BMS 113 · Common Core (Tronc commun)Section 2.1 · Sources of drugs
Section 5 of 31
Plants as a source of drugs
Explain how the different parts of a plant yield drugs, and match key plant-derived active ingredients to their medicinal uses.
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Why this matters
Without this, you may treat a plant name as if it guaranteed a medicine, and miss that the species, the part used and standardisation decide the dose the patient receives. These ideas return in Unit 2.4, where plant molecules are modified by chemistry, and in Unit 2.6, where the active ingredient receives its generic name.
What you need to know first
You should already know that a drug is any substance that changes how the body works, and that a medicine is given in a measured dose. Unit 1.2 explains what a drug is, and Unit 1.4 lists the five rights, including the right dose.
Words to know (4)
Active ingredient
The chemical in a plant or medicine that actually changes the body; the other substances around it do not produce the medicinal effect. Quinine is the active ingredient of Cinchona bark.
Pharmacognosy
The study of medicines that come from natural sources, which identifies and checks the raw plant or animal material before it is used. Pharmacognosy confirms that a dried leaf really is Artemisia annua.
Standardisation
Testing each batch of a natural material so that a fixed weight always contains the same measured amount of active ingredient. A standardised leaf extract states how much active ingredient each tablet holds.
Crude drug preparation
A simple preparation of dried, powdered or partly extracted plant material that still contains most of the plant substances, active and inactive. A traditional bark decoction is a crude drug preparation.
One idea at a timeIdea 1 / 4
In one sentence One plant can supply several different medicines, because each part may hold its own active ingredient.
A whole plant is a pharmacy of parts
The part used is part of a medicine’s identity, rather than a harvesting detail. Seeds, bark, roots, stems, fruits, leaves and sap are chosen because the active ingredient is concentrated there, so two medicines can come from one species when one uses bark and another leaves. This matters for identification, harvesting at the right moment, and the dose the patient finally receives.
For example
Cinchona bark yields quinine for malaria and foxglove leaves yield digoxin for heart failure, while Panax ginseng root is put to several medicinal uses.
Pause and check yourself
Why can the bark and the leaves of one plant give two different medicines?
Show the answer
Because the active ingredient is concentrated in particular parts, so each part is harvested and used for a different medicine.
Read detailed explanations
Alkaloids, glycosides and the active ingredient
Active ingredients from plants belong to chemical families that behave in recognisable ways. Alkaloids are nitrogen-containing plant substances with powerful effects, often on the nervous system; morphine — isolated from opium in 1805 by Friedrich Sertürner and the first alkaloid — atropine, quinine and caffeine all belong to this family. Glycosides combine a sugar with another chemical group; digoxin from foxglove is a cardiac glycoside, which is why it acts on the heart, and aloin is the laxative glycoside of aloe. Resins and gums form in the sap. These families matter in practice for three reasons. First, they are potent, so small quantities produce large effects and precise dosing is essential. Second, they are chemically active, so they can interact with other drugs. Third, they are extracted by different methods, so the same plant may need a different process to give a pure medicine. When you read that a drug is plant-derived, ask which family its active ingredient belongs to; that tells you how it is likely to behave.
From willow bark to a tablet: what standardisation adds
Traditional plant use and modern plant-derived drugs share a source but differ in control. A traditional preparation is a crude drug preparation: bark, leaf or root dried, powdered or partly extracted, retaining most of the active and inactive compounds and, if present, the toxic ones. A modern plant-derived medicine isolates the active ingredient, measures it, and puts a known quantity into a dosage form. Standardisation is the step in between: testing each batch for identity, purity and potency so that one tablet equals another. Without it, the same weight of plant material can contain very different amounts of active ingredient, and the prescriber cannot predict either the benefit or the harm. This is also why pharmacology insists on the species name and the part used: standardisation is impossible if the raw material has not first been identified. The clinical consequence is straightforward — with an unstandardised preparation, neither the patient nor the health worker can know what dose has actually been taken, which makes both under-treatment and poisoning possible.
Explore the diagram
Look at the whole drawing first. Then select one numbered part and connect it to the idea you just read.
Where plant drugs come from
A stylised plant with the parts used as drug sources, beside a table that pairs each source with its active ingredient and medicinal use.
New to diagrams? Read this first
Look at the whole drawing once without clicking anything. Ask yourself only: what kind of picture is this (a comparison, a route, a sequence, a table)?
Press Play, or use the arrows, to move through the numbered parts in order. The drawing stays still; only the highlight moves.
Click a number, or its name in the list below, to read what that part is and why it matters. Use “Hide labels” when you want to test your memory.
More controls
01 / 6Bark
If the drawing is wider than this panel, scroll it sideways. Labels stay readable; the numbered explanations below show the same ideas.
Bark
Bark is the protective outer layer of the trunk and it yields two of the oldest plant medicines: the salicylates of Salix alba, which became aspirin, and the quinine of Cinchona, still used against malaria. Which part is harvested therefore fixes which active ingredient is obtained.
Schematic plant drawing; not to scale. The parts are gathered on one plant for teaching, but each drug comes from a different species and the active ingredient is concentrated and standardised from the harvested part.
Key points to remember
✓A plant is a source, not a medicine: the medicine is the active ingredient, measured in a known amount.
✓Different parts of one species can give different drugs, so the species and the part must both be named.
✓Plant active ingredients reach nearly every therapeutic class, from pain and fever to malaria and heart failure.
✓An unstandardised plant material cannot guarantee a dose, so it can under-treat or poison.
Go deeper
Worked example
Building a plant-drug table from the sources. You are asked to prepare a teaching table of plant drugs. For each entry you must give the active ingredient, the species and part it comes from, and its medicinal use, then explain to a first-year student why the part matters.
Start with the active ingredient, because that is the substance the body responds to: aspirin, artemisinin, aloin, digoxin, atropine, caffeine, quinine and morphine.
Attach the source: bark of Salix alba for aspirin, Artemisia annua for artemisinin, Aloe vera for aloe and aloin, leaves of foxglove for digoxin, Atropa belladonna for atropine, Coffea arabica seeds for caffeine, Cinchona bark for quinine, and the poppy for morphine.
Name the part harvested: bark for aspirin and quinine, leaves for digoxin and tea, seeds for coffee and coca, root for ginseng, and the poppy for morphine.
State the use: analgesic, antipyretic, anti-inflammatory and anti-platelet for aspirin; antimalarial for artemisinin and quinine; laxative for aloin and for fibres; heart failure for digoxin; drying of gastrointestinal secretions and anti-emetic for atropine; central nervous system stimulant for caffeine; analgesic for morphine.
Explain why the part matters: the active ingredient is concentrated in one part, so identification and harvesting protect the dose — the same species can give an inactive stem and an active leaf.
A plant is not a drug until its active ingredient, species, part and use have all been named. The table turns folk knowledge into a reproducible medicine.
Common misconception
Because a substance is natural, many beginners assume it is automatically gentle and safe. In fact plant material contains potent active ingredients — morphine, digoxin, atropine and quinine are all plant-derived — and an unstandardised preparation can deliver too little or far too much.
Check your reasoning
Why is it not enough to know that a medicine “comes from a plant”?
Reveal an answer
Because the plant name alone says nothing about how much active ingredient the patient receives. Two samples of the same species can differ with season, soil and drying, and different species can share a common name. Only precise identification of species and part, plus standardisation of the active ingredient, makes the dose predictable.
Where this comes from
BMS 113 General Pharmacology I lecture notes · Unit 2, pp. 10–11
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About this course and its sources
Bilingual lessons organised from the supplied BMS 113 General Pharmacology I lecture notes. Diagrams are original educational schematics; pharmacology, nursing and French-language review are pending.