Physiology is the study of how organisms function within the limits of their anatomy. In multicellular organisms, physiology is the study of organ systems and how they work. In microorganisms, it is the study of biochemistry, for these simple life forms operate primarily at a biochemical level.
The simplest microorganisms are viruses. They consist of single strands of RNA or DNA, containing just a few genes. These genes carry the instructions for taking over the cells of higher organisms and causing them to manufacture more viruses. It all happens at a chemical level. Viruses are inactive until they find a host. They cannot carry out normal cell metabolism on their own but need to invade a host cell to do this. That is why they are considered only half alive. Once they have invaded a cell, their DNA takes over the cell, which then functions solely for the purposes of viral survival and reproduction. As one author put it: "Their 'life cycle' is an interesting study of deception, pillage, and piracy."*
In bacteria, the physiological processes are more complicated but still basically biochemical in nature. Bacteria have no nuclei in there cells, nor are there other organelles such as ribosomes or mitochondria. Even so, bacteria can still carry out normal cellular physiological processes such as transport of food and wastes into and out of the cell, reproduction, as well as the manufacture of complex organic molecules such as fats, proteins, carbohydrates, DNA and RNA.
Bacteria use their DNA instruction sets to create the cell membranes and proteinaceous cytoplasm that form the bodies of these creatures. Those instructions enable each cell to carry out basic heterotrophic functions: movement, growth, food capture and digestion, respiration and reproduction. All these processes are primarily biochemical in nature. Respiration, for instance, involves the chemical breakdown of food to release energy, producing carbon dioxide as a waste product. All the chemicals necessary for the bacteria to live are manufactured and float freely in the cytoplasm.
True cells are more complicated physiologically and anatomically than bacteria. True cells have organelles where specific processes occur, which makes the cells far more efficient. Protozoans have a nucleus whose physiological function is cell control. DNA is stored there in chromosomes. All the functions of the cell, such as growth, movement and reproduction, begin with instructions from the genes. These are biochemical blueprints for making all the chemicals necessary for the normal functioning of the organism: proteins for cytoplasm; proteins, fats and carbohydrates for cell membranes and a number of other organelles with specific physiological roles in the cell.
Ribosomes, for instance, are the structures where proteins are manufactured. When the cell needs proteins for growth, the nucleus provides the genetic instructions and the ribosomes do the work. The new proteins are then deployed where they are needed the most. Another cellular organelle is the endoplasmic reticulum which acts as a 'highway' in the cell to move materials from one part of the cell to another.
Mitochondria are the powerhouses of the cell, where the biochemical process of respiration takes place. This is where food is broken down to release energy stored in ATP molecules. Food stored as glucose is converted to water and carbon dioxide and the energy released is used by the cell for movement and other physiological processes. The food has already been broken down by lysosomes, which use enzymes to break up large molecules. Lysosomes can also break down waste products prior to ejection through the cell's membrane.
When one first looks at microbes, they seem to be so simple in structure that it is easy to think that they must also be simple in function. But microbial physiology is complicated because it is all happening at a biochemical level and this requires an understanding of biochemistry, which is neither simple nor straightforward. Microbes have been evolving as biochemical factories and powerhouses for billions of years. They can do things that we are only just learning how to do in laboratories. It took years to unravel the anatomy of a DNA molecule and years more to figure out how the genes work, how they pass those instructions on to other parts of the cell, and how those instructions are used to make all the chemicals necessary for those cells to carry out all their diverse functions.
Similarly the physiological processes of photosynthesis and respiration have taken years of study to unravel and understand: How cells use pigments and enzymes to make sugars; how energy is stored and released; how it is used and how the waste products are removed. Cellular physiology is complicated and we are only just beginning to understand how these seemingly simple organisms operate.
References: * http://www.infoplease.com/cig/biology/viruses.html