# DBT JRF 2020 — Question 68

Exam: DBT JRF 2020 · Section: B · Topic: Biotechnology · Difficulty: Easy

## Question

The temperature (°C) of liquid nitrogen used for cryopreservation of plant samples is

## Options

- **A.** -100°C
- **B.** -120°C
- **C.** -170°C
- **D.** -196°C

## Answer

Option D

## Explanation

Liquid nitrogen (LN $_2$ ) boils (liquefies) at $-195.79°C \approx -196°C$ (77.36 K = 77 K) at standard atmospheric pressure (1 atm = 101.325 kPa). It is produced industrially by fractional distillation of liquefied air. At $-196°C$ , biological processes, enzymatic reactions, ice crystal formation, and molecular diffusion are effectively halted — providing indefinite preservation of cells, tissues, seeds, embryos, and microorganisms. Vitrification (ice-free cryopreservation using cryoprotectants like DMSO or glycerol) prevents damaging ice crystal formation. The International Society for Biological and Environmental Repositories (ISBER) recommends storage in vapour phase ( $-190°C$ ) rather than liquid phase ( $-196°C$ ) to prevent cross-contamination.

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