How can we distinguish living organisms from non-living things in biology? It is explained by the characteristics of life which are listed below.

  • Cellular organization: Living things are made of cells. Cells are the most basic unit of life.

Cells → Tissues → Organs → Organ Systems → Organism

Cardiac Muscle Cell → Cardiac Muscle Tissue → Heart → Circulatory System → Human

Eukaryotic cell is shown in the figure.

The Characteristics of Life - Eukaryotic cell

  • Energy utilization: Energy is needed for life sustaining chemical reactions, like movement, growth and repair. Metabolism is the sum of all these reactions. Metabolizing or eating can be considered to be alive. Metabolism is the total chemical activity of an organism.


  • Metabolism: Metabolism manages the energy and resources in a cell. There are two basic types. Catabolic reactions are breakdown of large molecules. For example; cellular respiration is the process that breaks down sugar molecules into small energy storing molecules. During those reactions energy is released. Anabolic reactions are responsible for building of larger molecules and energy is used during anabolic reactions. For example, protein synthesis is the building of large proteins from smaller molecules called amino acid.


  • Growth and Development: Growth is the increase in size and mass of living matter over a period of time. Development is the process of changing from earlier stages into mature adult. For example, a fertilized egg develops into an entire organism.


  • Response to Stimuli: Stimulus is a change in the environment. Organisms receives and respond to the changes around them. For example, animals run from predator.


  • Heredity: Organisms transfer hereditary information in genes to their offspring. Genes are encoded in the molecule called DNAallows for adaptations and changes through time.


  • Reproduction: If something is alive, it must be capable of reproducing. Reproduction is vital for the survival of the species.Multicellular organisms such as humans reproduce sexually. Whereas, unicellular life forms like bacteria reproduce asexually by duplicating themselves and then dividing into 2 daughter cells. Daughter cells are identical unless mutation occurs. Mutation is the raw material of biological evolution.


  • Homeostasis: Homeostasis is the tendency of an organism to maintain the stable internal condition with respect to the changingenvironmental conditions.For example, whatever the environment temperature is, our body temperature is 37℃ / 98.6 ℉. This is controlled and achieved by nervous and endocrine system. If the weather is too hot, we sweat in order to increase heat loss by evaporation.  If homeostasis cannot be achieved, our body temperature goes up.


  • Biological Evolution & Adaptation:Biological evolution can be defined as genetic alteration in the population. Since resources are limited in the nature, organisms best fitted that is adapted to the environment reproduce more than their peers and survive. As a result of this, those organisms transfer their genetic properties to the next generations which results in the genetic change of the population.


Virus is the boundary of what is alive or not. Because virus does not possess all the characteristics of life. It completely depends on external element to perform energy utilization, there is no internal metabolism.


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