Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- The plasma membrane separates the cell’s internal environment from the outside. It is selectively permeable because its interior is hydrophobic.
- The nonpolar hydrocarbon tails block ions and polar molecules. An ion would have to leave its water shell to enter the tails.
- Small nonpolar molecules (N₂, O₂, CO₂) cross freely. Small polar, uncharged molecules (H₂O, NH₃) cross in small amounts.
- Hydrophilic substances, such as ions and large polar molecules (glucose), cross through embedded channel and transport proteins.
- Cell walls (Bacteria, Archaea, Fungi, plants) are outside the membrane. They are a structural boundary, a barrier to some substances and protection from osmotic lysis. Animal cells have no wall.
Key relationships
| Substance | Crosses the bilayer? | Route |
|---|---|---|
| N₂, O₂, CO₂ | freely | through the tails |
| H₂O, NH₃ | small amounts | through the tails |
| glucose and other large polar molecules | hardly | transport proteins |
| Na⁺, K⁺, Cl⁻, H⁺ | no | channel and transport proteins |
| Group | Wall material |
|---|---|
| Bacteria | peptidoglycan |
| Archaea | varied, not peptidoglycan |
| Fungi | chitin |
| Plants | cellulose |
| Statistics | Rule |
|---|---|
| SE = s ÷ √n | s = sample standard deviation, n = number of values |
| mean ± 2 SE | roughly a 95% confidence interval |
| bars do not overlap | reject H₀ (no difference) |
| bars overlap | fail to reject H₀ |
Assumptions behind the conclusions
- Artificial vesicles are a model; real membranes contain many proteins and may offer several routes.
- The ± 2 SE rule is an approximation. It works best with several repeats.
- Data support a hypothesis or lead you to reject or fail to reject H₀; they do not prove anything.
Mistakes to avoid
- Saying ions are blocked by size. Their charge blocks them.
- Saying water cannot cross a bilayer at all.
- Saying the cell wall chooses what enters; the membrane does the fine selecting.
- Saying the wall stops water entering; it stops the cell expanding and bursting.
- Dividing s by n instead of √n for the standard error.
- Writing “accept the null hypothesis” or “prove the hypothesis”.
Quick self-check
- Why does CO₂ cross a pure bilayer but Cl⁻ does not? (CO₂ is small and nonpolar, so it passes through the tails; Cl⁻ is charged and held in its water shell.)
- Values 10, 12 and 14: what is the SE? (Mean 12, s = 2, SE = 2 ÷ √3 = 1.15; interval 12 ± 2.3.)
- Why does a plant cell not burst in pure water? (The wall resists expansion; turgor pressure builds until there is no net entry of water.)
Next: practice questions.