Simple diffusion and osmosis
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Question
What does 'passive transport' mean?
Answer
Movement across a membrane that needs **no energy (ATP)** — particles move **down a gradient** on their own.
Question
Define simple diffusion.
Answer
The **net movement of small or non-polar particles** down their concentration gradient, **straight through the phospholipid bilayer**.
Question
Define osmosis.
Answer
The net movement of **water** across a **partially permeable membrane**, from **higher water potential (dilute) to lower (concentrated)**.
Question
Which molecules cross the bilayer easily by simple diffusion?
Answer
**Small, non-polar** molecules — e.g. **O₂, CO₂** — and **lipid-soluble** molecules such as **steroid hormones**.
Question
Why do non-polar molecules pass straight through the membrane?
Answer
The bilayer's core is **non-polar / hydrophobic**, so non-polar molecules are **not repelled** — they dissolve in and pass through.
Question
Why can't charged or large molecules use simple diffusion?
Answer
They are **repelled by the hydrophobic core** (or too large), so they need a **protein** to cross.
Question
Which way does water move in osmosis?
Answer
From the **more dilute** solution to the **more concentrated** one (high → low **water potential**).
Question
What is water potential?
Answer
A measure of how free the water is to move. **Pure water is highest**; adding solute lowers it. Water moves from **high to low** water potential.
Question
What is an aquaporin and what does it do?
Answer
A **channel protein** that lets water cross **quickly**, speeding up osmosis. It uses **no ATP** and doesn't change the direction.
Question
Is osmosis active or passive?
Answer
**Passive** — it uses no ATP, even when aquaporins speed it up.
Question
How does the concentration gradient affect the rate of diffusion?
Answer
A **steeper** gradient gives a **faster** rate of net diffusion; a shallower one gives a slower rate.
Question
On a data graph, what does a rising cell mass tell you?
Answer
Water is **entering** the cell (net water movement in), so the outside solution is **more dilute / hypotonic**.
Question
On a data graph, what does a falling cell mass tell you?
Answer
Water is **leaving** the cell (net water movement out), so the outside solution is **more concentrated / hypertonic**.
Question
Besides the gradient, what else raises the rate of diffusion?
Answer
A **higher temperature** and a **larger membrane surface area**; a **thicker** membrane slows it down.
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Topic 2.3 hub
Membranes and membrane transport
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