AP Biology

## Learning Outcomes

- Explain how the phospholipid bilayer creates selective permeability.
- Predict whether a molecule can cross a membrane by simple diffusion based on size, charge, and polarity.
- Describe how concentration gradients affect the direction and rate of diffusion.
- Explain why surface area-to-volume ratio limits cell size and affects exchange with the environment.
- Distinguish simple diffusion from facilitated diffusion.
- Describe how channel proteins, carrier proteins, and aquaporins affect diffusion across membranes.
- Explain how cell walls add support and act as an extra barrier to movement of materials.

## AP Biology Syllabus

In AP Biology Unit 2, membrane permeability connects cell structure to cell function. You are expected to explain how the fluid mosaic structure of the plasma membrane allows cells to maintain internal conditions different from their surroundings. Questions often ask you to predict movement of molecules, explain experimental results, or connect membrane structure to selective permeability.

- Cell membranes separate the internal environment of a cell from the external environment.
- The plasma membrane is selectively permeable because of its phospholipid bilayer and embedded proteins.
- Small nonpolar molecules such as $$\mathrm{O_2}$$, $$\mathrm{CO_2}$$, and $$\mathrm{N_2}$$ can cross the bilayer easily.
- Ions and large polar molecules require transport proteins to cross the membrane.
- Water can cross slowly through the bilayer, but most rapid water movement occurs through aquaporins.
- Diffusion and facilitated diffusion are passive processes because substances move down their concentration gradients without direct ATP input.
- Surface area-to-volume ratio affects how efficiently cells exchange materials with their environment.
- Cell walls in plants, fungi, and prokaryotes provide support and act as an additional barrier outside the plasma membrane.

## 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 diffuse through the plasma membrane more easily than sodium ions?
2. How does increasing cell size affect the surface area-to-volume ratio and the efficiency of diffusion?
3. Why is facilitated diffusion considered passive transport?
4. What role do aquaporins play in water movement across membranes?
5. Why can the rate of facilitated diffusion become saturated, while simple diffusion usually does not?

## Introduction

Diffusion across membranes is one of the main ways cells exchange materials with their environment. Cells need oxygen, nutrients, water, ions, and signaling molecules, but they also need to remove wastes such as carbon dioxide. The plasma membrane controls this exchange. Its structure allows some substances to pass easily while restricting others.

Membrane permeability depends on several related factors: the structure of the membrane, the size and chemical properties of the molecule, the concentration gradient, the available surface area, and whether transport proteins are present. In AP Biology, you should be able to use these factors to predict movement across membranes and explain data from diffusion or osmosis experiments.

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

### The phospholipid bilayer as the main permeability barrier

The plasma membrane is made mainly of a phospholipid bilayer. Each phospholipid has a hydrophilic phosphate head and hydrophobic fatty acid tails. In water, the hydrophilic heads face the aqueous environments outside and inside the cell, while the hydrophobic tails face inward, forming a nonpolar interior.

This arrangement explains why membranes are selectively permeable. The central hydrophobic region allows small nonpolar molecules to pass through easily, but it blocks most charged and strongly polar substances.

> **Key Term: phospholipid bilayer**
> A double layer of phospholipids with hydrophilic heads facing water and hydrophobic tails facing inward.

Small nonpolar gases such as $$\mathrm{O_2}$$, $$\mathrm{CO_2}$$, and $$\mathrm{N_2}$$ cross the bilayer by simple diffusion. They dissolve in the hydrophobic interior of the membrane and move between phospholipids. This follows the idea that nonpolar substances mix more easily with nonpolar environments.

In contrast, ions such as $$\mathrm{Na^+}$$, $$\mathrm{K^+}$$, $$\mathrm{Ca^{2+}}$$, and $$\mathrm{Cl^-}$$ do not cross the hydrophobic core easily. Their charges interact strongly with water, and the nonpolar membrane interior is an unfavorable environment for them. Large polar molecules also cannot pass freely because they are too large and interact poorly with the hydrophobic interior.

> **Key Term: simple diffusion**
> Passive movement of molecules directly through the phospholipid bilayer from an area of higher concentration to an area of lower concentration.

The fluid mosaic model also matters. The membrane is not a rigid sheet. Phospholipids and many proteins can move laterally, and the membrane includes proteins, glycoproteins, glycolipids, and steroids such as cholesterol in animal cells. These components affect the membrane’s function. Transport proteins provide routes for substances that cannot cross the lipid bilayer directly.

> **Test Tip:**
> If an AP question asks what crosses a membrane most easily, look for a small nonpolar molecule. Oxygen and carbon dioxide are classic answers. If the option is an ion or a large polar molecule, it usually needs a protein.

Cell walls add another layer of structure outside the plasma membrane in plants, fungi, and prokaryotes. Plant cell walls are made mainly of cellulose, fungal cell walls often contain chitin or glucans, and bacterial cell walls contain peptidoglycan. Cell walls provide rigidity and protection, especially against osmotic changes, but they do not replace the selective role of the plasma membrane. Materials must still cross the plasma membrane to enter the cytoplasm.

### Concentration gradients, distance, and surface area

Diffusion is driven by random molecular motion. Over time, particles tend to move from areas where they are more concentrated to areas where they are less concentrated. The larger the difference in concentration across a membrane, the faster the net diffusion at the start.

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

For example, if oxygen concentration is high outside a cell and low inside because the cell is using oxygen during cellular respiration, oxygen will diffuse into the cell. If carbon dioxide concentration is high inside the cell because it is produced by respiration, carbon dioxide will diffuse out.

Diffusion is passive. It does not require direct input of ATP because substances move down their concentration gradients. This is different from active transport, where substances are moved against a gradient and energy is required.

> **Key Term: passive transport**
> Movement of substances across a membrane down their concentration gradient without direct input of cellular energy.

The rate of diffusion is affected by diffusion distance. Substances diffuse more quickly over short distances than long distances. This is one reason cells are small. A molecule entering a very large cell would take longer to reach the center, and the cell’s surface might not provide enough exchange area for the whole volume.

Surface area-to-volume ratio is therefore a major factor in membrane exchange. As a cell grows, its volume increases faster than its surface area. The surface area is where materials cross the membrane, but the volume represents the cytoplasm that must be supplied.

For a spherical cell:

$$
\text{Surface area} = 4\pi r^2
\text{Volume} = \frac{4}{3}\pi r^3
$$

So:

$$
\text{Surface area-to-volume ratio} = \frac{3}{r}
$$

As radius $$r$$ increases, the ratio decreases. A smaller cell has more membrane surface per unit volume than a larger cell. This allows more efficient exchange of nutrients, gases, and wastes.

Cells and organs often increase surface area by folding membranes or forming projections. Cristae in mitochondria increase the surface area available for electron transport chains. Villi and microvilli in the small intestine increase surface area for nutrient absorption. Root hairs increase surface area for water and mineral uptake.

> **Exam Warning:**
> Do not say that large cells have more surface area and are therefore always better at exchange. Large cells may have more total surface area, but they have less surface area per unit volume. The ratio is what limits efficient diffusion.

Temperature can also affect diffusion because molecules move faster at higher temperatures. However, in living systems, temperature changes can also alter membrane fluidity and protein shape. AP Biology questions are more likely to test concentration gradient, molecule properties, surface area-to-volume ratio, and protein availability than temperature alone.

### Molecular size, polarity, charge, and lipid solubility

The properties of the substance crossing the membrane are often the most direct clue in exam questions. The main factors are size, polarity, charge, and lipid solubility.

Small nonpolar molecules diffuse through the bilayer most easily. Oxygen, carbon dioxide, and nitrogen are common examples. Steroid hormones are also nonpolar enough to pass through membranes. This is why many steroid hormones can bind receptors inside target cells rather than needing a receptor on the cell surface.

Small uncharged polar molecules can sometimes cross the membrane, but much more slowly than nonpolar molecules. Water is the key example. Water is polar, so the hydrophobic core resists its movement, but water is small enough to pass through the bilayer in limited amounts. In cells, most rapid water movement occurs through aquaporins.

Large polar molecules, such as many sugars, and charged particles, such as ions, require membrane proteins. Glucose is polar and relatively large, so it does not diffuse directly through the bilayer at a useful rate. Sodium and potassium ions are small, but their charge prevents free movement through the hydrophobic core.

> **Key Term: hydrophobic core**
> The nonpolar interior of the phospholipid bilayer formed by fatty acid tails, which restricts polar and charged substances.

A useful AP Biology ranking is:

- Easiest: small nonpolar molecules, such as $$\mathrm{O_2}$$ and $$\mathrm{CO_2}$$
- Limited movement: small uncharged polar molecules, such as water
- Requires proteins: large polar molecules, such as glucose
- Requires proteins: ions, such as $$\mathrm{Na^+}$$, $$\mathrm{K^+}$$, and $$\mathrm{Cl^-}$$

This ranking helps with multiple-choice questions. If a question asks why a hydrophilic ligand cannot enter a cell, the answer is that the hydrophobic membrane interior prevents it from crossing unassisted. Hydrophilic signaling molecules usually bind to plasma membrane receptors. Hydrophobic signaling molecules can often cross the membrane and bind intracellular receptors.

> **Test Tip:**
> When asked to justify membrane permeability, connect the molecule’s property to the membrane’s structure. For example: “The ion is charged, so it cannot pass through the hydrophobic core and must use a channel protein.”

Size matters too. Even if a molecule is nonpolar, very large molecules do not simply slip through the phospholipid bilayer. Very large particles are moved by bulk transport processes such as endocytosis and exocytosis, not diffusion through the membrane. Those processes require membrane remodeling and energy, so they are outside simple diffusion.

### Transport proteins, aquaporins, and saturation

Many biologically useful substances cannot cross the phospholipid bilayer directly. Cells solve this problem with transport proteins embedded in the membrane. When a substance moves down its concentration gradient through a protein, the process is called facilitated diffusion.

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

Facilitated diffusion is still diffusion because the substance moves from higher concentration to lower concentration. It does not directly require ATP. The protein provides a pathway through the membrane that avoids the hydrophobic core.

There are two broad types of proteins involved in facilitated diffusion: channel proteins and carrier proteins.

Channel proteins form hydrophilic passages through the membrane. Ions often move through channels. Because ions are charged, they cannot cross the lipid bilayer directly, but they can move through channels that shield them from the hydrophobic core. Ion channels may be selective, allowing only certain ions through.

> **Key Term: channel protein**
> A membrane protein that forms a hydrophilic passage through which specific substances can diffuse.

Aquaporins are channel proteins specialized for water movement. Water can cross the membrane in small amounts, but aquaporins allow water to move much faster down its water concentration gradient. This is especially significant in cells that experience large water movement, such as kidney cells and plant cells.

> **Key Term: aquaporin**
> A membrane channel protein that allows rapid passive movement of water across a membrane.

Carrier proteins bind a specific molecule and change shape to move it across the membrane. A common example is a glucose transporter that allows glucose to enter a cell down its concentration gradient. The carrier does not add energy to push glucose uphill; it simply provides a route for passive movement.

Facilitated diffusion can become saturated. This means that once all available transport proteins are occupied and working at their maximum rate, increasing the concentration of the transported substance will not increase the rate much further. The limiting factor becomes the number and activity of the proteins.

Simple diffusion through the lipid bilayer does not have the same type of protein saturation because it does not depend on a fixed number of protein binding sites. Its rate is more directly affected by the concentration gradient, membrane surface area, membrane thickness, and permeability of the molecule.

> **Exam Warning:**
> Facilitated diffusion does not mean active transport. If the substance moves down its gradient through a protein, it is passive. Active transport moves substances against their gradient and requires energy, often from ATP or from another gradient.

The sodium-potassium pump is a helpful contrast. It moves $$\mathrm{Na^+}$$ out of the cell and $$\mathrm{K^+}$$ into the cell against their gradients, using ATP. For every three sodium ions pumped out, two potassium ions are pumped in. This is active transport, not diffusion. AP questions often include this pump as a distractor when testing passive transport.

Cell membranes also contain proteins involved in signaling, anchoring, recognition, and enzyme activity. Glycoproteins and glycolipids help with cell recognition. These are not usually the direct route for diffusion, but they are part of the membrane’s fluid mosaic structure and may appear in questions about membrane components.

### Applying diffusion factors to AP-style scenarios

Many AP Biology questions ask you to apply membrane permeability to a new situation. The best approach is to identify the substance, the membrane structure, and the direction of the gradient.

> **Question:**
> A cell is placed in an environment with a high oxygen concentration. The oxygen concentration inside the cell is low. Predict the movement of oxygen and justify your answer.

Oxygen will diffuse into the cell. It is small and nonpolar, so it can cross the phospholipid bilayer directly. It moves down its concentration gradient from outside to inside.

> **Question:**
> A molecule irreversibly binds to transport proteins in the membrane. Predict the effect on glucose uptake by a cell that uses facilitated diffusion for glucose.

Glucose uptake will decrease. Glucose is large and polar enough that it requires transport proteins. If those proteins are blocked, glucose cannot cross the membrane efficiently by facilitated diffusion.

In experimental questions, look for data showing a rate plateau. A plateau in transport rate as concentration increases often suggests protein-mediated transport with saturation. If diffusion rate continues rising proportionally with concentration difference, simple diffusion may be involved.

Cell size questions often connect diffusion to surface area-to-volume ratio. A small cell can exchange materials more efficiently because it has more surface area relative to its volume. A large cell may not be able to move enough materials across its membrane to support its internal volume. This helps explain why many organisms are made of many smaller cells rather than a few very large ones.

Plant cell questions may add the cell wall. The wall supports the cell and protects against osmotic stress, but substances still must cross the plasma membrane to enter the cytoplasm. In a dehydrated plant, the central vacuole loses water, turgor pressure decreases, and the plant wilts. Although this connects more directly to osmosis and water potential, it is built on the same membrane permeability principles.

## Key Point Checklist

This article has covered the following key knowledge points:

- The plasma membrane is selectively permeable because of its phospholipid bilayer and embedded proteins.
- Hydrophilic heads face water, while hydrophobic tails form the membrane’s nonpolar interior.
- Small nonpolar molecules such as $$\mathrm{O_2}$$, $$\mathrm{CO_2}$$, and $$\mathrm{N_2}$$ cross the membrane easily by simple diffusion.
- Ions and large polar molecules cannot cross the hydrophobic core without membrane proteins.
- Diffusion moves substances down their concentration gradients and does not directly require ATP.
- A steeper concentration gradient usually increases the initial rate of diffusion.
- Smaller cells have a higher surface area-to-volume ratio, allowing more efficient exchange.
- Membrane folding and projections increase surface area for transport and other cellular processes.
- Water can cross membranes slowly, but aquaporins allow rapid passive water movement.
- Facilitated diffusion uses channel or carrier proteins but remains passive if movement is down the gradient.
- Facilitated diffusion can become saturated because there are limited transport proteins.
- Cell walls provide support and an extra barrier, but the plasma membrane remains the main selectively permeable boundary.

## Key Terms and Concepts

- selective permeability
- phospholipid bilayer
- simple diffusion
- concentration gradient
- passive transport
- hydrophobic core
- facilitated diffusion
- channel protein
- aquaporin


Membrane permeability - Factors affecting diffusion across membranes