BIOCHEMISTRY - L. Stryer - 1984

VOLUME 2

PART II GENERATION AND STORAGE OF METABOLIC ENERGY

CHAPTER 19. PHOTOSYNTHESIS

All of the Free energy utilized by biological systems originates from solar energy, which is captured during The process of Photosynthesis. At first glance, the overall equation for photosynthesis appears simple:

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(СН₂O) in this equation represents a carbohydrate. The Mechanism of photosynthesis is complex and requires the concerted action of many macromolecules and small molecules. In green plants, photosynthesis takes place within METABOLISM/14.html">Chloroplasts, which are specialized Organelles. The energy-Transduction apparatus is an integral component of the membrane system within the chloroplast thylakoids (Fig. 19.1). The initial step in photosynthesis is the absorption of light by a chlorophyll molecule. Energy is transferred from one chlorophyll molecule to another until it reaches a specialized molecule at a site designated as the reaction center. The conversion of light into chemically usable energy occurs within two distinct types of reaction centers. In fact, photosynthesis requires the cooperation of two light reactions. One of these, termed Photosystem I, generates reducing power in the form of NADPH, whereas the other, termed Photosystem II, splits Water with the release of O₂ and generates a reductant. A proton gradient across the thylakoid membrane is generated when O₂ is evolved and when electrons flow through the electron-transport chain linking the Two Photosystems. The synthesis of ATP, much like in Oxidative Phosphorylation, is driven by this proton gradient. ATP can also be formed independently of simultaneous NADPH generation.

Fig. 19.1. Electron micrograph of a portion of a chloroplast from a spinach leaf. Thylakoid membranes form stacks known as grana

The NADPH and ATP generated via light-driven reactions are subsequently utilized to reduce СO₂ into CARBOHYDRATES through a series of dark reactions known as The Calvin Cycle. These reactions take place within the soluble compartment of the chloroplasts. The first step involves the reaction of СO₂ with ribulose 1,5-bisphosphate, yielding two molecules of 3-phosphoglycerate. Through The Gluconeogenesis pathway, 3-phosphoglycerate is converted into hexoses, and subsequently, via the action of transketolase, aldolase, and several Other Enzymes, ribulose bisphosphate is regenerated. During one turn of the cycle, the reduction of СО₂ to hexose phosphate consumes 3 ATP and 2 NADPH.

Electron micrograph of an entire chloroplast from a spinach leaf

19.1. Discovery of the Overall Equation of Photosynthesis

The overall equation for photosynthesis could be formulated, at least in a first approximation, as early as the late 18th century. The evolution of oxygen during photosynthesis was discovered by Joseph Priestley in 1780. He found that plants were able to "restore air that had been fouled by a burning candle." Priestley placed a mint sprig inside an inverted Glass jar submerged in a vessel of water and, after several days, observed that "the air inside the jar did not extinguish a candle, and when I placed a mouse in the jar, it did no harm to it." This prominent 18th-century chemist was also an English nonconformist minister. His primary interests lay in theology, philosophy, and politics. In 1791, owing to his sympathies for the French Revolution, Priestley was forced to leave England. He moved first to France and later to the United States, where he passed away in 1804 after spending several peaceful years on the banks of the Saskatchewan. Quite different was The Fate of Priestley's contemporary, who laid the foundation for elucidating the fundamental process of photosynthesis. Antoine Lavoisier developed Methods for gas research and formulated THE CONCEPT OF oxidation alongside the law of conservation of mass in Chemical Reactions. Because of his ties to royalists, he was executed in 1794 by French revolutionaries. The judge who pronounced the sentence famously remarked that "the Republic has no need of scientists."

Fig. 19.2. Priestley's classic experiment on photosynthesis

The next major contribution to The Study of photosynthesis was made by Jan Ingenhousz, a Dutch physician to the Austrian empress. Ingenhousz was a man of the world who loved visiting London. Having heard a Structure/133.html">Discussion of Priestley's Experiments on the restoration of air by plants, he became so captivated by the problem that he resolved to perform some experiments at his very first opportunity. That opportunity presented itself six years later, when Ingenhousz rented a villa near London and spent the summer feverishly conducting more than five hundred experiments. He discovered the crucial role of light in photosynthesis:

"I discovered that plants not only possess the power to correct foul air over six or ten days when growing in it, as Dr. Priestley's experiments indicate, but that they perform this vital function completely within a few hours. This remarkable operation is due not to the growth of the plant, but rather to The Influence of sunlight upon it."

Ingenhousz hurried to publish his observations, fearing that someone else might beat him to it. At the end of the summer, he released a book entitled *Experiments Upon Vegetables, Discovering Their Great Power of Purifying the Common Air in Sunshine, and of Injuring It in the Shade and at Night*.

This fear of competition was entirely justified. Similar experiments were being conducted in Geneva by the Swiss pastor Jean Senebier. He made an outstanding contribution to the study of photosynthesis by demonstrating that the process involves the consumption of "fixed air," specifically СO₂. The Role of water in photosynthesis was likewise established in Geneva by Théodore de Saussure. He found that the total weight of organic matter produced by plants, together with the evolved O₂, significantly exceeds the weight of the consumed СO₂. Based on Lavoisier's law of conservation of mass, de Saussure concluded that another substance must be utilized. Since the only components of his system were СO₂, water, and light, de Saussure deduced that this re-

agent must be water.

The final contribution toward uncovering the overall equation of photosynthesis was made nearly half a century later. In 1842, Julius Robert Mayer, a German surgeon, formulated the law of conservation of energy. Mayer established that plants convert solar energy into chemical free energy: "Plants consume one form of energy—light—and transform it into another form of energy—chemical energy."

The amount of energy stored through photosynthesis is immense. More than 10¹⁷ kcal of free energy is captured globally each year via photosynthesis, which corresponds to the assimilation of over 10¹⁰ tons of carbon incorporated into carbohydrates and Other forms of organic matter.