Living systems are incredibly diverse in size, shape, environment, and behavior. It is estimated that there are between 10 million and 100 million different species. Despite this wide variety of organisms, it remains difficult to define what it means to say something is alive. Which of the following can be described as the smallest living unit?
A.) Organelle
B.) DNA
C.) Protein
D.) Cell

Cells are the structural and functional unit of life. Irrespective of species, all the organisms are made of one or more cells. Unicellular organisms such as bacteria have a single cell in their body while the body of multicellular organisms has many cells. A cell is able to perform vital life processes such as reproduction, nutrition, metabolism, excretion, growth, and sensitivity to surroundings.

These life processes are not performed by non-living things. Cells have organelles and macromolecules such as DNA, proteins, etc. However, these organelles and molecules do not exhibit vital life processes unless they are present in the cells. Therefore, cells are the smallest unit of life.