Are Viruses Alive? Reading Comprehension Questions 1. Describe Dmitri Ivanovski’s experiment and summarize his conclusion. 2. Explain why Wendell Stanley was puzzled. 3. What are the two main parts make up all viruses? 4. List all the characteristics that a living thing needs to have in ordered to be considered alive. a. e. b f c. g. d. h. 5. What is the basic unit structure of all living things?
The Correct Answer and Explanation is:
Answers:
- Dmitri Ivanovski’s Experiment and Conclusion:
Dmitri Ivanovski filtered sap from infected tobacco plants and discovered that the filtered liquid still caused disease, even though bacteria were removed. He concluded that a new type of infectious agent, smaller than bacteria, was responsible. - Why Wendell Stanley Was Puzzled:
Wendell Stanley was puzzled because when he crystallized the tobacco mosaic virus, it behaved like a chemical and not a living organism, which challenged the idea of whether viruses are truly alive. - Two Main Parts of All Viruses:
- Genetic material (DNA or RNA)
- Protein coat (capsid)
- Characteristics of Living Things:
a. Made of cells
b. Obtain and use energy
c. Grow and develop
d. Respond to environment
e. Maintain homeostasis
f. Reproduce
g. Based on genetic code (DNA/RNA)
h. Evolve over time - Basic Unit Structure of All Living Things:
- The cell
Explanation
The question of whether viruses are alive has long fascinated scientists. This debate began with experiments like those of Dmitri Ivanovski, who found that disease could be spread by sap filtered of bacteria. His discovery suggested that a new type of pathogen—later identified as a virus—was responsible. This tiny agent passed through filters designed to trap bacteria, hinting it was much smaller and fundamentally different.
Wendell Stanley advanced this understanding when he crystallized the tobacco mosaic virus in 1935. He expected a living organism to resist crystallization, a property of chemicals, not life forms. The virus behaved like a chemical, not a cell-based life form, which confused scientists. How could something that replicated in living cells behave like a non-living molecule outside them?
All viruses consist of two essential parts: a nucleic acid core (DNA or RNA) and a protein coat called a capsid. Some may have an envelope, but the core and capsid are universal. Despite having genetic material, viruses cannot carry out life processes on their own. They do not have cells, cannot grow, don’t respond to stimuli independently, and can’t reproduce without a host cell.
Living things, by contrast, meet eight criteria: they are made of cells, obtain and use energy, grow, respond to their environment, maintain homeostasis, reproduce, have a genetic code, and evolve. Viruses meet only a few—especially reproduction and evolution—but only within a host, not independently.
Because viruses lack the cell—the basic unit of life—and cannot independently perform life processes, many scientists classify them as non-living. However, their ability to evolve and reproduce in hosts continues to fuel debate, showing how viruses blur the line between living and non-living matter.
