Facilitated diffusion is passive transport that uses channel or transport proteins to move substances that cannot cross the hydrophobic core of the membrane on their own, like charged ions and large polar molecules. It moves substances down their concentration gradient with no energy input.
Why This Matters for the AP Biology Exam
This topic connects directly to a skill the AP Biology exam tests often: explaining how the structure of a molecule affects whether it can pass through the plasma membrane. You will see questions that ask you to predict how a substance crosses (directly or through a protein) based on its size, polarity, and charge, and to explain why ions need channel proteins. Because membrane transport ties into selective permeability, osmosis, and gradients, the reasoning here shows up in both multiple-choice questions and free-response explanations where you justify movement using concentration gradients and membrane structure.
Key Takeaways
- Facilitated diffusion is passive: substances move down their concentration gradient with no direct energy input.
- It needs channel or transport (carrier) proteins because the moving substances cannot cross the hydrophobic membrane core alone.
- Charged ions like Na⁺ and K⁺ require channel proteins to cross the membrane.
- Large polar molecules also use facilitated diffusion to move across the membrane without energy.
- Aquaporins are channel proteins that move large quantities of water across membranes quickly.
- The movement of ions across the membrane can polarize it, creating an uneven charge distribution.
Why Some Molecules Need Help Crossing
The membrane has a hydrophobic interior made of phospholipid fatty acid tails. That interior blocks charged and large polar substances. Small nonpolar molecules (like O₂, CO₂, and N₂) slip through the bilayer directly by simple diffusion, but anything charged or large and polar gets stuck without help.
That is where facilitated diffusion comes in. It still moves substances from high concentration to low concentration, just like simple diffusion, but it relies on proteins embedded in the membrane to provide a pathway. No ATP is spent, because the substance is moving down its gradient.
How molecular structure decides the route:
- Small and nonpolar: passes directly through the bilayer.
- Large and polar (like glucose): needs a transport protein.
- Charged ion (like Na⁺ or K⁺): needs a channel protein.
Channel Proteins and Ion Movement
Charged ions cannot pass through the hydrophobic core on their own because of their charge. Ions such as Na⁺ (sodium) and K⁺ (potassium) require channel proteins to move through the membrane during facilitated diffusion.
When ions move across the membrane through these channels, the membrane can become polarized. Polarization happens because moving charged ions creates an uneven distribution of electrical charge across the membrane, so one side becomes more positive or negative than the other.
Aquaporins and Water
Water can cross the membrane in small amounts on its own, but that is slow. Aquaporins are channel proteins that transport large quantities of water across membranes much faster than simple diffusion would allow. They give cells a way to move water quickly when it is needed.
How Facilitated Diffusion Compares
| Transport Type | Energy Required? | Direction | Examples |
|---|---|---|---|
| Simple Diffusion | No | High to Low | O₂, CO₂ |
| Facilitated Diffusion | No | High to Low | Glucose, ions, water through aquaporins |
| Active Transport | Yes (ATP) | Low to High | Na⁺/K⁺ pump, calcium pumps |
Active transport, endocytosis, and exocytosis are covered in other topics. Just remember that those require energy, while facilitated diffusion does not. The key difference between simple and facilitated diffusion is the protein, not the direction or energy use.
How to Use This on the AP Biology Exam
MCQ
Expect questions that hand you a molecule's properties (size, polarity, charge) and ask how it crosses the membrane. Match the structure to the route: nonpolar and small goes straight through, large polar and charged needs a protein. Watch for choices that wrongly attach energy use to facilitated diffusion.
Free Response
When you explain movement, name two things: the concentration gradient (high to low) and the reason a protein is needed (the substance cannot cross the hydrophobic interior). For ions, state that channel proteins are required and that moving ions can polarize the membrane. Tie your explanation back to membrane structure instead of just naming the process.
Common Trap
If a question describes movement down the gradient but through a protein, that is still passive transport. Do not call it active transport just because a protein is involved.
Common Misconceptions
- Facilitated diffusion does not use energy. Using a protein does not mean ATP is spent. The substance still moves down its gradient.
- Proteins do not change the direction of movement. In facilitated diffusion, substances always move from high to low concentration; the protein only provides a path.
- Channel and transport proteins are not the same as the pumps used in active transport. Pumps move substances against the gradient and need energy.
- Water does cross membranes without aquaporins, just slowly. Aquaporins speed up water movement; they are not the only way water gets across.
- Charged ions need channel proteins not because they are large, but because their charge keeps them out of the hydrophobic core.
Related AP Biology Guides
Vocabulary
The following words are mentioned explicitly in the College Board Course and Exam Description for this topic.Term | Definition |
|---|---|
aquaporins | Channel proteins that facilitate the rapid transport of water molecules across cell membranes. |
channel protein | Membrane proteins that form pores or channels allowing specific ions or molecules to pass through the plasma membrane. |
charged ion | Atoms or molecules that have gained or lost electrons and carry an electrical charge, such as sodium (Na⁺) and potassium (K⁺). |
concentration gradient | A difference in the concentration of a substance across a membrane, with higher concentration on one side and lower concentration on the other. |
facilitated diffusion | The passive transport of substances across a membrane through protein channels or carriers, requiring no energy input but moving down the concentration gradient. |
membrane polarization | The development of an electrical charge difference across a membrane caused by the movement of ions. |
plasma membrane | The selectively permeable membrane that surrounds the cell, composed of phospholipids, proteins, and other molecules that regulate what enters and exits the cell. |
polar molecule | Molecules with uneven distribution of electrical charge that generally cannot cross the hydrophobic membrane interior without assistance. |
transport protein | Embedded membrane proteins that facilitate the movement of hydrophilic substances across the membrane. |
Frequently Asked Questions
What is facilitated diffusion in AP Biology?
Facilitated diffusion is passive transport that moves substances down their concentration gradient through membrane proteins. It does not require ATP.
Why do some molecules need facilitated diffusion?
Charged ions and large polar molecules cannot cross the hydrophobic interior of the phospholipid bilayer on their own, so they need channel or carrier proteins.
Is facilitated diffusion active or passive transport?
Facilitated diffusion is passive transport because substances move from high concentration to low concentration without direct energy input.
What is the difference between simple diffusion and facilitated diffusion?
Simple diffusion moves small nonpolar molecules directly through the bilayer. Facilitated diffusion uses proteins to move substances that cannot cross the bilayer alone.
What are aquaporins?
Aquaporins are channel proteins that let water move across membranes quickly. Water can cross slowly without them, but aquaporins greatly increase the rate of movement.
How is facilitated diffusion tested on AP Biology?
AP Biology questions may ask you to predict how a molecule crosses a membrane, explain why proteins are needed, or distinguish facilitated diffusion from active transport.