AP Biology 2.4 Membrane Permeability Study Notes - New Syllabus Effective 2025
AP Biology 2.4 Membrane Permeability Study Notes- New syllabus
AP Biology 2.4 Membrane Permeability Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
Explain how the structure of biological membranes influences selective permeability.
Key Concepts:
- Membrane Permeability
2.4.A – Membrane Structure & Selective Permeability
🧱 What Is Selective Permeability?
- The membrane controls what enters and exits the cell
- Some molecules pass easily, others are blocked
- This helps maintain internal balance (homeostasis)
🧬 How Structure Affects It
| Membrane Feature | How It Affects Permeability |
| Phospholipid bilayer | Only small, nonpolar molecules (O₂, CO₂) pass freely 🟢, blocks large or charged molecules (glucose, ions) 🔴 |
| Transport proteins | Allow polar or large molecules (like H₂O, ions, glucose) to cross through channels or carriers |
| Cholesterol | Maintains fluidity → keeps membrane flexible for transport |
| Carbs (glyco-) | Involved in recognition, not direct transport – help interact with external signals |
✅ Summary
- The membrane’s structure – especially the phospholipid bilayer and embedded proteins — creates a selective barrier.
- It lets useful molecules in, keeps harmful ones out, and maintains homeostasis.
2.4.A.1 – Selective Permeability & the Hydrophobic Membrane Interior
Selective permeability comes from the hydrophobic interior of the plasma membrane.
🌐 Membrane = Boundary
The plasma membrane creates a clear division between:
- 🧬 Inside of the cell (cytoplasm)
- 🌍 Outside environment
This separation allows the cell to control what enters and exits.
🔒 Why It’s Selectively Permeable
The phospholipid bilayer has:
- Hydrophilic heads (face outward)
- Hydrophobic tails (face inward → form membrane’s interior)
The hydrophobic interior:
- ✅ Allows small, nonpolar molecules (O₂, CO₂)
- ❌ Blocks large or polar/charged molecules (like glucose, ions, H₂O without help)
✅ Summary
The plasma membrane separates the cell from its surroundings. Its hydrophobic core makes it selectively permeable, meaning it lets in what the cell needs and keeps out what it doesn’t – protecting the internal environment.
2.4.A.2 – How Different Molecules Cross the Membrane
Small nonpolar molecules move freely through the membrane, large or polar molecules need help.
🚀 Molecules That Move Freely
| Type | Examples | How They Cross |
| Small nonpolar | O₂, CO₂, N₂ | Slip directly through the lipid bilayer (no protein needed) |
✔ They are uncharged and lipid-soluble, so they pass easily through the hydrophobic interior.
🚫 Molecules That Need Help
| Type | Examples | How They Cross |
| Large polar | Glucose, water | Use channel/carrier proteins |
| Charged ions | Na⁺, K⁺, Cl⁻, Ca²⁺ | Pass through ion channels or pumps |
❌ These molecules are repelled by the hydrophobic core and cannot pass unaided.
✅ Summary
- 🟢 Small nonpolar molecules = move freely
- 🔴 Large polar or charged molecules = need transport proteins
This ensures that only the right substances enter or exit, maintaining cell balance.
2.4.A.3 – How the Membrane Blocks or Allows Molecules
🔥 Hydrophobic Tails = The Real Barrier
The nonpolar fatty acid tails in the phospholipid bilayer form the membrane’s core.
They repel:
- ❌ Ions (Na⁺, Cl⁻, etc.)
- ❌ Large polar molecules
Why? → These substances can’t mix with the nonpolar, oily interior.
🌫️ Exceptions: Small Polar Molecules
| Molecule | Can It Pass Through? | Why? |
| Water (H₂O) | Yes, but only in small amounts | It’s small and uncharged |
| Ammonia (NH₃) | Passes slowly | Also small and uncharged |
⚠️ These can cross on their own but slowly – faster movement happens via aquaporins (water channels) or transport proteins.
✅ Summary
- The hydrophobic tails of phospholipids make the membrane selectively permeable by blocking ions and polar molecules,
- While small uncharged polar molecules like water and ammonia sneak through in very limited amounts.
2.4.B – Role of the Cell Wall in Structure & Function
🧩 What Is the Cell Wall?
A rigid outer layer found in:
- 🪴 Plant cells
- 🍄 Fungi
- 🦠 Some prokaryotes
Lies outside the plasma membrane
Made of cellulose (plants), chitin (fungi), or peptidoglycan (bacteria)
🛡️ Functions of the Cell Wall
| Function | What It Does |
| Structural support | Maintains cell shape and prevents collapse |
| Protection | Shields the cell from physical damage, pathogens, or bursting |
| Prevents lysis | Stops the cell from bursting in hypotonic solutions (water in) |
| Filtering role | Allows small molecules to pass but blocks harmful large ones |
| Turgor pressure | Works with vacuole to maintain firmness in plant cells |
✅ Summary
The cell wall gives strength, shape, and protection to cells especially in plants helping them stay upright, resist bursting, and interact safely with their environment.
2.4.B.1 – Cell Walls in Different Organisms
🌍 Who Has Cell Walls?
| Group | Main Component |
| Plants | Cellulose |
| Fungi | Chitin |
| Bacteria | Peptidoglycan |
| Archaea | Varies (e.g., pseudopeptidoglycan) |
🧩 Functions of the Cell Wall
| Function | Explanation |
| Structural boundary | Keeps cell shape and supports structure 🧱 |
| Permeability barrier | Regulates what enters/exits the cell; not as selective as membranes |
| Prevents osmotic lysis | Stops the cell from bursting when water flows in (especially in hypotonic environments) 💧 |
✅ Summary
Cell walls in bacteria, archaea, fungi, and plants help maintain shape, limit what moves across, and protect cells from bursting due to water intake essential for survival in changing environments.