Biology is studied at a wide variety of scales, although not to the same degree as physics and chemistry. Biology is, in a sense, "downstream" from physics and chemistry, insofar as biology is limited by the laws of chemistry and physics, but they are not limited by biology. In a simpler perspective, biology is the study of life, and is therefore limited to the scale of life itself.
- Molecular biology studies life at the level of its composite molecules, such as proteins, DNA, ligands, sugars and so forth. The scale of this perspective is on the order of nanometers (10^-9 meters) The importance of this level is that these are the mechanisms by which life functions; it can be seen as a way of understanding the "alphabet" of biology, which is then constructed into more complicated "language" by the living organism and the processes it undergoes.
- Cellular biology studies cells, which can include single-celled organisms, or specialized cells in a multicellular eukaryote. Physically this spans a fairly large range of sizes because cells themselves can be several hundred times different in size, in addition to eukaryotic cells being significantly more complex and varied. Cellular biology helps us to understand how life organizes its molecular machinery into task-oriented functions.
- Anatomy and physiology is something of a bridge between the microscopic and macroscopic scales, to the point where you can talk about things that are visible and familiar to most of us, such as bones. At this point you can begin to integrate human experiences directly instead of by analogy; for example I wouldn't need to use my "alphabet" analogy to talk about bones, because I can just show you one and we can experiment directly upon it.
- Ecology studies entire ecosystems, which can range in size up to portions of a continent. At this scale, biology necessarily looks at influences from other factors, such as weather. Since this not only incorporates all of the other preceding scales of biology, but also other sciences, it can be difficult to clearly identify cause and effect relationships.
- Evolution and ecology also look into the perspective of time; since biology currently exists only on the Earth, this could be seen as the equivalent of the "universe" scale found astronomy or physics.
Several of these realms overlap and merge depending upon what is being studied; for example, molecular evolution combines molecular biology with evolution in order to study exactly how certain molecules (DNA, proteins etc) may have changed over time in order to lead to the physiological structures studied in cell biology or anatomy.