AP Biology

## Learning Outcomes

- Explain how membrane structure creates selective permeability.
- Distinguish passive transport from active transport using concentration gradients and energy requirements.
- Identify examples of simple diffusion, facilitated diffusion, osmosis, and active transport.
- Describe how transport proteins, channel proteins, aquaporins, and pumps affect movement across membranes.
- Explain how the sodium-potassium pump uses ATP and contributes to membrane potential.
- Predict how changes in gradients, ATP supply, or transport protein availability affect transport rates.

## AP Biology Syllabus

For AP Biology Unit 2, membrane transport links cell structure to cell function. You are expected to explain how the plasma membrane controls exchange with the environment and how this helps cells maintain homeostasis. Exam questions often ask you to classify a transport process from a description, predict movement across a membrane, or interpret data from experiments involving gradients, ATP, or transport proteins.

- The plasma membrane is selectively permeable because of its phospholipid bilayer and embedded proteins.
- Small nonpolar molecules such as $$\mathrm{O_2}$$ and $$\mathrm{CO_2}$$ can cross the bilayer by simple diffusion.
- Ions and large polar molecules usually require membrane proteins to cross.
- Passive transport moves substances down their concentration gradient without direct energy input.
- Facilitated diffusion is passive transport through a membrane protein.
- Active transport moves substances against a concentration gradient and requires energy.
- The $$\mathrm{Na}^{+}/\mathrm{K}^{+}$$ pump uses ATP to move three sodium ions out and two potassium ions in.
- Endocytosis and exocytosis require energy to move large materials into or out of cells.

## 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. A molecule moves from high concentration outside a cell to low concentration inside a cell through a channel protein. Is this passive or active transport?
3. What does it mean for facilitated diffusion to be saturable?
4. How does the sodium-potassium pump use ATP to maintain ion gradients?
5. Why would blocking ATP production affect active transport more directly than simple diffusion?

## Introduction

Cells must exchange materials with their surroundings while maintaining internal conditions that support life. Nutrients must enter, wastes must leave, signaling molecules must be detected, and ion concentrations must be regulated. The plasma membrane makes this possible because it is not equally permeable to all substances.

Membrane transport is mainly classified by two questions: does the substance move down or against its gradient, and is energy required? Passive transport moves substances down gradients without energy input. Active transport uses energy to move substances against gradients or to move large materials by vesicles.

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

### Selective permeability sets the rules for transport

The plasma membrane is a phospholipid bilayer. Each phospholipid has a hydrophilic phosphate head and hydrophobic fatty acid tails. In an aqueous environment, the hydrophilic heads face the watery exterior and cytoplasm, while the hydrophobic tails face inward. This creates a hydrophobic interior that blocks many polar or charged substances.

Small hydrophobic molecules cross the membrane most easily. Examples include oxygen, carbon dioxide, and nitrogen gas. These molecules can move between phospholipids because they are nonpolar and compatible with the hydrophobic interior of the bilayer. This explains why oxygen can diffuse into cells during cellular respiration and carbon dioxide can diffuse out as a waste product.

Large polar molecules and ions cannot easily pass through the lipid bilayer unassisted. Ions such as $$\mathrm{Na}^{+}$$, $$\mathrm{K}^{+}$$, $$\mathrm{Ca}^{2+}$$, and $$\mathrm{Cl}^{-}$$ are charged, so they interact strongly with water and are not able to pass through the hydrophobic membrane interior. Many larger polar molecules also need help from membrane proteins.

Water is small and polar, so a limited amount can cross the bilayer directly. However, most water movement across membranes occurs through aquaporins, which are specialized channel proteins that allow rapid water transport.

> **Key Term: aquaporin**
> A membrane channel protein that allows water molecules to move across a membrane, usually down their water concentration gradient.

Membrane proteins are not all the same. Some form channels, some bind substances and change shape, and some use energy to pump substances across the membrane. In AP Biology questions, the presence of a membrane protein does not automatically mean active transport. You must check whether the substance is moving down or against its gradient and whether energy is being used.

Cells with cell walls, such as plant, fungal, and prokaryotic cells, also have an additional outer layer beyond the plasma membrane. Plant cell walls contain cellulose, fungal cell walls often contain chitin or glucans, and bacterial cell walls contain peptidoglycan. Cell walls provide support and protection, but the plasma membrane remains the main structure controlling selective movement into and out of the cytoplasm.

> **Test Tip:**
> If an AP question asks which molecules cross the membrane most easily, choose small nonpolar or hydrophobic molecules. Ions and large polar molecules usually need a transport protein.

### Passive transport moves down gradients without ATP

Passive transport is movement from an area of higher concentration to an area of lower concentration. This movement is described as going “down” a concentration gradient. Because the particles are moving in the direction they naturally tend to move, passive transport does not require ATP.

> **Key Term: concentration gradient**
> A difference in the concentration of a substance between two regions.
>
> **Key Term: passive transport**
> Movement of substances across a membrane down a concentration gradient without energy input from the cell.

Simple diffusion is the most direct form of passive transport. In simple diffusion, molecules move through the phospholipid bilayer without the help of transport proteins. Oxygen moving into a red blood cell from the alveoli of the lungs is a classic example. Carbon dioxide moving out of cells is another.

> **Key Term: diffusion**
> The net movement of particles from an area of higher concentration to an area of lower concentration.

Osmosis is the diffusion of water across a selectively permeable membrane. Water moves down its own concentration gradient. In AP Biology, osmosis is often discussed with water potential, tonicity, and plant cell turgor pressure, but the key classification here is that osmosis is passive transport.

> **Key Term: osmosis**
> The diffusion of water across a selectively permeable membrane.

Facilitated diffusion is also passive transport, but it uses a membrane protein. It is needed when a substance cannot easily pass through the phospholipid bilayer on its own. Examples include water moving through aquaporins and ions moving through ion channels. In neurons, sodium ions moving through voltage-gated channels down their electrochemical gradient are using facilitated diffusion, not active transport.

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

Facilitated diffusion can be saturable. This means the transport rate is limited by the number of available transport proteins. If all the channels or carriers are occupied, adding more solute will not increase movement at the same rate. Simple diffusion through the bilayer does not depend on a specific number of transport proteins, so it does not show the same protein-limited saturation pattern.

A useful comparison is glucose uptake through certain carrier proteins. If external glucose concentration rises, glucose transport may initially increase. Eventually, the transport proteins become fully occupied, and the rate levels off. By contrast, oxygen diffusion depends largely on the gradient and membrane permeability rather than a limited set of oxygen transporters in the membrane.

> **Exam Warning:**
> Do not classify every protein-assisted process as active transport. Facilitated diffusion uses proteins but does not require ATP because the substance moves down its gradient.

Passive transport continues until equilibrium is reached, unless the cell maintains gradients through other processes. At equilibrium, particles still move, but there is no net movement in one direction. For example, if oxygen concentrations become equal on both sides of a membrane, oxygen molecules still cross both ways, but the overall concentrations remain stable.

### Active transport uses energy to move against gradients

Active transport moves substances from low concentration to high concentration. This is movement “against” the concentration gradient, so it requires energy. The energy may come directly from ATP or indirectly from another gradient.

> **Key Term: active transport**
> Movement of substances across a membrane against a concentration gradient using energy.

Primary active transport directly uses ATP. A transport protein hydrolyzes ATP, changes shape, and moves a substance across the membrane against its gradient. The sodium-potassium pump is the key AP Biology example.

> **Key Term: sodium-potassium pump**
> A membrane protein that uses ATP to pump three $$\mathrm{Na}^{+}$$ ions out of the cell and two $$\mathrm{K}^{+}$$ ions into the cell.

The sodium-potassium pump moves $$\mathrm{Na}^{+}$$ out of the cell even though sodium concentration is already higher outside. It also moves $$\mathrm{K}^{+}$$ into the cell even though potassium concentration is already higher inside. For every ATP used, the pump exports three sodium ions and imports two potassium ions:

$$3\ \mathrm{Na}^{+}\ \text{out} + 2\ \mathrm{K}^{+}\ \text{in}$$

Because more positive charge leaves the cell than enters, the pump helps maintain a difference in charge across the membrane. This contributes to membrane potential, with the inside of many cells being relatively negative compared with the outside. This is especially significant in nerve and muscle cells, where ion gradients are used in electrical signaling.

> **Key Term: membrane potential**
> A difference in electrical charge across a membrane, caused by unequal distribution of ions.

Secondary active transport does not use ATP directly at the transport protein doing the work. Instead, it uses energy stored in an ion gradient that was usually created by primary active transport. For example, if a sodium gradient has been built using ATP, sodium can move back down its gradient through a cotransporter, helping move another substance against its gradient. The second substance is being actively transported because its movement depends on stored gradient energy.

A common AP-style idea is that ATP supply affects active transport. If glycolysis or cellular respiration is inhibited, ATP levels may fall. Pumps such as the sodium-potassium pump may slow or stop because they need ATP. Simple diffusion of oxygen or carbon dioxide can still occur because it does not require ATP. Facilitated diffusion may also continue if a gradient exists, although it depends on functional transport proteins.

> **Test Tip:**
> When asked to justify a prediction about active transport, mention both the direction of movement against the gradient and the need for energy, often ATP.

Active transport also includes vesicle-based movement of large materials. Endocytosis brings substances into the cell by forming vesicles from the plasma membrane. Exocytosis removes substances when vesicles fuse with the plasma membrane and release contents outside the cell. These processes require energy and are used for materials too large to pass through channels or carriers.

> **Key Term: endocytosis**
> Energy-requiring uptake of material into a cell by folding the plasma membrane around the material to form a vesicle.
>
> **Key Term: exocytosis**
> Energy-requiring release of material from a cell when a vesicle fuses with the plasma membrane.

Types of endocytosis include phagocytosis, pinocytosis, and receptor-mediated endocytosis. Phagocytosis is “cell eating,” used to take in large particles. Pinocytosis is “cell drinking,” used to take in extracellular fluid. Receptor-mediated endocytosis uses specific receptors, often associated with coated pits, to bring selected molecules into the cell.

### Distinguishing transport types in AP-style questions

AP Biology questions often describe a situation rather than naming the transport process. Train yourself to classify transport using evidence.

First, identify the substance. Small hydrophobic molecule? It may cross by simple diffusion. Ion? It probably needs a protein. Large particle or macromolecule? Vesicle transport may be involved.

Second, identify the direction of movement. If the substance moves from high concentration to low concentration, the process is passive unless energy is clearly required for another reason. If it moves from low concentration to high concentration, it is active transport.

Third, look for ATP or energy language. “Uses ATP,” “ATP hydrolysis,” “energy is required,” or “pump” usually points to active transport. However, “channel” usually points to facilitated diffusion if the substance moves down its gradient.

Fourth, watch for rate patterns. If a transport rate levels off as concentration increases, that suggests a limited number of transport proteins. This is common for facilitated diffusion and some active transport systems. If a chemical irreversibly binds to transport proteins, transport that depends on those proteins should decrease or stop. Simple diffusion of small nonpolar molecules may be less affected.

> **Question:**
> A student observes that chloride ions move from a high concentration outside a cell to a low concentration inside the cell through a membrane protein. The process stops when a toxin blocks the protein, but no ATP is used. What type of transport is this?

This is facilitated diffusion. Chloride is an ion, so it needs a membrane protein. It moves down its gradient, so it is passive. The blocked protein stops transport because the ion cannot pass through the lipid bilayer unassisted.

> **Question:**
> A bacterial cell uses ATP to pump an antibiotic out of the cytoplasm even though the antibiotic concentration is already higher outside the cell. What type of transport is this?

This is active transport. The substance is being moved against its gradient, and ATP is used.

A common trap is confusing endocytosis with diffusion because both can bring material into the cell. Endocytosis is not movement through a protein channel and is not movement through the bilayer. It uses the membrane to engulf material and form a vesicle, so it requires energy.

Another trap is assuming water movement is always through the phospholipid bilayer. Although water is small, most water movement in cells occurs through aquaporins. Since water moves down its gradient through a channel, this is facilitated diffusion and still passive.

Surface area also affects transport. Cells exchange materials across their surface. Smaller cells generally have a higher surface area-to-volume ratio, making exchange more efficient. Membrane folds, such as intestinal villi, increase surface area for absorption. While this is not the same as changing passive transport into active transport, it can affect how much exchange can occur.

## Key Point Checklist

This article has covered the following key knowledge points:

- The plasma membrane is selectively permeable because of its phospholipid bilayer and membrane proteins.
- Small hydrophobic molecules such as oxygen and carbon dioxide cross membranes easily by simple diffusion.
- Ions and large polar molecules usually require transport proteins.
- Passive transport moves substances down their concentration gradient and does not require ATP.
- Diffusion is passive movement from high concentration to low concentration.
- Osmosis is passive diffusion of water across a selectively permeable membrane.
- Facilitated diffusion is passive transport through a membrane protein.
- Facilitated diffusion can be saturable because transport rate depends on available proteins.
- Active transport moves substances against a concentration gradient and requires energy.
- The sodium-potassium pump uses ATP to move three $$\mathrm{Na}^{+}$$ out and two $$\mathrm{K}^{+}$$ in.
- The sodium-potassium pump helps maintain ion gradients and membrane potential.
- Endocytosis and exocytosis require energy and move large materials using vesicles.

## Key Terms and Concepts

- selective permeability
- aquaporin
- concentration gradient
- passive transport
- diffusion
- osmosis
- facilitated diffusion
- active transport
- sodium-potassium pump
- membrane potential
- endocytosis
- exocytosis


Membrane transport - Passive versus active transport