MadSci Network: Agricultural Sciences

Re: How can you measure the amount of pigment in plants

Area: Agricultural Sciences
Posted By: Jack Paxton, Faculty Crop Science Emeritus, University of Illinois
Date: Tue Dec 17 02:55:22 1996
Message ID: 850157309.Ag

Plant pigments include chlorophylls [generally green], carotenoids [yellow], anthocyanins [often red, blue, orange, or yellow, and common in flowers]. These pigments are generally measured by determining the amount and wavelength of light that they absorb. This is best done by separating them from one another by extraction with solvents and then chromatographing the extracts.

Some pigments [such as carotenoids] are more fat soluble and extract more easily into liid solvents like gasoline, corn oil, or cleaning fluid. Other pigments are more water soluble [such as chlorophyll, the green pigment that is necessary for photosynthesis and enables plants to capture light energy from the sun].These polar/water soluble pigments extract more easily into water or alcohols such as methanol or isopropanol [rubbing alcohol]. Chromatography allows you to separate these pigments from one another. This separation is necessary since there are several chlorophylls that resemble one another chemically, and several carotenoids and anthocyanins that also are chemically similar but not identical.

Once separated from one another, the pigments are put into a colorimeter which can precisely determine which wavelength of light the pigment absorbs [thus giving it the color you see with your eye, by absorbing the opposite wavelengths]. And the colorimeter can tell you how much light is absorbed. Some pigments are intensely colored and therefore absorb most of the light [photons] at a particular wavelength whereas other pigments may absorb little of the light at that particular wavelength. Therefore the wavelength and amount of light absorbed by a pigment can help tell you what pigment you have and how much of it is there.

Jack Paxton,  Professor Emeritus Univ. of Illinois, 
Agriculture is like a bus. When you ask were you are going and the reply is
"Faster!" it is time to get off,  before the bus crashes or the wheels fall
Admin note:
If you're interested in trying some 'primitive chromatography' you can experiment with the following demo.

You'll need:

  1. Some heavy filter paper cut into strips - 1cm X 5cm is fine.
  2. Rubbing alcohol, water, and a lipophilic or 'fat soluble' solvent - corn oil as above or kerosene/gasoline. If you use gasoline, be certain to have an adult help you use it.
  3. Plant material - some green leaves, carrots or other samples of interest.
First, grind up your plant material, chop it, grate it, mince it, whatever you prefer. For your 'water soluble' extractions, boil the material in a small volume of water for 20-30 minutes. For 'lipid soluble extractions' place the material in the lipophilic solvent (gasoline will probably work best), but don't heat it up! Stir occasionally, until you see the solvent becomes colored by what you added (carrots would be a good initial try).

Take the solution you generated, and place a few drops on one end of your strips of filter paper, say 1/2 cm from the end. Let it dry. You can reapply more drops, preferably until you can see a small colored spot. When the paper has dried, you're ready to start your chromatography - incidentally this is called paper chromatography since you're using paper.

Now take your solute - if you have mutliple strips of paper with the same sample, you'll probably find it interesting to see the effects of different solvents. Pour some solvent (water, rubbing alcohol, etc..) into the bottom of a glass or other clear object - just enough to cover the bottom. Stand a strip of filter paper + sample on the bottom with the sample placed close to the bottom of the glass. The paper will absorb the solvent. Depending on the solvent, and the properties of the different pigments, they should separate along the length of the strip of paper. 'Lighter' pigments (in terms of weight, not color) will move faster in the solvent than heavier ones, and so travel to higher locations on the strip of paper.

Ball-point pen ink provides a good control for your experiments. Draw and fill in a small circle on one end of a strip of paper and place it in your solvent of choice (rubbing alcohol will probably be most effective).

You could also compare and contrast the effects from solvents you use for extractions with those you use for the chromatography. What happens if you extract with water, but run with gasoline? or extract with gasoline and run with water? etc..

Have fun!

-L. Bry, MadSci Admin

Current Queue | Current Queue for Agricultural Sciences | Agricultural Sciences archives

Return to the MadSci Network

MadSci Home | Information | Search | Random Knowledge Generator | MadSci Archives | Mad Library | MAD Labs | MAD FAQs | Ask a ? | Join Us! | Help Support MadSci

MadSci Network
© Copyright 1996, Washington University. All rights reserved.