What are the characteristics of life? There are a number of features that could be used to characterize life.
Are all seven of these essential characteristics? Some are debatable, such as item D. While it is definitely of adaptive value to have a stable internal environment, it is probably not absolutely essential. Small organisms tend to have a less stable internal environment than large organisms, yet they are definitely alive.
There are three that stand out by appearing in everything we call alive: Growth, metabolism, and reproduction. One could argue that this is an artificial, prejudicial definition of life that automatically allows the entry of some organized levels of matter, and excludes others. If we wrote a different definition, we could include things we do not now think of as being alive. And so the debate goes on.
2. Biology states that the cell is the smallest unit of life. Organized bits of matter smaller than a cell, or without most typical cell structures, do not have enough of the characteristics of life to be accepted as alive. There are three possible exceptions, of which by now you are aware, and which are considered in question three.
3. Although most biologists do not consider viruses, mitochondria, or chloroplasts to be alive, a case may be made for their inclusion. Mitochondria and chloroplasts, in particular, have many of the characteristics of an independently living cell, and they resemble bacteria. Because they have double cell membranes, their own DNA, and reproduce themselves by dividing in two, they are believed to have originated as free living bacteria and became endosymbionts. Therefore, perhaps one could say that mitochondria and chloroplasts are themselves living organisms. However, at least one reason we don't says they are alive is because they are not found independently in nature. They are not found outside of a cell. While they were likely free living bacteria at one time in the ancient past history of life, they are now an integral part of a cell. Chloroplasts are essential components of plant cells, and mitochondria are equally essential in both plant and animal cells.
Viruses have very few of the characteristics of life, but they do have enough to cause some trouble in trying to decide whether or not they are truly alive. They have a specific size and shape, they contain DNA and protein, but otherwise seen to have little going for them by way of being alive. Some say that since viruses reproduce they therefore must be alive. But viruses don't reproduce themselves directly. They cause the host cell to produce more of the invading virus. Viruses don't grow in accordance with our understanding of the process of growth, and they appear to have no metabolism. At present, in agreement with the three essential criteria for aliveness we accepted (Or did we?) in our discussion of question one, viruses are definitely not alive. It wouldn't require much of a rewrite of our biological definition of life to be able to include viruses.