Unit One. The Study of Life
1. The Science of Biology
1.3. Organization of Life
The organisms of the living world function and interact with each other at many levels, from the very small and simple to the large and complex.
A Hierarchy of Increasing Complexity
A key factor in organizing these interactions is the degree of complexity. We will examine the complexity of life at three levels: cellular, organismal, and populational.
Cellular Level. Following down the first section of figure 1.4, you can see that structures get more and more complex—that there is a hierarchy of increasing complexity within cells.
1. Atoms. The fundamental elements of matter are atoms.
2. Molecules. Atoms are joined together into complex clusters called molecules.
3. Macromolecules. Large complex molecules are called macromolecules. DNA, which stores the hereditary information in all living organisms, is a macromolecule.
4. Organelles. Complex biological molecules are assembled into tiny compartments within cells called organelles, within which cellular activities are organized. The nucleus is an organelle within which the cell’s DNA is stored.
5. Cells. Organelles and other elements are assembled in the membrane-bounded units we call cells. Cells are the smallest level of organization that can be considered alive.
Organismal Level. At the organismal level, in the second section of figure 1.4, cells are organized into four levels of complexity.
6. Tissues. The most basic level is that of tissues, which are groups of similar cells that act as a functional unit. Nerve tissue is one kind of tissue, composed of cells called neurons that are specialized to carry electrical signals from one place to another in the body.
7. Organs. Tissues, in turn, are grouped into organs, which are body structures composed of several different tissues grouped together in a structural and functional unit. Your brain is an organ composed of nerve cells and a variety of connective tissues that form protective coverings and distribute blood.
8. Organ systems. At the third level of organization, organs are grouped into organ systems. The nervous system, for example, consists of sensory organs, the brain and spinal cord, neurons that convey signals to and from them, and supporting cells.
9. Organism. Separate organ systems function together to form an organism.
Populational Level. Organisms are further organized into several hierarchical levels within the living world, as you can see in the third section of figure 1.4.
10. Population. The most basic of these is the population, which is a group of organisms of the same species living in the same place. A flock of geese living together on a pond is a population.
11. Species. All the populations of a particular kind of organism together form a species, its members similar in appearance and able to interbreed. All Canada geese, whether found in Canada, Minnesota, or Missouri, are basically the same, members of the species Branta canadensis. Sandhill cranes are a different species.
12. Community. At a higher level of biological organization, a community consists of all the populations of different species living together in one place. Geese, for example, may share their pond with ducks, fish, grasses, and many kinds of insects. All interact in a single pond community.
13. Ecosystem. At the highest tier of biological organization, a biological community and the soil and water within which it lives together constitute an ecological system, or ecosystem.
Figure 1.4. Levels of organization.
A traditional and very useful way to sort through the many ways in which the organisms of the living world interact is to organize them in terms of levels of organization, proceeding from the very small and simple to the very large and complex. Here we examine organization within the cellular, organismal, and populational levels.
At each higher level in the living hierarchy, novel properties emerge, properties that were not present at the simpler level of organization. These emergent properties result from the way in which components interact, and often cannot be guessed just by looking at the parts themselves. You have the same array of cell types as a giraffe, for example. Yet, examining a collection of its individual cells gives little clue of what your body is like.
The emergent properties of life are not magical or supernatural. They are the natural consequence of the hierarchy, or structural organization, that is the hallmark of life. Water, which makes up 50%-75% of your body’s weight, and ice are both made of H2O molecules, but one is liquid and the other solid because the H2O molecules in ice are more organized.
Functional properties emerge from more complex organization. Metabolism is an emergent property of life. The chemical reactions within a cell arise from interactions between molecules that are orchestrated by the orderly environment of the cell’s interior. Consciousness is an emergent property of the brain that results from the interactions of many neurons in different parts of the brain.
Key Learning Outcome 1.3. Cells, multicellular organisms, and ecological systems each are organized in a hierarchy of increased complexity. Life's hierarchical organization is responsible for the emergent properties that characterize so many aspects of the living world.