What sequence correctly lists the different levels of biological organization from the smallest and simplest to the largest and most complex?

Learning Outcomes

• Order the levels of organization of living things

Living things are highly organized and structured, following a hierarchy that can be examined 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 one or more chemical bonds. Many molecules that are biologically important are macromolecules, large molecules that are typically formed by polymerization (a polymer is a large molecule that is made by combining smaller units called monomers, which are simpler than macromolecules). An example of a macromolecule is deoxyribonucleic acid (DNA) (Figure 1), which contains the instructions for the structure and functioning of all living organisms.

Figure 1. All molecules, including this DNA molecule, are composed of atoms. (credit: “brian0918″/Wikimedia Commons)

Some cells contain aggregates of macromolecules surrounded by membranes; these are called organelles. Organelles are small structures that exist within cells. Examples of organelles include mitochondria and chloroplasts, which carry out indispensable functions: mitochondria produce energy to power the cell, while chloroplasts enable green plants to utilize the energy in sunlight to make sugars. 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 the reproductive mechanism of 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 or colonial organisms that do not have membrane-bound nuclei or organelles; in contrast, the cells of eukaryotes do have membrane-bound organelles and a membrane-bound nucleus.

In larger organisms, cells combine to make tissues, which are groups of similar cells carrying out similar or related functions. Organs are collections of tissues grouped together performing 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. Mammals have many organ systems. For instance, the circulatory system transports blood through 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 pine trees. All of these pine trees represent the population of 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 sum 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. Keep in mind that the community level only consists of living organisms. The forest itself is an ecosystem; this is the first level that contains non-living aspects of a given area that impact the living things in that environment. An ecosystem consists of all the living things in a particular area together with the abiotic, non-living parts of that environment such as nitrogen in the soil or rain water. At the highest level of organization (Figure 2), the biosphere is the collection of all ecosystems, and it represents the zones of life on earth. It includes land, water, and even the atmosphere to a certain extent.

From a single organelle to the entire biosphere, living organisms are parts of a highly structured hierarchy.

Figure 2. The biological levels of organization of living things are shown. From a single organelle to the entire biosphere, living organisms are parts of a highly structured hierarchy. (credit “organelles”: modification of work by Umberto Salvagnin; credit “cells”: modification of work by Bruce Wetzel, Harry Schaefer/ National Cancer Institute; credit “tissues”: modification of work by Kilbad; Fama Clamosa; Mikael Häggström; credit “organs”: modification of work by Mariana Ruiz Villareal; credit “organisms”: modification of work by “Crystal”/Flickr; credit “ecosystems”: modification of work by US Fish and Wildlife Service Headquarters; credit “biosphere”: modification of work by NASA)

Which of the following statements is false?

1. Tissues exist within organs, which exist within organ systems.
2. Communities exist within populations, which exist within ecosystems.
3. Organelles exist within cells, which exist within tissues.
4. Communities exist within ecosystems, which exist in the biosphere.

Contribute!

Did you have an idea for improving this content? We’d love your input.

Improve this pageLearn More

Updated October 07, 2019

By Nikole Yearout

Biology is the study of life. Since life is such a broad topic, scientists break it down into several different levels of organization to make it easier to study. These levels start from the smallest unit of life and work up to the largest and most broad category.

The levels, from smallest to largest, are: molecule, cell, tissue, organ, organ system, organism, population, community, ecosystem, biosphere.

Molecules are made of atoms, the smallest unit of chemical elements. They can be found in all matter, living and non-living. Molecules make up the most basic structures of living beings. Two biological disciplines that focus on this level are biochemistry and molecular biology.

A cell is the basic unit of life. There are two kinds of cells: plant cells, which have a rigid cell wall made of cellulose molecules, and animal cells, which have flexible cell membranes. Cell biologists consider questions such as metabolism and other questions about structure and function within and between cells.

Tissue is made of cells that work together to perform a certain task. Muscle tissue, connective tissue, and neural tissue are some types of tissue. Histologists are an example of biologists who work at this level.

An organ is a system of tissues that work together on a larger scale to do certain jobs within an animal's body. Examples of organs are the brain, heart and lungs. Anatomy is an example of a biology specialty concerned with this level.

An organ system is a group of organs that work together to perform specific bodily functions. The respiratory system, for example, uses the lungs, airways and respiratory muscles to inhale oxygen and release carbon dioxide in animals. Physiologists study the function of parts of the body as they work together. Though physiologists can work at any level of biological organization, they often answer questions related to organ systems.

An organism is a recognizable, self-contained individual. Organisms can be unicellular organisms such as bacteria or amoebae, or multi-cellular organisms comprised of organs and organ systems. A human being is an example of a multi-cellular organism.

A population is a group of multiple organisms of the same species within a specific area. For example, a pride of lions in Kenya, Africa, is a population.

A community consists of all the different species within a certain area. The population of lions in Kenya, plus the populations of gazelles, giraffes, elephants, dung beetles, and all other species in that area, add up to a community.

An ecosystem is made up of all the communities in a certain area, as well as all the non-living, physical components of the environment. Rocks, water and dirt are a part of an ecosystem. Ecologists may study populations, communities, or whole ecosystems.

The biosphere is all of the ecosystems on Earth added together. Every animal, plant, bacteria, rock, and molecule is a part of the Earth's biosphere. Non-biologists, such as meteorologists and geologists, may join biologists to answer questions at this level of biology organization.