The Chemistry of Plants

Perfumes, Pigments and Poisons

Margareta Sequin

• Publisher: Royal Society of Chemistry
• Year: 2015
• ISBN:
  9781782625568
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Organic Chemistry

This book is an introduction to organic chemistry and its compounds as related to plants. Chemistry tends to be seen as a field that is hard to comprehend and that has few connections with the living world. This book fills a gap as it eases access to organic chemistry by connecting it with plants and includes numerous photos and other illustrations. The book is a combination of organic chemistry with the living world of plants and is an introduction to organic plant compounds for the non-chemist. It starts with a review of basic concepts of chemistry as they relate to plant life, followed by an introduction to structures of organic compounds, which prepares the reader for the following chapters on primary metabolites and on plant fragrances, pigments, and plant defensive compounds. The final chapter relates plant compounds to human life, with subchapters on foods from plants, medicines, psychoactives, fibers, and dyes. Historic discoveries of plant compounds and their developments to contemporary uses, like modern pharmaceuticals, and a section on genetically modified plants, connect with topics of recent interest. The book leads the serious reader from chemistry basics to complex plant substances and their human uses and plant photos and stories accompany chemistry topics and chemical structures to aid understanding. The author, an organic chemist and plant enthusiast, has taught popular undergraduate college level courses on plant chemistry to non-chemistry majors and numerous field seminars to the general public for more than fifteen years. The book's topics and contents have been taught for many years and have proved successful in providing an understanding of plant compounds, organic compounds, and their importance. The book provides a basis for a better understanding of chemistry and its connections to the world of plants, the natural world in general, and to daily life. It is aimed at non-chemistry undergraduate students and to people in general who are interested in plants and who would like to learn more about them. It addresses an audience with little previous chemistry knowledge, yet, leads the serious reader to an understanding of sometimes complex plant compounds, by providing an introduction to chemistry basics, combining the chemistry with pictures and stories, and using simple, clear language. The book 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 developed a wealth of shapes and sizes and with them a rich array 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 panoply 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 depend on (and 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 to their organic chemistry. As a preparation for the later descriptions of the chemistry of plant odors, colors, and defensive plant compounds, this first 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 the wealth of carbon-based organic molecules.

Elements link up by chemical bonds to form compounds. As plants are mostly aqueous systems, a special section addresses the unusual properties of the compound water. The distinct structures of water molecules 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. Only with the proper nutrients and conditions can plants make all the amazing compounds shown later. A discussion of soils leads to a more detailed look at ions, their mineral nutrients, and their acid or alkaline nature. Some soil compositions are also described that make 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. These can be highly restrictive conditions. Yet, plants are able to function under such 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 and toxins. Respiration, the set of reactions in which sugars and fats are broken down, provide the energy for further reactions in plants.

The chapter concludes with an introduction to organic compounds and how to understand their structures. Only 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 hydrocarbons.

Just as chemical structures exactly describe the composition of a plant substance and pinpoint which compound is addressed (caffeine, vanillin, or vitamin C?), systematic names of plants, also known as scientific names 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 or instead may describe a tree. 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 here. The glossary at the end of this book provides explanations of key expressions (emphasized in the text) 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 which 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 (see inside back cover) lists all the elements known to this day. But only a select few are essential for plant growth, meaning they must be available to plants for survival. Table 1.1 lists them 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.^1–3 Some of their major functions in plants are shown there 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 enzyme in nitrogen metabolism.	Ni^2+

Plant nutrients are required in different amounts, and the quantities needed vary at different stages of plant life. Nutrient elements are considered to be either macronutrients that are required in relatively large amounts, or micronutrients that are needed in much smaller amounts, sometimes traces only. Nevertheless, all the nutrients must be available. The growth of plants is greatly challenged if too much of a micronutrient or too little of a macronutrient is supplied, or if toxic additional elements are present in the soil. Plants that are able to grow in such environments need to have special adaptations. Some examples are described in Box 1.1.

The macronutrients carbon (C), hydrogen (H), oxygen (O) and nitrogen (N) assemble most of a plant structure. They are parts of compounds in which the elements’ atoms are linked to each other by chemical bonds to form molecules.⁴ A large portion of hydrogen and oxygen atoms are tied up in water molecules (H₂O), as plants are mostly aqueous systems. Carbon atoms are a required part of all organic compounds. In addition, the ...

Table of contents

• Cover image
• Title page
• Copyright
• Preface
• Contents
• Chapter 1 Basic Plant Chemistry Concepts
• Chapter 2 The Molecular Building Blocks
• Chapter 3 Perfumes, Fragrant or Foul
• Chapter 4 Colorful Plant Pigments
• Chapter 5 Poisons and Other Defenses
• Chapter 6 Plants and People
• Epilogue
• Glossary
• Photo Credits
• Subject Index