Plant Chemistry Quiz

Unless they’re food on the table or part of captivating scenery, plants are under-appreciated. Even fewer people care for the intricacy of their biochemistry or realize how in tune they are with their environment. Here’s a short quiz written for nonspecialists who nevertheless have a curiosity for what goes on below the surface of leaves and flowers. Especially since there are no student grades at stake, if one can anesthetize the ego, one can trade in a little time for precious botanical knowledge. By clicking on “explanation” below each question, you will find the answer with more insight into the topic.

1.  Which of these compounds do plants assemble “from scratch”—- in other words just from water, carbon dioxide and ions from the soil?

(A) glucose

(B) amino acids

(C) flavonoids (used as a “sunscreen”, pigments and have many other functions)

(D) defence compounds

(E) all of the above

(F) none of the above


2. What compound, found in willow bark, can be reacted with acetic anhydride in the presence of mineral acid to make aspirin?

(A) paracetamol
(B) ibuprofen

(C) salicylic acid

(D) caffeine


3. Which of these plants, a member of the buttercup family, has one of the most poisonous roots due to 3 nasty alkaloids(aconitine, mesaconitine & jesaconitine)?

(A) monkshood (B) mountain ash

(C) opium poppy

(D) milkweed


4. Fermentation often occurs inside a cantaloupe. Small volumes of which alcohols are produced?

(A) butanol and ethanol

(B) ethanol and benzyl alcohol

(C) methanol and benzyl alcohol

(D) methanol and butanol



5. The glandular hairs of wormwood leaves, Artemisia annua , yields the important drug artemisinin? What disease does the drug treat?

(A) malaria (B) sleeping sickness

(C) elephantiasis

(D) ebola


6. At least 400 compounds contribute to its beautiful scent including these:

Which flower are we referring to? (look carefully at the structures!)

(A) iris

(B) peony

(C) rose

(D) violet


7. Compounds like tomatin make the leaves poisonous to many insects. Solanine makes the leaves of related plants also toxic to many insects. Compared to tomatin, solanine is considerably more toxic to humans. What plants are we referring to, respectively?

(A) tomatoes and red peppers

(B) tomatoes and potatoes/eggplant

(C) potaotes and red peppers

(D )potatoes or eggplant/ tomatoes


8. What common substance is crucial to photosynthesis because it is the ultimate source of electrons, which must be returned to excited chlorophyll?

(A) potash fertilizer

(B) nitrate fertilizer

(C) water

(D) carbon dioxide


9. Which plant’s roots provides sugars and leghemoglobin to bacteria in exchange for ammonium ion?

(A) bean

(B) radish

(C) tomato

(D) lettuce


10. What are the most common elements in a plant?

(A) N, C, H, O, P and Mn

(B) N, C, H, O and S

(C) C, H, O, N and Mg

(D) C, H, O, P, N and K


whitespace#Answer1 –E–Plants don’t eat. They only absorb water and ions from the soil and carbon dioxide from the air. With these substances they produce not only glucose but all of the amino acids, nucleic acids and secondary compounds that they require.

whitespace#Answer2 –C– The first two are tylenol and advil. Salicylic acid, upon reacting with acetic anhydride in the presence of a catalyst(acid), will create acetylsalicylic acid, which is aspirin.

Caffeine is an alkaloid, whose structure is unrelated to aspirin.

whitespace#Answer3 –A– If one is familiar with the buttercup family(Ranunculacae), one realizes from the shape of the leaves of the four plants shown, that only the monkshood is related to the buttercup. The opium poppy produces morphine in its pod. The fruits of the mountain ash in large amounts can cause stomach irritation and pain, vomiting, queasiness, diarrhea, kidney damage, and other side effects. Finally the milkweed concentrates its poison in the above ground parts.

whitespace#Answer4 –B– Technically an alcohol is any compound in which the oxygen atom is sandwiched between a hydrogen and a carbon atom from a hydrocarbon group. Cantaloupe produces small amounts of ethanol (CH3CH2OH), the same alcohol produced from fermentation of apples, hops and grapes to make cider, beer and wine. According to this source benzyl alcohol (C6H5CH2OH) can also be produced.  The latter is not at all toxic in low doses and has a sweet, pleasant odor. Not surprisingly, it is also found in hyacinth and jasmine flowers.

whitespace#Answer5 –A– Notice that malaria and one form of sleeping sickness are directly caused by protozoans(Plasmodium and Trypanosoma respectively), but in both cases the microorganism needs a vector, another organism that carries it from one victim to the next. Elephantiasis has no vector, and ebola is the only one out of the four above diseases to be caused by a virus.

From Center of Disease Control
From Center of Disease Control


For more on the use of artemesin and its “green” extraction, see this Periodic Table Video: artemisinin , which is sparingly produced by Artemisia annua and also difficult and expensive to artificially make from scratch.

whitespace#Answer6 –C– The name of the first compound gives it away, cis-rose

whitespace#Answer7 –B–
The name tomatin gives away the 1st part of the answer. Potatoes and eggplant leaves are toxic to humans becuase of solanine. Those of tomatoes are free of solanine and since tomato leaves have low concentrations of a much milder alkaloid, they are not toxic, contrary to popular belief. In fact tomatin is also found in green tomatoes.

whitespace#Answer8 –C–
Plants need water for a variety of reasons but especially to release electrons after sunlight ejects electrons from the photosystems of chloroplasts. Oxygen is produced when water dissociates, and the hydrogen ions that are also released set up a +/- gradient and thus a voltage that provides the chloroplast with the energy it needs to incorporate carbon dioxide.

whitespace#Answer9 –A–
From the list only the bean is a legume, which thanks to mutualism, can get ammonium ion from Rhizobium bacteria, which use nitrogen from the air to produce it.

whitespace#Answer10 –D– Along with N, C, H, O, and P,  sulfur is one of the six most common elements of life. Sulfur is part of four amino acids and is also key to many biochemical reactions. Having said that, S only makes up 0.1% of dry plant matter. From recalling the components of industrial fertilizer (N-P-K) one could correctly surmise that potassium(K) would be more common than sulfur in plants. And that would be correct. It makes just under 1% of plant dry matter, on average. Plants need potassium ion for proper transcription of many genes and to control the activity of many enzymes.


One Man’s Weed Is Another Man’s Science

When I was a teenager, my father would sooner believe that I was an incompetent lawn mower than even entertain the possibility that I was motivated to preserve what he considered weeds, those suburban symbols of lassitude and irresponsibility.


If I’d see a patch of flowering clover or the tiny star-like flowers of stitchwort (Stellaria),I felt compelled to mow around them, come back to the patches with lawn clippers to cut protruding blades of grass and try to make the weeds as inconspicuous as possible, hoping to sneak my conservation effort past my father’s critical eye.  But it never worked. Even though I tried to point out that stitchwort was rich in vitamin C, in his eyes there was a place for everything, and only grass should be grown in lawns.

I discovered that one of my father’s many kindred spirits lives on my street. A few nights ago while walking my dog, I noticed he was destroying an attractive patch of black medic from his lawn that bordered the sidewalk.

With the friendliest tone I could manage, I asked him, “Did you know that plant is a member of the legume family, and it makes its own fertilizer?”

“Yes, but it’s not nice,”  he replied.

I sensed it was hopeless, but my compulsion to explain science obliged me to continue. “The nitrogen from the air gets converted to useful ammonium by a helpful bacterium in that plant’s roots,” I said.

“Yes, but it’s not nice,”  he repeated, still friendly.

For the sake of neighborly relations, now seemed like a good time to let go, so I smiled and concluded, “Oh well…it’s your property.”

black medic

So, alas, unable to spark a street discussion of the intricacies of black medic, I turn once again to the laptop keyboard. Medicago lupilina, is a member of the pea family (Leguminosae or Fabaceae). A close look at the cluster of yellow flowers reveals a set of miniature pea-like flowers. The bases, sepals and stamens are fused together into a cup-like structure, and you have to lift the cup and have exceptional closeup eyesight to see its reproductive parts.

A key ecological feature of this family is the nitrogen-fixing ability shared by most of its 18 to 20 thousand members. It happens through a symbiotic association with Rhizobium, which infect legume roots and form nodules, where in exchange for sugars, the bacteria use an anaerobic reaction to convert diatomic nitrogen into ammonium ion (NH4+). In the reaction, the nitrogen is “fixed”, because plants cannot convert relatively inert N2 into needed amino acids, but ammonium will do the trick. But free nitrogen gas has an oxidation number of zero, and it gets converted into  NH4+ , whose nitrogen atom has an oxidation number of (-3). This reduction process would not occur if the common oxidizer in air, oxygen, would come into contact with the nitrogen-fixing enzymes.

root nodules of black medic

But the problem is that the natural form of nitrogen fixation has something in common with the industrial version (Haber Process): it is an energy-demanding reaction,and what better way is there to release energy from sugars than through cellular respiration, which needs oxygen?

The bacteria get around this dilemma by making use of leghemoglobin(LEG), a pigment similar in structure to our hemoglobin but with a higher affinity for oxygen. In the diagram, notice that the pigment is actually in the nodule, outside of the bacterial cell wall, away from the enzyme complex (NC).

Nfixing_1The small amount of oxygen is then delivered to the bacteria’s respiratory chain (RC), allowing several ATP molecules to be fed into the enzyme complex, where the reducing agent, NADH, converts the nitrogen into ammonium. The latter is released in aqueous form into the host cell , where it is converted into glutamine, asparagine and urea derivatives of the general form,R-CO-NH-CO NH2, where R can be a different hydrocarbon group.

These products are then transported to the rest of the plant through the xylem (not with sugars, which are distributed by the phloem), and with the abundance of these protein-building blocks, it is not surprising that the seeds of the black, ripe pods of black medic, like those of beans, lentils and other legumes, are rich in proteins.

seed of black medic in my hand

Compared to other plants, legumes seem to be more sensitive to increases in carbon dioxide levels.(Incidentally, we have surpassed the global average concentration of 400 ppm,and you could see how it varies globally here.) Some species produce bigger seeds when the atmosphere is CO2-enriched, and in general, at least in soybeans, extra carbon facilitates the fixation of nitrogen, provided that there are no other stresses such as limited nutrient availability or drought.

I guess that’s another reason why my neighbor was digging up his black medic—better get to it before climate change amplifies his problem.


Will Elevated Carbon Dioxide Concentration Amplify the Benefits of Nitrogen Fixation in Legumes?1
Plant Physiology November 2009 vol. 151 no. 3 1009-1016  (available in its entirety, free of charge)
The Botanical Garden Volume II: Perennials and Annuals. Roger Phillips and Martyn Rix. Firefly. 2002

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