Levels of Biological Organization
Living things are highly organized and structured, following a scale hierarchy from small to large (Figure 1). The atom is the smallest and most fundamental unit of matter. It consists of a nucleus surrounded by electrons. Atoms combine to form molecules, which are chemical structures consisting of at least two atoms held together by a chemical bond. In plants, animals, and many other types of organisms, molecules come together in specific ways to create structures called organelles. Organelles are small structures that exist within cells and perform specialized functions. As discussed below, all living things are made of one or more cells.
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.
All the individuals of a species living within a specific area are collectively called a population. A community is a set of different populations inhabiting a common area. For instance, all of the forest’s trees, flowers, insects, and other populations 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, representing the life zones on Earth. It includes land, water, and portions of the atmosphere.
What is the basic building block of that wall? It is a single brick, of course. Like a brick wall, your body is composed of basic building blocks, and the building blocks of your body are cells. Your body has many cells, each specialized for a specific purpose. Just as a home is made from various building materials, the human body is constructed from many cell types. For example, bone cells help to support and protect the body. Cells of the immune system fight invading bacteria. And red blood cells carry oxygen throughout the body. Each cell type plays a vital role during the body’s growth, development, and day-to-day maintenance. Despite their enormous variety, however, all cells share certain fundamental characteristics.
The microscopes we use today are far more complex than those used in the 1600s by Antony van Leeuwenhoek, a Dutch shopkeeper with great skill in crafting lenses. Despite the limitations of his now-ancient lenses, van Leeuwenhoek obs