Two molecules thick, assembled by nothing but water's refusal to touch a hydrocarbon, and everything a cell knows about the outside world arrives through it.
The membrane is two molecules thick and does half the work in the course: barrier, gateway, sensor, name badge and workbench, all in the same sheet.
Amphipathic phospholipids arrange themselves: heads to the water, tails away from it. No assembly required.
Integral proteins span the sheet; peripheral ones sit on a face. Transport, catalysis, reception, recognition.
Fluid because components drift; mosaic because the proteins are scattered rather than ordered.
Drop them in and watch. Nothing arranges them and nothing spends energy: each one simply ends up where fewest of its tails touch water. What forms depends only on how many there are.
Tap each component. The pink dashed band is the part that does the excluding.
Tap the bilayer, the hydrophobic core, a channel, a carrier, cholesterol, the glycoprotein or the peripheral protein.
Five molecules meet the same membrane. Two rules decide every outcome: how big, and how polar.
Permeability is not a yes or no. It is a spectrum spanning several orders of magnitude, and a cell's control sits at the far end of it.
Nonpolar dissolves into the core and crosses. Polar does not. Charge is the strongest block of all.
Small helps. Water is polar but tiny, so a little gets through anyway; a protein never will.
In plants, fungi and bacteria a wall sits outside the membrane. It is fully permeable and purely mechanical: selectivity stays with the membrane.
Oxygen crosses freely, glucose queues for a carrier, and sodium never gets in at all. The counters keep score.
It is almost always taught as the fluidity buffer, and that is only half of it. The same wedging that steadies the tails also plugs the gaps between them, which changes what can cross.
A single hydroxyl group is hydrophilic and sits up among the phosphate heads; the rigid steroid rings and short tail are hydrophobic and sit among the fatty acid tails. That is what holds it in position rather than letting it drift into the core.
Warm: the rings restrain phospholipid movement, so the membrane does not become too loose. Cold: they hold tails apart, so it does not set into a gel. A buffer in both directions.
Filling the transient gaps between moving tails leaves fewer routes for ions and small polar molecules, so the bilayer leaks less. Distinct from fluidity, and frequently confused with it.
Tap a node to light up its connections. The pink dashed link is the one that explains why membranes need proteins at all.
The bilayer says no. The proteins say who.
Drag a term into a gap, or tap a term and then tap a gap. Two terms belong nowhere.
The membrane is described by the model, because phospholipids and proteins drift laterally within a scattered arrangement. Each phospholipid is , with a polar head and two nonpolar tails, and the tails create a core through the middle. Wedged between those tails, buffers the fluidity against temperature change, while on the outer face act in cell recognition. Because different substances cross at very different rates, the membrane is described as . An ion crosses through a hydrophilic , whereas glucose binds to a protein that changes shape.
Single best answer. You get the reasoning as soon as you commit.
Biology by Bradford