Glyphosate kills plants by blocking a single enzyme they need to build three essential amino acids. Without these building blocks, plants can’t make the proteins and compounds required to grow, and they slowly starve at the cellular level. It’s the active ingredient in Roundup and hundreds of other herbicide products, and its effectiveness comes from both its precise molecular target and its ability to travel throughout an entire plant after a single spray.
The Enzyme It Targets
Glyphosate zeroes in on an enzyme called EPSP synthase. This enzyme handles a critical step in a seven-step metabolic process called the shikimate pathway, which converts simple sugar-derived molecules into chorismate, the precursor for three aromatic amino acids: phenylalanine, tyrosine, and tryptophan. These amino acids are the starting materials for proteins, hormones, and a range of other compounds plants need to function.
The way glyphosate blocks the enzyme is specific. EPSP synthase normally binds a molecule called shikimate-3-phosphate (S3P) first, then accepts a second substrate called PEP to carry out its reaction. Glyphosate mimics PEP closely enough to slip into the enzyme’s active site after S3P is already bound, forming a dead-end complex that locks the enzyme in place. Because glyphosate competes directly with PEP for the same binding spot, the enzyme simply can’t do its job as long as glyphosate is present in sufficient concentration.
The shikimate pathway doesn’t just produce amino acids. It also feeds into the production of folate, certain vitamins, and a wide array of secondary metabolites that plants use for defense, pigmentation, and structural support. Shutting down this single enzymatic step has cascading effects across the plant’s entire metabolism.
Why It Only Targets Plants and Microbes
The shikimate pathway exists in plants, bacteria, fungi, and algae, but not in animals. Humans and other mammals get phenylalanine and tryptophan entirely from food (which is why they’re called “essential” amino acids for us). Because our cells never run this pathway, glyphosate has no equivalent enzyme to block in human tissue. This absence is the core reason glyphosate was considered to have a favorable safety profile compared to many older herbicides.
How It Moves Through the Plant
Glyphosate is a systemic herbicide, meaning it doesn’t just damage the leaf it lands on. After being sprayed on foliage, it enters through green tissue and is rapidly transported through the plant’s vascular system to wherever growth is most active. Young shoots, root tips, and developing buds all act as sinks that pull glyphosate toward them.
This translocation is a major part of what makes glyphosate effective against perennial weeds with deep root systems. The chemical is relatively stable inside most plant species, so it isn’t quickly broken down before reaching those distant tissues. It accumulates in roots and can even be released into the surrounding soil from root tissue. For the plant, this means there’s no safe zone: the herbicide reaches the very growing points that would otherwise allow the weed to regenerate.
Commercial formulations enhance this process by including surfactants, most commonly a class of compounds called polyethoxylated tallow amines (POEA). These act like detergents, helping glyphosate penetrate the waxy cuticle on leaf surfaces and absorb into the plant more efficiently.
What Happens to the Plant
Glyphosate doesn’t cause instant burn or visible contact damage the way some herbicides do. Because it works by cutting off amino acid production, the effects build gradually. Within a few days of application, actively growing tissue begins to yellow (a process called chlorosis) as the plant can no longer synthesize the proteins and pigments it needs. New growth is affected first since those tissues have the highest demand for fresh amino acids.
Over the following one to two weeks, chlorosis progresses to browning and tissue death (necrosis). The exact timeline varies with the plant species, its growth stage, temperature, and how much glyphosate was applied. Fast-growing annual weeds in warm conditions may collapse within a week, while larger perennials can take two to three weeks to show full effects. Because the herbicide has already spread to the roots by that point, regrowth is far less likely than with herbicides that only damage above-ground tissue.
How Resistant Crops Survive It
Genetically modified “Roundup Ready” crops carry a gene borrowed from a soil bacterium, Agrobacterium strain CP4, which was originally isolated from a waste column at a glyphosate production facility. This bacterium produces its own version of EPSP synthase that glyphosate simply can’t shut down. The difference comes down to a single amino acid position in the enzyme’s active site: the bacterial version has an alanine residue where most plant enzymes have a glycine. That one substitution changes the shape of the binding pocket just enough that glyphosate folds into a compact, non-inhibitory position instead of its usual extended shape that blocks the active site.
When this bacterial gene is inserted into a crop plant’s genome, the plant produces the resistant enzyme alongside (or instead of) its own vulnerable version. The shikimate pathway keeps running normally even when glyphosate is present, so the crop survives while surrounding weeds die. The bacterial enzyme is also unusually stable across a wide range of temperatures and pH levels, which helps it function reliably in different crop tissues and growing conditions.
The Gut Microbiome Question
Because gut bacteria do possess the shikimate pathway, researchers have asked whether dietary glyphosate residues could harm the human microbiome. A large computational study examining 734 paired samples of gut microbial DNA and gene activity found that, while genetic traces of the shikimate pathway appeared in 98% of gut microbiome samples, the pathway was actively being used in only about 35% of them. Most gut bacteria appear to scavenge aromatic amino acids from food rather than manufacturing their own, which would make them largely indifferent to glyphosate’s mechanism of action.
Among the bacterial species that do carry the relevant enzyme, roughly 18% of the total microbial population in the gut was predicted to have the naturally resistant (class II) form, similar to the one used in GM crops. The remaining species technically carry the sensitive form, but because they aren’t actively running the pathway, the practical significance is unclear. One analysis concluded that the aromatic amino acids present in a normal diet would be more than sufficient to prevent any shortfall, even if glyphosate did suppress some bacterial production. Overall, experimental evidence for meaningful effects of glyphosate on the human gut microbiome remains limited.