Beyond Biology - Plants in STEM subjects: Chemistry

Kate Whittington, kate.whittington@bgci.org | 19/07/13 | London

Always looking to increase and inspire the up-take of STEM (science, technology, engineering and maths) subjects in schools, botanic gardens provide great outdoor learning sites to use plants to encourage inquiry-based learning for a wide range of subjects. The relevance of plants to the study of biology on the school curriculum is obvious, but plants also have surprising links to other STEM subjects.

Following on from last week’s post about the relationship between plants and mathematics, this week looks at how plants can be used to teach or provide an introduction to several aspects of chemistry.

As organic chemistry and biology are so intrinsically linked there is a great deal of crossover, and it doesn’t take much to discover ways in which plants relate to the chemical sciences. This post presents some fairly recognizable concepts but there are a few more unusual examples – such as how plants can “communicate” using chemical signals. And, as always, there are lots of useful links to resources for using plants to teach chemistry. As a starting point, this video gives a brief introduction of “How Plants Matter to Chemistry”.

'Every plant is a chemical factory for complex substances which exceeds any human capability. In their poisons, antibiotic agents, prickles and foul tastes, they developed defences against attack long before human stockades and pesticides.'

- Anthony Huxley, Green Inheritance, 1984

Medicine/Pharmaceuticals

From caffeine to morphine, over a hundred chemical substances have been derived from plants for use as drugs or medicines. Not only does this highlight the importance of plant conservation in preserving potentially untapped plant products, but it also provides an opportunity to explore the process of extracting and/or synthesising plant-derived chemicals.

Salix albaAs an example, extract of willow bark has been used for many hundreds, even thousands of years. Ancient Asian records suggest it was used as much as 2400 years ago, and the famous Greek physician, Hippocrates, recommended a brew made with willow leaves to treat labour pains as early as 400 years BC!

The active compound responsible for these properties of willow bark is called Salicin. The first scientific study of this herbal medicine was carried out in 1763 when Reverend Edward Stone, an English clergyman, gave ground up willow bark to 50 parishioners suffering from rheumatic fever and described his observations.

As is often the case, once these drugs become in high demand there is not enough of the plant-derived chemical available, so scientists look to chemical synthesis as an alternative source.

So, in 1897, Felix Hoffmann of Bayer pharmaceutical company found a way to synthesise a derivative of Salicin – acetyl salicylic acid – which is now commonly known as Aspirin!

Many of us will have taken aspirin at some point, and approximately 35,000 metric tonnes of it are produced and consumed each year – that’s over 100 billion standard tablets! Not only is aspirin used as a pain-killer but it’s also thought to reduce incidence of heart disease. 

The Royal Society of Chemistry’s “Learn Chemistry” resources have a downloadable practical session aimed at 16-18 year olds where they can synthesise their own aspirin and analyse its purity! Whilst the practical might be a tad pricey for many schools, the notes still provide a good explanation of the process and introduce several different concepts on the chemistry curriculum including laboratory procedures, molecular electronic transition spectroscopy and mass spectrometry.

Plant-derived pharmaceuticals are arguably one of the best ways to promote the importance of plants and their relevance to human society. Other examples include Digitalin – derived from purple foxglove (Digitalis purpurea) and used in the treatment of heart conditions; Vinblastine – derived from Madagascar periwinkle (Catharanthus roseus) and used to treat several types of cancer; and Galanthamine – derived from snowdrops (Galanthus woronowii) and used as an anti-Alzheimer’s drug.

Chemical communications

The notion that plants can “communicate” via chemicals is only a recently accepted ecological phenomenon. Plant signalling using chemical cues occurs both within and between plants, as well as with pollinators, fruit dispersers, herbivores and even the predators of herbivores.

One plant which has several fascinating adaptations involving chemical signalling is the wild coyote tobacco (Nicotiana attenuata).

As you may have guessed, this plant contains the toxin nicotine, which is harmful to most herbivorous predators. But there are some which are immune to the plants toxicity and for those it has developed other defences…

Hawkmoth’s are key pollinators of Nicotiana attenuate, and are attracted by both the smell (a chemical signal) and sight (visual signal) of the plant’s flowers. Their larvae (tobacco hornworm (Manduca sexta)), on the other hand, are not such welcome visitors as they feed on the leaves of the tobacco plant, unaffected by the nicotine they contain.

Once they begin to feed, however, they produce an oral secretion which is then detected by the plant, causing it to release jasmonic acid into the air. This acts as a signal to the same individual plant to alter it’s flowering patterns. In response, it switches from producing flowers which open at night (thereby attracting the hawkmoths) to ones that open during the day (which attracts hummingbirds to act as pollinators instead). Not only does the time at which the flowers open change, but the plant also alters the floral scent, the nectar produced and the morphology of the flower! The resulting decline in hawkmoth visitors means that they are less like to deposit eggs on the plant, meaning fewer larvae predators.

But that’s not all, as Nicotiana attenuate has another trick up it’s sleeve to rid itself of the hungry hornworms. In response to the larvae’s saliva, the tobacco plants also manufacture methyl jasmonate (MeJ). This then induces the plant to produce volatile terpenes (a type of organic compound) which attracts the tobacco hornworm’s natural predator – the big-eyed bug (Geocoris pallens) – which consumes both the larvae and the eggs of the hawkmoth.

And as if that weren’t enough, the plant also generates chemical volatiles in response to the depositing of herbivore eggs on wounded leaf areas, which attracts egg parasitoid wasps which then attack the eggs! It also thought that these volatile organic compounds can be detected by certain neighbouring plants which will in turn boost their own defences against herbivores, for example increasing production of a toxic substance in their leaves to deter predation.

 

Phytomining

Phytomining is an innovative new method of extracting copper from low-grade ores using plants, which also limits the environmental impact of traditional mining methods. It uses “hyperaccumulating” plants which absorb copper compounds through their roots and then concentrate these compounds within their tissues.  These can then be burnt to produce an ash which contains copper compounds.

As a classroom practical, brassica plants (such as Indian Mustard – Brassica juncea) which are relatively fast-growing, can be grown in compost with added copper sulphate, or sprayed with copper sulphate solution. The plants can then be ashed in a fume cupboard, and students can obtain metal from the ashes by adding sulphuric acid, filtering, and then separating the metal using either displacement or electrolysis (as described in the AQA exam boards GCSE chemistry specification).

Alternatively, the plants can be grown hydroponically and students can then measure copper levels during the test period and calculate the absorption of copper by the Brassica seeds. Instructions for this practical are available from SAPS (Science and Plants for Schools).

More plant practicals for the chemistry classroom…

Make your own pH indicator from cabbage leaves! Red cabbage leaves release pigment molecules called anthocyanins which change colour depending on the pH of the environment they are in. You can download resources from the Royal Society of Chemistry.

Investigate the effects of acid rain on plants – The AQA specification mentioned earlier has several different chemistry topics and suggested experiments related to plants. In this case students would grow cress from seeds in various concentrations of sodium metabisulfate solution and observe the differences in growth to see how acid rain can affect plants.

There are many other ways in which chemical sciences relate to plants – you can find some more practicals involving plants here and here. If you know of any other examples, particular those involving hands-on or outdoor experiments and activities please share your ideas and experiences in the comments section below!

 

Photo credits:
Plants in test tubes: from Flickr by CIAT International Center for Tropical Agriculture
Salix alba: from Flickr by HermannFalkner/sokol
Nicotiana attenuata: from Flickr by randomtruth
Manduca sexta: from Flickr by Chris Bede
Manduca sexta with parasitic wasp eggs: from Flickr by http://www.flickr.com/people/13113152@N03/
Red cabbage: from Flickr by rob patrick

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