The Chemistry of Plants

Perfumes, Pigments and Poisons

Margareta Séquin

• Publisher: Royal Society of Chemistry
• Year: 2021
• ISBN:
  9781839162688
• Edition: 2
• Topic: Physical Sciences
• Subtopic: Organic Chemistry

Why are some plants so important to humans? The chemistry of the plants has a lot to do with it!

The plant world offers a fascinating way to explore basic chemistry concepts. The spectacular variety of colors, fragrances and other characteristics of plants are driven by the seemingly subtle differences in the structure and properties of organic compounds. Well-known flowers, like daffodils and narcissus, are examples of plants that provide ample perfumes, pigments and poisons as part of their intricate and fascinating chemistry.

This second edition retains it accessibility, expanding on the first edition and combining scientific concepts with colorful pictures and stories in simple, clear language. Readers will find introductory information on some chemistry and plant biology. This prepares them for the more complex chemical structures that compose plant substances, many of them of vital importance to humans. The final chapter has been expanded, in particular the sections on medicinal plants and on genetic modification. The end-of chapter references have been thoroughly updated with articles, books, and relevant websites that illustrate the topics discussed.

Dr Margareta Sequin, an organic chemist and plant enthusiast, has taught popular undergraduate college level courses on plant chemistry to non-chemistry majors and has led numerous field seminars for the general public. The comments and questions from these audiences and the topics that especially captured people's interest have greatly shaped this book.

The Chemistry of Plants addresses an audience with little previous chemistry knowledge, but will appeal to the expert reader looking for an understanding of more complex plant compounds. It can be used both as a text to introduce organic chemistry as it relates to plants and as a text of reference for more advanced readers.

Chapter 1

Basic Plant Chemistry Concepts

1.1 Introduction

For millions of years, plants have evolved a wealth of shapes and sizes and with them an abundance of highly diverse substances that help them stay alive and reproduce. Sweet fragrances from wild roses, bright pigments in fall leaves, and potent poisons, like those of the deadly nightshade plant, are all part of the huge range of compounds that plants produce to attract, protect, and repel (Figure 1.1). Most importantly, plants contain green chlorophyll, capable of trapping portions of sunlight. With chlorophyll's help, plants generate the basic chemicals that humans could not live without, like oxygen, sugars, fats, amino acids, and vitamins.

Figure 1.1 Perfumes, pigments, and poisons. (a) A fragrant wild rose (Rosa rugosa). (b) Colorful fall foliage of a grapevine (Vitis cultivar). (c) Branch of a deadly nightshade plant (Atropa belladonna). Photo by Ruth Marent.

This book is an introduction to the chemistry of plants, especially concerning their organic chemistry. As a preparation for the descriptions of the chemistry of plant smells, colors, and defensive plant compounds, this introductory chapter reviews some basic chemistry concepts as they relate to plants.

We begin with a look at elements and their atoms. Plants need to have a set of elements available as nutrients, and in useful form. (It is a set that is not so different from human needs.) Just a couple of these elements—carbon, hydrogen, oxygen, nitrogen, and a few others—assemble to form the abundance of carbon-based organic molecules.

Elements link up by chemical bonds to form compounds. Plants, like all living things on Earth, require the compound water to live and grow. Therefore, a special section addresses the distinct structures of water molecules. They determine the unusual properties of this vital compound and affect how water moves through plants, how minerals are transported in aqueous saps, and where pigments are stored in plant cells.

Every gardener knows that growing plants starts with the right soil. Aside from suitable growing conditions, plants must have a sufficient supply of essential nutrients to be able to synthesize all the compounds that enable them to grow and live. With the proper nutrients and the right growing conditions, plants can produce alluring smells, enticing colors, or defensive substances. A look at the composition of soils leads to a more detailed description of ions, mineral nutrients in soils, and the acid or alkaline nature of growing media. Some soil compositions are also described that make understanding plant life truly challenging.

With a basic knowledge of elements, ions, and compounds in hand, we continue to study how plant compounds interact in chemical reactions that assemble new plant compounds or break them down. Plants must be able to perform these reactions in conditions dictated by their environment, namely at ambient temperatures and mostly in water. However, the environment can also entail highly restrictive conditions. Plants are able to function under harsh conditions thanks to elaborate enzymes and lots of time. Suitable nutrients, with light as the source of energy and with the help of the pigment chlorophyll, allow plants to undergo the numerous reaction steps of photosynthesis. These reactions produce oxygen and simple sugars like glucose. The sugars, in turn, are needed to compose all other organic compounds in plants: cellulose for plant structures, starch in bulbs to store energy, fats and amino acids, as well as plant fragrances, pigments, and toxins. Respiration, the set of reactions in which sugars and fats are broken down, provides the energy for further reactions in plants.

The chapter ends with an introduction to organic compounds and how to understand their structures. A few simple rules are needed to assemble basic organic molecules. Hydrocarbons, consisting of carbon and hydrogen only, will provide an introduction to organic structures. They will be illustrated with some examples of plants that contain specific hydrocarbons.

Just as chemical structures exactly describe the composition of a plant substance and pinpoint which compound is addressed (e.g. caffeine, vanillin, or vitamin C), systematic names of plants, also known as scientific or binomial names, clearly identify a plant. Common names vary regionally, and the same name may describe different plants. For example, the common name “hemlock” can refer to a poisonous, herbaceous plant native to the Mediterranean region or may describe a coniferous tree growing in North America. Add to this native and foreign language names, and the confusion is complete. The scientific name of a plant, on the other hand, describes universally which plant is meant (although it can change sometimes, too, because of new studies of plant relationships). Therefore, scientific names are included with the plant examples. The glossary at the start of this book provides explanations of key expressions (emphasized in the text with italics) and a brief introduction to the structure of scientific plant names.

1.2 Plants and Their Elements

For successful growth, productivity, and good survival, plants must have a continuous supply of specific nutrients they can then transform into sugars, starch, plant structural materials, colorful pigments, and all the substances that make plant life possible. These nutrients are a collection of elements in various forms and combinations. The periodic table of elements (see the front of this book and Box 1.1) lists all the elements known to this day.^1,2 But only a select few are essential for plant growth, i.e. they must be available to plants for survival. Table 1.1 lists the essential elements and shows some of the forms or combinations in which these elements have to be available so that plants can make use of them as nutrients.^3–5 Some of their major functions in plants are provided as well.

Table 1.1 Essential elements in plants.

Element	Some major functions	Sources
Macronutrients
Carbon	Essential component of organic compounds	CO2
Oxygen	Major component of organic compounds	H2O, O2
Hydrogen	Major component of organic compounds	H2O
Nitrogen	Component of nucleic acids, proteins, chlorophyll, alkaloids	NO3^−, NH4^+
Sulfur	Component of some amino acids, proteins, coenzymes	SO4^2−
Phosphorus	Component of nucleic acids, ATP, phospholipids, coenzymes	H2PO4^−, HPO4^2−
Potassium	For osmotic balance, operation of stomata; enzyme activator	K^+
Calcium	Required for formation and stability of membranes; activation of some enzymes	Ca^2+
Magnesium	Component of chlorophyll; activates many enzymes	Mg^2+
Micronutrients
Iron	In chlorophyll synthesis, activates some enzymes	Fe^3+, Fe^2+
Chlorine	For ion balance; in water-splitting process of photosynthesis	Cl^−
Boron	Cofactor in chlorophyll synthesis	H2BO3^−
Manganese	Activates enzymes; in chlorophyll synthesis	Mn^2+
Copper	Involved in redox reactions	Cu^2+, Cu^+
Zinc	Activates enzymes	Zn^2+
Molybdenum	Essential for nitrogen fixation	MoO4^2−
Nickel	Cofactor for enzymes in nitrogen metabolism	Ni^2+

Box 1.1 A Brief History of the Periodic Table of Elements.

In 1869, Dmitri Mendeleev, professor of general chemistry at the St. Petersburg University, Russia, published a table of the then-known somewhat more than 60 elements. In this table he sorted the elements according to increasing atomic weights. (Mendeleev did not yet know details about atomic structures, of course, like numbers of protons and electrons.) He listed the elements with symbols, like C for carbon or Mg for magnesium, a practice that is still used. As he was aware of regular patterns (or “periodicities”) of recurring chemical affinities among elements, he placed elements with similar properties into the same groups. He also realized that there were elements yet unknown and predicted their properties. As a consequence, elements that had not been known were rapidly discovered in the years following the publication of Mendeleev's first periodic table.

The alignment of elements in Mendeleev's first periodic table and its shape were different from the current periodic table. But the basic idea of sorting elements acco...