📝 Cell fractionation method (8 MCQs)
📖 From Principles of Biochemistry • 1. The Foundations of Biochemistry • 8 questions available
What is Cell fractionation method?
Definition:
Cell fractionation is a laboratory technique used to separate and isolate different cellular components, such as organelles and macromolecules, based on their physical properties like size, shape, density, and charge, by disrupting the cell membrane and applying differential centrifugation or density gradient centrifugation to obtain purified fractions for biochemical and functional studies.
Working:
This method works by first homogenizing tissues to break cell membranes, then subjecting the homogenate to a series of centrifugation steps at increasing speeds, where the centrifugal force is given by , with being mass, angular velocity, and radius, causing larger organelles like nuclei to pellet at low speeds, while smaller organelles like ribosomes require higher speeds, and density gradient centrifugation can further purify specific organelles.
Example:
A simple example is isolating mitochondria from liver tissue, where the tissue is homogenized in a buffer, centrifuged at 600g for 10 minutes to remove nuclei and cell debris, then the supernatant is centrifuged at 10,000g for 20 minutes to pellet mitochondria, which can be resuspended for assays measuring mitochondrial enzyme activity or oxygen consumption.
Reason:
Cell fractionation is a fundamental tool in cell biology because it allows researchers to study organelle function, isolate specific proteins and enzymes, and understand cellular processes in isolation, enabling discoveries in metabolism, signaling, and disease mechanisms, and is essential for biotechnology and pharmaceutical development.
📝 All Cell fractionation method MCQs
Q1. A researcher wants to isolate intact mitochondria from a homogenized eukaryotic cell mixture. Which strategy is most appropriate as the initial separation step?
📖 Explanation: Differential centrifugation is appropriate for the initial fractionation because cellular components differ substantially in size and density. Low-speed centrifugation can remove larger structures first, while progressively higher speeds pellet smaller organelles. This preserves a logical size-based separation sequence.
Q2. Two fractions are obtained after cell fractionation. Fraction X contains many membrane-bound organelles, whereas Fraction Y contains mostly soluble enzymes. Which conclusion is best supported?
📖 Explanation: A soluble enzyme-rich fraction is consistent with cytosolic material remaining in the supernatant after membrane-containing structures are removed. In contrast, membrane-bound organelles tend to sediment during appropriate centrifugation steps. The conclusion does not require every enzyme to originate exclusively from cytosol.
Q3. A laboratory needs to compare mitochondrial enzyme activity between healthy and damaged cells. Damaged cells release mitochondrial enzymes into the surrounding cytosol before fractionation. What result would most likely complicate interpretation?
📖 Explanation: If mitochondrial enzymes leak into the cytosol before fractionation, subsequent separation redistributes those enzymes into different fractions. Therefore, measuring only the mitochondrial fraction could falsely suggest decreased mitochondrial activity, even when the total amount of enzyme in the cell has changed little.
Q4. A student claims, If an organelle is found in the pellet after centrifugation, that pellet must contain a pure preparation of that organelle. What is the main flaw in this reasoning?
📖 Explanation: A pellet obtained during centrifugation is not necessarily pure. Organelles and other structures with overlapping sedimentation behavior can sediment together. Additional fractionation steps, such as carefully chosen centrifugation conditions or density-based separation, may be needed to improve purity.
Q5. A graph shows that the percentage of mitochondria recovered in the pellet increases rapidly as centrifugal force rises from low to moderate values, then increases only slightly at higher forces. Which interpretation is most reasonable?
📖 Explanation: The plateau indicates that most recoverable mitochondria have already sedimented by the moderate-force range. Increasing centrifugal force further produces only a small additional recovery. Importantly, recovery and purity are different properties, so the graph does not prove that the pellet is pure.
Q6. A researcher performs fractionation on two cell types. Cell type P has larger organelles than cell type Q, but their organelles have similar densities. If identical centrifugation conditions are used, what outcome is most plausible?
📖 Explanation: Sedimentation depends on several physical properties, including particle size, density, and the characteristics of the surrounding medium. When density is similar, larger organelles generally sediment more readily under comparable conditions. Thus, identical centrifugation settings can produce different fractionation patterns in different cells.
Q7. A scientist measures a marker enzyme for a target organelle across several fractions. The marker activity is highest in Fraction 3, while a cytosolic marker is also unexpectedly elevated in Fraction 3. What is the best interpretation?
📖 Explanation: High activity of the target-organelle marker suggests enrichment of that organelle in Fraction 3, but elevated cytosolic marker activity indicates contamination. Fractionation separates components imperfectly, so marker profiles should be interpreted together rather than treating one marker as definitive evidence of purity.
Q8. A researcher wants a fraction enriched in a specific organelle while minimizing contamination. An initial centrifugation produces a pellet containing the desired organelle plus several similarly sized structures. Which refinement would most directly improve the separation?
📖 Explanation: When particles have similar sedimentation behavior, simply repeating the same centrifugation condition may not substantially improve separation. A density-based or otherwise selective separation method can exploit a physical difference that distinguishes the desired organelle from contaminants, producing a more enriched fraction.