AP Biology

## Learning Outcomes

- Explain why polar molecules and ions usually cannot cross the phospholipid bilayer without help.
- Define facilitated diffusion as a type of passive transport down a concentration gradient.
- Compare channel proteins and carrier proteins in structure, function, specificity, and rate.
- Describe how aquaporins and ion channels support movement of water and ions across membranes.
- Explain why facilitated diffusion is saturable and how this affects transport graphs.
- Distinguish facilitated diffusion from simple diffusion, osmosis, and active transport in AP-style questions.
- Predict how changes in transport protein number or function affect movement across membranes.

## AP Biology Syllabus

In AP Biology Unit 2, membrane transport is tested as part of cell structure and function, selective permeability, and homeostasis. For this subtopic, focus on how the phospholipid bilayer limits movement of charged and polar substances, and how membrane proteins allow passive transport without ATP. Exam questions often ask you to predict transport direction, interpret transport-rate graphs, or identify whether a process is passive or active.

- The plasma membrane is selectively permeable because of its phospholipid bilayer.
- Small nonpolar molecules, such as oxygen and carbon dioxide, can cross by simple diffusion.
- Ions and many polar molecules need membrane proteins to cross the hydrophobic interior of the membrane.
- Facilitated diffusion moves substances down their concentration or electrochemical gradient.
- Channel proteins form selective passageways, often for ions or water.
- Carrier proteins bind specific solutes and change shape to move them across the membrane.
- Facilitated diffusion does not require ATP, but it is limited by the number and activity of transport proteins.
- AP questions may compare facilitated diffusion with active transport, such as the sodium-potassium pump.

## 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 chloride ions?
2. What is the difference between a channel protein and a carrier protein?
3. Why does the rate of facilitated diffusion eventually level off as solute concentration increases?
4. A sodium ion enters a neuron through a voltage-gated sodium channel during an action potential. Is this active transport or facilitated diffusion?
5. Predict what would happen to glucose uptake if a chemical blocked glucose carrier proteins in the plasma membrane.

## Introduction

Cells must exchange materials with their surroundings, but the plasma membrane does not allow every substance through equally. The membrane is made mainly of phospholipids, with hydrophilic phosphate heads facing the watery environments inside and outside the cell and hydrophobic fatty acid tails forming the membrane interior. This structure allows small nonpolar molecules, such as $$\mathrm{O_2}$$, $$\mathrm{CO_2}$$, and $$\mathrm{N_2}$$, to move through the bilayer relatively easily. In contrast, ions and many polar molecules cannot pass through the hydrophobic interior without assistance.

Facilitated diffusion solves this problem without using cellular energy. Specific membrane proteins provide routes for substances that cannot directly cross the lipid bilayer. Two major types are channel proteins and carrier proteins. Both allow movement down a gradient, but they work in different ways and have different effects on transport rate.

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

### Why polar molecules and ions need transport proteins

The phospholipid bilayer is selectively permeable. This means it allows some substances to cross more easily than others. Size, polarity, and charge are the main factors that determine whether a molecule can move through the membrane unaided.

Small hydrophobic molecules cross the bilayer easily because the membrane interior is also hydrophobic. For example, oxygen enters cells by simple diffusion as it is used in cellular respiration, and carbon dioxide leaves cells as it is produced. These molecules do not need membrane proteins.

Large polar molecules and charged particles face a barrier. Glucose is polar and relatively large, so it cannot easily dissolve in the hydrophobic membrane interior. Ions such as $$\mathrm{Ca^{2+}}$$, $$\mathrm{Cl^-}$$, $$\mathrm{Na^+}$$, and $$\mathrm{K^+}$$ are charged, so they are strongly excluded by the hydrophobic core of the membrane. To cross, they need proteins embedded in the membrane.

> **Key Term: concentration gradient**
> A difference in the concentration of a substance between two regions, such as the inside and outside of a cell.

Passive transport moves substances from higher concentration to lower concentration. This movement is often described as moving “down” a concentration gradient. Diffusion is the general term for this movement, and osmosis is the diffusion of water across a membrane. Facilitated diffusion is still diffusion because the substance moves down its gradient; the only difference is that a membrane protein assists the process.

For ions, the direction of movement depends on both concentration and charge. A positively charged ion may be affected by where it is more concentrated and by the electrical difference across the membrane. The combined effect is called an electrochemical gradient.

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

This distinction matters on the AP Biology exam. If an ion moves through a channel from high concentration to low concentration, no ATP is required. If an ion is pumped from low concentration to high concentration, ATP or another energy source is needed.

> **Test Tip:**
> Do not classify a transport process as active just because a membrane protein is involved. Facilitated diffusion uses proteins but is still passive because movement is down a gradient and does not require ATP.

Cell walls also affect transport in some organisms. Plants, fungi, and prokaryotes have cell walls outside the plasma membrane. These walls provide support and can act as an additional barrier, but the plasma membrane remains the main structure controlling selective movement into and out of the cell. Plant cell walls are made mainly of cellulose, fungal cell walls often contain chitin or glucans, and bacterial cell walls contain peptidoglycan.

### Channel proteins as selective hydrophilic passageways

Channel proteins form hydrophilic passageways through the membrane. They are useful for substances that are small enough to pass through a pore but too charged or polar to cross the lipid bilayer directly. Many channel proteins are highly selective, allowing only certain ions or molecules through.

> **Key Term: channel protein**
> A membrane protein that forms a passageway allowing specific ions or molecules to diffuse across a membrane.

A useful example is an ion channel. A calcium channel may allow $$\mathrm{Ca^{2+}}$$ to move across the membrane, while a chloride channel may allow $$\mathrm{Cl^-}$$ to pass. Selectivity depends on features such as channel size, shape, charge, and binding sites inside the pore. The cell benefits from this specificity because different ions have different roles in signaling, membrane potential, and homeostasis.

Some channels are open most of the time, while others are gated. A gated channel opens or closes in response to a stimulus. Common gating mechanisms include changes in membrane voltage, binding of a molecule, or mechanical stress. For example, voltage-gated sodium channels in neurons open during an action potential, allowing sodium ions to move into the cell down their electrochemical gradient. Although this ion movement is part of nerve signaling, the movement through the channel itself is passive.

Water provides another high-yield AP example. Water is small and polar, so a limited amount can cross the bilayer directly. However, most rapid water movement across cell membranes occurs through aquaporins, which are specialized channel proteins.

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

Aquaporins are especially useful in cells that need fast water movement, such as kidney cells in animals and many plant cells. Water movement is still passive: water moves down its water concentration gradient, often described in AP Biology through osmosis and water potential. Aquaporins do not pump water; they provide a route for water to diffuse more quickly.

Channel proteins can move substances very rapidly because, when open, they provide a continuous passageway. However, transport is still limited by the number of channels in the membrane and by how many are open at a given time. If all available channels are already occupied or open at their maximum rate, adding more solute will not increase movement indefinitely.

> **Exam Warning:**
> A channel is not a nonspecific hole in the membrane. AP questions often test that channels are selective, and many only allow one ion or a narrow group of related ions to pass.

### Carrier proteins and shape changes during transport

Carrier proteins work differently from channels. Instead of forming an open pore, a carrier protein binds a specific molecule on one side of the membrane, changes shape, and releases the molecule on the other side. This makes carriers especially useful for larger polar solutes such as glucose.

> **Key Term: carrier protein**
> A membrane transport protein that binds a specific solute and changes shape to move it across the membrane.

A common example is a glucose transporter. Glucose is polar and cannot cross the hydrophobic membrane interior efficiently. A glucose carrier protein can bind glucose outside the cell, change shape, and release glucose into the cytoplasm. If glucose is moving from higher concentration to lower concentration, this is facilitated diffusion.

Carrier proteins show specificity. A glucose carrier does not usually transport chloride ions, amino acids, or water. The binding site on the carrier matches the transported solute in a way that allows selective binding. This is similar to the specificity seen in enzymes, although carrier proteins are not always enzymes.

Carrier proteins also explain why facilitated diffusion is saturable. Each carrier must bind a solute, change shape, release the solute, and return to its original shape. At low solute concentration, increasing solute concentration increases the transport rate because more carrier proteins are being used. At high solute concentration, nearly all carriers may be working as fast as they can. Once this happens, the transport rate reaches a maximum.

> **Key Term: saturation**
> A condition in which all available transport proteins are occupied or working at maximum rate, so transport rate no longer rises much as solute concentration increases.

This differs from simple diffusion through the lipid bilayer. For a small nonpolar molecule crossing directly through the membrane, transport rate often increases more directly as the concentration gradient increases. For facilitated diffusion, the number of transport proteins places an upper limit on the rate.

Carrier proteins can be involved in passive or active transport, depending on the direction of movement and energy use. This is a common source of confusion. A carrier that moves a solute down its gradient without ATP is carrying out facilitated diffusion. A membrane pump that moves solutes against their gradients using ATP is active transport.

The sodium-potassium pump is a classic active transport example. It uses ATP to move sodium ions out of the cell and potassium ions into the cell against their gradients. For every three sodium ions moved out, two potassium ions are moved in. This helps maintain membrane potential. Because ATP is required and ions are moved against gradients, the sodium-potassium pump is not facilitated diffusion.

> **Test Tip:**
> When classifying transport, ask two questions: “Which way is the substance moving relative to its gradient?” and “Is energy such as ATP required?” Protein use alone is not enough to decide.

### Rate limits, inhibitors, and AP-style data questions

AP Biology questions often present experiments involving membrane transport. You may be asked to interpret a graph, predict the effect of a treatment, or justify a claim using data. Facilitated diffusion has several predictable patterns.

First, the direction of net movement depends on the gradient. If there is more solute outside the cell than inside, and the correct transport protein is present, the solute will tend to enter the cell. If there is more solute inside, the solute will tend to leave. If concentrations are equal, molecules may still move in both directions, but there is no net movement.

Second, the rate depends on the steepness of the gradient. A larger difference in concentration usually causes a faster net movement, at least until the transport proteins become saturated. For ions, remember to consider the electrochemical gradient, not just the concentration gradient.

Third, the rate depends on the number and condition of membrane proteins. A cell with more relevant channels or carriers can usually move the substance faster than a cell with fewer. If a mutation changes the shape of a carrier’s binding site, the carrier may no longer bind its solute effectively. If a toxin blocks a channel, the ion that depends on that channel will move more slowly or may stop moving.

A typical AP-style graph may compare simple diffusion and facilitated diffusion. Simple diffusion of a small hydrophobic molecule may show a steady increase in rate as concentration difference increases. Facilitated diffusion may rise at first but then level off as transport proteins become saturated. The plateau represents a maximum transport rate set by the number and speed of available proteins.

> **Exam Warning:**
> If a graph of transport rate levels off, think about protein saturation. Do not automatically assume the cell has run out of ATP; facilitated diffusion does not require ATP.

Consider this sample reasoning:

> **Question:**
> An experiment measures uptake of a polar solute by cells. When the outside solute concentration increases, uptake rises at first but then reaches a plateau. A chemical is added that irreversibly binds to membrane transport proteins. Predict the result.
>
> **Answer:**
> Uptake of the solute would decrease, and the maximum transport rate would be lower. The polar solute depends on transport proteins for facilitated diffusion. If some of those proteins are blocked, fewer functional proteins remain, so the cell cannot move the solute across the membrane as quickly.

This type of question rewards clear prediction and justification. A good prediction states the expected change in the data. A good justification links the change to membrane structure and transport mechanism.

For example, if the solute is glucose, blocking glucose carriers should reduce glucose uptake. If the substance is oxygen, blocking transport proteins should have little direct effect because oxygen can cross the phospholipid bilayer by simple diffusion. If the substance is water, blocking aquaporins would reduce rapid water movement, although a small amount of water may still cross the membrane directly.

AP questions may also ask you to identify controls. In a transport experiment, useful controls could include cells without the inhibitor, cells exposed to a solvent only, or comparison of a molecule that does not require a transport protein. The independent variable might be solute concentration, inhibitor presence, temperature, or protein expression level. The dependent variable might be rate of uptake, concentration change inside the cell, or ion movement across the membrane.

Facilitated diffusion is also tied to homeostasis. Cells regulate which transport proteins are present in the membrane, how many are present, and whether channels are open or closed. By controlling these proteins, cells control the movement of water, ions, sugars, and other solutes without always spending ATP directly on each movement.

A final point for exam success: be precise with vocabulary. “Diffusion” is not the same as “facilitated diffusion,” although facilitated diffusion is a type of diffusion. “Osmosis” refers specifically to water movement. “Active transport” requires energy and moves substances against a gradient. “Transport protein” is a broad term that can include channels, carriers, and pumps, so you must use the context to decide the mechanism.

## Key Point Checklist

This article has covered the following key knowledge points:

- The plasma membrane is selectively permeable because of its phospholipid bilayer.
- Small nonpolar molecules such as oxygen and carbon dioxide can cross the bilayer by simple diffusion.
- Ions and many polar molecules need membrane proteins because the membrane interior is hydrophobic.
- Facilitated diffusion is passive transport through a membrane protein down a concentration or electrochemical gradient.
- Channel proteins form selective passageways for substances such as ions and water.
- Aquaporins are channel proteins that allow rapid water movement across membranes.
- Carrier proteins bind specific solutes and change shape to move them across the membrane.
- Glucose transporters are common examples of carrier proteins used in facilitated diffusion.
- Facilitated diffusion is saturable because the number of transport proteins is limited.
- A plateau in a transport-rate graph often indicates saturation of channels or carriers.
- The sodium-potassium pump is active transport, not facilitated diffusion, because it uses ATP to move ions against gradients.
- Inhibitors or mutations that block transport proteins reduce movement of substances that depend on those proteins.

## Key Terms and Concepts

- facilitated diffusion
- concentration gradient
- electrochemical gradient
- channel protein
- aquaporin
- carrier protein
- saturation
- passive transport
- selective permeability
- phospholipid bilayer
- simple diffusion
- osmosis
- active transport
- sodium-potassium pump


Facilitated diffusion - Channel and carrier proteins