CDS (II) 2022 General Knowledge · Question 62 of 120

Correct — A, Liquid nitrogen. Cryopreservation of blood, tissue, organs and semen is carried out at about minus 196 degrees Celsius, which is the boiling point of nitrogen, and liquid nitrogen is the bath that holds a sample there. Below roughly minus 130 degrees Celsius no liquid water remains and no chemical or biological reaction proceeds at any meaningful rate, so a cell held at minus 196 is effectively suspended in time and can be revived years later. Nitrogen is chosen over every other cold liquid for practical reasons rather than physical ones: it makes up most of the air, so it is cheap and inexhaustible; it is chemically inert, so it will not react with the sample or with the container; and it is not flammable and does not support combustion.

• (b)Solid carbon dioxide — Solid carbon dioxide, or dry ice, sublimes at about minus 78·5 degrees Celsius. That is cold enough to transport frozen food or a sample for a day, but far above the temperature at which biological activity truly stops, so it cannot be used for long-term storage of semen, cells or organs.
• (c)Liquid air — Liquid air is a mixture, not a pure substance. As it boils the nitrogen leaves first and the remaining liquid becomes enriched in oxygen, which turns the bath into a powerful oxidiser and a serious fire hazard around organic material. Its temperature also drifts as the composition changes.
• (d)Helium — Helium is a gas at room temperature and cools nothing on its own. Liquid helium exists, at about minus 269 degrees Celsius, but it is far colder than tissue banking needs, is enormously more expensive and boils away rapidly; it is reserved for superconducting magnets, not for sample storage.

Cryopreservation is the storage of living material at temperatures low enough to halt every process that would otherwise degrade it. The hard part is not the cold but the journey to it: as water freezes it forms ice crystals that puncture membranes, so samples are treated with cryoprotectants such as glycerol or dimethyl sulphoxide and cooled at a controlled rate, or cooled so fast that the water sets as a glass rather than as crystals. Once at liquid nitrogen temperature the sample is stable almost indefinitely.

This is a question about a working temperature, and the reliable route to it is to carry the numbers for the four common coolants: dry ice minus 78·5, liquid nitrogen minus 196, liquid oxygen minus 183, liquid helium minus 269. Only nitrogen sits in the window that is cold enough to stop biology and cheap enough to use by the tank. The application is worth knowing beyond the laboratory: liquid nitrogen containers are what carry bull semen for artificial insemination to village cattle-breeding centres, and the same tanks hold blood components, stem cells and biological samples in hospitals and gene banks.

• Liquid nitrogen boils at about minus 196 degrees Celsius, the standard temperature for long-term cryopreservation.
• Cryopreservation is carried out typically at minus 80 degrees Celsius or minus 196 degrees Celsius using liquid nitrogen.
• Solid carbon dioxide, or dry ice, sublimes at about minus 78·5 degrees Celsius — useful for transport, not for storage.
• Liquid helium at about minus 269 degrees Celsius is used to cool superconducting magnets, not biological samples.
• Cryoprotectants such as glycerol and dimethyl sulphoxide are used to stop ice crystals from rupturing cell membranes.

• Picking dry ice because it is the coolant most often seen; minus 78·5 degrees Celsius is not cold enough for biological storage.
• Treating liquid air as interchangeable with liquid nitrogen; it is a mixture that turns oxygen-rich as it boils.
• Reading 'helium' as liquid helium; the option as printed names the gas.

As a which-coolant item for storing biological material, or as a temperature-matching item across dry ice, liquid nitrogen and liquid helium.

• practice — not a real PYQ

  Dry ice is the solid form of which one of the following?

  • (a)Water
  • (b)Carbon dioxide
  • (c)Nitrogen
  • (d)Ammonia

  Answer(b) Carbon dioxide — it sublimes at about minus 78·5 degrees Celsius without passing through a liquid stage at ordinary pressure.

• practice — not a real PYQ

  Cryoprotectants such as glycerol are added before freezing biological samples mainly to

  • (a)raise the freezing point of the sample
  • (b)prevent ice crystals from damaging cell membranes
  • (c)sterilise the sample
  • (d)increase the sample's density

  Answer(b) prevent ice crystals from damaging cell membranes — crystal formation, not cold itself, is what kills the cell.