1Why the core is the gatekeeper

A substance crossing the bilayer on its own has to pass through the layer of fatty acid tails in the middle. That layer is nonpolar, with no water in it.

• A nonpolar molecule is at home there. It dissolves in the core, crosses it, and leaves on the other side.
• A polar molecule or an ion is surrounded by a shell of water molecules attracted to its partial or full charges. To enter the core it would have to leave that water behind and sit among nonpolar tails that cannot attract it. That costs a lot of energy, so it very rarely happens.

So the core blocks ions and polar molecules, and the bigger and more charged a particle is, the more completely it is blocked. Figure 1 sorts the main examples.

2Three groups of substances

Two details catch students.

• Water is polar but small and uncharged, so some does get through the bilayer. It is not enough for most cells, which is why many have aquaporins, channels for water (2.6).
• Size and charge together decide it. A glucose molecule is uncharged but large and covered in polar –OH groups, so it is effectively blocked. An Na⁺ ion is tiny but charged, so it is blocked too.

The membrane's interior is hydrophobic, so nonpolar molecules cross it and ions and polar molecules need proteins.

3Why selective permeability matters

Because the membrane blocks ions and polar molecules, the cell can keep their concentrations inside different from those outside: more K⁺ in, more Na⁺ out, glucose held in, proteins kept in. Those concentration gradients are how a cell stores energy and controls its chemistry (2.5 and 2.8). The cell decides what crosses by deciding which channel and transport proteins it puts in the membrane, and when they are open. A leaky membrane would let every gradient run down.

4Cell walls

The cells of bacteria, archaea, fungi and plants have a cell wall outside the plasma membrane. Animal cells do not. The walls differ in what they are made of (cellulose in plants, chitin in fungi, peptidoglycan in bacteria), but they share three jobs.

• Structural boundary. The wall is rigid enough to give the cell its shape and support.
• Permeability barrier for some substances. The wall is far more open than the membrane; water and small solutes pass through it easily. But it can keep out some large molecules and particles.
• Protection from osmotic lysis. When a cell sits in water that has fewer dissolved solutes than its cytoplasm, water enters by osmosis (2.7). A cell with no wall swells and can burst; that bursting is osmotic lysis. A wall resists the swelling: the cell presses outward, the wall pushes back, and the cell stays whole and firm.

That is why some antibiotics, such as penicillin, kill bacteria: they stop bacteria building the wall properly, and the weakened cells burst in the watery environment of a body.

5Testing a claim with data: the null hypothesis

AP expects you to use data to decide whether to reject or fail to reject a null hypothesis. The null hypothesis states that there is no difference between the groups being compared, or no effect of the variable being tested.

To decide, compare the means and their error bars. AP data usually show error bars of ±2 standard errors of the mean (SE), using $SE_{\bar{x}} = \dfrac{s}{\sqrt{n}}$ from the formula sheet. As a rule of thumb, if the ±2 SE bars of two groups do not overlap, the difference is unlikely to be due to chance and the null hypothesis is rejected. If they overlap, you fail to reject it.

Worked example. An invented experiment uses liposomes, tiny artificial spheres of phospholipid bilayer. One batch is pure bilayer; the other has a K⁺ channel protein inserted. The rate of entry of K⁺ and of O₂ is measured, with 9 samples per group.

For K⁺ without the channel: $SE = 0.15/\sqrt{9} = 0.05$, so ±2 SE = ±0.1 and the interval is 0.2 to 0.4. With the channel: $SE = 0.6/3 = 0.2$, ±2 SE = ±0.4, interval 4.4 to 5.2. The intervals do not overlap, so reject the null hypothesis "the channel protein has no effect on K⁺ entry".

For O₂: $SE = 1.2/3 = 0.4$, ±2 SE = ±0.8 for both. Intervals 11.3 to 12.9 and 11.6 to 13.2 overlap, so fail to reject the null hypothesis for O₂.

The biology agrees: K⁺ is an ion and needs a channel; O₂ is small and nonpolar and crosses the bilayer anyway, so adding a channel changes nothing. Notice the wording: you never "prove" or "accept" the null hypothesis; you fail to reject it.