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.
