2. This problem examines some physiological data from the laboratory of Professor Carol Beuchat. Animals have evolved different mechanisms for excreting waste nitrogen. The principle means of excreting nitrogen are uric acid, urea, and ammonia. Unfortunately, the latter two are toxic so require larger volumes of water for excretion. Uric acid uses less water, but it requires more energy (ATP) to produce. Thus, animals must weigh their needs of water versus energy when selecting a means of excretion.

a. Here we only examine the amount of urea excreted. First, a standard is run to determine the absorbance at 570 nm as a function of the concentration of urea. (This is a standard technique using spectrophotometry.) The data are listed below:

Use the EXCEL's trendline feature on a scatterplot to find the best straight line through the data, where

is the straight line describing absorbance (A) as a function of the concentration of urea (U) with EXCEL determining the slope (m) and intercept (b). Write the equation for the best linear model, and show the graph with both the data and this linear model.

b. Find the expected absorbance for a sample containing 53 mg/dl of urea and a sample containing 176 mg/dl of urea.

c. In practice, one will use the spectrograph to measure the absorbance, and use the relationship between the two to calculate the urea levels. In order to do this you must now solve for U as a function of A. That is find a function f(A) such that

d. In Professor Beuchat's laboratory they found that the urine from a hummingbird kept at 10°C had an absorbance of 0.142. When the hummingbird was kept at 20°C, the absorbance for a urine sample was 0.201, while at 40°C the absorbance was 0.29. Find the corresponding values of concentration of urea for each temperature from the results in part c. (Don't forget to include the correct units on your answer.) Can you explain your results with regard to either energy or water conservation by these hummingbirds?

e. We would like to see if the analysis of urine samples tells us about other species. The table below lists different animals and the corresponding absorbances measured.