- The diversity of organisms is determined by the organization of their genes and their expression patterns.
- Organisms share characteristics; they consist of units called cells, each of which is surrounded by a membrane consisting mainly of a phospholipid bilayer; they self-replicate using a genetic material called DNA; they respond to external stimuli; they synthesize adenosine triphosphate (ATP), an energetic material, and live and grow using the energy thus produced.
- Organisms are roughly classified into the three categories (i.e., domains) of bacteria, archaea and eukarya (eukaryotes). The former two do not have a clearly defined nucleus, and are also known as prokaryotes.
- It is believed that during the process of evolution, bacteria capable of aerobic respiration and cyanobacteria with photosynthetic ability started living symbiotically with primitive eukaryotes, subsequently adapted to the cellular environment of these eukaryotes by discarding their unnecessary genes, and finally became organelles - mitochondria and chloroplasts - respectively (as per endosymbiotic theory).
- Among the materials that make up cells, the most abundant is water (normally representing 70-80% of the total volume), in which many substances are dissolved. The next most common are proteins and lipids, while carbohydrates and minerals also play important roles.