Channel proteins for facilitated diffusion
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Question
What is facilitated diffusion?
Answer
The **passive** movement of **ions and large polar molecules** across a membrane through a **channel or carrier protein**, **down the concentration gradient** (no ATP).
Question
Why can't glucose or ions cross the bilayer directly?
Answer
They are **large/polar or charged**, so they are repelled by the **hydrophobic (water-hating) core** of the bilayer — they need a transport protein.
Question
What is a channel protein?
Answer
A membrane protein with an **open water-filled pore** that lets specific ions or polar molecules pass **straight through**.
Question
What is a carrier protein?
Answer
A membrane protein that **binds** a specific molecule and **changes shape** to move it across the membrane.
Question
How does a channel protein differ from a carrier protein?
Answer
A channel is an **open pore** (fast; ions, water); a carrier **binds and changes shape** (slower; glucose, fructose).
Question
In which direction does facilitated diffusion move particles?
Answer
**Down the concentration gradient** — from **high** to **low** concentration.
Question
Does facilitated diffusion use ATP?
Answer
**No** — it is **passive**, because particles move down their concentration gradient.
Question
What is the only difference between simple and facilitated diffusion?
Answer
The **route**: simple diffusion goes **straight through the bilayer**; facilitated diffusion goes **through a protein**. Both are passive and down the gradient.
Question
Which type of protein typically moves ions like Na⁺ and K⁺?
Answer
A **channel protein** (an open pore).
Question
Which type of protein typically moves sugars like glucose and fructose?
Answer
A **carrier protein** (it binds and changes shape).
Question
Why does the rate of facilitated diffusion level off at high concentration?
Answer
The transport proteins become **saturated** — every channel/carrier is occupied, so the rate reaches a **maximum** and cannot rise further.
Question
How do aquaporins relate to facilitated diffusion?
Answer
Aquaporins are **channel proteins** for **water** — water crosses through them by facilitated diffusion (a fast, passive route).
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Membranes and membrane transport
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