A student watches a pulsing microbe through the lens of her microscope. Another slices open the pickled body of a fetal pig. Yet another notes the red spots that have spread across a petri dish filled with agar. Scenes such as these unfold daily in middle- and high-school biology labs around the world. For some students, such classes are the only laboratory experience they will ever have. For others, it is the first step in a lifetime of biological research. In either case, the hands-on adventure of experimental biology can forever change the way students view their world and analyze what they see and hear around them.
What is experimental biology? It is a way of studying life and testing our ideas about it according to scientific principles. On one level, people have been engaging in experimental biology for as long as humans have been nibbling on potential foods to distinguish the tasty from the nasty. Today, we have experimental biology to thank for thousands of lifesaving drugs and medical treatments, as well as our awareness of the dangers of toxic chemicals in our environment and food supply. Indeed, experimentation and lab work lie at the heart of virtually all modern-day biological research.History of Experimental Biology. Ironically, the world's first biologists had little interest in experimentation. More than 2,300 years ago, the recognized "father" of biology—the Greek philosopher-scientist Aristotle—exalted the application of pure logic to solve scientific problems. Logical thinking alone could reveal all truth, he taught. Unfortunately, what Aristotle failed to realize was that the human mind can come up with correct answers only when given all the facts.
Consequently, superficial observations combined with rigid Aristotelian logic produced many bizarre beliefs such as "horsehair dropped in a pail of milk produces snakes." This bit of twisted thinking came from the observation, by Aristotle or one of his pupils, that A) snakes do not normally emerge from milk; and B) a snake was seen to emerge from milk after a horsehair dropped into it. What Aristotle and his students did not do was to carefully test their conclusions with systematic experimentation.
Aristotelian logic continued to dominate scientific thinking for nearly 2,000 years. Only after the Middle Ages did Renaissance thinkers begin challenging and testing the ideas that had so long been held sacred. Prominent among them was the 17th-century Italian scientist and physician Francesco Redi.
In his most famous experiment, Redi disproved the widely held belief that maggots arise spontaneously from rotting meat (with no need for parent flies). Specifically, he devised an experiment in which he placed samples of meat in openmouthed jars, then covered some jars with mesh to exclude flies. As Redi predicted, only the meat in the uncovered jars became maggoty. Redi showed that the maggots that appear on rotting meat do not arise spontaneously, but in fact hatch from tiny eggs laid by passing flies.
Most importantly, Redi's clever demonstration gave biology the "controlled experiment," designed to isolate and test the effect of changing a single variable—in this case, the flies' access to the meat. For this invaluable contribution, Redi has been widely recognized as the founder of experimental biology. Today, experiment remains a core part of the scientific method, which in turn is the basis for all reliable scientific inquiry.Principles of Scientific Inquiry. Experimental biology rests on three basic assumptions, or principles, that distinguish scientific investigation from more-subjective, or personal, studies such as art and religion. The first of these principles is that of objectivity—namely, the assumption that different people will perceive the results of a well-designed experiment in generally the same way. For example, if the end result of an experiment is a strip of pH paper turning pink or blue (to indicate relative acidity), the scientist assumes that everyone looking at the pH paper can agree on the result. Contrast this with the personal value systems involved in judging the quality of a painting or the morality of a behavior.
The second principle underlying experimental science is that of natural causality—the assumption that the results of an experiment trace to natural causes. This contrasts with belief systems that attribute events to supernatural forces, as in sickness being the consequence for a curse or sin. The principle of natural causality also assumes a level of integrity on the part of those involved in a given experiment—namely, that no one has deliberately falsified the results of that experiment.
The final principle underlying experimental science is the uniformity of space and time. We must assume that the laws of nature apply everywhere, anytime. So an experiment performed in one laboratory will have the same result as an identical experiment performed in a comparable laboratory anywhere else. Similarly, we assume that the same laws of nature apply today as they did in the past. This becomes vital, for example, when a biologist extrapolates findings about a present-day population to comparable populations of the past.PERFORMING AN EXPERIMENT
The Scientific Method. When scientists prepare to answer questions through experimentation, they do so following a rigorous set of steps designed to produce reliable and informative results. Known together as the scientific method, the steps include observation, hypothesis, experimentation, and conclusion.
Biologists begin by observing a phenomenon and asking how it came to be. Next, they develop one or more hypotheses, or possible explanations for the phenomenon. For example, the scientists might notice a variation in size among a group of potted pea plants in a greenhouse. They propose various explanations for the variation, such as differences in soil nutrients, sunlight, water, insect pests, and so on.
Next, they must test their hypotheses through experimentation. They must isolate and alter the one factor they believe could be the cause of the observed growth variation and see what happens. From this, the biologists can proceed to the scientific method's fourth and final step—drawing a conclusion from the results.Controls. By design, an experiment must isolate the variable being studied from other possible variables. In the experiment described above, let us say that the experimenters decide to examine whether the amount of soil-nitrogen content was the determining factor behind variations in plant growth. They must isolate nitrogen's effect from the effects of other growth factors.
To do this, the scientists must have a control—that portion of the experiment in which all possible variables are kept the same. This is for comparison against the experimental portion, in which the biologists alter the variable under study. In the example above, the biologists will want a control group of plants alike in every way except for the addition of nitrogen fertilizer to their soil. The control plants will receive the same amount of water, sunlight, and other soil nutrients, such as potassium and phosphorus, as do the experimental plants, which also receive a set amount of nitrogen fertilizer.
A familiar experimental control is the placebo, or dummy treatment, used in medical trials. By giving a control group of patients a placebo such as a sugar pill, researchers can determine that any benefits experienced by the experimental group are due to the experimental drug they are receiving, and not simply a consequence of their receiving medical attention.Observation and Measurement. The ultimate success of any given experiment depends heavily on two factors: careful observation and accurate measurement. An experiment comparing plant growth, for example, relies on accurate calculation of that growth. This may be as simple as measuring the height of seedlings with a ruler, or it may involve weighing each plant after carefully extracting it from its pot without losing roots or adding dirt in the process. More-sophisticated biology experiments may require the measurement of chemical changes through methods such as titration, gas chromatography, and mass spectroscopy. In all cases, accurate measurement depends on the experimenter knowing her instruments, using them correctly, and carefully recording the information they convey.
Virtually all scientific measurements and laboratory devices today employ metric units—meters for length, grams for mass, liters for liquid volume, and so on. The biology student and the professional biologist must be familiar with such units and their increments (such as micrograms and kilograms). The standard scientific units for temperature are degrees Celsius.Reliable and Reproducible. Experimental biologists seldom compare a single control case with a single experimental case. They compare many specimens or run several trials, for an important reason. When working with a small number of cases, an unseen variable (such as an infertile seed) can skew results. Generally, the larger the number of subjects or repetitions in an experiment, the greater the reliability of its results. Sheer numbers are what separate a potential coincidence such as "I sneezed after I petted a cat" from an experimentally validated observation such as "I sneeze every time I pet a cat" (therefore, I may be allergic to cat dander). A valid biology experiment also must be reproducible. Any competent person should be able to obtain the same or similar results following the same procedures.
Analyzing Data. At times, the experimenter must mathematically analyze data through the methods of statistics. Statistics has been called the art of drawing inferences from the raw data of observation. An inference is a conclusion derived by logic and reasoning based on evidence—in this case, experimental data. Statisticians working in biology may be professionally designated as biostatisticians or biometricians. So they use data to wring meaning from otherwise ambiguous or complicated experimental results.
Unexpected Results. Careful observation, accurate measurement, and sufficient numbers of subjects all help minimize the chances of experimental error—that is, misleading or inconsistent results. Yet even successful experiments often produce unexpected results. For starters, the results of an experiment may prove the initial hypothesis wrong. Far from representing failure, a negative result directs the biologist to look for other variables to explain the phenomenon being studied.
Experimental biology's primary arena is the laboratory, and biology laboratories everywhere have certain basic pieces of equipment and require the mastery of certain important techniques. Tools of the trade include microscopes for viewing small specimens, test tubes and beakers for holding liquids, graduated cylinders and pipettes for measuring liquids, balances and scales for weighing dry materials, petri dishes for culturing microbes in a suitable medium, cutting and pinning instruments for dissection, and spectrometers for analyzing biological reactions involving color change. Biology lab work also involves the careful use of preservatives, reagents, and other chemicals, some that can be dangerous if mishandled.The Microscope. The magnifying device known as the microscope remains as important to the science of biology as the stethoscope is to medicine. The familiar compound light microscope uses visible light, typically from a small built-in lamp, to view a specimen, be it a live microbe, stained cells, or a thin piece of tissue. Many laboratories are also equipped with stereoscopes, which give a greater sense of depth to the specimen; other microscopes work with color or contrast (see also the article "Seeing the Very Small").
Live Culture. In the context of the biology lab, the term "culture" refers to the growth of microorganisms or tissue cells on a prepared medium. Agar, a clear, semiliquid algae extract, is one common growing medium. Typically, it is liquefied and sterilized by heating, and then poured into test tubes or petri dishes before being inoculated with a pure sample of some microorganism. Sterile, or germ-free, technique is especially crucial to the preparation of pure cultures. Nonsterile instruments and exposure to open air both can contaminate a culture by introducing unwanted microbes, or "germs."
Dissection. The time-honored practice of dissection involves taking an organism apart "to see how it works." Worms, frogs, and fetal pigs stand out as the most common subjects for biology students. In addition to their convenient tabletop size, fetal pigs have the benefit of an anatomy quite similar to a human's. So dissecting a pig is the closest most students come to getting a look inside their own bodies.
Dissecting tools include scalpels and laboratory scissors for cutting, and forceps for pinching and pulling. A maxim to remember when dissecting is to never cut blindly or deeply, but instead to use repeated, shallow strokes until you can see what is inside each layer. In this way, you can study how organs and other structures look in their natural positions before they have been dissected out of place. As one wise laboratory instructor once advised: "Although you won't be expected to put it all back together, dissect so that you could do so."
Many school systems allow students to decline participation in hands-on classroom dissections. For such students, the class requirement may be met using a constructed freestanding model or three-dimensional computer visualization of the animals.Scientific Drawing. Whether viewing cells through a microscope, studying a fungal culture, or dissecting an animal, students—like all biologists—must draw what they see. Scientific drawing does not require artistic ability so much as it demands close attention to detail, including proportion, shape, and texture. Most importantly, this type of drawing forces the student to examine a subject thoroughly. (A word of advice: resist the temptation to draw what you think you are supposed to see.)
Computer Modeling. Computers have emerged as a vital part of the modern biology laboratory. Computer technology allows the biologist to simulate processes such as, for example, the growth of a population under different conditions or even the impact of introducing a predator or other disturbance into a given ecosystem.
Given the vast amounts of research data being generated today, scientists welcome computer technology as an aid for computing and organizing material as well as for its ability to model complex structures and processes. Medical researchers use computer models to predict how drugs might interact. Such predictive models are made possible through computer programming based on mathematical equations that accurately describe cause-and-effect relationships. Computer models can likewise serve as stand-ins for live subjects, as when a student chooses to "dissect" a three-dimensional model of a human or an animal to study its anatomy or studies a computer model of a complex biological molecule such as hemoglobin.