AP Biology

## Learning Outcomes

- Explain why some molecules require membrane proteins to cross the phospholipid bilayer.
- Distinguish facilitated diffusion from simple diffusion and active transport.
- Describe how channel proteins, carrier proteins, and aquaporins function in passive transport.
- Predict how concentration gradients and electrochemical gradients affect the direction of facilitated diffusion.
- Interpret experimental data showing saturation of transport proteins.
- Identify common AP Biology mistakes involving ATP, ion channels, and membrane permeability.

## AP Biology Syllabus

In AP Biology Unit 2, membrane transport connects cell structure to homeostasis. Facilitated diffusion is tested as part of selective permeability, passive transport, and the role of membrane proteins. You should be able to explain transport direction, energy use, molecular properties, and how experimental results reveal whether a molecule crosses by simple diffusion or through a protein.

- The phospholipid bilayer is selectively permeable because its interior is hydrophobic.
- Small nonpolar molecules such as $$\mathrm{O_2}$$, $$\mathrm{CO_2}$$, and $$\mathrm{N_2}$$ can cross membranes by simple diffusion.
- Ions and many polar molecules cannot cross the membrane unassisted.
- Facilitated diffusion uses membrane proteins but does not require ATP.
- Molecules move down their concentration gradient, or ions move down their electrochemical gradient.
- Aquaporins allow rapid water movement across membranes.
- The rate of facilitated diffusion can become limited by the number of available transport proteins.
- Data showing a transport-rate plateau often indicates protein-mediated transport.

## Test Your Knowledge

Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision.

1. Why can oxygen cross the plasma membrane more easily than sodium ions?
2. Does facilitated diffusion require ATP? Explain your answer using the direction of movement.
3. How does an aquaporin help water cross a membrane?
4. Why does the rate of facilitated diffusion often level off at high solute concentrations?
5. A chemical irreversibly binds to membrane transport proteins. Which transport process is more likely to be affected: simple diffusion of $$\mathrm{CO_2}$$ or facilitated diffusion of glucose?

## Introduction

Cells must exchange materials with their surroundings while maintaining an internal environment suitable for life. The plasma membrane helps achieve this by being selectively permeable: some substances cross easily, while others need help. This selectivity comes from the structure of the phospholipid bilayer. Hydrophilic phosphate heads face the watery exterior and cytoplasm, while hydrophobic fatty acid tails form the membrane interior.

Facilitated diffusion is one of the main ways cells move substances that cannot pass freely through the lipid bilayer. It is still diffusion, so it is passive and moves substances down a gradient. The “facilitated” part means a membrane protein provides a route through the hydrophobic membrane interior.

> **Key Term: facilitated diffusion**
> Passive movement of a substance across a membrane down its concentration or electrochemical gradient through a membrane protein.

### Why the Phospholipid Bilayer Blocks Many Substances

The plasma membrane is made mostly of phospholipids and proteins. Because phospholipids have polar heads and nonpolar tails, they form a bilayer with a hydrophobic center. This structure explains why membrane permeability depends strongly on size, polarity, and charge.

Small nonpolar molecules, such as $$\mathrm{O_2}$$ and $$\mathrm{CO_2}$$, can dissolve into and pass through the hydrophobic core of the membrane. This is simple diffusion. These molecules do not need a channel or carrier protein.

By contrast, ions such as $$\mathrm{Na^+}$$, $$\mathrm{K^+}$$, $$\mathrm{Ca^{2+}}$$, and $$\mathrm{Cl^-}$$ are charged. Large polar molecules, such as glucose, also do not pass easily through the hydrophobic membrane interior. These substances need membrane proteins if they are to cross by passive transport.

> **Key Term: selective permeability**
> The property of a membrane that allows some substances to cross more easily than others.

Water is a special case. Water is small and polar, so a small amount can cross the lipid bilayer slowly. However, most rapid water movement across many cell membranes occurs through aquaporins, which are specialized channel proteins.

> **Key Term: aquaporin**
> A membrane channel protein that allows water molecules to move rapidly across a membrane.

Facilitated diffusion is therefore linked directly to membrane structure. The bilayer creates the barrier, and transport proteins create specific pathways through that barrier.

> **Test Tip:**
> When an AP question asks why a molecule needs facilitated diffusion, mention both the molecule’s properties and the membrane’s hydrophobic interior. For example: “The ion is charged, so it cannot pass through the hydrophobic core of the phospholipid bilayer and must move through a membrane protein.”

### Movement Down Concentration and Electrochemical Gradients

Diffusion is the net movement of particles from an area of higher concentration to an area of lower concentration. This movement does not require metabolic energy because it follows the natural tendency of particles to spread out.

> **Key Term: concentration gradient**
> A difference in the concentration of a substance between two regions.

Facilitated diffusion follows the same rule. If glucose is at a higher concentration outside a cell than inside, and the cell has a suitable glucose transport protein, glucose can move into the cell by facilitated diffusion. If the concentration is higher inside than outside, net movement would be out of the cell instead, assuming the protein allows that direction.

For ions, direction is affected by both concentration and charge. Because ions are charged, they respond not only to differences in concentration but also to differences in electrical potential across the membrane. Together, these form an electrochemical gradient.

> **Key Term: electrochemical gradient**
> The combined effect of a concentration gradient and an electrical charge difference on the movement of an ion.

For example, if $$\mathrm{Na^+}$$ is more concentrated outside a neuron and the inside of the cell is relatively negative, both the concentration gradient and electrical gradient favor movement of $$\mathrm{Na^+}$$ into the cell. If sodium channels open, $$\mathrm{Na^+}$$ can diffuse inward through those channels. This is facilitated diffusion, not active transport, because sodium moves down its electrochemical gradient and ATP is not directly used by the channel.

This point is a frequent source of wrong answers. Students often see an ion channel and assume “active transport” because ions are involved. That is not correct. The key question is whether the substance moves down or against its gradient.

> **Exam Warning:**
> Do not classify transport as active just because a protein is used. Facilitated diffusion uses proteins but is still passive because movement is down a gradient and does not require ATP.

Passive transport includes simple diffusion, osmosis, and facilitated diffusion. Osmosis is the diffusion of water across a selectively permeable membrane. When water moves through aquaporins, it is both osmosis and facilitated diffusion.

Active transport is different. It moves substances against their gradients and requires energy. The sodium-potassium pump is a classic example: it uses ATP to move three $$\mathrm{Na^+}$$ ions out of the cell and two $$\mathrm{K^+}$$ ions into the cell, against their gradients. This helps maintain membrane potential. Ion flow through an open channel down a gradient is not the same as ion pumping against a gradient.

### Channel Proteins, Carrier Proteins, and Specificity

Membrane proteins used in facilitated diffusion are not all identical. The two main categories are channel proteins and carrier proteins.

> **Key Term: channel protein**
> A membrane protein that forms a hydrophilic passageway through which specific ions or molecules can diffuse.

Channel proteins act like selective tunnels. They allow certain substances to pass through the membrane without contacting the hydrophobic core of the bilayer. Many ion channels are highly specific. A potassium channel, for example, may allow $$\mathrm{K^+}$$ to pass while excluding ions with different size or charge properties.

Some channels are always open, while others are gated. Gated channels open or close in response to a signal, such as a change in voltage, binding of a ligand, or mechanical stimulation. Voltage-gated sodium channels in neurons are a common AP Biology example. When these channels open, $$\mathrm{Na^+}$$ enters the neuron down its electrochemical gradient, contributing to an action potential.

Aquaporins are channel proteins specialized for water. They are especially useful because water is polar and because cells often need rapid water movement to maintain water balance. Plant cells, animal cells, fungi, and prokaryotic cells all rely on controlled transport across membranes, though cells with walls also have an extra structural barrier outside the plasma membrane.

> **Key Term: carrier protein**
> A membrane transport protein that binds a specific substance, changes shape, and releases the substance on the other side of the membrane.

Carrier proteins do not usually form an open tunnel across the membrane. Instead, they bind a solute on one side, change shape, and release it on the other side. Glucose transporters in many animal cells are good examples. Glucose is polar and relatively large, so it cannot freely pass through the lipid bilayer. A glucose transporter can allow glucose to move down its concentration gradient by facilitated diffusion.

Specificity matters. A transport protein usually recognizes only certain molecules or ions. This helps cells regulate what enters and leaves. A cell membrane may contain many different proteins, each with a role in transport, signaling, adhesion, recognition, or enzymatic activity.

> **Test Tip:**
> If a question asks why facilitated diffusion is selective, refer to the structure of the transport protein. Channels and carriers have shapes, charges, and binding sites that allow certain substances through while excluding others.

Facilitated diffusion can occur into or out of the cell. The protein does not decide direction by “wanting” to import or export. Direction depends on the gradient. If the gradient reverses, net movement can reverse, unless the protein is regulated or absent.

### Saturation and Experimental Evidence for Facilitated Diffusion

A major AP Biology skill is interpreting data from transport experiments. Facilitated diffusion often produces a different graph from simple diffusion because it depends on a limited number of membrane proteins.

> **Key Term: saturation**
> A condition in which all available transport proteins are occupied, so increasing solute concentration no longer increases transport rate much.

In simple diffusion, the rate generally increases as the concentration difference across the membrane increases. If more $$\mathrm{CO_2}$$ is outside a cell than inside, increasing the outside concentration tends to increase the rate of diffusion into the cell, assuming the membrane remains intact and permeable.

In facilitated diffusion, the rate also increases at first as the concentration gradient increases. However, once most or all transport proteins are occupied, the rate reaches a maximum. Adding more solute cannot make transport much faster unless more transport proteins are added to the membrane. This is why facilitated diffusion is described as saturable.

Imagine two molecules, A and B, are tested in a cell transport experiment. As the concentration of each molecule outside the cell increases, molecule A enters faster and faster with no clear plateau. Molecule B enters faster at first, but then the rate levels off. A reasonable interpretation is that molecule A crosses by simple diffusion, while molecule B uses facilitated diffusion. The plateau for molecule B suggests that transport proteins have become limiting.

A second experiment could add a chemical that irreversibly binds to transport proteins. If molecule A uses simple diffusion, its movement should be largely unaffected. If molecule B requires a transport protein, its entry should decrease or stop. This type of reasoning appears often in AP-style free-response questions.

> **Exam Warning:**
> Do not say facilitated diffusion stops because the concentration gradient “runs out” if the graph shows a plateau while a gradient is still being maintained. The better explanation is that the number of available transport proteins limits the rate.

Protein number is not the only factor affecting transport rate, but it is central for this subtopic. Other factors can include the steepness of the gradient, whether a channel is open or closed, membrane surface area, temperature effects on molecular motion, and the presence of inhibitors or mutations affecting the transport protein.

Surface area can also affect exchange. A cell with more membrane surface area has more space for transport proteins and more area for diffusion. This is one reason small cells, folded membranes, and structures such as intestinal villi are useful for exchange. However, for a single type of facilitated diffusion, the number and activity of the relevant proteins are often the key rate-limiting factors.

### Comparing Facilitated Diffusion with Other Membrane Transport

Facilitated diffusion is easiest to remember when compared with nearby processes.

Simple diffusion:

- Does not require a protein.
- Moves down a concentration gradient.
- Does not require ATP.
- Works best for small nonpolar molecules such as $$\mathrm{O_2}$$ and $$\mathrm{CO_2}$$.

Facilitated diffusion:

- Requires a membrane protein.
- Moves down a concentration or electrochemical gradient.
- Does not require ATP.
- Transports ions, water through aquaporins, and larger polar molecules such as glucose.

Active transport:

- Requires a membrane protein or transport system.
- Moves against a concentration or electrochemical gradient.
- Requires energy, often from ATP.
- Includes the sodium-potassium pump.

Bulk transport:

- Moves large particles or large quantities of material using vesicles.
- Includes endocytosis and exocytosis.
- Requires energy and membrane remodeling.

A common AP multiple-choice pattern gives four scenarios and asks which is active transport. For example, oxygen diffusing into red blood cells is simple diffusion. Sodium ions entering a neuron through voltage-gated channels is facilitated diffusion. Water moving through aquaporins is osmosis and facilitated diffusion. The sodium-potassium pump using ATP is active transport.

Another common pattern asks which molecules cross the membrane most easily. The best answer is usually small nonpolar molecules. Charged ions, even if small, do not cross the hydrophobic membrane core without a protein. Large molecules also need help or vesicle-based transport.

Facilitated diffusion supports homeostasis by helping cells import needed molecules and export wastes while preserving membrane selectivity. For example, cells need glucose for cellular respiration, but glucose cannot simply dissolve through the hydrophobic bilayer. Transport proteins solve this problem without ATP when the glucose gradient favors movement. Ion channels help cells produce electrical signals, regulate volume, and control cell responses.

> **Key Term: passive transport**
> Movement of substances down a concentration or electrochemical gradient without direct input of metabolic energy.

For exam answers, keep wording precise. “No energy” can be acceptable in many contexts, but “no direct ATP input” or “does not require metabolic energy to move down the gradient” is safer. Cells may use energy elsewhere to create or maintain gradients, but the actual movement through a facilitated diffusion protein is passive.

## Key Point Checklist

This article has covered the following key knowledge points:

- The phospholipid bilayer has a hydrophobic interior that restricts polar and charged substances.
- Small nonpolar molecules such as $$\mathrm{O_2}$$ and $$\mathrm{CO_2}$$ cross membranes by simple diffusion.
- Facilitated diffusion is passive transport through a membrane protein.
- Facilitated diffusion moves substances down a concentration gradient or electrochemical gradient.
- Facilitated diffusion does not require ATP.
- Ions such as $$\mathrm{Na^+}$$, $$\mathrm{K^+}$$, $$\mathrm{Ca^{2+}}$$, and $$\mathrm{Cl^-}$$ require transport proteins to cross membranes.
- Aquaporins are channel proteins that allow rapid water movement.
- Channel proteins form selective hydrophilic passageways through membranes.
- Carrier proteins bind specific solutes and change shape to move them across membranes.
- Facilitated diffusion is saturable because transport rate is limited by the number of available proteins.
- A transport-rate plateau at high solute concentration suggests protein-mediated transport.
- Active transport differs from facilitated diffusion because it moves substances against gradients using energy.

## Key Terms and Concepts

- facilitated diffusion
- selective permeability
- aquaporin
- concentration gradient
- electrochemical gradient
- channel protein
- carrier protein
- saturation
- passive transport
- simple diffusion
- osmosis
- active transport
- phospholipid bilayer
- hydrophobic
- hydrophilic
- transport protein
- ion channel
- gated channel
- sodium-potassium pump
- membrane potential


Membrane transport - Facilitated diffusion