ANALYSIS OF PLANT COMPOSITION AND SPECTRA OF PHOTOSYNTHETHETIC PIGMENTS

Copyright © 1986, D. B. Fankhauser, Professor of Biology and Chemistry 
University of Cincinnati Clermont College,
Batavia OH 45103
 

This page has been accessed Counter times since 2 February 2006.
 Created1986, rvsd 9Jan 06, 2 Feb 06

Chlorophyll and accessory pigments are used by leaves to collect energy from light and transduce it to chemical energy be used to synthesize sugar.  These pigments can be separated by chromatography in which a solvent system travels through paper by capillary action, carrying the pigments with it.  Because each pigment has a different solubility and/or affinity for paper, they move at different rates, and separate along the paper.  Pigments separated in the way in a previous lab have varying absorbtion spectra which may be demonstrated using the technique for determining the absorbtion spectra.

Alternatively, simple analysis of the composition of plants may be performed by drying, extracting with various organic solvents, and determining the absorbtion spectra of each extract.  In this experiment, we will determine percent water, and relative pigment concentrations in a variety of plant materials, notably salad greens.
 

NOTE: It is required that you know how to use a spectrophotometer and correctly plot data on a graph to complete this lab.  Ask the instructor for previous protocols if you lack this knowledge.

Questions:
 Which solvent is most effective extracting chlorophyll?
 How does the chlorophyll content compare among salad greens?
 What differences do you note when comparing spectra of various greens?
 How does the absorption spectrum of  plant extract compare with known dyes?
 Might the concentration of photosynthetic pigments correlate with the nutritional quality of these salad greens?
 
 

SUPPLIES EQUIPMENT
Variety of salad greens, fresh and unfaded 
Organic solvents: 
      95% ethyl alcohol
      petroleum ether
      acetone, etc.
Whatman #1 filter paper
Diluted standard microbiological stains:
   Hucker’s: 1.0 uL into 10 mL dH2O
   Safranin O: 3.0 uL into 3 mL dH2O
   Methylene Blue: 2.0 uL into 3 mL dH2O		 large strainers with feet
drying oven, 80°C
balance
mortar and pestle
16 x 150 mm test tubes with corks
filter funnel
Spectrophotometer
5 cuvettes in plastic rack
lens paper

PROTOCOL:
 

1) Weigh out 10.0 g of stemmed fresh salad green, record type and exact weight.
2) Dry: Arrange in a monolayer in a strainer, dry in drying oven (80°C) until a stable weight is achieved (About 1 hour). 
Record dry weight, calculate % water.
3) Pulverize: Crush dried leaves to fine powder in a mortar and pestle.
4) Weigh out 0.25 g powdered leaves into a 16 x 150 mm test tube.
5) Extract: Add 10 mL of solvent (record whether  95% alcohol, pet ether, acetone, etc).
6) Securely cork, shake 10 minutes to dissolve.
7) Filter through #1 Whatman filter paper in filter funnel into clean 16 x 150 mm test tube supported in a rack.
8) Dilute filtered extract 1:10: 5.0 mL EtOH + 0.555 mL extract (we are ignoring effect of diluting alternate solvents into EtOH.).
9) Prepare cuvettes: Rinse five cuvettes with 95% EtOH.  Fill each with 3.00 mL EtOH, polish outside, read A350.  If the difference is >than 0.005, clean and polish again.  Mark the cuvette with the lowest A350 as “B” (blank), the others S1, S2, etc (S = sample).
10) Read absorbencies at 350 nm.
 11) Dilute the extract to adjust absorbency so that A350 ~ 0.800 to 1.000
12)  Read absorption spectra for all group samples every 25 nm from 350-800 nm.  (Read all samples at a given wavelength, then re zero and reblank for the next wavelength and read all samples at that wavelength, etc).  Rotate reading and recording roles. Enter into the computer as instructed.
13) Graph the data: Graph all data on the same graph, plotting wavelength (on the X axis) versus absorbance (on the Y axis), noting maxima for each of the samples.  Follow graphing protocol previously distributed.  Note maxima for each of the solutions tested.  Which of the original questions can you answer?  What conclusions do you draw?