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Coming from two Greek words, "bios," meaning "life," and "logos," meaning "study," biology refers to the study of life and living organisms. Therefore, biology studies the characteristics of plants, animals, and microorganisms, and their functions. Now why do these plants, animals, and microorganisms have life? These organisms have life because of all of them can grow, are structurally composed of cells, have the ability to use energy to sustain life, hold the capacity to adapt to changes in the environment, respond to stimuli, and can produce offspring. These characteristics are manifestations that an organism is living or has life.
With these characteristics of life, the following five basic principles serve as the foundation of biology: cell theory, gene theory or genetics, evolution, homeostasis, and thermodynamics.
Cell theory, gene theory, and evolution are very much related. Cell theory discusses that all living organisms, plants, animals, and even microorganisms, are made up of cells. Cells are therefore considered the basic unit of life. Every cell contains organelles that are involved in metabolism and in cell division. And one of these organelles, which is called nucleus in most organisms, contains hereditary information known as deoxyribonucleic acid (DNA). Subunits of DNA are known as genes in which traits are inherited from the parent to its offspring. This principle encompasses gene theory.
The characteristics or traits present in the genes are not constant. Changes due to certain internal and environmental circumstances may occur in the genes of a certain organism which can be inherited to succeeding generations. This theory of evolution also believes that all living organisms came from a common ancestor.
Some of the cell's organelles, as mentioned earlier, are involved in metabolism. These contribute to various systemic functions a living organism is undergoing. This is to achieve homeostasis, another basic principle in biology which refers to the ability of a living organism to regulate its internal environment to make it constant despite changes in the external environment.
Lastly, the law of thermodynamics is also known as the principle of energy in some references. This principle simply means that constant energy is needed in order to sustain life. Some bodily functions also use and give off energy to help the organism to adapt to its surroundings.
There are many branches in biology, since this field has a very wide scope. Some of them are categorized according to the type of organism studied. Included in this category are botany, which deals with the study of plants; zoology, the study of animals; microbiology, the study of microorganisms; and mycology, the study of fungi.
There are also branches that are focused on the study of the five basic principles of biology. Cellular biology refers to the study of life at the cellular level and this includes the characteristics and functions of the cells and its organelles. Meanwhile, genetics deals with heredity and how traits are inherited from one generation to another. Physiology, which is the study of bodily functions in a living organism, can be utilized in examining how an organism is able to achieve homeostasis.
Some branches are considered subdisciplines under a broader branch. For example, entomology, the study of insects; mammalogy, the study of mammals; and ornithology, the study of birds may fall under zoology. On the other hand, some branches are classified according to the link of biology with other fields of science. Biochemistry (study of life at the chemical level), bioinformatics (use of technology in analyzing biological data), and biophysics (study involving application of principles of physics to the study of life) are examples of this.
So far, biology is discussed as the study of life and living organisms. At present, knowing that until now life does not have a clear-cut definition, biologists and even ordinary people continue their quest to discover life.