AP® Biology review sheet from Aim for Five (aimforfive.com/bio/units/2/2-4)
Unit 2 · Topic 2.4
2.4 Membrane Permeability
The plasma membrane is selectively permeable: it lets some substances cross freely and blocks others. Its hydrophobic interior is the reason, because small nonpolar molecules dissolve through it easily while ions and large polar molecules can't. Cell walls in plants, fungi, bacteria and archaea add a strong outer layer that supports the cell and keeps it from bursting.
Key terms
- selective permeability
- nonpolar molecule
- polar molecule
- transport protein
- cell wall
Why the membrane is selective
The plasma membrane separates the cell's internal environment from the outside. It's selectively permeable, meaning it lets some substances through and keeps others out. That selectivity comes from the hydrophobic core of the bilayer: the fatty acid tails (1.5).
A molecule crossing the membrane on its own has to pass through that oily middle. Nonpolar molecules can dissolve into it. Charged ions and polar molecules are attracted to water and are pushed back by the nonpolar tails.
What crosses and what doesn't
Two things make crossing harder: charge and size. Charge matters most. Even a tiny ion like H⁺ can't cross the bilayer on its own, because its charge is strongly attracted to water. Water itself is polar but very small, so a little slips through; cells that need to move a lot of water use channel proteins called aquaporins (2.6).
So membrane proteins are the cell's way of letting in exactly the hydrophilic substances it needs. Channels and transport proteins give ions and large polar molecules a hydrophilic path across.
| Type of substance | Examples | Crosses the bilayer on its own? |
|---|---|---|
| Small nonpolar molecules | O₂, CO₂, N₂ | Yes, freely |
| Small polar, uncharged molecules | H₂O, NH₃ (ammonia) | Only in small amounts |
| Large polar molecules | Glucose, sucrose | Essentially no; needs a transport protein |
| Ions (charged) | Na⁺, K⁺, Cl⁻, H⁺ | No; needs a channel or transport protein |
Cell walls
Many cells have a cell wall outside the plasma membrane. Plants build walls mainly of cellulose (1.4), fungi use chitin, and most bacteria use peptidoglycan, a polymer of sugars and short amino acid chains. Most archaea also have walls, but made of different materials than bacteria use. Animal cells don't have cell walls.
A cell wall does three things. It gives the cell a structural boundary and support. It blocks or slows the passage of some substances, although most plant cell walls let water and small molecules through easily. And it protects the cell from osmotic lysis, which is bursting when too much water rushes in (2.7). When water enters a plant cell, the cell swells and presses against the wall, and the wall pushes back. That pressure keeps the cell firm instead of letting it burst.
Worked examples
Try each one yourself first, then open the solution.
- Example 1
Ranking how easily molecules cross
Rank these substances from fastest to slowest at crossing a pure phospholipid bilayer with no proteins: glucose, CO₂, Na⁺, H₂O. Justify your ranking.
Show the solutionHide the solution
- Step 1: CO₂ is small and nonpolar, so it dissolves straight through the hydrophobic core. Fastest.
- Step 2: H₂O is polar, but very small and uncharged, so a small amount slips through. Second.
- Step 3: Glucose is polar and much larger, so it barely crosses without a transport protein. Third.
- Step 4: Na⁺ carries a full charge, which is strongly attracted to water and blocked by the nonpolar tails. Slowest; it essentially can't cross alone.
Answer: CO₂ > H₂O > glucose > Na⁺, because crossing gets harder as molecules become more polar, larger and especially charged.
- Example 2
Why penicillin kills bacteria
Penicillin blocks bacteria from building new peptidoglycan for their cell walls. Bacteria exposed to penicillin while they're growing in a watery environment often burst. Explain why.
Show the solutionHide the solution
- Step 1: Recall the wall's job: the cell wall resists the pressure that builds when water flows into the cell, protecting it from osmotic lysis.
- Step 2: Identify the environment: the inside of a bacterium usually has more dissolved solutes than its surroundings, so water tends to flow in (2.7).
- Step 3: Apply the drug: growing bacteria need to keep adding peptidoglycan. Without it, the wall develops weak spots.
- Step 4: Conclude: water keeps entering, the weakened wall can't push back, and the cell bursts.
Answer: Without new peptidoglycan, the wall weakens and can't resist the pressure from water entering by osmosis, so the cell undergoes osmotic lysis.
Common mistakes
- Saying water can't cross the membrane because it's polar. Water is small enough that a little crosses the bilayer directly; aquaporins let much more through.
- Saying the cell wall is selectively permeable like the membrane. The plasma membrane does the fine selecting; walls mainly provide support and protection.
- Giving animal cells a cell wall, or saying all walls are made of cellulose. Plants use cellulose, fungi chitin and most bacteria peptidoglycan.
- Ignoring charge. Small ions like H⁺ still can't cross the bilayer alone, because charge matters more than size.
On the exam
- Expect questions that give a molecule's size and polarity and ask how it will cross, or ask you to predict which molecule crosses fastest.
- When explaining selective permeability, name the cause: the hydrophobic interior of the bilayer made of fatty acid tails.
Connected topics
Videos
Check yourself
4 questions on 2.4 Membrane Permeability. Pick an answer to see if you got it, and why.
Which of the following substances would cross a pure phospholipid bilayer (with no proteins) most rapidly?
When an animal cell and a plant cell are both placed in distilled water, the animal cell bursts but the plant cell does not. Which of the following best explains this difference?
| Substance | Size and polarity | Relative rate of crossing (glucose = 1) |
|---|---|---|
| Oxygen (O₂) | Small, nonpolar | 10,000,000 |
| Water (H₂O) | Small, polar | 10,000 |
| Glycerol | Small, polar | 100 |
| Glucose | Large, polar | 1 |
| Chloride ion (Cl⁻) | Charged | 0.001 |
| Sodium ion (Na⁺) | Charged | 0.0001 |
Approximate relative rates at which substances cross an artificial phospholipid bilayer that contains no proteins.
Which of the following claims is best supported by the data?
Based on the data, which of the following best explains why cells need membrane proteins to move Na⁺ and Cl⁻ in useful amounts?
0 of 4 answered