244 ❯ STEP 4. Review the Knowledge You Need to Score High
break surface tension). The little disks float on top of the water, which introduces the chal-
lenging part of the lab: getting them to sink. A vacuum must be created in the syringe to
pull out the atmospheric gases from the spongy mesophyll layer in the leaf tissue. This takes
a bit of trial and error, but once it is accomplished, the disks will slowly drift to the bottom
of the syringe. The contents of the syringe are dumped into a cup filled with a sodium
bicarbonate solution and put under light. As more photosynthesis occurs, more oxygen is
produced:
6CO 2 +6H 2 O→C 6 H 12 O 6 +6O 2
If you look closely, you can see tiny bubbles forming on your leaf disks! Once enough
oxygen is produced (and caught within the internal leaf space), the little disks will begin to
rise to the top very slowly.
Here’s a question: if your leaf pieces are photosynthesizing, where are they getting the
necessary carbon dioxide? When sodium bicarbonate ionizes in water, it provides an alter-
native source of carbon dioxide for the plant.Results
The investigative part of the lab allows you to explore variables that you think might influ-
ence photosynthesis in your leaf disks. Do the levels of CO 2 effect photosynthesis? How
about amount of light? Regardless of your choice of variable, perform the same leaf disk
analysis and compare the amount of time it takes for half of your disks to rise (ET 50 , or esti-
mated time it takes 50 percent of the disks to float) in both your control and experimental
groups.Key Concepts- A lot of photosynthesis means a lot of oxygen production.
- Light increases the rate of photosynthesis.
- Carbon dioxide was provided by dissolved sodium bicarbonate.
Investigation 6: Cellular Respiration
If you would like to see how different environmental conditions affect an organism’s respi-
ration rate, this is the lab for you! In this investigation you will use a respirometer (or
microrespirometer) to track the respiration rate of seeds. Based on the equation for cellular
respiration, C 6 H 12 O 6 +6O 2 →6CO 2 +6H 2 O, how would you measure respiration rates?
There are, in fact, three ways to measure respiration:- Oxygen consumption:how much O 2 is actually consumed
- Carbon dioxide production:how much CO 2 is actually produced
- Energy released during respiration:how much energy is released
Basic Setup
This experiment examines germinating peas by measuring the volume of gas that surrounds
the peas at certain intervals in an effort to determine the rate of respiration. Two gases con-
tribute to the volume around the pea: O 2 and CO 2. How can we use the amount of oxygen
consumed during respiration as our measuring point if CO 2 is present as well? Something
needs to be done with the CO 2 released during respiration. Otherwise we would not get a
true representation of how much the volume is changing as a result of oxygen consumption.
The CO 2 would skew the numbers by making it appear as if less O 2 were being consumed.KEY IDEABIG IDEA 2
Cellular
Processes:
Energy,
Communication,
and Homeostasis