Laboratory #8 - PHOTOSYNTHESIS

Photosynthesis is one of the most fundamentally important biological processes. All green plants can produce their own "food" simply by using the energy in sunlight to convert carbon dioxide (present in the surrounding air) into a carbohydrate. Although sugar is the initial product of photosynthesis, it is usually stored in the form of starch (e.g., potatoes) and later used when the plant can't photosynthesize as much as normal. The chemical reactions involved in photosynthesis are quite complex and will be emphasized more in lecture. After completing this lab, you should be able to: determine what light is best for photosynthesis; identify chlorophyll as well as accessory pigments; and test for the presence of starch as an indicator of whether or not photosynthesis has taken place.



PAPER CHROMATOGRAPHY OF PHOTOSYNTHETIC PIGMENTS (procedure 8.1):

Chromatography is a fairly simple technique used to separate and help identify unknown chemical compounds. This technique is based on the idea that different chemicals have a different "attractiveness" for the paper and the solvent used. In lab, spinach was ground up and blotted on the bottom of a piece of chromatography paper. The tip of the paper was placed in a solvent which travelled up the paper by capillary action. The various pigments in the spinach leaves separated out according to their affinity for the paper. The resulting chromatogram looks something like this.


Chlorophyll b was the first pigment to separate out. Above that is Chlorophyll a, then xanthophyll, and finally at the very top is carotene.


UPTAKE OF CO2 (procedure 8.3):

Elodea and phenol red were added to a test tube. Phenol red is yellow when acidic and red when alkaline. The pictures below show three tubes taken at successive time intervals. Notice the color of the phenolphthalein changes as the Elodea photosynthesizes. Can you account for the color change from yellow to red?




REQUIREMENT OF LIGHT (procedure 8.6):

Geranium leaves were used to illustrate the importance of light in photosynthesis. A few leaves were partially covered for one week to that part of the leaves were shaded and thus presumably did not photosynthesize.


The leaves were boiled in ethanol (to remove the chlorophyll) and stained with iodine. Regions of the leave that photosynthesized would turn a dark brown color due to the presence of starch. Variegated Coleus leaves were also used. Only certain parts of these leaves contain chlorophyll and are thus able to photosynthesize. Observe and explain the following results.

Geranium leaves:
normal leaf
whole leaf exposed to light
half of leaf covered



Variegated Coleus Leaves:
normal leaf
leaf after stained with iodine


This concludes the review for Lab 8 - Photosynthesis.




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