Charge beats size. If it dissolves in oil, it crosses. If it's charged, it needs a protein.
A membrane is picky because its middle is oily. Whether a molecule crosses on its own depends more on charge and polarity than on size: oil-friendly molecules slip through, and ions need a protein.
Key terms
- Selective permeability — the membrane lets some substances cross freely, some slowly, and some only with help from proteins.
- Hydrophobic core — the layer of fatty acid tails in the middle of the bilayer. It acts like a thin film of oil.
- Nonpolar molecule — has no charge and few polar groups, like O2, CO2, or a steroid hormone. It dissolves into the core.
- Ion — a particle with a net charge, like Na+, K+, or Cl−. It almost never crosses a bare bilayer.
- Hydration shell — water molecules clustered around an ion's charge. Shedding them to enter the oily core costs a lot of energy.
- Cell wall — a tough, porous layer outside the membrane in plants (cellulose), fungi (chitin), and most bacteria (peptidoglycan).
What gets through, and what changes it
- Small nonpolar molecules like O2, CO2, and N2 dissolve into the core and cross fast. Steroid hormones cross the same way, even though they're heavier than glucose.
- Small polar molecules with no charge, like water, urea, and glycerol, cross slowly. Water moves much faster through channel proteins called aquaporins.
- Larger polar molecules like glucose and amino acids barely cross the bare bilayer. They need transport proteins.
- Ions come last. Their charge and water shell make the oily core nearly impossible to enter, however small they are. They cross through proteins.
- Loose packing makes a leakier membrane. More double bonds or shorter tails → more gaps for small polar molecules to slip through.
- Strong heat disorders the tails and unfolds membrane proteins. With enough heat, the membrane leaks what it normally holds in.
Why it matters
Every transport topic after this one starts here. Whatever the bilayer blocks, the cell must move with a protein or a vesicle, and that is where the cell gets control. Exam tables often list mass, charge, and crossing rate. Find two rows that differ in only one property and compare their rates. The property that tracks the rate is your answer.
Exam traps
| Trap | Actually |
|---|
| Smaller molecules always cross faster. | Charge and polarity come first. A steroid hormone crosses easily, while a far smaller Na+ ion barely crosses on its own. |
| The charged phosphate heads repel ions at the surface. | The barrier is the oily core. An ion pays a huge energy cost to leave water and enter it. |
| Heated beet slices leak red pigment because warm water dissolves it better. | The pigment can't cross intact membranes. Leakage means heat damaged the membranes holding it in. |
| The cell wall decides what enters a plant cell. | The wall is porous and blocks only very large particles. The plasma membrane does the fine sorting. |
Worked example
A protein-free artificial bilayer separates two chambers. Equal amounts of four solutes are added to one side: glucose (180 g/mol), O2 (32 g/mol), glycerol (92 g/mol), and chloride ions, Cl− (35 g/mol). Rank them from fastest to slowest crossing.
- Sort by charge first. Cl− is the only ion, so it goes last, even though it's lighter than glycerol and glucose.
- Next, polarity. O2 is nonpolar, so it dissolves into the core and crosses fastest.
- Glycerol and glucose are both polar and uncharged. Glycerol is smaller and has fewer OH groups (3 versus 5), so it crosses faster.
Fastest to slowest: O2, glycerol, glucose, Cl−.
Charge carries the biggest penalty, so the ion is last whatever its size. Among uncharged molecules, less polar and smaller wins. The near-miss ranks by mass and puts Cl− second. The oily core sorts by charge and polarity before it sorts by size.