The effect of temperature on insect respiration rates

Aim:

To determine how changing the surrounding temperature influences the rate of respiration of an insect, in this case a locust.

Hypothesis:

Increasing the temperature should raise the rate of respiration as it will increase enzyme activity and therefore cell metabolism. This in turn will cause the cells of the locust to use up oxygen more quickly increasing carbon dioxide levels in the cells whose detection will quicken the breathing rate, the observation of the inspiration of air will be used to determine the rate of respiration.
As oxygen is used up, the locust notices this drop by the accumulation of lactic acid in its muscles. When oxygen is not present (anaerobic conditions) the first stage of respiration, glycolysis, and takes place readily but the link reaction in the matrix of the mitochondria is different from that in aerobic respiration. Instead of pyruvate being converted into acetyl coenzyme A, as under aerobic conditions, it is converted to lactic acid. As lactic acid can harm the muscles in large quantities the locust detects a need for greater oxygen consumption, to provide this change the spiracles on the insect's exoskeleton open for longer periods of time and they are moistened to increase the rate of oxygen diffusion into the insect's body. When undertaking activities with high metabolic needs e.g. flying other methods increase the rate of respiration but considering that the locust being used will be at rest these methods will have no effect on the experiments conducted.
Particle theory dictates that increasing the temperature provides kinetic energy to all molecules so causing them to move more quickly so react more often, so enzyme activity increases and the Q10 law states that increasing the temperature by 10ºC should double enzyme activity. Considering that enzyme activity is directly linked to the rate of respiration it can be assumed that increasing the temperature by 10ºC should lead to the doubling of the respiration rate if the Q10 law is to be observed.

Method:

A test tube had some small stones placed in the bottom of it with some cotton wool on top of it, a locust was then placed head down in to the tube. The test tube was placed in a water bath at varying temperatures and the rate of respiration measured at each temperature until a steady value was obtained. The rate of respiration will be measured over one minute periods and will be determined by the number of times the abdomen of the locust moves up and down (the clearest indication of breathing).
The five temperatures used were 10, 20, 30 and 40ºC with the addition of room temperature, which was recorded as 22ºC. The temperatures in all water baths were maintained by occasional heating with a Bunsen flame.

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Results:

The following table shows the recorded respirations over five various temperatures:

Temperature ( ºC) Respiration rate (inspirations/minute) Mean (x) Standard deviation
10 27
18
5 5.33 0.47
6
5
20 53
41
17 17.33 0.47
17
18
22 40
30
24
20 19.67 0.47
19
20
30 27
26 27 0.82
28
40 41
39 40 0.82
40

The first three sets of results have an initial increase before settling down (ringed entries), this is due to the locust becoming unsettled as it was transferred between water baths, but in the last two result sets the locust was given a settling time of 5 minutes so that it had time to adjust to its altered surroundings. The means and standard deviations were calculated only for the rates once they had been assumed to settle, so as to take into consideration the accuracy and consistency of the sets of results that will mainly be used.

To show the adverse effects of including the settling times a scatter graph of all the raw data is presented below.

As can be seen the data varies widely and so for the following two graphs it was decided upon to remove the settling time values to create a more refined graph that would be easier to interpret.

The following graph shows the temperature changes recorded but to ensure an accurate depiction the 'settling' time recordings have been removed to present a smooth gradual uprising in comparison to a greatly fluctuating line.

As can be seen, this graph does not provide a clear overall picture of the data so a clearer graph of the mean respiration rates of against temperature was drawn up (below).

Conclusions:

The basic overall conclusion that can be drawn is that there is a positive correlation between temperature and the rate of respiration and that this increase is proportional as the graph of mean respiration rates is roughly straight.
This increase was hypothesised as a rise in temperature provides enzymes kinetic energy to metabolise at a faster rate so use up oxygen up more quickly, the oxygen used up does not cause an increase in breathing rate as the locust is only sensitive to carbon dioxide levels. As carbon dioxide levels rise, due to it being the by-product of cell metabolism, the locust senses it needs to breathe rapidly to decrease the carbon dioxide levels. It does this by ventilating - rapidly moving its abdomen up and down, this process is repeated each time the carbon dioxide levels inside the locust rise too high.
The ventilation movements compress the air sacs in the locust's tracheal system causes an increase in pressure inside the locust so air moves in through the spiracles by a system of mass flow, this air then supplies oxygen to all parts of the locust's body via the tracheal network. This system is only applicable to smaller organisms as they have a relatively small volume to provide oxygen to so a diffusion system can function without the necessity of a more complex circulatory system.
The Q10 law was not upheld by the results as increases in temperature by 10ºC did not lead to doubling the respiration rate but varied greatly from quadrupling to less than doubling; although there did appear to be a rise of 10-15 inspiration per minute between all 10ºC intervals.

The results obtained were as expected with no clearly anomalous results, the only factor that could have affected the results was that although the water baths were heated to various temperatures it was assumed that the cells of the locust were metabolising at the set temperatures. This fact was not known for certain as locusts are relatively large invertebrates and the heat may not have been distributed throughout the whole organism.