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Sound Writing

Subsection 5.18.2 Introductions

The introduction presents (1) the primary/secondary literature or known scientific background that contextualizes the experiment and (2) the objectives of your current experiment. Introductions generally follow what’s called an ‘inverted triangle’ format: they begin by broadly introducing the topic, then zoom in on the specific focus, then identify the unresolved question you’re trying to answer, and end with the objectives of the experiment you performed to answer that question. In biology, the introduction should discuss published peer-reviewed studies in this line of research, and how their results lead into the experiment you did. In chemistry, intro level lab reports may only require background information from your textbook, while higher level reports should include primary literature. Introductions in biology often start with a broader focus, while in chemistry they come more quickly to the specifics of the experimental work.

Note 5.18.8. Literature for intro-level lab reports.

For lab reports in intro-level science classes, your experiment probably doesn’t make ground-breaking contributions to the field, so justifying the pressing questions it addresses can feel slightly forced. However, scientific literature that will give context to your work definitely exists; if you’re having trouble finding it, ask your professor, a tutor, or a librarian.

Note 5.18.9. Quoting in lab reports..

Avoid quotes in lab reports unless copying the exact language used by your source is vital. Most evidence comes in the form of paraphrase and numerical results instead. Remember that you still have to cite paraphrased information!

Note 5.18.10. Variation within fields.

Some courses, like biochemistry and molecular biology, may combine approaches from biology and chemistry. These biology courses, for example, are more methodologically focused and reports may be less broad. If you’re uncertain what’s expected, always ask your professor!

Example 5.18.11. Introduction: Biology.

The primary source of energy for nearly all life comes from the sun. Plants use photosynthesis to transform light radiation from the sun, carbon dioxide, and water into usable chemical energy stored in sugars. Photosynthesis is a process involving light-independent and light-dependent reactions that occur in the chloroplast of a plant cell. In light-dependent reactions, the pigment chlorophyll absorbs light energy in the form of a photon. When the energy is absorbed by chlorophyll, it causes an electron to “jump” to its next orbital. This causes photosystem II to begin. Through PSII, water molecules are split to replace the electron and oxygen is released as a waste product. The electron is then used to provide energy to create a H+ gradient that ATP Synthase uses to transfer energy in the form of ATP. The next step in light-dependent reactions is through photosystem I. In PSI, an electron from PSII replaces the “lost” electron. This electron then moves through an electron transport chain where its last electron acceptor is NADP, becoming energy in the form of NADPH. The ATP and NADPH produced through these light-dependent reactions are then used to complete the light-independent stage two of photosynthesis, or the Calvin cycle (Campbell et al. 2009). For the purposes of this experiment, we will focus solely on the light-dependent reactions that occur during photosynthesis.
This first paragraph begins broadly by establishing the context and significance of photosynthesis as a biological process. In biology papers, breadth is important, because biology is concerned with complex living systems. Quickly, the author transitions into providing more focused background information on the details of photosynthesis that are relevant to this experiment. Arguably, some of the details included here aren’t actually important to the experiment, which just requires an understanding of the importance of light and production of Oxygen.
Past research has shown that an increase in light intensity has a positive effect on photosynthetic activity specifically in six species of marine phytoplankton. Researchers varied the amount of light each phytoplankton was exposed to and respective \(\text{O}_2\) levels were recorded. The results showed a direct relationship between the increase in light intensity and \(\text{O}_2\) production up to a certain point. After this peak was reached, \(\text{O}_2\) levels leveled off. This suggests that light does not always directly equal photosynthetic activity (Falkowski and Owens, 1978).
This paragraph reviews published, peer reviewed literature related to the lab topic, which can be found through your university library. This example does a good job summarizing the main take-aways of the study instead of leaving readers to try to figure them out themselves and also uses paraphrasing effectively. While the citation used here is correct, the writer could also choose to credit the authors of the study more directly. This might look like opening the paragraph with the phrase “In their 1978 study, Falkowski and Owens showed that an increase in light intensity…”
For an intro-level lab like this, only citing one study may be just fine. However, for a 200+ level class, find several articles that lead to your research question. What unknowns or gaps in literature does your experiment address? Maybe the relationship between light intensity and photosynthesis has been thoroughly studied in phytoplankton, but your experiment contributes by studying it in plants instead. Even if your experiment is investigating things that are already well understood in existing science, published studies provide context. Be sure to make the connections between studies you cite and your own experiment clear. A more advanced lab report would have several paragraphs about different elements of important background literature, each containing information from a few published studies that lead to the question proposed by the lab.
In this study, we examined photosynthetic activity at different light intensities in the aquatic plant Elodea canadensis. Because light energy is essential to begin photosynthesis, we hypothesize that increasing the light intensity will subsequently increase photosynthetic activity, showing an increase in the level of dissolved oxygen. Conversely, the plants exposed to no light in this experiment should be forced to use oxygen to survive, showing a decrease in the level of dissolved oxygen. Additionally, we hypothesize that the plants may reach their peak production capacity making the increase in light intensity no longer advantageous after this peak and therefore will not increase photosynthetic activity past this point.
The final paragraph introduces the study you did, your hypothesis, and your predictions. While you don’t need to include details of the methods, you should include a quick overview and definitely introduce your study system (or the plant/animal/cell type/other you worked with). This example introduces the study system (Elodea canadensis) but it would be stronger if it included some justification of why this is a good study system for the experiment at hand. This could be super simple, like “ the aquatic plant Elodea canadensis, which was selected because it is well-studied and easy to care for in the lab (citation needed).”
This student explains the reasons for their predictions and connects photosynthesis to the variable measured, dissolved oxygen. The prediction is directly related to the results of the published study cited above, but also novel because it uses a different organism. This paragraph could be more clear about what the study will achieve, although this can be difficult to articulate for labs that aren’t actually novel research. An example would be adding this sentence to the end: “This research improves our understanding of the limits of photosynthesis for plants like Elodea Canadensis and will help determine sunlight needs and optimum growing conditions for this species.”

Tip 5.18.12.

Including a quick overview of the methods of studies you cite helps the reader understand similarities and differences with the current experiment.

Example 5.18.13. Introduction: Chemistry.

Gasses can be difficult to identify due to their similarities in appearance. However, various physical characteristics of gases are well-documented and can be used for identification. These include molar mass, flammability, smell, and reactivity with other chemicals.
Determining the molar mass of a gas is possible thanks to the ideal gas law, PV=nRT, which was created by Émile Clapeyron in 1834 by combining previous gas laws (Flowers et al. 2015). The ideal gas law can be used to determine how many moles of a known gas or gas mixture, like air, fit into a flask. Italian chemist Amedeo Avogadro determined that an equal volume of two different gasses at the same temperature and pressure would consist of the same number of moles, so by calculating how many moles of a known gas fit in a container we can assume that the same number of moles of an unknown gas fit in the same container under equivalent environmental conditions (Flowers et al 2015). By determining the number of moles and the mass we can determine the molar mass, an important clue to identify the gas.
In this lab, we will use standard techniques including flame tests and determining molar mass to examine the properties of three unknown gasses. The properties of the unknowns will be compared to literature values for different gasses in order to identify them.
For lab reports in general Chemistry, you may not be expected to write an introduction. This is because these labs are focused on learning experimental methods and chemical principles, so less background is required to place them in the context of scientific literature in the field. This introduction focuses on giving relevant background information on the techniques used and laying out the objective of the lab, and is notably shorter than the biology example. The only cited source is the textbook. For upper division chemistry work, you should provide more substantive background information, including citing the original papers that pioneered this synthesis, experiment, etc. Also look into how the experiment applies to real life of the field more broadly: does it have functional applications, or is it used in making everyday products? Still, introductions for chemistry lab reports will generally start less broad than those for biology.
The last paragraph addresses the objective of the lab. In this example, hypotheses or predictions are not included because there aren’t yet any clues as to the identity of the gasses.