State whether the following is true or false.
Culture medium turning yellow generally indicates that the medium has become acidic.
Answer: B
Phenol red is yellow below about pH 6.8, and the usual causes are cells that need feeding or a bacterial contamination. Either way the flask requires attention promptly. Purple at the other extreme suggests carbon dioxide has been lost.
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A flask containing 5 millilitres of medium at 2 million cells per millilitre is split 1 in 10. How many cells are in the new flask?
Answer: C
The flask holds 10 million cells in total, and a tenth of those is 1 million. Split ratios are quoted in terms of the fraction transferred rather than the final volume. A one in ten split therefore discards nine tenths of the cells.
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Cells double every 24 hours. Starting from 1 million, roughly how many days until the culture reaches 16 million?
Answer: D
Reaching sixteen times the starting number needs four doublings, since two to the fourth is 16, and four doublings at one a day is four days. Cultures are usually split before they reach that density. Exponential growth means the last doubling adds the most cells.
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Confluence in a culture of adherent cells describes
Answer: A
Cells are usually passaged before they reach full coverage, since crowding slows growth and can change their behaviour. It is judged by eye under a microscope rather than measured. Judging it consistently takes a little practice.
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State whether the following is true or false.
Adherent cells should always be allowed to reach full confluence before passaging.
Answer: B
Overgrowth causes contact inhibition, altered behaviour and sometimes death, so cells are normally split at around eighty per cent coverage. Waiting longer damages the culture rather than improving it. Cells are usually split at around eighty per cent coverage.
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Cells are frozen for long-term storage in the presence of a cryoprotectant such as DMSO in order to
Answer: C
Ice crystals forming inside a cell tear its membranes apart, and the cryoprotectant limits their growth. Cooling slowly and thawing quickly are both part of the same strategy. Both rates are chosen to give ice as little chance to form as possible.
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Assertion (A): Cells are frozen slowly but thawed quickly.
Reason (R): Slow cooling limits intracellular ice formation, and quick thawing limits crystal growth.
Answer: D
Both are true and the reason gives the rationale for each half: the danger at both stages is ice, and the two rates are chosen to give it as little opportunity as possible to form or grow. Ice, not cold, is what actually destroys a frozen cell.
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A haemocytometer is used to
Answer: A
Correct answer: count cells in a known volume of suspension
A haemocytometer is a slide with a precisely etched grid and a chamber of known depth, so counting the cells within a defined number of squares gives a concentration directly. It remains standard because it is cheap and, with trypan blue, reports viability at the same time - dead cells take up the dye and living ones exclude it.
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Trypan blue is used in cell counting because it
Answer: B
An intact membrane keeps the dye out, so blue cells are dead and clear ones alive. The test measures membrane integrity rather than any deeper notion of viability. A cell can be dying and still exclude the dye for some hours.
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State whether the following is true or false.
A cell that excludes trypan blue is certainly healthy.
Answer: C
Exclusion shows only that the membrane is intact, and a cell can be committed to apoptosis while still excluding the dye. The test is a useful screen rather than a verdict on the state of the cell. Apoptotic cells commonly exclude it until quite late in the process.
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