Biology · Rung 2 of 6

Membranes & transport

The membrane is a selective barrier made mainly of lipids and proteins. Diffusion produces net flow down a gradient. Osmosis is the movement of water across a selective membrane. Transporting substances against their gradient requires an energy source.

Where this sits

Comes after
The cell

Exercises from this rung

Generated by the app, from easiest to hardest

  1. There is 10% CO₂ inside a cell and 5% outside. CO₂ crosses the membrane unaided. Which way does it go overall?

    • It does not move without spending ATP
    • Outwards: from where there is more to where there is less
    • Inwards: from where there is more to where there is less
    • It spreads out evenly in an instant

    Why Outwards: from where there is more to where there is less. Molecules cross in both directions, but more cross from the 10% side than from the 5% side, and the net flow runs down the gradient until it evens out. It costs no energy: it is the same randomness that spreads a perfume through a room. And it is not instant, which is why cells are small.

    easier

  2. In water, how do membrane phospholipids usually arrange themselves?

    • All form one rigid chain without separation
    • Tails face water; heads hide in the centre
    • Heads face water; tails face the bilayer's interior

    Why Polar heads interact with water. Nonpolar tails gather away from it, forming the bilayer's interior. This structure creates a selective barrier, with proteins that help transport substances.

    in between

  3. Glucose enters through a protein, down its gradient, without spending ATP. What kind of transport is that?

    • Simple diffusion
    • Active transport
    • Osmosis
    • Facilitated diffusion

    Why Facilitated diffusion: down the gradient, spending no ATP, but through a protein, because glucose is large and polar and the bilayer will not let it through on its own. The cell regulates how many proteins it puts out.

    harder