Plant Constituents

Primary & Secondary Metabolism

Plants make thousands of molecules, but they don’t all serve the same purpose.

Primary metabolites are needed for basic growth and survival. They include carbohydrates, amino acids, proteins, lipids, nucleic acids, vitamins, and related compounds.

Secondary metabolites are more specialised.

Plants use them to repel herbivores, fight microbes, attract pollinators, survive ultraviolet light, communicate with other organisms, and compete with neighbouring plants.

Most of the chemistry we associate specifically with herbal medicine comes from this second group.

A bitter alkaloid may have evolved to discourage an insect.

A volatile terpene may attract a pollinator or repel a grazer.

A resin can seal damaged tissue and reduce microbial invasion.

Humans are borrowing chemistry that evolved for the plant’s own purposes.

Terpenoids

One of the largest and most diverse groups in plant chemistry — 668 of the compounds indexed here.

Terpenoids are built from repeating five-carbon isoprene units and range from highly volatile essential-oil constituents to large, non-volatile molecules.

This group includes:

At the lighter end are compounds such as menthol that readily evaporate.

At the heavier end are compounds such as steroidal and triterpenoid saponins that behave very differently in both the plant and the body.

Phenolics

A huge group built around aromatic rings bearing hydroxyl groups — 664 compounds here.

Phenolics are involved in pigmentation, defence, astringency, signalling, ultraviolet protection, and interactions with microbes.

Major groups include:

They’re often grouped together as “antioxidants,” but that describes only a small part of what they do.

Alkaloids

Nitrogen-containing compounds that are often bitter and pharmacologically potent — 402 of them here.

Some of the most familiar plant-derived drugs belong here: morphine, atropine, quinine, caffeine, and many others.

Important groups include:

This is also one of the classes where dose matters most. Medicinal and toxic alkaloids often sit very close together pharmacologically.

Carbohydrates

Not all active plant chemistry works by binding to receptors.

Carbohydrates include:

Some work mainly through their physical properties.

Mucilage coats and protects tissue.

Soluble fibres change the viscosity of gut contents.

Other polysaccharides interact with the microbiome or immune system.

These compounds are often dosed in grams rather than milligrams because bulk is part of the mechanism.

Lipids & Fatty Acids

This group includes the fixed oils concentrated in seeds, nuts, and oily fruits, along with the fatty acids and other lipid-soluble compounds they contain.

They’re especially sensitive to extraction method.

A water infusion leaves most fixed oils behind.

Cold pressing or lipid extraction concentrates them.

Amino Acids & Proteins

Plants also contain free amino acids, proteins, enzymes, and peptides.

Some free amino acids have direct pharmacological effects.

L-theanine, for example, is absorbed intact and acts in the nervous system.

Larger proteins and peptides are different because many are broken apart during digestion before reaching circulation.

Organic Acids

Small acids that influence pH, mineral binding, solubility, metabolism, preservation, and the behaviour of other compounds around them.

They rarely define a herb by themselves, but they can strongly influence the chemistry of a preparation.

Vitamins & Minerals

Some medicinal plants work partly because they’re nutritious.

Vitamins are organic compounds made by living organisms.

Minerals are elements taken up from the soil.

That distinction matters because mineral content can vary enormously with growing conditions, while vitamins are produced and degraded according to the plant’s metabolism, storage, and processing.

Isoprenoid Pathways

The mevalonate and MEP pathways produce the five-carbon building blocks used to make terpenoids.

From those same basic units, plants build everything from volatile monoterpenes to sterols and carotenoids.

Shikimate & Phenylpropanoid Pathways

The shikimate pathway produces aromatic amino acids that feed into a huge amount of plant chemistry.

From there, the phenylpropanoid pathway gives rise to many flavonoids, phenolic acids, lignans, coumarins, and related compounds.

Amino-Acid-Derived Chemistry

Many alkaloids begin with amino acids.

Tryptophan feeds many indole alkaloids.

Tyrosine feeds many isoquinoline alkaloids.

Ornithine contributes to tropane alkaloids.

Knowing the pathway helps explain why chemically related compounds tend to appear together.

Presence Isn’t Dose

Modern analytical chemistry can detect extremely small quantities.

A compound may appear on a constituent list while occurring at far too little to matter pharmacologically.

Extraction Determines What Reaches the Dose

A molecule can be abundant in the raw plant and nearly absent from a tea.

Another may become heavily concentrated in an alcohol extract or essential oil.

The preparation is part of the chemistry.

Absorption Matters

Even a large oral dose means little if the compound isn’t absorbed.

Curcumin is the familiar example: turmeric contains plenty of it, but free curcumin has poor oral bioavailability.

Metabolism Can Change the Molecule

The compound swallowed isn’t always the compound that acts.

Gut bacteria, intestinal enzymes, and the liver can transform plant constituents into metabolites with very different activity.

The Isolated Compound Isn’t the Whole Plant

Evidence for an isolated constituent tells you about that constituent.

It doesn’t automatically tell you what the whole herb does.

The dose may be different by orders of magnitude, and the rest of the plant’s chemistry may alter absorption, metabolism, toxicity, or effect.

The reverse matters too: a whole-herb result can’t automatically be assigned to whichever constituent happens to be best known.

Standardisation & Marker Compounds

Commercial extracts are often standardised to a particular constituent.

That might mean an extract contains a defined amount of curcuminoids, ginsenosides, silymarin, or another chemical marker.

Sometimes the marker is central to the pharmacology.

Sometimes it was chosen because it’s stable, distinctive, and easy to measure.

Standardisation is mainly a tool for consistency.

It tells you something about what’s in the extract. It doesn’t always tell you which compound is responsible for the effect.

Isolation

The molecule is separated from everything else in the plant.

Morphine from opium and digoxin from foxglove are examples.

Isolation makes the dose much more precise, but the resulting medicine is no longer chemically equivalent to the whole plant.

Standardised Extracts

The plant is still present as a mixture, but manufacturing concentrates or controls part of its chemistry.

This sits somewhere between traditional whole-herb preparations and isolated molecules.

Semi-Synthesis

The organism produces a complicated molecule or precursor, and chemistry modifies it into the final medicine.

Paclitaxel is a useful example of how natural-product chemistry can become the starting material for industrial drug production.

Synthetic Analogues

Once chemists understand the structure responsible for an effect, they can alter it.

A synthetic analogue may be made more stable, more selective, longer acting, easier to absorb, or easier to manufacture.

At this point the original plant may have supplied the lead compound, but the resulting molecule may never have existed in nature.

Natural vs Synthetic

“Natural” and “synthetic” describe where something came from.

They don’t tell you whether it’s safe.

A molecule has the same chemical structure whether a plant made it or a laboratory did.

Plants also produce some exceptionally toxic substances, including aconitine, coniine, and α-amanitin.

What matters pharmacologically is the molecule, dose, route, preparation, and person taking it.

The poisonous plants & fungi index covers many of the natural compounds where that distinction matters most.