Free MCAT-style practice, 2A
MCAT Organelles, Protein Sorting and Cytoskeleton Practice Questions
Ten free MCAT-style practice questions on the nucleolus, rough and smooth ER, Golgi sorting, lysosomes, peroxisomes, mitochondria and the cytoskeleton, with full explanations.
Quick concept primer
Organelle questions usually ask where a molecule is made and where it goes next. Proteins bound for secretion, the plasma membrane or lysosomes are made on ribosomes attached to the rough ER, travel in vesicles to the Golgi, and are sorted there by tags: mannose 6-phosphate sends enzymes to lysosomes, and everything without a retention or targeting signal moves on toward the cell surface. The smooth ER lacks ribosomes and handles lipid and steroid synthesis, detoxification, and calcium storage. The nucleolus is not membrane bound; it transcribes rRNA and assembles ribosomal subunits. Peroxisomes break down very long chain fatty acids and destroy hydrogen peroxide with catalase. Mitochondria keep their own circular DNA and ribosomes, evidence for their bacterial ancestry.
The cytoskeleton has three systems. Microfilaments (actin) drive cytokinesis, muscle contraction and cell crawling. Microtubules (tubulin) form the spindle, cilia and flagella, and tracks for kinesin (toward the plus end, usually outward) and dynein (toward the minus end). Intermediate filaments such as keratin, vimentin, desmin and lamins resist tension and mark the cell type.
Traps: the nucleolus has no membrane, and cytoskeletal motors read filament polarity, not cell geography.
10 practice questions
Question 1
Pancreatic cells were given a brief pulse of radioactive amino acids, then incubated with unlabeled amino acids. The percent of total labeled protein found in each compartment was measured.
| Time after pulse | Rough ER | Golgi | Secretory vesicles |
|---|---|---|---|
| 3 min | 86 | 9 | 5 |
| 20 min | 18 | 67 | 15 |
| 90 min | 4 | 14 | 82 |
What do these results show?
- Secretory proteins are made in the Golgi and then sent back to the rough ER
- Secretory proteins move from the rough ER to the Golgi and then into secretory vesicles
- Secretory vesicles deliver proteins to the rough ER for folding
- Labeled protein is degraded in the Golgi, so total label falls over time
Show answer and explanation
Answer: B. Secretory proteins move from the rough ER to the Golgi and then into secretory vesicles
Label appears first in the rough ER, where ribosomes make secretory proteins, then peaks in the Golgi at 20 minutes, then accumulates in secretory vesicles by 90 minutes. That sequence shows forward movement through the pathway. Nothing in the table shows reverse flow, and the values are percentages of label, so they cannot show degradation.
Question 2
A cell line has a defective enzyme that adds the mannose 6-phosphate tag in the Golgi. What is the most likely result?
- Lysosomal hydrolases are secreted from the cell instead of reaching lysosomes
- Mitochondrial proteins fail to enter mitochondria
- Ribosomes can no longer bind the rough ER
- Cytosolic proteins become trapped in the nucleus
Show answer and explanation
Answer: A. Lysosomal hydrolases are secreted from the cell instead of reaching lysosomes
Mannose 6-phosphate is the Golgi sorting tag recognized by receptors that package acid hydrolases into vesicles bound for lysosomes. Without it, the enzymes follow the default pathway to the cell surface and are secreted, while lysosomes fill with undigested material. Mitochondrial import, ER binding of ribosomes and nuclear transport use different signals.
Question 3
Peroxisomes carry out each of the following functions EXCEPT:
- Beta oxidation of very long chain fatty acids
- Breakdown of hydrogen peroxide by catalase
- Synthesis of plasmalogen phospholipids
- ATP synthesis by oxidative phosphorylation
Show answer and explanation
Answer: D. ATP synthesis by oxidative phosphorylation
Oxidative phosphorylation uses the electron transport chain and ATP synthase of the inner mitochondrial membrane, which peroxisomes lack. Peroxisomes shorten very long chain fatty acids, generate hydrogen peroxide in oxidase reactions and break it down with catalase, and help build plasmalogens, an ether phospholipid abundant in myelin.
Question 4
Which cell type would be expected to have the most extensive smooth ER relative to its rough ER?
- A plasma cell secreting large amounts of antibody
- A pancreatic acinar cell secreting digestive enzymes
- An adrenal cortex cell producing cortisol
- A mature red blood cell
Show answer and explanation
Answer: C. An adrenal cortex cell producing cortisol
Cortisol is a steroid, and steroid synthesis enzymes sit in the smooth ER, so steroid-producing cells are packed with it. Plasma cells and acinar cells make large amounts of secreted protein, which requires abundant rough ER. Mature red blood cells have lost their ER and other organelles entirely.
Question 5
Each of the following observations supports the endosymbiotic origin of mitochondria EXCEPT:
- Mitochondria contain a circular genome not packaged with histones
- Mitochondrial ribosomes resemble bacterial ribosomes more than cytosolic ones
- Mitochondria divide by a fission process resembling bacterial division
- Mitochondria are most abundant in cells with high energy demand
Show answer and explanation
Answer: D. Mitochondria are most abundant in cells with high energy demand
The abundance of mitochondria in muscle or kidney tubule cells reflects their role in ATP production and says nothing about where they came from. A circular histone-free genome, bacteria-like ribosomes that are sensitive to some antibacterial drugs, and division by fission all resemble free-living bacteria and support the endosymbiotic theory.
Question 6
Colchicine binds tubulin dimers and prevents microtubule polymerization. Which process is most directly blocked in a dividing cell treated with colchicine?
- Movement of chromosomes by the mitotic spindle
- Formation of the contractile ring that pinches the cell in two
- Anchoring of neighboring cells through desmosomes
- Synthesis of new phospholipids for the membrane
Show answer and explanation
Answer: A. Movement of chromosomes by the mitotic spindle
The mitotic spindle is built from microtubules, so blocking their polymerization stops chromosome alignment and separation, and cells stall in mitosis. The contractile ring is made of actin and myosin, desmosomes connect to intermediate filaments such as keratin, and phospholipid synthesis happens in the smooth ER.
Question 7
A man has recurrent sinus and lung infections and is infertile because his sperm are immotile, although sperm counts are normal. A defect in which protein best explains both findings?
- Kinesin
- Keratin
- Axonemal dynein
- Actin
Show answer and explanation
Answer: C. Axonemal dynein
Cilia lining the airways and the sperm flagellum share the 9 + 2 microtubule axoneme, which bends when dynein arms slide neighboring doublets past each other. Without functional dynein, cilia cannot clear mucus and sperm cannot swim. Kinesin carries cargo along microtubules but does not bend the axoneme, keratin is an intermediate filament, and actin is not part of the axoneme.
Question 8
A kinesin motor carrying a vesicle down an axon takes one 8 nm step along a microtubule for each ATP it hydrolyzes. How many ATP molecules are used to carry the vesicle 4 micrometers, and in which direction does kinesin move?
- 50 ATP, toward the plus end
- 500 ATP, toward the minus end
- 2000 ATP, toward the minus end
- 500 ATP, toward the plus end
Show answer and explanation
Answer: D. 500 ATP, toward the plus end
Four micrometers is 4000 nm, and 4000 / 8 = 500 steps, one ATP per step, so 500 ATP. Kinesin walks toward the plus end of microtubules, which in an axon points away from the cell body, so it carries cargo outward toward the terminal. Dynein is the motor that moves toward the minus end, back toward the cell body.
Question 9
A tumor biopsy stains strongly for cytokeratin but not for desmin or vimentin. From which tissue type did the tumor most likely arise?
- Epithelium
- Smooth muscle
- Fibroblasts of connective tissue
- Skeletal muscle
Show answer and explanation
Answer: A. Epithelium
Intermediate filament proteins are characteristic of cell type: keratins in epithelial cells, desmin in muscle cells, and vimentin in fibroblasts and other mesenchymal cells. A tumor that keeps cytokeratin and lacks desmin and vimentin most likely came from epithelium, which is why cytokeratin staining helps identify carcinomas.
Question 10
A drug that disrupts the nucleolus is added to growing cells. Which process is most directly impaired?
- Replication of telomeres
- Production of new ribosomal subunits
- Splicing of pre-mRNA in the cytoplasm
- Packaging of lysosomal enzymes
Show answer and explanation
Answer: B. Production of new ribosomal subunits
The nucleolus is the site where RNA polymerase I transcribes rRNA genes and where rRNA joins ribosomal proteins to form the small and large subunits, which are then exported to the cytoplasm. Telomere replication is unrelated, splicing occurs in the nucleus rather than the cytoplasm, and lysosomal enzyme packaging occurs in the Golgi.