AP Biology 2.6 Facilitated Diffusion Study Notes - New Syllabus Effective 2025
AP Biology 2.6 Facilitated Diffusion Study Notes- New syllabus
AP Biology 2.6 Facilitated Diffusion Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
Explain how the structure of a molecule affects its ability to pass through the plasma membrane.
Key Concepts:
- Facilitated Diffusion
2.6.A – How Molecular Structure Affects Membrane Transport
🧬 Key Factors That Affect Movement
| Property | Effect on Membrane Transport |
| Size | 🔹 Small = can pass more easily 🔸 Large = needs protein help or vesicle |
| Polarity | 🔹 Nonpolar = moves freely through lipid bilayer 🔸 Polar = needs help (facilitated diffusion or active transport) |
| Charge | 🔹 Uncharged = easier movement 🔸 Charged ions (Na⁺, Cl⁻) = blocked by hydrophobic interior, need protein channels |
🧪 Examples
| Molecule | Movement Type | Reason |
| O₂, CO₂ | Simple diffusion | Small, nonpolar |
| H₂O | Osmosis (slow, or via aquaporins) | Small, polar |
| Glucose | Facilitated diffusion | Large, polar |
| Na⁺ / K⁺ | Ion channels or active transport | Charged |
✅ Summary
A molecule’s size, polarity, and charge decide how it crosses the membrane.
- Small + nonpolar = pass freely
- Large or charged = need help from proteins or energy
2.6.A.1 – Facilitated Diffusion of Ions
Charged ions like Na⁺ and K⁺ need protein channels to cross the membrane.
🚫 Why Ions Can’t Cross Freely
- The hydrophobic interior of the membrane blocks charged particles.
- Ions like Na⁺, K⁺, Ca²⁺, Cl⁻ are polar and charged → they can’t diffuse directly.
🚪 How They Cross → Facilitated Diffusion
- Uses channel or transport proteins (no energy needed).
- Allows specific ions to pass through the membrane selectively.
⚡ Membrane Becomes Polarized
- When ions move across, it creates an electrical difference (charge separation).
- The inside of the membrane is often slightly negative.
- This is called membrane polarization – crucial for nerve signals & muscle contraction.
✅ Summary
Charged ions like Na⁺ and K⁺ require channel proteins for transport via facilitated diffusion.
Their movement helps polarize the membrane, enabling essential cell functions like communication and nerve impulse transmission.
2.6.A.2 – Facilitated Diffusion of Large Polar Molecules
Large polar molecules move through membranes using proteins – no energy needed!
🧬 What Is Facilitated Diffusion?
- A type of passive transport
- Moves substances down the concentration gradient (🔺 High → 🔻 Low)
- No ATP or energy required
🚫 Why They Need Help
- Large polar molecules (like glucose and amino acids)
- ❌ Cannot pass through the hydrophobic membrane interior
- ✅ Solution: Use carrier or channel proteins to cross
🚪 Examples
| Molecule | How It Moves |
| Glucose | Carrier protein (GLUT transporter) |
| Amino acids | Specific carrier proteins |
✅ Summary
Facilitated diffusion allows large, polar molecules to cross membranes without energy, using transport proteins, as long as they move with the concentration gradient.
2.6.A.3 – Aquaporins & Water Transport
Aquaporins help move lots of water quickly across the membrane.
🚪 What Are Aquaporins?
- Aquaporins = special channel proteins
- Specifically made to transport water (H₂O)
- Allow fast, efficient movement of water across the membrane
- → Much faster than simple diffusion alone!
🧠 Why Are They Important?
- Water is polar → moves slowly through the hydrophobic membrane
- Cells need rapid water flow for:
- 💧 Maintaining osmotic balance
- 🪴 Plant water uptake
- 🧍♂️ Kidney water reabsorption
✅ Summary
Aquaporins are protein channels that speed up water movement across cell membranes helping organisms balance water quickly and efficiently.