The plasma membrane is selectively permeable because of its hydrophobic interior, formed by the fatty acid tails of phospholipids. Small nonpolar molecules like O₂ and CO₂ cross freely, while ions and large polar molecules need transport proteins to get through.
Why This Matters for the AP Biology Exam
Membrane permeability connects structure to function, which is exactly the kind of reasoning the AP Biology exam rewards. You may be asked to explain why a specific molecule can or cannot cross a membrane, predict how substances move based on membrane structure, or describe how a cell wall protects a cell. This shows up in multiple-choice questions, in diagram-based questions where you interpret membrane structure, and in evidence-based written responses where you connect membrane features to selective permeability and cell survival.
Key Takeaways
- Selective permeability comes from the hydrophobic interior of the membrane, made of phospholipid fatty acid tails.
- Small nonpolar molecules (N₂, O₂, CO₂) pass through the membrane freely.
- Ions and large polar molecules cannot cross the hydrophobic core on their own and need channels or transport proteins.
- Small polar, uncharged molecules like H₂O and NH₃ can slip through in small amounts.
- Cell walls in Bacteria, Archaea, Fungi, and plants provide structure, act as a permeability barrier for some substances, and protect against osmotic lysis.
How Membrane Structure Creates Selective Permeability
The plasma membrane is a phospholipid bilayer. Each phospholipid has two regions: a hydrophilic (water-loving) phosphate head and a hydrophobic (water-fearing) fatty acid tail.
The hydrophilic heads face outward toward the watery environments inside and outside the cell. The hydrophobic tails point inward toward each other, forming a nonpolar core. That core is the key to permeability: it acts as a barrier that repels charged and polar substances.
Proteins are embedded in the membrane, and their structure follows the same logic. The hydrophilic regions of a protein sit in the watery areas or face the cytosol, while the hydrophobic regions interact with the fatty acid tails in the membrane interior. The membrane also includes steroids (such as cholesterol in vertebrates), glycoproteins, and glycolipids that can move within the membrane. This is the idea behind the fluid mosaic model: the membrane is fluid, and its scattered proteins and attached molecules form a mosaic.
What Can and Cannot Cross the Membrane
Selective permeability means some substances cross easily, some need help, and some are mostly blocked. What determines this is a molecule's size, polarity, and charge.
Cross freely (small and nonpolar):
- N₂ (nitrogen gas)
- O₂ (oxygen)
- CO₂ (carbon dioxide)
Cross in small amounts (small, polar, uncharged):
- H₂O (water)
- NH₃ (ammonia)
Need channels or transport proteins (ions and large polar molecules):
- Charged ions
- Large polar molecules
The nonpolar fatty acid tails are what block ions and polar molecules. Because the interior is hydrophobic, charged particles cannot easily pass through it. To move these substances in meaningful quantities, the cell relies on embedded channel and transport proteins.
Cell Walls: Extra Structure and Protection
A cell wall is a structural boundary outside the plasma membrane found in certain organisms. It is not the same as the plasma membrane, and it does not replace selective permeability.
| Organism group | Cell wall material |
|---|---|
| Bacteria | peptidoglycan |
| Archaea | various polymers (not peptidoglycan) |
| Fungi | chitin |
| Plants | cellulose |
Cell walls do three main jobs:
- Structural boundary: They help maintain cell shape and provide mechanical strength.
- Permeability barrier: They block some substances from reaching the plasma membrane.
- Protection from osmotic lysis: They keep cells from bursting when water moves in, such as in a hypotonic environment.
This is why plant cells can take in water without rupturing. The rigid cell wall lets water and small dissolved substances pass through to reach the plasma membrane while limiting over-expansion. The wall and the membrane work together: the membrane controls permeability based on molecular properties, and the wall adds protection and structure.
How to Use This on the AP Biology Exam
Multiple Choice
Expect questions that give you a molecule and ask whether it can cross the membrane. Sort by three traits: size, polarity, and charge. Small and nonpolar means it crosses freely. Charged or large and polar means it needs a transport protein. Small, polar, and uncharged (like water) means it crosses, but only in small amounts on its own.
Data and Diagrams
If you get a membrane diagram, identify the hydrophilic heads facing the water and the hydrophobic tails facing inward. Be ready to point to the hydrophobic core as the reason ions and polar molecules cannot pass freely.
Written Responses
When asked to explain selective permeability, connect structure to function in your answer: the hydrophobic interior repels charged and polar molecules, so those substances need channels or transport proteins, while small nonpolar molecules diffuse across on their own. For cell wall questions, tie the wall to protection from osmotic lysis and structural support, and make clear it is separate from the membrane's selective permeability.
Common Trap
Do not say the cell wall is selectively permeable in the way the membrane is. The cell wall is a structural boundary and a permeability barrier for some substances, but the plasma membrane is what provides true selective permeability based on molecular properties.
Common Misconceptions
- "Water cannot cross the membrane at all." Water is small, polar, and uncharged, so it can cross in small amounts on its own. To move large quantities, cells use water channel proteins.
- "All proteins in the membrane are fully hydrophilic." Membrane proteins have both regions. Hydrophobic parts interact with the fatty acid tails, while hydrophilic parts face the watery environment or the cytosol.
- "The cell wall and plasma membrane are the same thing." They are different structures. The plasma membrane controls selective permeability; the cell wall adds structure and protection against bursting.
- "Charged ions just slip through the membrane because they are small." Charge, not just size, matters. Ions are blocked by the hydrophobic core and require channel or transport proteins.
- "All organisms have cell walls." Cell walls are found in Bacteria, Archaea, Fungi, and plants, but animal cells do not have them.
Related AP Biology Guides
Vocabulary
The following words are mentioned explicitly in the College Board Course and Exam Description for this topic.Term | Definition |
|---|---|
cell wall | A rigid structural layer outside the cell membrane that provides support, maintains cell shape, and acts as a permeability barrier in bacteria, archaea, fungi, and plants. |
hydrocarbon tail | The nonpolar, hydrophobic portions of phospholipids that form the interior of the membrane and prevent ion and polar molecule movement. |
hydrophilic substance | Water-loving polar molecules and ions that cannot easily cross the hydrophobic membrane interior and require transport proteins or channels. |
hydrophobic interior | The nonpolar region in the middle of the phospholipid bilayer that repels water and polar molecules. |
ion | Charged particles that cannot freely cross the hydrophobic membrane interior and require transport proteins or channels for movement. |
nonpolar molecule | Molecules that lack a net electric charge and can pass freely across the hydrophobic interior of the membrane. |
osmotic lysis | The rupture of a cell membrane due to water entering the cell in a hypotonic environment; prevented by the cell wall in plant cells, bacteria, archaea, and fungi. |
permeability barrier | A selective barrier that controls which substances can pass between the internal and external cellular environments. |
phospholipid | Amphipathic molecules with hydrophilic phosphate heads and hydrophobic fatty acid tails that form the basic structure of the cell membrane. |
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. |
selective permeability | The property of a membrane that allows certain substances to pass through while restricting the passage of others. |
structural boundary | The physical barrier provided by the cell wall that defines and maintains the shape and integrity of the cell. |
transport protein | Embedded membrane proteins that facilitate the movement of hydrophilic substances across the membrane. |
Frequently Asked Questions
What is membrane permeability in AP Biology?
Membrane permeability is the ability of substances to pass through the plasma membrane. In AP Biology Topic 2.4, the key idea is selective permeability: the membrane lets some substances cross easily while others need transport proteins because of the hydrophobic interior of the phospholipid bilayer.
Why is the plasma membrane selectively permeable?
The plasma membrane is selectively permeable because phospholipids form a bilayer with hydrophilic heads facing water and hydrophobic tails facing inward. That hydrophobic core blocks ions and many polar molecules but allows small nonpolar molecules to pass through more easily.
Which molecules can cross the plasma membrane freely?
Small nonpolar molecules such as N2, O2, and CO2 can cross the plasma membrane freely. Small polar uncharged molecules, including H2O and NH3, can pass in small amounts. Ions and large polar molecules generally need channels or transport proteins to move across the membrane.
Why do ions need transport proteins to cross the membrane?
Ions are charged, so they do not move easily through the nonpolar hydrophobic core of the membrane. Channels and transport proteins provide a pathway that lets ions and other hydrophilic substances cross without passing directly through the fatty acid tails.
How does the cell wall relate to membrane permeability?
The cell wall is outside the plasma membrane in Bacteria, Archaea, Fungi, and plants. It provides structure, acts as a barrier for some substances, and helps prevent osmotic lysis. It does not replace the plasma membrane, which is still the main structure controlling selective permeability.
What should I know for AP Biology Topic 2.4?
For Topic 2.4, be ready to explain how membrane structure influences selective permeability. Connect the hydrophobic interior to molecule movement, know which substances cross freely or need proteins, and describe how cell walls help maintain structure and protect some cells from osmotic lysis.