Lab 1 Assignment: Comparing Measured and Expected Protein Concentration

Each weekly writing assignment in this course is designed to develop a different scientific thinking and communication skill. Rather than repeating a standard lab report, each assignment focuses on a distinct part of professional scientific writing—such as interpreting data, writing precise methods, evaluating techniques, or proposing original hypotheses. You are expected to approach each assignment with attention to its specific writing and analysis goals, which are introduced clearly in each assignment prompt.

Lab 1 Assignment objectives: To practice interpreting quantitative results by comparing your measured protein concentration to an expected value, evaluating the statistical significance of any difference, and communicating your interpretation clearly and concisely in the format of a short Results and Discussion section.

Key vocabulary. Be sure to use the following words precisely in your writing.

  • Analyte: A substance being measured by means of a laboratory procedure or test. For example, protein is the analyte for the Bradford assay.      
  • Assay: An assay is a method for measuring the amount of a specific analyte in a sample with reference to a standard. The word "assay" may also refer to each individual experiment. For example, in the Bradford assay, each test tube reaction is an assay. Avoid the common novice mistake of calling each assay a sample.
  • Sample:  A sample is a representitve portion of a larger substance. For example, you were given 1.00 mL of a larger milk sample to analyze. In turn, you took multiple samples from that sample for your analysis. As a style tip, avoid calling your samples "unknowns", simply because the analyte concentration was unknown to you. The word "unknown" is usually reserved for samples of unidentified composition.
  • Standard:  A standard is a solution or specimen containing a known quantity of an analyte. For example, you prepared a series of assays containing BSA standards. These known amounts of BSA allowed you to make a standard curve. Even though your standards are samples in the sense that your standard assays were prepared by sampling a BSA standard solution, you should avoid refering to standards as "samples" to reduce possible confusion in your writing. 

Writing a Results and Discussion Section

The expected structure of this assignment is explained in the Writing a Results and Discussion Section of the Writing Guidelines.

Your "Results and Discussion" section should:

  • Only present the data and analysis of the protein concentration measurement. Any mention of the pipetting and buffer exercises will not be graded.
  • Always begin your Results and Discussion section by reminding the reader of your objectives by affirmatively stating what specific work was accomplished. If your results have high uncertainty, then you should open by stating your aspirational objectives and be clear about the limits of what can be concluded from your results. As a matter of scientific ethics, your reader should be informed upfront if you were unsuccessful or have low confidence in your results, rather than discover this in the concluding paragraphs.
  • Briefly describe what was done to obtain the data and refer to Table 1 (described below). Always place a table or figure after the paragraph of first mention.
  • Table 1. Present a table containing the sample name, absorbance, micrograms measured, concentration of each sample, and undiluted concentration concentration of each (if applicable). The final row should give the mean and standard deviation of the undiluted concentrations. A template for this table layout is given below. Use this format to present replicate data in future labs in this class.
Sample Abs at 595 nm Milk Protein Amount (μg) Protein Concentration Diluted (μg/μl) Sample Protein Concentration (μg/μl)
A 0.### #.## #.## #.##
B 0.### #.## #.## #.##
C 0.### #.## #.## #.##
D 0.### #.## #.## #.##
Mean #.# ± 0.#

    • All tables must be identified by a unique number (e.g. Table 1) followed by a descriptive title. In scientific writing, the identifier and title are above the table.
    • Round your uncertainty to one significant digit and limit the significant digits of your value based on the uncertainty (this applies to all values in your technical writing, not just the table).
    • For more details about numerical presentation, read Measured Values in the Data tab of the Writing Guidelines.
    • For more details about table formatting, read Table Format in the Data tab of the Writing Guidelines.
  • Briefly explain to the reader what Figure 1 (described below) reveals.
  • Figure 1. Present a plot of your standard curve with trendline.
    • All graphics must be identified by a unique number (e.g. Figure 1) followed by a descriptive title and a caption. In scientific writing, the identifier, title, and caption are below the graphic.
    • Your descriptive title should include the concepts "standard curve", "Bradford Assay", and "BSA".
    • Your caption must clearly describe what is plotted by identifying the protein and wavelengths; include trendline equations and R2; and a key to any symbols or line styles.
    • For requirements about what to include in captions for future experiments visit the Captions-Methods tab of the Writing Guidelines.
    • The plot should only present the data for your standards. Do not include attempt to include your unknown milk samples the standard curve plot.
    • Do not include gridlines or legends on your plots in this course. For more details about figure formatting, read Figure Format in the Data tab of the Writing Guidelines.
  • Describe the quality of the Figure 1 data. If outliers were dropped, you must state this and justify your reasons for doing so. If the data were poor, provide an specific explanation (e.g. "human error" is not specific).
  • State your protein concentration result (mean and uncertainty with units).
  • Comment on the quality of the protein concentration with reference to the standard deviation (or whatever measurement of uncertainty you used). When evaluating uncertainty values, you should always consider the relative standard deviation (RSD) rather than the absolute standard deviation.
    $$ RSD = \frac{\textit{standard deviation}}{value} \times 100\% \tag{1} $$ Measurements in the biological sciences tend have higher uncertainty than measurements in the physical sciences due to the inherent complexity and variability of biological systems. In the biological sciences (including biochemistry), <10% RSD is generally precise enough and <5% RSD is excellent.
    • Example 1: The value 4 ± 2 U has 50% error and 4.0 ± 0.3 U has 8% error. Although both values are the same, the high relative error for the former value indicates a poorer quality result.
    • Example 2: The value 4 ± 1 U has 25% error (RSD) and 40 ± 1 U has 2.5% error. Although the SD is the same for both measurements, the precision differs significantly because the values differ. This fact is clearly communicated by the RSD values.
  • Compare the result of your measurement to the the value reported by the producer of the milk.
    • Conclude by discussing any potential sources of error that may have influenced the disparity, such as experimental limitations or uncertainties. You must be specific; simply stating "human error" or "bad pipetting technique" is not sufficient. Instead, demonstrate your understanding of the methodology by specifying the impact that a particular problem or limitation has on the results.
    • Suggest a future improvement or follow-up experiment (if you see no need for improvement).
    • If you are suggesting an improvement, you should be specific about what went wrong in your experiment and explain why your improvement would reduce the effect of the problem you encountered.

Submit by next lab meeting

  • Physical submission:
    • Writing Assignment 1
  • Digital submissions:
    • Your notebook pages including answers to the two questions asked at the end of the procedure.
    • Writing Assignment 1
    • Your Excel spreadsheet 

General tip: Be sure to use the Grading Rubric as a final checklist before submitting any assignment for this class.