Although this may come as a surprise to many, the introduction section of a report should be one of the last sections written. In writing the Materials and Methods, Results, and Discussion sections, you have outlined the issues that your report discusses. The introduction sets the framework for the entire report and shows the readers (and your professors) that you understand the purpose of the study you have done. Explaining the overall purpose of the experiment is the most important part of the introduction and is generally used to conclude this section (Pechenik, p. 95). In the examples presented here, the reports are written on experiments rather than studies. Experiments always involve the testing of a specific hypothesis, whereas studies do not. Insect collections, simple observations, and any work done that does not require manipulation is referred to as a study rather than an experiment (Pechenik, p.97).

Stating the question

One very important part of the introduction section is outlining the purpose of the experiment as concisely as possible. Stating the question or questions that are to be answered by the experiment can easily be introduced with the phrase "In this experiment" or "In this study" and then explaining from there. These statements should be as specific as possible to demonstrate a clear understanding of the experiment. The purpose of these statements is to explain what the experiment does and how the results will be interpreted. The use of the personal plural (we) is acceptable to use in the introduction and present or past tense can be used in the introduction section. Either active voice (we measured) or passive voice (it was measured) can be used depending on which the professor prefers.

Once the question that the experiment attempts to answer has been stated, the background information (p. 2) needs to be given to show why the question was asked. The general guidelines for writing about the background information can be found in A Short Guide to Writing about Biology pp. 98-101, so refer to this book for further examples and explanations.

General guidelines for writing the background information of an introduction section

1. Back all statements of fact with a reference to your textbook, laboratory manual, outside reading, or lecture notes. Some form of internal citation is generally used for this.

2. Define specialized terminology. Any terms that are used within the report that are necessary for understanding the report should be defined within the introduction. For more basic biology classes, most of the scientific terms need to be defined because they are new to the writer. In higher level biology courses the terms that are assumed to be understood do not require definition. A good rule of thumb--if you don't understand a certain term or concept, you need to explain it in your introduction!

3. Never set out to prove, verify, or demonstrate the truth about something. Rather, set out to test, document, or describe. Nothing can be "proven" indisputably in science, and it is important to keep an open mind when interpreting the results of you experiment. If it were not for people looking for the new and unexpected, nothing would ever be discovered!

4. Be brief. Only information that is relevant to the experiment should be presented in the introduction. Any description and explanation that is necessary for understanding the purpose of the experiment should also be included.

5. Write an introduction for the study that you ended up doing. If an experiment is altered by the professor in any way, the introduction and the entire report should be about the experiment actually performed. Be sure to take careful note of any changes made during the experiment as well because this could change the overall purpose of the experiment which the introduction section describes.

The following text includes two samples of an introduction for an enzyme kinetics lab. Italicized words are links to explanations of why that particular part of the introduction is important and what makes the sentences appropriate or in need of improvement.

Sample 1: This study, "Enzyme Kinetics," focuses on the study of enzymes and what makes them work. Enzymes are an important part of every living organism and many studies have been performed on them to try to learn more about how they work. Enzymes are involved in a lot of the digestion processes in the human body. The object of this experiment, however, is to get the substrate, catechol, to the product, benzoquinone, by way of the enzyme, catecholase. Experiment one alters the amount of enzyme to prove that the more enzyme you have, the faster the reaction takes place and a greater amount of product results. Experiment two adds ascorbic acid to lower the pH. The goal of this is to prove that increased acidity stops a reaction. (?)

Sample 2: It is well known that enzymes are catalytic proteins which function to accelerate reactions by lowering the activation energy (Campbell, 1996). An enzyme is very specific in the reactions in which it undergoes: it contains an active site that allows only certain reactants, known as substrates, to bind to it (Campbell, 1996). In the first experiment, referred to as the variable enzyme experiment, we examined the rate of reaction of catechol and oxygen to form benzoquinone when the amounts of the enzyme (catecholase) were varied. We hypothesized that enzyme amount affects reaction rates and thus we expected that reactions with increased amounts of enzyme relative to the amount of substrate will have a greater net conversion of substrates than those reactions with a lesser ratio of enzyme to substrate.

Likewise, in order to maintain its specific function, an enzyme must retain the specialized shape of its active site (Campbell, 1996). Environmental factors such as ionic concentration and pH have been known to alter the conformation of a protein and subsequently its active site conformation. In this experiment, referred to as the variable pH experiment, we examined the rate of reaction of catechol and oxygen again, but this time when the pH was varied. It was expected that the reactions that occurred in a fairly neutral pH would convert more substrates than those reactions which were in an acidic environment of pH 4.

Explanations of the Example Links

Enzymes: In sample one the writer only refers to enzymes as an "important part of every living organism." This gives no information about the later use of the terms such as enzyme and substrate and this type of specialized terminology should be defined in an introduction. (return to Sample 1)

Digestion: This sentence does not belong in the introduction section because the experiment does not deal with any type of digestive enzymes, nor does it matter that other studies have been performed on enzymes unless they directly relate to this particular experiment. All information presented in an introduction should be relevant to the report. (return to Sample 1)

Experiment: This sentence is an attempt to state the question that the experiment tries to answer; however, it only summarizes what the experimenters actually did rather than what the purpose of the experiment was. It is true that the experiment altered the concentration of the enzyme, but the reason behind it was to observe the effects of those changes on reaction rates. Information pertaining to the purpose of the experiment is the type of information that this statement should contain. (return to Sample 1)

Goal: The writer makes a serious mistake by assuming that the experiment is going to prove something about enzymes. In biology nothing is proven, especially not by one experiment, so in writing a report it needs to be explained that the experimenters only observed the experimental results and then interpreted them. (return to Sample 1)

No reference: In the report writing sample one there are no references to any outside sources, whereas sample two refers often to a text by Campbell. All factual statements should be backed with references to show that the information has been obtained from a credible source. (return to Sample 1)

An enzyme: This is a good example of defining specialized terms that are important to the experiment. This definition of an enzyme gives enough information so that the reader can understand the purpose of the experiment, but not so much information that it does not apply to the experiment. (return to Sample 2)

In the first experiment: This is a very concise statement of the question that the experiment attempts to answer, and it begins with that most commonly used convention of "in this experiment." This is an appropriate statement because it is specific about the experiment and demonstrates a clear understanding that the purpose is not only to alter the amounts of catecholase but, in addition, to observe how these changes effect reaction rates. (return to Sample 2)

Envirnmental: This explanation of the relationship between the shape of a protein and the utilization of its active site is important to understanding how pH could affect enzyme activity. Introductions should always contain the information necessary to understanding the entire experiment and report. This may depend on the level of the course because in beginner biology classes the professors will want explanations of more terms and techniques that are considered assumed knowledge in higher level courses. When in doubt, ask your professor how specific you should be in the introuction section. (return to Sample 2)

In this experiment: This is also a very good example of stating the purpose of the experiment because it is specific about the experiment, varying pH, and it shows that the expected results would be a change in reaction rate. (return to Sample 2)

Return to "Biology" opening page

All citations from Pechenik, Jan A. A short guide to writing about Biology. pp. 54-102, Tufts University: Harper CollinsCollege Publishers. 1993.

For additional instruction on writing in biology, see John Bishop's page.