Biology » The Study of Life » Themes and Concepts of Biology

Summary and Main Ideas

Summary of Lessons So Far

Biology is the science of life. All living organisms share several key properties such as order, sensitivity or response to stimuli, reproduction, growth and development, regulation, homeostasis, and energy processing.

Living things are highly organized parts of a hierarchy that includes atoms, molecules, organelles, cells, tissues, organs, and organ systems. Organisms, in turn, are grouped as populations, communities, ecosystems, and the biosphere.

The great diversity of life today evolved from less-diverse ancestral organisms over billions of years. A diagram called a phylogenetic tree can be used to show evolutionary relationships among organisms.

Biology is very broad and includes many branches and subdisciplines. Examples include molecular biology, microbiology, neurobiology, zoology, and botany, among others.

Glossary of Words


smallest and most fundamental unit of matter


study of the chemistry of biological organisms


collection of all the ecosystems on Earth


study of plants


smallest fundamental unit of structure and function in living things


set of populations inhabiting a particular area


all the living things in a particular area together with the abiotic, nonliving parts of that environment


organism with cells that have nuclei and membrane-bound organelles


process of gradual change during which new species arise from older species and some species become extinct


ability of an organism to maintain constant internal conditions


large molecule, typically formed by the joining of smaller molecules


study of the structure and function of microorganisms


chemical structure consisting of at least two atoms held together by one or more chemical bonds

Molecular Biology

study of biological processes and their regulation at the molecular level, including interactions among molecules such as DNA, RNA, and proteins


study of the biology of the nervous system


collection of related tissues grouped together performing a common function

Organ system

level of organization that consists of functionally related interacting organs


small structures that exist within cells and carry out cellular functions


individual living entity


study of life’s history by means of fossils

Phylogenetic tree

diagram showing the evolutionary relationships among various biological species based on similarities and differences in genetic or physical traits or both; in essence, a hypothesis concerning evolutionary connections


all of the individuals of a species living within a specific area


single-celled organism that lacks organelles and does not have nuclei surrounded by a nuclear membrane


group of similar cells carrying out related functions


study of animals

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