<h2 data-type="title">Levels of Organization</h2>
<p>Living things are highly organized and structured, following a hierarchy that can be examined on a scale from small to large. In general, the <strong>atom</strong> is the smallest and most fundamental unit of matter. It consists of a <a data-lesson="nucleus-2" data-subject="biology" data-tutorial="cell-biology" href="https://app.ulearngo.com/biology/cell-biology/nucleus-2">nucleus</a> surrounded by electrons. Atoms form molecules. A <strong>molecule</strong> is a chemical structure consisting of at least two atoms held together by one or more chemical bonds.</p>
<p>Many molecules that are biologically important are <strong>macromolecules</strong>, 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) (<a href="#attachment_95059">see image below</a>), which contains the instructions for the structure and functioning of all living organisms.</p>
<div class="wp-caption alignnone" id="attachment_95059" style="width: 544px"><a href="https://data.ulearngo.com/assets/081ea91c-6247-47fe-9e63-0da54c174812"><img alt="dna-molecule" class="size-full wp-image-95059" height="940" src="https://data.ulearngo.com/assets/081ea91c-6247-47fe-9e63-0da54c174812" width="544"/></a><p class="wp-caption-text">All molecules, including this DNA molecule, are composed of atoms. Image credit: “brian0918”/Wikimedia Commons</p></div>
<h3>Organelles and Cells</h3>
<p>Some cells contain aggregates of macromolecules surrounded by membranes. These are called <strong>organelles</strong>. Organelles are small structures that exist within cells. Examples of organelles include <a data-lesson="mitochondria" data-subject="biology" data-tutorial="introducing-the-cell" href="https://app.ulearngo.com/biology/introducing-the-cell/mitochondria">mitochondria</a> and chloroplasts, which carry out indispensable functions: mitochondria produce energy to power the cell, while chloroplasts enable green <a data-lesson="nutritional-adaptations-of-plants" data-subject="biology" data-tutorial="soil-plant-nutrition" href="https://app.ulearngo.com/biology/soil-plant-nutrition/nutritional-adaptations-of-plants">plants</a> to utilize the energy in sunlight to make sugars.</p>
<p>All living things are made of cells. The <strong>cell</strong> 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.)</p>
<p>Some organisms consist of a single cell and others are multicellular. Cells are classified as <a data-lesson="summarizing-prokaryotic-cell-division" data-subject="biology" data-tutorial="cell-reproduction" href="https://app.ulearngo.com/biology/cell-reproduction/summarizing-prokaryotic-cell-division">prokaryotic</a> or eukaryotic. <strong>Prokaryotes</strong> are single-celled or colonial organisms that do not have membrane-bound nuclei. In contrast, the cells of <strong>eukaryotes</strong> do have membrane-bound organelles and a membrane-bound nucleus.</p>
<h3>Tissues, Organs and Organ Systems</h3>
<p>In larger organisms, cells combine to make <strong>tissues</strong>, which are groups of similar cells carrying out similar or related functions. <strong>Organs</strong> are collections of <a data-lesson="complex-tissue-structure" data-subject="biology" data-tutorial="introducing-animal-diversity" href="https://app.ulearngo.com/biology/introducing-animal-diversity/complex-tissue-structure">tissues</a> grouped together performing a common function. Organs are present not only in animals but also in plants.</p>
<p>An <strong>organ system</strong> is a higher level of organization that consists of functionally related organs. Mammals have many organ systems. For instance, the <a data-lesson="circulatory-system" data-subject="biology" data-tutorial="human-biology" href="https://app.ulearngo.com/biology/human-biology/circulatory-system">circulatory system</a> transports <a data-lesson="blood-2" data-subject="biology" data-tutorial="human-biology" href="https://app.ulearngo.com/biology/human-biology/blood-2">blood</a> through the body and to and from the lungs; it includes organs such as the <a data-lesson="heart" data-subject="biology" data-tutorial="human-biology" href="https://app.ulearngo.com/biology/human-biology/heart">heart</a> and blood vessels.</p>
<h3>Organisms, Populations and Communities</h3>
<p><strong>Organisms</strong> are individual living entities. For example, each tree in a forest is an organism. Single-celled <a data-lesson="needs-of-prokaryotes" data-subject="biology" data-tutorial="prokaryotes-bacteria-archaea" href="https://app.ulearngo.com/biology/prokaryotes-bacteria-archaea/needs-of-prokaryotes">prokaryotes</a> and single-celled <a data-lesson="characteristics-of-eukaryotes" data-subject="biology" data-tutorial="protists" href="https://app.ulearngo.com/biology/protists/characteristics-of-eukaryotes">eukaryotes</a> are also considered organisms and are typically referred to as microorganisms.</p>
<p>All the individuals of a <a data-lesson="summarizing-community-ecology" data-subject="biology" data-tutorial="population-community-ecology" href="https://app.ulearngo.com/biology/population-community-ecology/summarizing-community-ecology">species</a> living within a specific area are collectively called a <strong>population</strong>. For example, a forest may include many pine trees. All of these pine trees represent the <a data-lesson="population" data-subject="biology" data-tutorial="ecology-overview" href="https://app.ulearngo.com/biology/ecology-overview/population">population</a> 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.</p>
<p>A <strong>community</strong> 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.</p>
<div class="wp-caption alignnone" id="attachment_95060" style="width: 345px"><a href="https://data.ulearngo.com/assets/9f95ff7e-b3d0-451a-b9f9-b6e09be3e8e6"><img alt="levels-of-organization" class="wp-image-95060 size-full" height="1000" src="https://data.ulearngo.com/assets/9f95ff7e-b3d0-451a-b9f9-b6e09be3e8e6" width="345"/></a><p class="wp-caption-text">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. Image Attribution: OpenStax Biology (<a href="https://creativecommons.org/licenses/by/4.0/">CC BY 4.0</a>)</p></div>
<h3>Ecosystems and the Biosphere</h3>
<p>The forest itself is an ecosystem. An <strong>ecosystem</strong> consists of all the living things in a particular area together with the abiotic, non-living parts of that environment such as <a data-lesson="nitrogen-cycle" data-subject="biology" data-tutorial="ecology-overview" href="https://app.ulearngo.com/biology/ecology-overview/nitrogen-cycle">nitrogen</a> in the soil or rain water.</p>
<p>At the highest level of organization (<a href="#attachment_95060">see image above</a>), the <strong>biosphere</strong> is the collection of all ecosystems. In general, it represents the zones of <a href="https://app.ulearngo.com/tutorials/introducing-study-of-life/what-is-life">life</a> on earth. It includes land, water, and even the atmosphere to a certain extent.</p>

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Levels of Organization of Living Things

Biology studies life and living organisms. Biology can be divided into different sections that cover the morphology, physiology, anatomy, behavior, and so on of living organisms.

Biology

Explore the foundations of biology as a science, including scientific reasoning, the scientific method, controlled experiments, and reporting scientific work to gain an understanding of the properties of life, levels of organization, and diversity of living things.


Levels of Organization of Living Things

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