3: Properties of Life

Properties of Life

Life is defined by a set of interdependent characteristics that distinguish living organisms from non-living matter. These properties collectively enable organisms to maintain their existence, adapt, and perpetuate their species.

1. Cellular Organization: All living things are composed of one or more cells—the basic structural and functional units of life. Unicellular organisms (e.g., bacteria) perform all life processes within a single cell, while multicellular organisms (e.g., plants, animals) exhibit specialized cell types organized into tissues and organs.

2. Metabolism: Living organisms transform energy and materials through chemical reactions. Catabolism breaks down molecules to release energy (e.g., cellular respiration), while anabolism uses energy to build complex molecules (e.g., protein synthesis). This constant energy flow sustains life processes.

3. Homeostasis: Organisms maintain stable internal conditions despite environmental fluctuations. Examples include temperature regulation (e.g., sweating in mammals) and pH balance. Failure to sustain homeostasis disrupts cellular functions and can be fatal.

4. Growth and Development: Growth involves an increase in size via cell division or enlargement. Development encompasses changes in structure or function (e.g., embryonic differentiation in animals or seed germination in plants), guided by genetic instructions.

5. Reproduction: Life perpetuates through reproduction. Asexual reproduction (e.g., binary fission in bacteria) produces genetically identical offspring. Sexual reproduction (e.g., fertilization in eukaryotes) combines genetic material from two parents, enhancing genetic diversity.

6. Response to Stimuli: Organisms detect and react to environmental cues. Motile organisms exhibit movement (e.g., plants bending toward light via phototropism); non-motile organisms respond at cellular levels (e.g., immune cells targeting pathogens).

7. Evolutionary Adaptation: Populations evolve traits that enhance survival and reproduction through natural selection. Genetic variations (e.g., antibiotic resistance in bacteria) accumulate over generations, allowing species to adapt to changing environments.

Viruses as a Boundary Case: Viruses possess genetic material (DNA/RNA) and evolve but lack cellular structure, independent metabolism, and the ability to reproduce without host cells. Thus, they are not considered alive but represent a unique biological entity blurring the line between living and non-living.

These seven properties form a unified framework for life. While no single trait is exclusive to living systems, their combined presence defines biological entities.