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Quiz: Cell Organization and Organelles

Test your understanding of cell architecture, organelle structure, and the cytoskeleton with these review questions.

1. What is the primary distinguishing feature between prokaryotic and eukaryotic cells?

  1. Presence of a cell wall
  2. Presence of a membrane-bound nucleus
  3. Presence of ribosomes
  4. Presence of a plasma membrane
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The correct answer is B. The defining distinction between the two cell types is whether DNA is enclosed within a membrane-bound nucleus. Prokaryotes keep their DNA in an unbound nucleoid region, while eukaryotes house it inside a double-membrane nuclear envelope. Cell walls, ribosomes, and plasma membranes are present in many prokaryotes as well, so they cannot serve as the distinguishing feature.

Concept Tested: Prokaryotic vs. eukaryotic cell architecture

2. Which organelle is the primary site of ATP production and is believed to have originated from an engulfed aerobic bacterium?

  1. Mitochondrion
  2. Chloroplast
  3. Golgi apparatus
  4. Peroxisome
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The correct answer is A. Mitochondria generate most of a eukaryotic cell's ATP through oxidative phosphorylation. Their circular DNA, 70S ribosomes, double membrane, and binary-fission reproduction all resemble bacterial features, supporting the endosymbiotic theory that they descend from an engulfed aerobic proteobacterium roughly 1.5–2 billion years ago.

Concept Tested: Mitochondrial structure and origin

3. Why do most cells remain small rather than growing very large?

  1. Large cells cannot synthesize enough DNA
  2. Large cells lack sufficient ribosomes
  3. The nucleus cannot expand beyond a fixed size
  4. A large cell's low surface-area-to-volume ratio limits nutrient and waste exchange
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The correct answer is D. As a cell grows, its volume increases faster (as the cube of the radius) than its surface area (as the square of the radius), so the surface-area-to-volume ratio falls. Because all nutrients and wastes must cross the plasma membrane, a low ratio means the interior cannot be supplied or cleared quickly enough, favoring small cell size or compartmentalization.

Concept Tested: Cell size and surface-area-to-volume ratio

4. What is the functional relationship between the rough ER and the Golgi apparatus?

  1. The rough ER degrades proteins, and the Golgi replaces them
  2. The Golgi synthesizes proteins that the rough ER stores
  3. The rough ER synthesizes and initially folds/glycosylates proteins, which the Golgi further modifies and sorts
  4. They perform identical, redundant functions
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The correct answer is C. Ribosomes on the rough ER synthesize secretory and membrane proteins co-translationally into the ER lumen, where initial folding and N-linked glycosylation occur. Vesicles then carry these proteins to the Golgi's cis face, where further glycosylation, sorting, and packaging into vesicles destined for the plasma membrane, lysosomes, or secretion take place.

Concept Tested: Endomembrane system protein processing

5. Which line of evidence best supports the endosymbiotic theory of mitochondrial origin?

  1. Mitochondria contain circular DNA, 70S ribosomes, and divide by binary fission, like bacteria
  2. Mitochondria are found only in animal cells
  3. Mitochondria are surrounded by a single membrane like the plasma membrane
  4. Mitochondria lack their own genetic material
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The correct answer is A. The endosymbiotic theory is supported by convergent evidence: mitochondria possess their own circular chromosome, bacterial-sized 70S ribosomes, and reproduce by binary fission independent of the host cell cycle. These features are inconsistent with an organelle that arose de novo within the eukaryotic cell and instead point to origin from an engulfed free-living bacterium.

Concept Tested: Endosymbiotic theory

6. A liver cell exposed to a high dose of a lipid-soluble drug shows a marked increase in the volume of one organelle responsible for detoxification. Which organelle is most likely enlarged?

  1. Rough ER
  2. Smooth ER
  3. Golgi apparatus
  4. Nucleolus
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The correct answer is B. The smooth ER contains cytochrome P450 enzymes that detoxify drugs, alcohol, and other lipid-soluble compounds by converting them into more water-soluble forms for excretion. Liver cells chronically exposed to such compounds respond by proliferating smooth ER membrane to increase detoxification capacity, unlike the rough ER, which specializes in protein synthesis rather than detoxification.

Concept Tested: Smooth ER function

7. A researcher treats a cell with a drug that specifically disables dynein motor proteins. Which cellular process would be most directly disrupted?

  1. DNA replication in the nucleus
  2. Formation of the phospholipid bilayer
  3. Glycosylation of proteins in the Golgi
  4. Transport of vesicles toward the minus end of microtubules
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The correct answer is D. Dynein is a motor protein that walks along microtubules toward their minus end, typically anchored at the centrosome, carrying vesicles, organelles, and other cargo inward. Disabling dynein would stall this centripetal transport, while kinesin-driven movement toward the plus end, DNA replication, membrane synthesis, and Golgi glycosylation depend on different molecular machinery.

Concept Tested: Microtubule motor proteins

8. A mutation eliminates the enzyme catalase from a cell's peroxisomes. What is the most likely direct consequence?

  1. Accumulation of toxic hydrogen peroxide within the peroxisome
  2. Failure of the cell to synthesize lipids
  3. Loss of the nuclear envelope's double membrane
  4. Inability of the Golgi apparatus to sort proteins
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The correct answer is A. Peroxisomes generate hydrogen peroxide (\(\ce{H2O2}\)) as a byproduct of oxidative reactions, and catalase normally breaks this toxic compound down into water and oxygen. Without catalase, hydrogen peroxide would accumulate within the peroxisome and damage cellular components, since no other organelle in this scenario is affected by the loss of this specific enzyme.

Concept Tested: Peroxisome function and catalase

9. A student compares two cells under the electron microscope: Cell X has a thick cellulose wall, a large central vacuole, and chloroplasts; Cell Y lacks a cell wall, has centrioles, and lacks chloroplasts. What can be concluded?

  1. Cell X is an animal cell and Cell Y is a plant cell
  2. Both cells are prokaryotic
  3. Cell X is a plant cell and Cell Y is an animal cell
  4. Both cells are the same cell type at different life stages
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The correct answer is C. A cellulose cell wall, large central vacuole, and chloroplasts are hallmark features of plant cells, while the absence of a cell wall and chloroplasts combined with the presence of centrioles are characteristic of animal cells. Neither cell shows the small size or lack of membrane-bound organelles that would indicate a prokaryote.

Concept Tested: Animal vs. plant cell structure

10. Which comparison correctly distinguishes the roles of microfilaments and microtubules in cell division?

  1. Microfilaments form the mitotic spindle; microtubules form the contractile ring for cytokinesis
  2. Microtubules form the mitotic spindle that separates chromosomes; microfilaments form the contractile ring that pinches the cell in two during cytokinesis
  3. Both microfilaments and microtubules perform identical roles in mitosis
  4. Neither filament type participates in cell division
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The correct answer is B. Microtubules radiating from the centrosomes assemble into the mitotic spindle, which attaches to chromosomes and pulls them to opposite poles. Actin microfilaments, together with myosin, instead form the contractile ring beneath the plasma membrane that constricts during cytokinesis to physically divide the cell into two daughter cells.

Concept Tested: Cytoskeleton roles in mitosis and cytokinesis