2.1 Levels of Organization of 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, containing protons and neutrons, surrounded by electrons (Figure 2.2). 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 biomolecules (also called macromolecules), which are large molecules that are typically formed by combining smaller units called monomers. An example of a biomolecule is deoxyribonucleic acid (DNA) (Figure 2.3), which contains the instructions for the functioning of the organism that contains it. Other important biomolecules used by living organisms besides nucleic acids include proteins, carbohydrates, and lipids.

Figure 2.3 A molecule, like this large DNA biomolecule, is composed of atoms. (credit: “Brian0918″/Wikimedia Commons). To see an animation of this DNA molecule, click here (http://openstaxcollege.org/l/rotating_DNA2) .

Some cells contain aggregates of biomolecules surrounded by membranes; these are called organelles (Figure 2.4). Organelles are small structures that exist within cells and perform specialized functions (ex- mitochondria make ATP, chloroplasts make glucose by photosynthesis). All living things are made of cells; the cell itself is the smallest fundamental unit of structure in living organisms. Cells have all of the properties of life, which include that they are composed of biomolecules, the ability to metabolize, composed of cells, maintain homeostasis, respond to external stimuli, grow and reproduce, and evolve. (Viruses are not considered living because 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 (Figure 2.5); in contrast, the cells of eukaryotes do have membrane-bound organelles and nuclei (Figure 2.4).

a. Animal Cell

b. Plant Cell

In most multicellular organisms, cells combine to make tissues, which are groups of similar cells carrying out the same function (ex- muscle tissue, nervous tissue, epithelial tissue, connective tissue). Organs are collections of two or more tissue types 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 (Figure 2.6), 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.

It is important to note that the structural arrangement and functional properties of one level in the hierarchy of biological organization are important to the structure and function of the next level. New properties emerge (called emergent properties) at the next level as the individual parts interact that are not seen at the levels below. For example, cells such as neurons do not have the property of thinking. But if the neurons are arranged into nervous tissue and then into the organ, the brain, the emergent property of thinking becomes evident.