|MadSci Network: Agricultural Sciences|
Glyphosate is a herbicide that looks a little like an amino acid. Remember this as it is important later. I'm not sure of your educational background so I'll start at the begining. Plants take carbon dioxide, water, and a few other nutrients and make more complex substances. To do this takes a series of chemical reactions. For the reactions to take place, energy must be expended. Initially the sun provides the energy for the water and carbon dioxide to make sugar and other high energy compounds. After that, plants make more complex substances using the energy in the sugars and other compounds (there are also other things like pH gradients and proton fluxes, but I will ignore these). Chemical reactions take a lot of energy. Some very complex compounds, called enzymes help reduce the energy required to make the reaction work. They do this in a number of ways, but basically, one compound binds to the enzyme, the enzyme stretchs, bends, and/or reduces or increases charges, such that a second compound can now easily combine with the first, with much less energy required. The new compound that is formed is released and then modified futher by another enzyme or used for something else by the plant. Enzymes are made up of simpler compounds called amino acids. There are twenty different kinds. Three of them have an aromatic group which is made up of a ring of 6 carbons. All three are made through a very similar chain of reactions. Glyphosate, sort of looks like these at a stage of their synthesis (making). Glyphosate binds to the enzyme (5-enolpyruvoyl shikimate phosphate synthase) which is one of the enzymes in the shikimic acid pathway. Once there, it stuffs up the enzyme. Glyphosate does not react with the second compound and it does not unbind. The plant no longer can make the 3 aromatic amino acids and after a week or two the plant plant yellows up and dies. Animals can't make these 3 amino acids and the shikimic acid pathway is not part of animal processes. Therefor glyphosate does not cause the same thing to happen in animals or humans which gives us some safety. Animals need those three amino acids however, and they get it by eating plants or eating animals which ate plants, or eating animals which ate animals who ate plants. For this information I used an old weed science text "Weed Science: Principles and Practices" by Ashton and Monaco. 1991 Wiley and Sons, Inc. There is a great web site for things weedy. http://ext.agn.uiuc.edu/wssa/ It has info on weeds, weed management, even videos of plants dying after being sprayed. It sound grim but it is great for determining what chemical killed your grapes when some farmer up the valley sprays and you think it was drift from is treatment that did it. There are also lots of links for the inquisitive.
Try the links in the MadSci Library for more information on Agricultural Sciences.