Chapter 13 Discussion

The Discussion section is where you interpret your results and explain what they mean. This section also is where you relate your results back to the larger body of scientific knowledge. By the end of the Discussion section, you should be talking in general, broad terms once again. Be careful not to get too broad though; your experiment is not the explanation for everything.

These are the questions your should try to answer in the Discussion.

  1. Did your results support your hypothesis?
  2. What is your biological explanation for why the manipulation you made produced the results you observed?
  3. How do your findings connect back to the information in your Introduction?
  4. How do your results and your explanation or interpretation compare with previously published studies?
  5. How do your results increase our understanding of the world around us?

13.1 Other Tips

  • Refer to published primary literature; it provides the supporting context for how you interpreted the results. Cite these sources using in-text citiations.
  • If you have evidence of an error in your experiment, say so, but do not make up possible sources of error if you do not think they actually occurred.
  • If appropriate, describe the alternative explanations of your data. If there is another possible explanation for your data, what future experiments are needed to determine the best explanation?
  • What is the next logical step for this study? Modify the procedure and repeat it? Try again, to increase the number of replicates with the existing methods? Test an entirely new hypothesis? Are there different methods or procedures you would use if you repeated the study again?

13.2 Examples of Poorly Written Discussion Sections

13.2.1 Example 1

This is the entire discussion submitted by the author, not an excerpt.

The hypothesis that we had was that those plants that grew in the presence of nitrogen would grow smaller than those that grew without the presence of oxygen. In order to test this, we measured those that were grown in the exact same conditions and looked to see what was the differences in the root to shoot ratio for the length of field peas.

This discussion is extremely disorganized, with no clear line of reasoning.

  1. They made no attempt to connect their study to others, or to summarize the results. The first sentence does not even make sense, switching from nitrogen to oxygen. The second sentence suggests they measured field peas that were grown in identical conditions, with no experimental treatment.
  2. The author basically ignores their results, and never discusses them.
  3. This discussion is so short that the GTA scored it as missing, which in our bins scoring model is an automatic “Unacceptable.”


13.2.2 Example 2

At first this author’s discussion looks very good, but it actually is not. Do you see the problem?

Our hypothesis that reduced sodium ion channel activity will increase the voltage required for threshold seemed to be supported by the increase in voltage required for muscle contraction in all frog legs. However, the change in values was not large enough to verify that this data was significant. The p- value determined by the paired t test was found to be 0.1583, which is much greater than the 0.05 p-value required to reject the null hypothesis that the threshold will be the same in each frog leg regardless of the presence of lidocaine. Although the difference in threshold appears notably different after injection, there was not enough change to prove a significant effect. It is possible that the increase in threshold for each frog leg was due to other factors such as fatigue as well(*). Frog leg 2 showed unusual responsiveness to lidocaine’s effects, but this may simply have been a frog with more sensitive neurons due to preexisting conditions. The effect of axon conduction and thresholds being unaffected by the presence of lidocaine can be seen in other studies as well(*). This is because other factors such as calcium ion channel activity are active in muscular contraction, but only sodium and potassium ions are involved in the transmission of sensations such as pain. This means the threshold for muscular contraction would only be partially be affected by sodium ion channel blockers(*). Despite lidocaine’s function of inhibiting sodium ion channels and use as a local anesthetic, its affect has been found to not significantly impact axon signaling for muscular contraction(*). This makes lidocaine effective as a local anesthetic, because complications involving loss of muscular function in important organs and skeletal muscle cannot occur, but the threshold for pain signaling is still increased. Future revisions to this experiment may do well to increase the time between stimulations of the frog legs so that fatigue has less of an effect on data collection. The effect of a calcium ion channel blocker in addition to the lidocaine may also be tested to determine if it has a more dangerous anesthetic than Lidocaine for inhibiting muscular contractions as would be expected(*). An understanding of the application of lidocaine and its function as an anesthetic without inhibiting muscular activity is important for improving the application of similar substances in the field of anesthesia and could allow for the development of low-risk anesthetics for surgeries to have fewer risks and negative consequences.

There are no references to outside sources that put this study into a larger context. Each item marked with an asterisk (*) is a potential point to connect this study to others.Under our bins grading system, this report would be marked “Unacceptable.”


13.2.3 Example 3

What is the problem with this discussion section?

The hypothesis that the frog legs will reach tetany at a higher voltage was supported as the p-value was less than 0.05. That means that the results we obtained for the experiment weren’t due to mere chance. The caffeine had an effect on the frog legs by allowing it to make room for more voltage needed to reach tetany. The caffeine caused the frog legs to have an “adrenaline effect” (Prado-Franceshi, 2002). The caffeine provided the frog muscle with abundant ATP and as a result, cross bridges continued to form in the frog leg that helped the muscle contract provided there was calcium. So the frog legs continued to contract and as a result became stiff as we observed. The frog leg had too much ATP that it continued to join the actin and myosin filaments together and so the leg wasn’t exhausted easily. This process might occur in humans as well as we use the same process to move our muscles. With this in mind, further research can be done to find the optimum dosage of caffeine humans can take in when we desperately need a boost. Studying muscles is still an ongoing process important for the well-being of humans and understanding how to improve muscle activity.

  1. The author did not describe their results precisely. Look at the sentence, “So the frog legs continued to contract and as a result became stiff as we observed.” The equipment used for this study was not measuring muscle stiffness, only contraction force and time.
  2. Their conclusions are overly broad and not supported by a strong argument connecting this experiment to humans.
  3. This last sentence is vague and not well connected to this study: “Studying muscles is still an ongoing process important for the well-being of humans and understanding how to improve muscle activity.”


13.2.4 Example 4

This is the complete discussion section, not an excerpt.

After conducting the experiment we can conclude that caffeine increased the voltage amplitude, which increases muscle tension. This causes a stronger action potential, resulting in a higher amplitude. There are more twitches in the muscle as well. Implications of this research are that increased muscle contractions can be stimulated with high levels of caffeine (Rosser, 2009). However, if we measured muscle fatigue, we would have seen faster fatigue in the caffeine injected muscles because caffeine causes a stronger contraction, but a muscle can not hold a strong contraction for as long.

  1. The author never refers back to their original hypothesis.
  2. The author does not talk about their own results in any depth.
  3. They represented the order of events in the muscle incorrectly, and talk about factors they did not measure. Voltage amplitude is an output measurement, not something they modified. Also, they did not measure the action potential strength, only muscle contraction strength.
  4. The author makes this statement of fact but they do not have any data or outside sources to back it up: “However, if we measured muscle fatigue, we would have seen faster fatigue in the caffeine injected muscles because caffeine causes a stronger contraction, but a muscle can not hold a strong contraction for as long.”


13.2.5 Example 5

What could be improved in this example?

Our hypothesis, that a calcium channel activator would cause increased muscle response in the Rana pipien frogs’ legs was not supported by our data. Our results, in fact, showed a trend in which there was a decrease in muscle response of the frog legs after the legs were treated with the A23187 calcium channel activator.

It is likely that we did not give the A23187 enough time to be evenly transported around the muscle because we only waited a minute after injection before re-stimulation of the nerve. If that was the case, the activators would not have had time to take effect, causing there to be very little difference between the controlled muscle responses and the A23187 treated muscle responses. It is also possible that not enough of the A23187 calcium channel activator was added to the gastrocnemius muscle, preventing the effect of the treatment from being observed. Additionally, only injecting into the gastrocnemius muscle may have prevented the activator from being transported efficiently to every part of the frog leg.

Upon researching Ca2+ in Rana pipiens independently, it became very clear that the effects of Ca2+ on the animals can actually be seen more clearly in the skin of the frogs because Ca2+ in frog skin is concentrated and easily extracted. Thus, in future it would be fruitful to try the experiment without deskinning the frog legs and observing how the movement of Ca2+ would move in the epithelium of the legs and see how that would change the responsivity (Stiffler, 1994).

  1. They only discuss their own results in the first two sentences, then start listing every reason why their results may be incorrect.
  2. The third paragraph is not a logical argument. How could calcium ions in skin affect muscle contractility? There is no anatomical connection that would allow ions to leave skin and go to the muscle. Arguments that make large jumps in thinking like this need to be laid out so that the reader can understand how the author came to this conclusion.


13.2.6 Example 6

What problems do you see here? What has been done well?

The results of this experiment reject the hypothesis that purple lettuce chloroplast samples would demonstrate a lesser change in absorbance over time than green leaves. The insignificant difference between the percent of initial absorbance after 30 minutes for purple and green samples with a high p value indicates that the presence of anthocyanins did not have a significant effect on the activity of photosystem II during photosynthesis.

The relatively consistent absorbance readings after each 10-minute interval for both samples show that DCIP was reduced by photosystem II at a similar rate in both green and purple lettuce chloroplasts. The results lead to the conclusion that anthocyanins may not play as significant of a role in photosynthesis as was predicted and indicated by previous studies.

However, it is essential to consider other possible interpretations of the data. It was observed that, following centrifugation, B-pellet samples for green and purple lettuce chloroplasts were nearly indistinguishable shades of green. It is possible that the process of centrifugation caused the anthocyanins to separate from the chloroplasts, preventing the anthocyanins from providing a protective mechanism against sunlight, and reducing photosynthetic rate. The results of the experiment would thus be invalid because the independent variable of purple pigmentation would no longer exist. In order to effectively analyze the difference in photosynthetic activity between purple and green lettuce, the experiment must be modified so that the anthocyanins are present in the purple lettuce chloroplast sample throughout analysis. Or, rather than isolating the chloroplasts, O2 production of purple lettuce plants could be compared to that of green lettuce leaves as a way to analyze photosynthetic activity. Further experimentation with anthocyanins in different fruits and vegetables, as well as other plants, can provide scientists with a better understanding of how anthocyanins evolved and what additional purposes they serve in plant cells.

One study conducted on Arabidopsis found that anthocyanins were produced in response to reactive oxygen species (ROS) that cause oxidative stress in the cells, demonstrating that anthocyanins may have purposes in the cell beyond shielding from radiation [Xu: 2018]. By observing the conditions under which anthocyanins are produced in cells, scientists can make conclusions about the functions of anthocyanin pigmentation.

  1. The first paragraph does not make sense.
  2. The author jumps to a conclusion in paragraph 2 then argues against it in paragraph 3. Paragraph 3 makes a very good point; they might have removed the anthocyanins accidentally. This is a point they can build on.
  3. Paragraph 4 reads like it was tacked on at the end as a way to get a literature citation into the discussion. The better strategy would be to drop paragraph 4, and provide literature citations to support their arguments in paragraph 3.


13.3 Examples of Well-Written Discussion Sections

13.3.1 Example 1

Our results supported our hypothesis that the root: shoot ratio will be larger in the plants given salt water for both weight and length than the plants given tap water because our results were statistically significant. The p-values we obtained showed that there was a significant difference in the root: shoot ratios between salt water plants and tap water plants. Other studies have shown that salt causes plants to grow slow and become stunted (Bernstein, 1975). Our results compare similarly to these studies, for we did not see much growth in our experimental plants given salt water. Because plants absorb water and nutrients through the roots, we expected the dehydrated plants to grow longer roots in search of water and nutrients by allocation their resources. Since the control plants were given tap water, they were able to grow tall and evenly making the root: shoot ratio fairly small. This is supported by the small averages of the root: shoot ratio for the weight and length of the tap water plants (Figure 1).

Water is crucial for the survival of plants, so allocating resources towards finding water is essential for growth. Some plants are capable of surviving in salt-affected soils however because they have a tolerance to the salinity. Plants like, tolerant grasses and legumes, “tend to improve soil structure in partially reclaimed soils through the beneficial action of roots, or through their incorporation into the soil as green manures” (Bernstein, 1975). However, some research suggests rye to be in the salt-tolerant category of plants (Francois, 1989). This could explain why we were able to see growth in our plants over the two-week period instead of dead plants. The tolerance that rye plants have allowed us to have enough data to obtain results, but not so tolerant as to hinder the experiment. Additionally, other research suggests that a small concentration of salt can actually improve certain plant growth, with only a few exceptions (Magistad, 1943).

The next step in this study could be to test what types of plants have more or less of a tolerance to salt in the soil because that tolerance could affect growth and therefore manipulate the root: shoot ratio for the length and weight. This experiment supports that salinity hinders plant growth, but a future study could also test whether a specific, smaller concentration of salt water would actually improve growth like some studies have found in the past.

What Is Particularly Good?

  1. The author says up front whether their original hypothesis was supported.
  2. They summarize the main findings, and compare their results to what was seen in an independent study.
  3. They make arguments then support them by referring to their own results.
  4. The second paragraph puts this experiment in a broader context of the literature, and uses the literature to help them understand their own results.
  5. The third paragraph proposes reasonable next steps.


13.3.2 Example 2

Our original hypothesis was supported in that the group that received an injection of caffeine had a significantly higher amplitude of contraction than the control group. In a similar study on human males, researchers found that consuming caffeine increased excitability in skeletal muscles of humans (Onslow, 2011). Although injecting caffeine would have a more immediate effect on the muscles, regularly consuming it likely has similar effects. Additionally, caffeine allows muscles to maximally contract and maintain contraction, indicating that it may be responsible for maintaining the binding of Ca2+ in the muscle and not allowing it to relax after fatigue (Kalmar, 1999). This would explain the rigidity of the muscle we saw following the removal of the stimulus. The muscle remained in a state of tetany, or constant contraction, for a longer period of time because the caffeine inhibited the return of Ca2+ to its storage site.

The p-value of 0.011 indicates there was a significant difference in contraction force between the control muscles and those injected with caffeine. The sample size was small (n=3), so one way to increase the validity of this experiment in the future could be to introduce more types of skeletal muscles from other species which have similar neuromuscular functions to humans. For example, a similar experiment described the effects of caffeine on the soleus of rats and found similar results, that the caffeine prolonged and intensified the contraction of the muscles (Fryer, 1989). If both rat and frog muscles were used in the experiment and their data combined, it would be interesting to see whether the control and treatment groups remain statistically significantly different among different species.

What Is Particularly Good?

  1. The author makes it clear whether their original hypothesis was supported.
  2. They connect their results to studies in humans, and use the findings from humans to help them interpret their own observations.
  3. They do not reject their own results, but allow that the study could be improved.
  4. They pose an interesting speculative question for the next study: will frog and rat muscles perform the same way if they are tested side by side?