2.2 Levels of Organization of Living Things

Learning Objectives

By the end of this section, you will be able to:

  • Describe the levels of organization among living things

Living things are highly organized and structured, following a hierarchy on a scale from small to large. The atom is the smallest and most fundamental unit of matter. It consists of a nucleus surrounded by electrons. Atoms form molecules. A molecule is a chemical structure consisting of at least two atoms held together by a chemical bond. Many molecules that are biologically important are macromolecules, large molecules that are typically formed by combining smaller units called monomers. An example of a macromolecule is deoxyribonucleic acid (DNA), which contains the instructions for the functioning of the organism that contains it.

Molecular model depicts a DNA molecule, showing its double helix structure.
Figure 2.6 A molecule, like this large DNA molecule, is composed of atoms. Credit: Brian0918 © Public Domain

Concept in Action

To see an animation of this DNA molecule, click here.

Some cells contain a collection of macromolecules surrounded by membranes; these are called organelles. Organelles are small structures that exist within cells and perform specialized functions. All living things are made of cells; the cell itself is the smallest fundamental unit of structure and function in living organisms. This requirement is why viruses are not considered living: they are not made of cells. To make new viruses, they have to invade and hijack a living cell; only then can they obtain the materials they need to reproduce. Some organisms consist of a single cell and others are multicellular. Cells are classified as prokaryotic or eukaryotic.

Prokaryotes are single-celled organisms that lack organelles surrounded by a membrane and do not have nuclei surrounded by nuclear membranes; in contrast, the cells of eukaryotes do have membrane-bound organelles and nuclei. In most multicellular organisms, cells combine to make tissues, which are groups of similar cells carrying out the same function. Organs are collections of tissues grouped together based on a common function. Organs are present not only in animals but also in plants. An organ system is a higher level of organization that consists of functionally related organs. For example vertebrate animals have many organ systems, such as the circulatory system that transports blood throughout the body and to and from the lungs; it includes organs such as the heart and blood vessels. Organisms are individual living entities. For example, each tree in a forest is an organism. Single-celled prokaryotes and single-celled eukaryotes are also considered organisms and are typically referred to as microorganisms.

All the individuals of a species living within a specific area are collectively called a population. For example, a forest may include many white pine trees. All of these pine trees represent the population of white pine trees in this forest. Different populations may live in the same specific area. For example, the forest with the pine trees includes populations of flowering plants and also insects and microbial populations. A community is the set of populations inhabiting a particular area. For instance, all of the trees, flowers, insects, and other populations in a forest form the forest’s community. The forest itself is an ecosystem. An ecosystem consists of all the living things in a particular area together with the abiotic, or non-living, parts of that environment such as nitrogen in the soil or rainwater. At the highest level of organization, the biosphere is the collection of all ecosystems, and it represents the zones of life on Earth. It includes land, water, and portions of the atmosphere.

 

A flow chart shows the hierarchy of living organisms. From smallest to largest, this hierarchy includes: 1 An atom, with protons, neutrons and electrons. 2 Molecules such as the phospholipid shown, made up of atoms. 3 Organelles, such as Golgi apparatus and nuclei, that exist inside cells. 4 Cells, such as a red blood cell. 5 Tissues, such as human skin tissue. 6 Organs such as the stomach and intestine make up the human digestive system, an example of an organ system. 7 Organisms, populations and communities. In a park, each person is an organism. Together, all the people make up a population. All the plant and animal species in the park comprise a community. 8 Ecosystems: The ecosystem of Central Park in New York includes living organisms and the environment in which they live. 9 The biosphere: encompasses all the ecosystems on Earth.
Figure 2.7 From an atom to the entire Earth, biology examines all aspects of life.“molecule”: modification of work by Jane Whitney; “organelles”: modification of work by Louisa Howard; “cells”: modification of work by Bruce Wetzel, Harry Schaefer, National Cancer Institute; “tissue”: modification of work by “Kilbad” © Public Domain;
“organs”: modification of work by Mariana Ruiz Villareal, Joaquim Alves Gaspar; “organisms”: modification of work by Peter Dutton; “ecosystem”: modification of work by “gigi4791″ © CC BY (Attribution) “biosphere”: modification of work by NASA © Public Domain

Glossary

abiotic: non living thing (ex. rock)

atom: a basic unit of matter that cannot be broken down by normal chemical reactions

biosphere: a collection of all ecosystems on Earth

biotic: living thing (ex. pine tree)

community: a set of populations inhabiting a particular area

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

eukaryote: an organism with cells that have nuclei and membrane-bound organelles

evolution: the process of gradual change in a population that can also lead to new species arising from older species

homeostasis: the ability of an organism to maintain constant internal conditions

macromolecule: a large molecule typically formed by the joining of smaller molecules

membrane: a thin, flexible layer of tissue that covers, lines, separates, or connects cells or parts of an organism, essentially acting as a barrier that controls what enters and exits a cell or cellular compartment

molecule: a chemical structure consisting of at least two atoms held together by a chemical bond

organ: a structure formed of tissues operating together to perform a common function

organ system: the higher level of organization that consists of functionally related organs

organelle: a small structure that exist within cells and perform specialized functions

organism: an individual living entity

population: all individuals within a species living within a specific area

prokaryote: a unicellular organism that lacks a nucleus or any other membrane-bound organelle

tissue: a group of similar cells carrying out the same function

References

Concepts of Biology – 1st Canadian Edition Copyright © 2015 by Charles Molnar and Jane Gair is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.© 2015 Charles Molnar and Jane Gair

Mt Hood Community College Biology 101 Copyright © 2016 by Lisa Bartee and Christine Anderson is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

License

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ABE 074: Biology Copyright © 2024 by Tacoma Community College is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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