PHOTOSYNTHESIS
This book tells the story of photosynthesis: plants use sunlight, water, CO2 to make food and oxygen, sustaining all life
The journey into understanding how life on Earth sustains itself begins with some deceptively simple questions. Why, for instance, does the planet's food supply not disappear, consumed by all the living things that depend upon it? Where does the constant replenishment of the atmosphere's oxygen come from? And, perhaps most fundamentally, how is the vast, incoming energy from the Sun transformed into something usable by living organisms? This book, part of a series dedicated to presenting the exciting world of science as only scientists know it, sets out to explore these vital questions by tracing the efforts of scientists to unravel the mysteries of photosynthesis. It's a look through a chemist's eyes at a process upon which life in all its forms is utterly dependent.
Science often progresses like a story, starting with broad observations and gradually filling in the details. The initial understanding involved recognizing a grand cycle, a fundamental exchange where life forms consume certain substances and energy, and in turn produce others that support different life forms or replenish the environment. Early scientists, like Hales in 1727, began studying the gases involved, realizing plants might take up gases just as animals do. Later, the crucial role of plants in producing a gas that supports life – what we now know is oxygen – was uncovered through experiments by Priestley and Ingenhousz. Ingenhousz made the momentous discovery that plants produced oxygen only in the presence of sunlight. This suggested that sunlight was essential, a form of energy driving the process where complex substances ("food") are built up from simpler ones (like carbon dioxide and water), alongside the production of oxygen. This initial understanding can be summarised as energy + water + carbon dioxide → oxygen + food.
To sharpen this picture, scientists needed to identify the specific "food" molecule. Experiments showed that starch, a complex substance, was produced in plant leaves exposed to sunlight. Since starch is known to be built from simpler glucose units, it seemed reasonable that glucose was the primary food substance formed first in photosynthesis. Substituting glucose for the vague term "food" gave a clearer view of the cycle. But understanding these molecules required delving into their structure – not just the types and number of atoms, but how they are arranged. The development of structural formulas, pioneered by chemists like Frankland and Kekulé, became essential tools for picturing these invisible building blocks of life. Glucose, a six-carbon sugar, was finally structurally elucidated later.
However, understanding the material side of the cycle is only part of the story; the energy side is equally crucial. The energy that makes all life possible ultimately comes from the Sun. The Sun's energy is released through nuclear fusion, a process that liberates enormous quantities of energy. This solar energy reaches Earth and, through photosynthesis, is converted into chemical energy stored in the glucose molecule.
The conversion and use of energy are governed by fundamental laws of physics, particularly the laws of thermodynamics. The first law, the conservation of energy, states that energy cannot be created or destroyed, only transformed. This law holds true even in living systems. The second law introduces the concept of entropy, a measure of disorder. Spontaneous processes, often called "downhill" reactions, involve an increase in entropy and a loss of "free energy" – the energy available to do work. Respiration, for instance, is a downhill process where glucose and oxygen react to release free energy. Photosynthesis, on the other hand, is an "uphill" process, requiring an input of energy to build complex molecules from simpler ones. This uphill climb is powered by solar energy captured by the plant.
Life has developed ingenious ways to manage energy, storing and transferring it in convenient packets. A key energy currency in living cells is ATP (adenosine triphosphate), a molecule that stores energy in its chemical bonds, particularly the "high-energy phosphate" bonds. Breaking these bonds releases free energy that can power the energy-consuming, uphill reactions necessary for life, such as building complex molecules. Processes like the breakdown of glucose (glycolysis and the Krebs cycle) are downhill processes that release energy, some of which is used to regenerate ATP from ADP (adenosine diphosphate). Photosynthesis is the ultimate uphill process that makes this entire energy cycle possible by capturing solar energy to create the glucose that fuels everything else.
Unravelling the detailed steps of photosynthesis, particularly how carbon dioxide is converted into glucose, was a significant challenge. Early hypotheses, like the formaldehyde hypothesis, proved incorrect. The breakthrough came with the development of new techniques, notably the use of radioactive isotopes as tracers. By using carbon dioxide containing a radioactive carbon atom (Carbon-11, initially, then the more stable Carbon-14), scientists could follow the path of carbon atoms as they were incorporated into organic molecules during photosynthesis. Coupled with the development of chromatography, a technique for separating complex mixtures, scientists could expose plant cells to radioactive carbon dioxide for very short periods, stop the process, and then identify the first molecules that contained the radioactive carbon. These experiments revealed the intermediate compounds formed on the way to glucose.
The story broadens beyond just the biochemical reactions within the plant cell, venturing into the grand scale of life and the universe. The sources make it clear that the Sun provides an immense amount of energy, far more than enough to support all life on Earth. Yet, the efficiency of photosynthesis in capturing this energy has its limits. This brings the story back to humanity, highlighting the stark reality that the amount of food available is ultimately limited by the total amount of glucose produced by plants annually. The increasing human population faces fundamental limitations imposed by the Earth's capacity to capture solar energy through photosynthesis, leading to Asimov's characteristic observations about the need for population control.
Finally, the book takes a philosophical turn, exploring the very beginning of life on Earth. Drawing on the understanding of the early Earth's atmosphere and the energy available (like ultraviolet light), scientists have conducted experiments showing that simple organic molecules, including amino acids (the building blocks of proteins), can form spontaneously under plausible early Earth conditions. This suggests that life might have arisen through purely natural, chemical evolution, driven by available energy, a view consistent with Asimov's perspective as a scientist exploring the unfolding view of the universe. The development of more complex molecules and structures, leading eventually to cell-like entities, is presented as a continuation of this process. In this narrative, photosynthesis emerges not just as a process sustaining modern life, but possibly as a pivotal development in life's history, enabling the shift from a dependence on less abundant energy sources to the vast power of the Sun.
This scientific saga, moving from simple questions to complex molecular mechanisms, from historical experiments to modern techniques, and expanding to encompass the vastness of the universe and the sweep of evolutionary time, exemplifies Asimov's skill in making intricate scientific subjects accessible and compelling. It tells the story of how scientists pieced together one of life's most fundamental processes, revealing not just the chemistry but the profound interconnectedness of energy, matter, and life itself, hinting at the possibility of similar stories unfolding on worlds far beyond our own.
