Name _____________________________ Date _______
Invitation to Inquiry: Respiration in Seeds
Introduction
This activity is an exploration of how science studies the cell respiration in seeds.
Cell respiration is an essential process for living things since it is the way in which
energy is released from food.
a.
A scientist was interested in the respiratory processes in germinating
seeds. She wanted to know if the processes were the same in different
species. He began his study with experiments to determine the ratio
between the amount of carbon dioxide produced and the oxygen used. (This
is called the respiratory ratio or quotient.)
First she placed germinating wheat grains in a suitable chamber equipped
with inlet and outlet tubes through which air could be circulated. Using
standard methods, she measured the amount of oxygen and carbon dioxide in
the air that entered the chamber and in the air that lefty the chamber.
For this information she could calculate the amount of oxygen used and
the amount of carbon dioxide produced by the germinating seeds. Some of
this data is presented in below.
Carbon dioxide produced
Oxygen used
(milliliters)
(milliliters)
Series 1
10.0
9.8
12.0
12.1
25.4
25.2
Series 2
4.1
4.4
24.5
22.1
25.4
26.2
Series 3
9.1
9.8
14.8
17.0
15.5
15.0
16.2
13.9
17.0
17.5
20.3
19.8
21.5
21.0
Plot these data on simple graph paper with the volume of produced carbon
dioxide on the x-axis (horizontal line) and the amount of oxygen used on the
y-axis (vertical line).
After you have plotted the data, answer the following two questions;
Do all the points fall on a straight line? What reasons can you give if
they do not?
b.
Draw the straight line that most nearly fits the points on your graph of
these data.
You may remember from mathematics that we can write an equation by
which we can determine other points on the line. In this case you can see
that according to this straight line 1 ml of carbon dioxide was liberated for
each milliliter of oxygen used. That is, x/y—the respiratory quotient—is 1.
Let us factor x/y = 1 into x = y
Because when all the points are plotted, they lie on a straight line, or
nearly so, we say that the relation here is “linear.” Many relations are linear,
however, in which y does not equal x, but equals some multiple or
fraction of x.
Next, the scientist collected similar data from germinating castor bean
seeds. The data for this series of experiments are presented in the
following chart.
Carbon dioxide produced
Oxygen used
(milliliters)
(milliliters)
5.1
7.5
4.0
5.0
13.0
17.5
9.0
13.3
2.5
4.0
5.0
6.8
10.0
15.0
19.0
27.0
Plot these values on graph paper as you did for the earlier experiments.
Again the points will not fall on a straight line, but the straight line can be
drawn which will come reasonably close to all the points.
Now compute the respiratory quotients for the pairs of data for the
castor bean.
What is the average of these ratios?
c. We plotted the amount of carbon dioxide on the x-axis and the amount of
oxygen on the y-axis. We now have an equation in the form of x/y = _____.
Replace this equation by the two equivalent equations x = ____ and
y = _____.
d. When we have a linear relation between x and y in the data for these two
series of experiments, we can say that y = kx, where k is a constant. In the
data for the germinating wheat grains, the value of k was 1; therefore we
could write y = x without mentioning k. For the data for the germinating
castor beans the value of k is 1.43. In this sort of relation, we say k is a
measure of the slope of the line, for in measures how steeply the line of the
slope above the x-axis.
Thus, in the case of wheat grains, for every unit extending the line
from left to right along the x-axis, there is a corresponding rise of 1 unit
along the y-axis. In the castor bean case, there is a rise of 1.43 units in the
value of y for every unit that the x value of the line is extended from left to
right.
Draw the curves for wheat and castor bean on a single sheet of
paper. Do it freehand, just to see the different slopes of the two lines.
Now draw another line beginning in the lower left corner, but just halfway
between the wheat line and the bottom axis.
Would the k for this line’s equation y = kx be greater or less than 1?
Now draw still another line halfway between the wheat line and the vertical
axis. Would the value of k for this line be greater or less than 1?
e. Now that the scientist has plotted her data, of what use are the plots?
f. We know that oxidation here consists of union of oxygen with some
foodstuff. In view of this fact, what factor might account for the
different respiratory quotients in the two species?
g. If you do not know, look up the chemical composition of starch and of fat.
Assume that each kind of seed was able to oxidize its food material
completely into carbon dioxide and water. Which kind of food, starch or
fat, will require the most oxygen to produce a given amount of carbon
dioxide? Explain your answer.
h. Suppose the scientist had determined the weight of carbon dioxide
produced and the weight of oxygen used instead of determining the volumes,
and that she obtained the data in the table below for the germinating wheat
seeds.
Plot these data on graph paper, again using the amount of carbon
dioxide on the x-axis and the amount of oxygen on the y-axis.
Carbon dioxide
produced (mg)
x
19.6
23.5
49.8
8.0
49.8
39.8
17.8
30.4
33.3
42.1
Oxygen
used (mg)
y
14.0
17.3
36.0
6.3
37.5
28.3
14.0
21.4
25.0
30.0
Oxygen/carbon
dioxide (mg)
y/x
0.71
0.74
0.72
0.79
0.75
0.76
0.79
0.70
0.75
0.71
You will notice that once again you obtain a straight line.
How would you describe the relationship between y and x, that is,
between oxygen used and carbon dioxide produced?
If we divide the number of milligrams of carbon dioxide produced, (y/x), we
obtain the values shown in the third column of the chart. The mean of these
values is approximately 0.74. Hence we may write: y = 0.74x
In your own words, what does this equation say?
After completing this exercise, what questions might you ask about either cell
respiration or about the process of investigating this process?
In a paragraph summarize what you learned from this activity.