Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Eukaryotic cells contain membrane-bound organelles: nucleus, rough and smooth ER, Golgi complex, vesicles, lysosomes, vacuoles, mitochondria and (in plants and algae) chloroplasts. The plasma membrane bounds the whole cell.
- Each compartment holds its own enzymes, substrates and conditions. This compartmentalization lets the cell run many reactions at once.
- Minimizing competing interactions: opposing pathways run in different places; each space keeps its own pH (lysosome about 4.5–5, cytosol about 7.2); harmful enzymes stay contained; enzymes and substrates are concentrated.
- Increasing surface area: many reactions happen on membrane proteins, so folds such as cristae (mitochondria), stacked thylakoids (chloroplasts) and the ER network add room for them without adding volume.
Key relationships
| Feature | How it helps | Example |
|---|---|---|
| separate membrane-bound space | keeps opposing reactions apart | building and breaking down molecules at the same time |
| proton pumps in a membrane | keep a different pH inside | acidic lysosome for digestive enzymes |
| enzymes kept inside | protects the rest of the cell | lysosomal enzymes work poorly in the cytosol |
| small volume | concentrates enzymes and substrates | faster reactions |
| folded inner membrane | more area for embedded proteins | cristae hold more ATP synthase |
| two membranes, two spaces | allows a difference across a membrane | H⁺ difference across the inner mitochondrial membrane (Unit 3) |
| Calculation | Rule |
|---|---|
| folded membrane area | unfolded area + area added by all folds |
| pH change | each 1 unit = factor of 10 in H⁺; higher pH = fewer H⁺ |
Assumptions behind the numbers
- Box and fold models ignore curvature; they compare areas, not exact sizes.
- “More membrane means more activity” assumes the same number of proteins per µm².
- Enzyme activity measured in a test tube may differ inside a cell.
Mistakes to avoid
- Saying folding makes the organelle bigger; it adds area, not volume.
- Forgetting the unfolded part of the membrane when totalling area.
- Saying higher pH means more H⁺ ions.
- Treating compartments as sealed; vesicles and transport proteins move material between them.
- Saying the nucleus makes proteins; ribosomes do, outside it.
- Describing an organelle’s job without saying why a separate compartment helps.
Quick self-check
- How many times more H⁺ is there at pH 5 than at pH 7? (10² = 100 times)
- A membrane of 2.0 µm² gains 10 folds of 0.30 µm² each. New total? (2.0 + 10 × 0.30 = 5.0 µm², 2.5 times the original)
- Predict one effect of blocking lysosome proton pumps. (Lysosome pH rises, digestive enzymes slow down, undigested material builds up.)
Next: practice questions.