2.6 Facilitated Diffusion
Keywords
| English Term | 中文翻译 | Definition & Explanation |
|---|---|---|
| Facilitated Diffusion | 协助扩散/易化扩散 | The passage of molecules or ions down their electrochemical gradient across a biological membrane with the assistance of specific transmembrane transport proteins, requiring no energy expenditure. |
| Channel Protein | 通道蛋白 | A transmembrane protein that provides a hydrophilic corridor for specific molecules or ions to cross the membrane. |
| Carrier Protein | 载体蛋白 | A transmembrane protein that undergoes a subtle change in shape to translocate a specific molecule across the membrane. |
| Aquaporin | 水通道蛋白 | A channel protein specifically designed to facilitate the rapid osmosis (diffusion) of water across the membrane. |
| Ion Channel | 离子通道 | A channel protein that allows a specific ion to diffuse across the membrane down its concentration or electrochemical gradient. |
1. What is Facilitated Diffusion?
As established in previous sections, passive transport is the net movement of molecules from regions of high concentration to regions of low concentration without the direct input of metabolic energy.
However, because the plasma membrane has a highly hydrophobic interior, many essential molecules—specifically large polar molecules and charged ions—cannot freely cross on their own. They need a "door" to pass through.
Facilitated diffusion is a specific type of passive transport. The molecules are still moving naturally down their concentration gradients (requiring no ATP), but they are doing so with the help of specific embedded transport proteins.
2. Types of Transport Proteins
Transport proteins are highly specific, meaning a specific transport protein will only allow a specific substance (or a small group of closely related substances) to cross the membrane. There are two main types of transport proteins involved in facilitated diffusion:
- Channel Proteins: Think of these as open tunnels. They provide a hydrophilic corridor that allows specific molecules or ions to pass quickly from one side of the membrane to the other.
- Carrier Proteins: Think of these as revolving doors. They bind to their specific "passenger" molecule, undergo a subtle conformational change in shape, and release the molecule on the other side of the membrane.
3. Crucial Examples in Biological Systems
The AP Exam frequently highlights specific examples of facilitated diffusion that are vital for cellular survival:
A. Aquaporins (Water Transport)
While small amounts of water (\(\ce{H2O}\)) can slowly slip through the lipid bilayer, cells that need to move massive quantities of water quickly (like human kidney cells or plant root cells) rely on aquaporins. Aquaporins are specialized channel proteins that facilitate the rapid diffusion of water (osmosis) across the membrane.
B. Ion Channels
Charged ions, including Sodium (\(\ce{Na+}\)), Potassium (\(\ce{K+}\)), Calcium (\(\ce{Ca^2+}\)), and Chloride (\(\ce{Cl-}\)), are completely blocked by the hydrophobic core of the membrane. They require specific ion channels to move through the membrane.
- Membrane Polarization: The movement of these charged ions across the membrane via facilitated diffusion is what allows membranes to become electrically polarized (which is the fundamental basis for how human nerve cells fire and transmit signals!).
Crucial Distinction: Facilitated Diffusion vs. Active Transport
It is easy to get confused when you see a protein pumping an ion like \(\ce{Na+}\).
- If the protein is moving \(\ce{Na+}\) from HIGH to LOW concentration, it is Facilitated Diffusion (Passive, no ATP).
- If the protein is forcing \(\ce{Na+}\) from LOW to HIGH concentration, it is an Active Transport pump (Requires ATP).
Quiz
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