The rate of photosynthesis can be affected by the amount of light available to plants. Since oxygen is a product of photosynthesis, measuring its production allows us to measure the rate of photosynthesis.
In this exercise, students at different tables varied the intensity of light responsible for photosynthesis in the spinach disks by varying the wattage of the bulbs. Recall that these disks were submerged in a sodium bicarbonate solution and subjected to a vacuum. This resulted in the utilization of carbon from the sodium bicarbonate as their primary source of carbon for photosynthesis (as opposed to carbon dioxide). The reasoning behind this is that the sunken disks in solution will produce oxygen as a product of photosynthesis and float to the surface. We should be able to count the number of disks as a representation of the rate of photosynthesis occurring in each experimental setup.
The resulting graph from this part of the lab should look similar to this:
Note that as we increase the watts from 40 to 150 (in columns 2 through 5) we can see an increase in the rate of photosynthesis.
An action spectrum defines the relative effectiveness of different wavelengths of light (colors) for light-dependent processes such as photosynthesis. In this experiment, you will test the effects of colors of light on photosynthesis in sections of spinach leaves.
This part of the experiment is very similar to the varying wattage exercise - except this time we're changing the wavelength of the light entering the Petri dishes instead of the intensity. All lights during this part used 150 watt bulbs. The resulting graph should look similar to this:
Red (610 - 700 nm) and blue (450 - 500 nm) wavelengths are most effective in promoting photosynthesis. Green (500 - 570 nm) light is least effective - it is not absorbed by plants but is reflected which is why green plants appear to be green. The conclusion: different wavelengths of light affect the photosynthetic process. Red and blue light support the highest rates of photosynthesis (although white light causes the most disks to float, remember that white is all wavelengths so it can be expected to result in the highest percentage).
The process of chromatography, which separates complex mixtures into their component parts based on their solubility in different kinds of solvents, can help identify some of the pigments used in photosynthesis.
For a good description of the varying types of pigments found in this part of the experiment, read the bottom of page 123 and the top of page 124 in your laboratory manuals. The experiment was prepared by streaking pigment extract to the bottom of chromatography paper and placing it in a tube with chromatography solvent in the bottom.
Note the movement of the solvent up the chromatography paper (click for a larger picture):
Chlorophyll b is olive green, chlorophyll a is a brighter green, xanthophyll is yellow, and carotene is a yellow-orange. Since both chlorophyll a and b are polar while the solvent is non-polar, they will stay lower on the strip and are attracted to the polar water molecules in the paper. Carotene and xanthophyll, however, are very non-polar and will thus dissolve in the solvent and travel further up the chromatography paper.
Having isolated and identified pigments found in chloroplasts, you can now determine the wavelengths of light transmitted by each pigment and then by all the pigments in the chloroplast extract. There was a slight change from the laboratory manual during this exercise because we used an extract containing all the pigments as opposed to working with each pigment separately. A graph was constructed using the absorbance of the extract at varying wavelengths. The goal of this exercise was to discover at what wavelengths do the chloroplast pigments absorb the greatest amount of light. An ideal graph would be similar to the following:
You can relate these results to Exercise A, Part 3. Remember how it was shown that wavelengths of red (610 - 700 nm) and blue (450 - 500 nm) resulted in the greatest amount of photosynthesis? The same is shown here in the graph - light is absorbed by the pigments better at those wavelengths as compared to others. Green, once again, is absorbed least. It ain't easy being green. ;-)