2 Chapter 2: Organization of Life
Before you begin to study the different structures and functions of the human body, it is helpful to consider its basic architecture; that is, how its smallest parts are assembled into larger structures. It is convenient to consider the structures of the body in terms of fundamental levels of organization that increase in complexity: atoms, molecules, macromolecules (e.g. carbohydrates, lipids, proteins, and nucleic acids), organelles, cells, tissues, organs, organ systems, and organisms. It is also important to realize that humans and other living organisms interact with one another and the environment as the hierarchy of biological organization continues upward from organisms to populations, communities, ecosystems, and the biosphere.
Learning Objectives
Organization BioConcept
- You will understand the basic classification system of life and how this system reflects evolutionary relationships. Levels of an organism are cell, tissue, organ, organ system, organism, population, ecosystem.
- You will understand how life is organized into a progressive, hierarchical, and increasingly complex set of systems from the molecular level to the ecosystem level.
- You will be able to provide three examples to explain the emergent properties that arise from these increasingly complex levels of organization.
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, compose