Characteristics and Classification of Living Organisms - Characteristics of living organisms

Living organisms are distinguished from non-living matter by seven fundamental characteristics, often remembered by the acronym MRS GREN.

Movement: An action by an organism or part of an organism causing a change of position or place. Most animals move their whole bodies, while plants may move parts toward light ($phototropism$).

Respiration: The chemical reactions in cells that break down nutrient molecules to release energy for metabolism. This is often represented by the oxidation of glucose: $C_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O$.

Sensitivity: The ability to detect or sense stimuli in the internal or external environment and to make appropriate responses. This involves receptors and effectors.

Growth: A permanent increase in size and dry mass by an increase in cell number or cell size or both. Dry mass is the mass of the organism after all $H_2O$ has been removed.

Reproduction: The processes that make more of the same kind of organism, ensuring the continuity of the species.

Excretion: The removal from organisms of the waste products of metabolism (chemical reactions in cells including respiration), toxic materials, and substances in excess of requirements (e.g., $CO_2$ and urea).

Nutrition: Taking in of materials for energy, growth, and development. Plants require light, $CO_2$, and $H_2O$ for photosynthesis; animals require organic compounds and ions.

Classification: Organisms are classified into groups based on features they share. The Binomial System uses a two-part Latin name: the first part is the Genus (capitalized) and the second is the species (lowercase), e.g., $Homo$ $sapiens$.