The Membrane & Getting Things Across
A cell is defined by its border. Two layers of fat decide what enters, what leaves, and what it costs.
Builds on: 0.1 What Is Alive, and What a Cell Is
Same idea, other course:⚗️ Catalyst 1.5 — why the fatty tails avoid water in the first place
A wall that builds itself
The cell membrane is made of phospholipids: molecules with a water-loving phosphate head and two water-hating fatty tails. Drop them in water and they arrange themselves — heads out towards the water on both sides, tails hidden in the middle. The result is a bilayer two molecules thick, and nobody has to assemble it. It forms because any other arrangement exposes the tails to water.
That is the same “like dissolves like” logic Catalyst covers under intermolecular forces, applied to the single most consequential structure in biology. The membrane is not a rigid skin either: the lipids drift sideways past each other like people in a crowd, with proteins floating among them. Hence the name for the standard picture — the fluid mosaic model.
What gets through, and what does not
The oily middle is the filter. Small and nonpolar passes easily; large or charged does not:
| Crosses freely | Needs help |
|---|---|
| O₂, CO₂, N₂ (small, nonpolar) | Glucose and amino acids (large, polar) |
| Water — slowly, and fast through aquaporins | Na⁺, K⁺, Cl⁻ (charged — the tails repel them) |
The membrane is therefore selectively permeable, and the things it blocks are exactly the things the cell wants to control.
Three ways across
Diffusion — molecules move randomly, so they spread from where they are crowded to where they are not. No energy is spent: the cell is simply exploiting the fact that random motion evens things out. This is how oxygen enters and CO₂ leaves.
Facilitated diffusion — still down the gradient, still free, but through a protein channel because the molecule cannot cross the oil. Glucose enters your cells this way.
Active transport — up the gradient, from low to high, which never happens by itself and therefore costs ATP. The sodium–potassium pump throws 3 Na⁺ out and hauls 2 K⁺ in per ATP, all day, in every one of your cells. It is thought to consume something like a fifth of your resting energy, and it is the reason your nerves can fire at all.
Osmosis, and why it matters at dinner
Osmosis is just diffusion of water across a selectively permeable membrane — but stated in terms of the solute, because that is what you can control: water moves towards the side with more dissolved stuff.
Put an animal cell in pure water and water floods in until the membrane fails — lysis. Put it in strong brine and it shrivels. Your blood is kept isotonic for exactly this reason, and a saline drip is mixed to match rather than to be pure.
A plant cell has a wall, so it cannot burst: it swells until the wall pushes back, and that pressure — turgor — is what holds a non-woody plant upright. Let it dry and the plant wilts. Salt a slug, salt a slice of aubergine, or watch lettuce go limp in dressing: same physics every time.
⚗️ Lab — The Osmosis Bench
One cell, one dial: how salty the outside is.
- Drag the concentration to zero — pure water. The animal cell bursts; switch to a plant cell and the wall saves it.
- Find the isotonic point, where the arrows balance and nothing moves on net.
- Push it to 0.9 M and watch the plant cell plasmolyse — the membrane peels off the wall.