Step 1 · Small and nonpolar?
It can usually cross the phospholipid bilayer easily.
Examples: oxygen, carbon dioxide
Where Selective Permeability Fits in Unit 2
The previous page, plasma membrane structure, explained membrane parts and proteins. This page applies that structure to crossing rules—what moves freely and what needs help.
After this page, you will study passive transport and diffusion and active transport, where you learn how substances actually move across the membrane.
Current concept
Selective Permeability
The membrane controls what crosses.
Learning Journey Checkpoint: For any molecule, name its size, polarity, and charge—then predict whether it crosses the bilayer or needs a protein.
- 1 Unit 2 Hub: Cell Structure and Function
- 2 Osmosis and Tonicity
- 3 Cell Structure and Function
- 4 Cell Organelles and Their Functions
- 5 Prokaryotic vs Eukaryotic Cells
- 6 Surface Area to Volume Ratio
- 7 Plasma Membrane Structure
- 8 Selective Permeability You are here
- 9 Passive Transport and Diffusion
- 10 Active Transport
- 11 Cell Compartmentalization
- 12 Unit 2 Practice Questions
What Is Selective Permeability in AP Biology?
Selective permeability AP Biology means the plasma membrane allows some substances to cross more easily than others. Small nonpolar molecules usually cross the phospholipid bilayer easily, while ions and many large polar molecules need membrane proteins.
For AP Biology, check size, polarity, and charge first—then name whether the bilayer, a protein, or vesicle transport handles crossing.
Say It Fast
- Selective = some substances cross
- Permeability = ability to pass through
- Small nonpolar molecules cross easily
- Ions usually need proteins
- Large polar molecules need help
- Membrane structure controls crossing
- Homeostasis depends on regulation
AP Exam Clue: If a question asks whether a molecule crosses easily, check size, polarity, and charge.
Can It Cross? The Membrane Decision Tool
When an AP Biology question names a molecule, run the three-part check: size, polarity, and charge. Those clues tell you whether it crosses the bilayer, needs a protein, or stays blocked.
Step 2 · Charged?
It usually needs a channel or carrier protein.
Examples: Na+, K+, Cl-, Ca2+
Step 3 · Large and polar?
It usually needs a transport protein.
Examples: glucose, many amino acids
Step 4 · Water?
Water can cross slowly, but aquaporins make movement faster.
Examples: osmosis scenarios
Step 5 · Very large?
It may need vesicle transport rather than crossing directly through the bilayer.
Examples: large proteins
AP Exam Tip: On AP questions, explain the molecule property and the membrane feature. Do not just say it can or cannot cross.
Molecule Passport Check: Can This Substance Enter?
Each molecule needs a membrane passport. The cell checks size, polarity, and charge to decide whether crossing is easy, protein-assisted, slow, or vesicle-based.
Oxygen
Crosses easilyAP reason: Small nonpolar molecules can pass through the hydrophobic interior.
Carbon dioxide
Crosses easilyAP reason: CO₂ can diffuse through the phospholipid bilayer.
Sodium ion
Needs a proteinAP reason: Charged ions cannot easily cross the hydrophobic interior.
Potassium ion
Needs a proteinAP reason: Ion channels or carriers provide controlled pathways.
Glucose
Needs a carrier proteinAP reason: Glucose is too large and polar to cross easily.
Water
Crosses slowly; aquaporins helpAP reason: Water can move across membranes, but aquaporins increase movement.
Steroid hormone
Can often crossAP reason: Lipid-soluble molecules can pass through the bilayer more easily.
Large protein
Needs vesicle transportAP reason: Large proteins do not cross directly through the phospholipid bilayer.
AP Exam Tip: A strong AP answer names the molecule property, the membrane barrier, and the crossing method.
AP FRQ Sentence Frame
Use this sentence pattern when AP Biology asks you to explain whether a substance can cross the membrane.
Sentence frame
“Because [molecule] is [small/nonpolar/charged/large/polar], it [can cross / needs a transport protein / crosses slowly / needs vesicle transport] because the phospholipid bilayer has a hydrophobic interior.”
Examples
- Example 1: Because oxygen is small and nonpolar, it can cross the phospholipid bilayer easily because it can move through the hydrophobic interior.
- Example 2: Because sodium is charged, it needs a transport protein because charged ions cannot easily cross the hydrophobic interior.
- Example 3: Because glucose is large and polar, it usually needs a carrier protein because it cannot easily pass through the phospholipid bilayer.
Why Is the Plasma Membrane Selectively Permeable?
The phospholipid bilayer is selectively permeable because hydrophilic heads face water while hydrophobic tails form a nonpolar core that repels many charged and large polar substances.
This section builds on plasma membrane structure. Review the bilayer there, then use crossing rules here without repeating full transport mechanisms.
AP Exam Clue: The hydrophobic interior is the main barrier for ions and many polar molecules.
What Can Cross the Cell Membrane Easily?
Small nonpolar molecules usually slip through the hydrophobic interior of the bilayer. Oxygen and carbon dioxide are the AP examples you should recognize instantly.
| Substance Type | Crosses Easily? | Why |
|---|---|---|
| Small nonpolar molecules | Yes | They pass through the hydrophobic interior |
| Oxygen | Yes | Small and nonpolar |
| Carbon dioxide | Yes | Small and nonpolar |
| Small uncharged polar molecules | Sometimes slowly | Polarity makes crossing harder |
| Water | Slowly; faster through aquaporins | Small but polar |
Tip: Scroll sideways to see the full table.
AP Exam Tip: Small does not always mean easy. Charge and polarity matter.
Which Substances Need Membrane Proteins?
Charged ions and many large polar molecules hit the hydrophobic barrier and usually need channels, carriers, or pumps—this page explains the rule; transport pages explain the movement.
| Substance Type | Protein Needed? | Reason |
|---|---|---|
| Ions | Yes | Charged particles cannot cross the hydrophobic interior easily |
| Large polar molecules | Usually yes | Too large and polar |
| Glucose | Yes | Large polar molecule |
| Amino acids | Often yes | Polar or charged groups |
| Water | Often uses aquaporins | Aquaporins increase water movement |
Tip: Scroll sideways to see the full table.
Transport pages explain movement in detail. Continue with passive transport and diffusion and active transport after you master crossing rules here.
AP Exam Clue: If an ion moves across a membrane, a membrane protein is almost always involved.
How Selective Permeability Supports Homeostasis
Homeostasis depends on the membrane letting in nutrients and signals while limiting what exits or enters unchecked—selective permeability is the gatekeeper for stable internal conditions.
- Nutrients enter when needed
- Wastes leave without losing control
- Ions are regulated for nerve and muscle function
- Water movement is controlled
- Harmful substances may be blocked
- Signals are received by membrane proteins
Water balance connects to osmosis and tonicity, where selective permeability and aquaporins shape cell responses.
AP Exam Tip: For homeostasis questions, explain what is regulated and why regulation matters.
Membrane Crossing Lab: Sort the Molecule
Predict how each molecule crosses, reveal the answer, and note the clue you used—size, polarity, or charge.
0 of 10 molecules sorted
Oxygen needs to enter a respiring cell.
Answer: Crosses easily.Oxygen is small and nonpolar, so it can pass through the hydrophobic interior.Clue used: small and nonpolar.
Carbon dioxide needs to leave a respiring cell.
Answer: Crosses easily.Carbon dioxide is small and nonpolar.Clue used: small and nonpolar.
Sodium ion (Na+) must enter or leave the cell.
Answer: Needs a protein.Sodium is charged and cannot easily cross the hydrophobic interior.Clue used: charged ion.
Potassium ion (K+) must be regulated inside the cell.
Answer: Needs a protein.Potassium is charged.Clue used: charged ion.
Glucose must enter the cell for energy.
Answer: Needs a transport protein.Glucose is large and polar.Clue used: large and polar.
Water must move quickly across the membrane.
Answer: Can cross slowly; aquaporins increase movement.Water is small but polar.Clue used: small but polar.
A large cytosolic protein must leave the cytoplasm.
Answer: Does not cross directly through the bilayer.Very large molecules usually need vesicle transport.Clue used: very large molecule.
A steroid hormone must reach an intracellular receptor.
Answer: Can often cross the membrane.Steroid hormones are nonpolar and lipid-soluble.Clue used: nonpolar and lipid-soluble.
Chloride ion (Cl-) must cross the membrane.
Answer: Needs a protein.Chloride is charged.Clue used: charged ion.
An amino acid must enter the cell.
Answer: Usually needs a transport protein.Many amino acids are polar or charged.Clue used: polar or charged.
AP Exam Tip: When sorting molecules, always justify the answer with size, polarity, or charge.
Common Mistakes About Selective Permeability
Replace vague phrases with molecule properties plus the membrane feature that explains crossing.
| Mistake | Better AP Biology Understanding |
|---|---|
| "All small molecules cross easily" | Charge and polarity also matter |
| "Ions cross because they are small" | Ions are charged and usually need proteins |
| "Water cannot cross membranes" | Water can cross slowly; aquaporins speed movement |
| "Proteins are only for active transport" | Proteins also help facilitated diffusion |
| "Selective permeability is random" | It depends on molecule properties and membrane structure |
| "The cell wall controls permeability" | The plasma membrane controls most selective crossing |
| "Everything polar is completely blocked" | Some small polar molecules cross slowly or use proteins |
Tip: Scroll sideways to see the full table.
AP Exam Clue: The most common trap is confusing size with permeability. Small charged ions still need help.
Selective Permeability Practice Questions
Answer all ten questions. Choices shuffle on reload—justify each pick with size, polarity, or charge.
Question 1 of 10
Start
Correct: 0
Answered: 0
Streak: 0
Accuracy: 0%
AP-Style FRQ Practice: Selective Permeability
Open each card, draft your response, then compare to the rubric. In selective permeability FRQs, link molecule property → membrane barrier → crossing method.
0 of 2 FRQs opened
Prompt
- Describe what selective permeability means.
- Explain why ions do not easily cross the phospholipid bilayer.
- Identify one membrane structure that helps ions cross.
- Explain how selective permeability helps maintain homeostasis.
Tip: Link the hydrophobic interior to ions, then name a protein pathway.
Scoring rubric (4 points)
- 1 pt — Selective permeability means some substances cross more easily than others.
- 1 pt — Ions are charged and cannot easily cross the hydrophobic interior of the bilayer.
- 1 pt — Accept channel protein, carrier protein, or pump.
- 1 pt — Selective permeability regulates entry and exit of substances, helping maintain stable internal conditions.
Sample response
A. Selective permeability means the plasma membrane allows some substances to cross more easily than others.
B. Ions are charged and cannot easily pass through the hydrophobic interior of the phospholipid bilayer.
C. A channel protein provides a passageway for specific ions across the membrane.
D. By controlling what enters and leaves, selective permeability helps the cell maintain homeostasis.
Self-check before you reveal
Status: Draft your answer first—then open the rubric or sample.
Prompt
- Predict whether oxygen crosses the membrane easily.
- Predict whether glucose crosses the membrane easily.
- Explain the difference using molecule properties.
- Explain why membrane proteins are important for cell function.
Tip: Oxygen = small nonpolar; glucose = large polar needing a protein.
Scoring rubric (4 points)
- 1 pt — Oxygen crosses easily because it is small and nonpolar.
- 1 pt — Glucose does not cross easily and usually needs a transport protein.
- 1 pt — Oxygen can pass through the hydrophobic interior; glucose is larger and polar.
- 1 pt — Membrane proteins allow specific substances to cross, supporting transport and homeostasis.
Sample response
A. Oxygen crosses the membrane easily.
B. Glucose does not cross easily and usually needs a transport protein.
C. Oxygen is small and nonpolar, so it can move through the hydrophobic interior; glucose is large and polar.
D. Membrane proteins provide selective pathways so the cell can control which substances cross, supporting homeostasis.
Self-check before you reveal
Status: Draft your answer first—then open the rubric or sample.
FRQ Tip
In selective permeability FRQs, identify the molecule property, the membrane feature, and the effect on transport.
FAQs About Selective Permeability in AP Biology
What is selective permeability in AP Biology?
Selective permeability means the plasma membrane allows some substances to cross more easily than others. On AP exams, connect this idea to molecule size, polarity, and charge—not to random blocking.
Why is the plasma membrane selectively permeable?
The phospholipid bilayer has a hydrophobic interior that blocks many charged or large polar substances. Hydrophilic heads face water, while tails cluster inward and create the barrier ions struggle to cross.
What molecules cross the membrane easily?
Small nonpolar molecules such as oxygen and carbon dioxide usually cross the phospholipid bilayer easily. They can move through the hydrophobic interior without a transport protein.
Why do ions need membrane proteins?
Ions are charged and cannot easily pass through the hydrophobic interior of the phospholipid bilayer. Channel, carrier, or pump proteins provide controlled pathways for specific ions.
Does water cross the membrane?
Water can cross the membrane slowly through the bilayer because it is small but polar. Aquaporin proteins allow water to move across much faster when the cell needs rapid water exchange.
Does glucose cross the membrane easily?
Glucose usually does not cross the membrane easily because it is large and polar. It often needs a carrier or channel protein for facilitated diffusion or active transport.
How does selective permeability help homeostasis?
Selective permeability lets cells regulate nutrients, ions, water, wastes, and signals that enter or leave. That control helps maintain stable internal conditions even when the environment changes.
Is selective permeability the same as passive transport?
No—selective permeability describes what the membrane allows through based on structure and molecule properties. Passive transport describes movement down a concentration gradient without ATP.
What is the biggest AP Biology mistake about selective permeability?
The biggest mistake is assuming all small molecules cross easily. Charge and polarity also determine whether a substance can cross the bilayer or needs a membrane protein.
Before You Move On
- Can you predict crossing using size, polarity, and charge?
- Can you explain why ions need proteins?
- Can you explain why oxygen crosses easily?
- Can you explain why glucose needs help?
- Can you connect selective permeability to homeostasis?
If yes, you are ready for Passive Transport and Diffusion.
Selective Permeability: Final Review
- Selective permeability means some substances cross more easily than others
- Small nonpolar molecules usually cross easily
- Ions usually need membrane proteins
- Large polar molecules usually need transport proteins
- Water can cross slowly and moves faster through aquaporins
- The hydrophobic interior blocks many charged or polar substances
- Membrane proteins add controlled pathways
- Selective permeability helps maintain homeostasis
- Strong AP answers use molecule size, polarity, and charge
You can now predict crossing from molecule clues. Continue with Passive Transport and Diffusion, or test yourself with Unit 2 practice questions.