increasing. Which of the following best explains the leveling off?
A nerve cell has a K⁺ concentration of about 140 mM inside and about 5 mM outside. If K⁺ channels in its membrane open, what will happen?
0 of 4 answered