Characteristics and Classification of Living Organisms

Biology: The Study of Life

Biology is the study of living things, exploring what they do, how they do it, and why. A key aspect of biology is understanding the relationship between an organism's structure, its function, and its adaptation to its environment.

Biology covers topics such as:

  • Population studies

  • Environmental issues

  • Health issues

Characteristics of Living Organisms

Living organisms possess several characteristics that distinguish them from non-living things. These include:

  1. Nutrition: The process by which organisms obtain energy and raw materials from nutrients such as proteins, carbohydrates, and fats.

  2. Respiration: The release of energy from food substances in all living cells.

  3. Movement: All living things move in various ways, even if the movement is slow and difficult to see.

  4. Excretion: The removal of toxic materials, waste products of metabolism, and excess substances from the body.

  5. Growth: The permanent increase in cell number and size.

  6. Reproduction: The ability to produce offspring.

  7. Sensitivity: The ability to sense and respond to stimuli such as light, temperature, water, gravity, and chemical substances.

While some non-living things may exhibit one or more of these characteristics, only living organisms possess all of them.

Classification of Living Organisms

Classification is the grouping of organisms according to their structural similarities. This helps to:

  • Impose order on the diversity of living things

  • Identify new organisms

  • Facilitate the study of organisms

The hierarchical classification system arranges organisms into groups from largest to smallest:

  • Kingdom

  • Phylum (plural phyla)

  • Class

  • Order

  • Family

  • Genus (plural genera)

  • Species

The species is the smallest group, with organisms sharing similar features and capable of breeding to produce fertile offspring. For example, horses and donkeys belong to the same kingdom, phylum, class, order, family, and genus but are different species. Their offspring, a mule, is infertile.

Hierarchical System Example: Human Being Classification

  • Kingdom: Animalia (all animals)

  • Phylum: Chordata (animals with a backbone)

  • Class: Mammalia (animals with a backbone and hair)

  • Order: Primate (mammals with hands and feet)

  • Family: Hominidae (apes, primitive humans, and modern humans)

  • Genus: Homo (primitive humans and modern humans only)

  • Species: sapiens (modern humans only)

Scientific Name: Homo sapiens

Two systems of classification

Classification can be based on two different systems, natural and artificial.

Natural classification

The hierarchical classification system is based on a natural classification system that uses common features shared by organisms. Natural classification is based on two ideas:

  • Homologous structures

  • Evolutionary relationships

Homologous structures

Homologous structures are features of organisms that are similar in structure but may look very different from each other and may be used for different purposes. Example a horse’s front leg, the human arm and a bat’s wing are all homologous structures. They have the same number and arrangement of bones and this means that they probably evolved from a single type of structure that was present in a common ancester millions of years ago.

A fly’s wing is not homologous with a bat’s wing. It may look similar and do the same job but it develops from a completely different origin. The fly’s wing has no bones and is not covered by feathers. A bat’s wing and a fly’s wing are termed analogous.

Evolutionary relationships

In a natural classification system, biologists group together organisms which are structurally similar and share common ancestors. Natural classification produces a branching set of relationships.

Artificial classification

With artificial classification you can use any grouping you like. Example, You could put all the animals that fly in the same group. This group would then include birds, bats and many insects.

Binomial System of Naming Species

Carl Linnaeus introduced the binomial system of naming species, giving each organism two names in Latin.

The binomial system of naming species means giving organisms two names in Latin (scientific names). The term binomial literally means two names – ‘bi’ means two and ‘nomial’ means name.

  • The genus name is written first, starting with a capital letter.

  • The species name is written second, starting with a small letter.

  • The scientific name is printed in italics or underlined when handwritten.

For example, the scientific name for a tiger is Panthera tigris.

Scientific names are universal, allowing biologists to communicate about organisms regardless of their native language.