Question

On page 2 of the textbook, a list of characteristics something must have in order to be considered "living" is provided. Which of these characteristics do viruses lack? Explain your answer.

Answer 1

Certain characteristics that define a living organism are as under:

1. Order: Living organisms have a highly coordinated and organized structure composed of cells (unicellular or multicellular) with these cells having small cellular organelles and inclusions to carry various activities required for survival.

2. Response to stimuli: All living organisms respond to stimuli either positively or negatively. For example, bacteria demonstrate chemotaxis and phototaxis, sunflower turns towards the direction of sunlight.

3. Reproduction: Living organisms reproduce to increase their species. Unicellular organisms replicate their DNA and then divide and multicellular organisms produce gametes. In essence, their main motive is to pass on genetic information to the next generation.

4. Growth and development: The genetic information inside the cells of living organisms encode for the growth and differentiation of cells and thus the growth of the whole organism.

5. Regulation: All living organisms have internal functions to maintain the internal environment like transport of nutrients, the flow of blood and oxygen to different organs, removal of waste products, etc.

6. Homeostasis: Organisms can maintain appropriate internal cellular temperature, pH, and concentration of certain chemicals even during a change of external environmental conditions through a process of homeostasis (steady-state). For example, organisms in extremely cold climate develop excess fur and fat to reduce the loss of heat from the body and thus maintain the proper internal temperature.

7. Energy processing: Living organisms process energy to perform metabolic activities like energy from the sun for photosynthesis.

Viruses have no cells, cellular organelles, or cytoplasm (no order). They carry out no metabolism or any of the tasks on their own (no response to stimuli, reproduction, growth, regulation, energy processing, or homeostasis). For performing these tasks they need to enter a living host until then they are non-living.

Some characteristics that are similar to living organisms are the presence of proteins and genetic material and they adapt according to their hosts.