Photosynthesis

Solar Energy for Life

Dmitry Shevela, Lars Olof Björn, Govindjee

• Publisher: WSPC
• Year: 2018
• ISBN:
  9789813223134
• Topic: Biological Sciences
• Subtopic: Botany
• Index: Biological Sciences

Photosynthesis has been an important field of research for more than a century, but the present concerns about energy, environment and climate have greatly intensified interest in and research on this topic. Research has progressed rapidly in recent years, and this book is an interesting read for an audience who is concerned with various ways of harnessing solar energy.

Our understanding of photosynthesis can now be said to have reached encyclopedic dimensions. There have been, in the past, many good books at various levels. Our book is expected to fulfill the needs of advanced undergraduate and beginning graduate students in branches of biology, biochemistry, biophysics, and bioengineering because photosynthesis is the basis of future advances in producing more food, more biomass, more fuel, and new chemicals for our expanding global human population. Further, the basics of photosynthesis are and will be used not only for the above, but in artificial photosynthesis, an important emerging field where chemists, researchers and engineers of solar energy systems will play a major role.

Contents:

• Introduction
• The Photosynthetic Apparatus
• Basics of Photosynthesis: Light-Dependent Reactions
• Basics of Photosynthesis: The Carbon Reactions
• Regulation of Photosynthesis
• Photosynthesis and Our Planet
• Anoxygenic Photosynthesis
• The Past, Present and the Future
• The Ultimate: Artificial Photosynthesis

Readership: Advanced undergraduate and graduate students of biology, biochemistry, biophysics, and bioengineering.
Key Features:

• To make the book readable to a wide readership, research methods are briefly described, concentrating on what is known about the biological process of photosynthesis
• Presents the most promising research results on natural photosynthesis and, to some extent, on artificial photosynthesis

Chapter 1

Introduction

1.1Why Study Photosynthesis?

At the dawn of photosynthesis research, the driving force was pure curiosity; it was truly a basic science. Today, we face several problems, which can only be solved by increased knowledge of climate change, energy demand, food supply, pollution, and much more!

Plants sequester the greenhouse gas carbon dioxide (CO₂) very efficiently from air. Can we learn from them and develop ways to clean the atmosphere more efficiently?

We have many questions before us…

How will plant behavior and plant photosynthesis change when the climate changes? How will these changes in photosynthesis provide feedback to the climate?

All food is produced directly or indirectly by photosynthesis. Can photosynthesis be increased to produce even more? Can it be modified to produce healthier food?

Mineral oil (from which gasoline/petroleum is produced) is a limited resource that was, ultimately, produced by photosynthesis long ago. In addition to serving as fuel, it is used to produce a number of chemicals, plastics, and other commodities. Can we modify photosynthetic organisms (plants, algae or bacteria) to produce the necessary raw materials for us?

Can we replace fossil fuel as an energy source with the products of artificial photosynthesis? Can electricity be produced directly by photosynthesis-inspired methods?

An increasing number of planets are being discovered outside of our own solar system. Do any of them harbor life? If there is photosynthetic life out there, can we detect it from the spectra of their light-catching pigments, or from the gas that they produce? In addition, we ask if there are alternatives to the terrestrial compounds involved in photosynthesis?

1.2History

1.2.1The discovery of photosynthesis

Contrary to the opinion of the Greek philosopher Aristotle (384–322 B.C.), the idea that much of the substance of a plant was derived from water can be traced to a 2nd century A.D. unidentified author, referred to as Pseudo-Clement [Howe, 1965]. The original text of his “Recognition” is now lost, and survives only in a Latin translation made by Rufinus, Presbyter of Aquileia, in the 4th century A.D.

The notion of water being an element remained for a long time. Nicholas of Cusa (1401–1464), a German philosopher, scientist and a Cardinal, wrote, in clear contradiction to Aristotle, in the book De Staticus Experimentis [1450; cf. Krikorian and Steward, 1968] that if one puts earth (soil) and a plant (or seeds) in a pot, and allows the plant to grow hundred-fold in size, one would find that the soil would have lost very little weight, and the plant would have gained all its weight from the added water. This book might have inspired the physician Jan Baptista van Helmont (1580–1644) in Brussels, Belgium, to conduct his famous experiment [van Helmont, 1648]:

I took an earthen pot and in it placed 200 pounds of earth which had been dried out in an oven. This I moistened with rainwater, and in it planted a shoot of willow [tree]which weighed five pounds. When five years had passed the tree, which grew from it, weighed 169 pounds and about three ounces. The earthen pot was wetted whenever it was necessary with rain or distilled water only. It was very large, and was sunk in the ground, and had a tin plated iron lid with many holes punched in it, which covered the edge of the pot to keep airborne dust from mixing with the earth. I did not keep record of the weight of the leaves, which fell in each of the four autumns. Finally I dried out the earth in the pot once more, and found the same 200 pounds, less about 2 ounces. Thus, 164 pounds of wood, bark, and roots had arisen from water alone.

Thus, van Helmont did not understand that a large part of the plant substance is derived from the air, which was still considered to be an individual element, just as water. The notion that air contains a gas that is important for life can be traced back to Michael Servetus (~1510–1553), who published the book “Christianismi Restitutio” which describes how blood, when passing through the lungs, changes color. The book was disliked by the authorities in Geneva (Switzerland), so both Servetus and most copies of his book were burnt; only one copy seems to have survived. In 1668, John Mayow, a former assistant to Robert Boyle [Severinghaus, 2014], published a book in which he showed that air was a mixture of two components of which one-fifth was essential for life; it is consumed during breathing, and in fueling fire, and thus provides both body heat and energy.

In 1771, Carl Wilhelm Scheele (1742–1786) from the then Swedish province of Pomeronia in what is now northern Germany, while working at Uppsala University, produced a gas by heating mercury oxide. He called it “fire air” since it supports fire even better than air. Because of several unfortunate circumstances, his findings were not published until 1777. By then Joseph Priestley (1733–1804), of England, had already replicated his earlier findings, giving the produced gas the name “oxigen” (“oxi” meaning acid), and discovered several other gases. On August 17, 1771, he placed a sprig of mint into the “noxious air” produced by a lit candle, in an airtight container. Ten days later, a candle burned in the air perfectly well. He carried out the experiment 10 times during the summer of 1771 and repeated this experiment in the summer of 1772. The best results were obtained when spinach was used, and this plant has remained a favorite material for photosynthesis researchers. On March 8, 1774, Priestley decided to test the effects of oxygen on a mouse. He found that a mouse could survive in “fouled air” after it had been “dephlogistonated” [dephlogisticated] by a green plant. Although this is commonly regarded as the discovery of photosynthesis, Priestley did not notice the importance of light or CO₂ (the latter had been discovered by the Scottish chemist Joseph Black [1728–1799], already in 1755). Surprisingly for us in the 21st century, Priestley continued to believe in the phlogiston theory for the rest of his life. It was the French chemist Antoine Laurent de Lavoisier (1743–1794) who disposed of the old ideas of air and water being elements by showing that water is made up of hydrogen and oxygen. Methods improve, and the death of old ideas becomes less painful than that of Michael Servetus, since, instead of being burnt alive, Antoine Laurent de Lavoisier was executed by the guillotine! Lavoisier had angered Jean Paul Marat (1743–1793), another scientist whose writings he had criticized. Marat, in turn, was stabbed to death in his bathtub by Charlotte Corday. During this time, the situation for Priestley grew very turbulent and he was forced to flee to America (USA).

In 1778, the Dutch physician Jan Ingenhousz (1730–1799) repeated Priestley’s experiments in a rented villa in England. He kept some jars and plants in darkness, and exposed others to sunlight. He found that a candle would burn longer, and a mouse would be revived only if the plant was exposed to sunlight. He clearly discovered the role of light in photosynthesis. In 1782, the Swiss botanist Jean Senebier (1742–1809), while modifying Ingenhousz’s experiments, proved that plants absorb CO₂.

Nicolas Théodore de Saussure (1767–1845) extended, in an admirably quantitative way, Senebier’s experiments. de Saussure described how plants quickly became yellow and wilted in light after he removed CO₂ from the air, which he did by using calcium hydroxide. Further, he noticed that plants evolved oxygen (O₂) only when CO₂ was present in the air surrounding them [de Saussure, 1804]. However, he drew the erroneous conclusion that CO₂ was decomposed so that free O₂ was released from it. In an experiment with periwinkle (Vinca minor), de Saussure found that a sample of air before the experiment contained 4199 cm³ nitrogen (N₂), 1116 cm³ O₂, and 431 cm³ CO₂, i.e., a total of 5746 cm³ gas. In another sample of 5746 cm³ of air in which he had kept plants illuminated for some time, he found 4338 cm³ of N₂, 1408 cm³ of O₂, and no CO₂. The increase of O₂ was 292 cm³, which was 139 cm³ less than the amount of CO₂ that had disappeared. de Saussure, once more, drew another erroneous conclusion that some CO₂ had been transformed to N₂. This example shows that experimental science is not always as straightforward as one would like to think. We have described this example here, since it is an early example of the use of a unit still in use in science (de Saussure called it “centim. cub.”). In other experiments, de Saussure showed that plants increased their content of carbon, just as CO₂ disappeared. He carried out experiments with many different plants, always trying to work as quantitatively as possible, and concluded that the amount of carbon gained matched what was obtained from the air, and that, together with water, gave rise to the plant material.

Ingenhouz in “An essay on the food of plants and the renovation of soils” (1796) translated the whole process of photosynthesis from the old phlogiston concept to the new chem...