Energy and the Human Journey: Where We Have Been; …
The end result of excitation of both photosystems is that electrons have been transferred from chlorophyll to NADP+, forming NADPH, and some of their energy has been used to generate ATP. While photosystem I gains electrons from photosystem II, the electrons lost by photosystem II have not been replaced yet. Its reaction center acquires these electrons by splitting water. During this process, the electrons in water are removed and passed to the reaction center chlorophyll. The associated hydrogen ions are released from the water molecule, and after two water molecules are thus split, the oxygen atoms join to form molecular oxygen (O2), a waste product of photosynthesis. The reaction is:
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In summary, today’s orthodox late-Proterozoic hypothesis is that the complex dynamics of a supercontinent breakup somehow triggered . The global glaciation was reversed by runaway effects primarily related to an immense increase in atmospheric carbon dioxide. During the events, oceanic life would have been delivered vast amounts of continental nutrients scoured from the rocks by glaciers, and the hot conditions would have combined to create a global explosion of photosynthetic life. A billion years of relative equilibrium between prokaryotes and eukaryotes was ultimately shattered, and oxygen levels began rising during the Cryogenian and Ediacaran periods toward modern levels. Largely sterilized oceans, which began to be oxygenated at depth for the first time, are now thought to have prepared the way for what came next: the rise of complex life.
Mass extinctions always have critical geophysical aspects to them, and often geochemical. Continental shelves under shallow seas, which are home to most marine life, are vulnerable to sea level and oceanic current changes. Stagnant waters, or waters that have too many nutrients dumped into them, can lose their oxygen, which triggers anoxic events that kill complex life. A continental shelf exposed to the atmosphere by a falling sea level would obviously lose its marine life, and that marine life might have had nowhere else to go. Sea levels can rise or fall for different reasons. The most obvious reason has been advancing and retreating ice sheets, as water is removed from or added to the oceans, but the aggregate continental landmass has always grown (possibly sporadically), continents can rise and can fall during the journeys of their tectonic plates, and the ocean’s collective basin has fluctuated in size, as water was hydrated into rocks, and also falling when and rising again as they fragmented. Generally, when , the continental shelves lost their marine life, and , anoxic conditions often accompanied them. There is evidence that the ozone layer has been periodically damaged, which stressed all plants and animals that the Sun directly shined on. The positions of the continents, both in relation to each other and their proximity to the equator or poles, can have dramatic effects, including impacts on global climate. Global climate changes and moving continents can turn rainforests into deserts and vice versa.
Plant Energy Transformations-Photosynthesis
photosynthesis , process in which green plants, algae, and cyanobacteria utilize the energy of sunlight to manufacture carbohydrates from carbon dioxide and water in the presence of chlorophyll. Some of the plants that lack chlorophyll, e.g., the , secure their nutrients from organic material, as do animals, and a few bacteria manufacture their own carbohydrates with hydrogen and energy obtained from inorganic compounds (e.g., hydrogen sulfide) in a process called . However, the vast majority of plants contain chlorophyll—concentrated, in the higher land plants, in the leaves.
In these plants water is absorbed by the roots and carried to the leaves by the xylem, and carbon dioxide is obtained from air that enters the leaves through the stomata and diffuses to the cells containing chlorophyll. The green pigment is uniquely capable of converting the active energy of light into a latent form that can be stored (in food) and used when needed.
The Photosynthetic Process
The initial process in photosynthesis is the decomposition of water (HO) into oxygen, which is released, and hydrogen; direct light is required for this process. The hydrogen and the carbon and oxygen of carbon dioxide (CO) are then converted into a series of increasingly complex compounds that result finally in a stable organic compound, glucose (CHO), and water. This phase of photosynthesis utilizes stored energy and therefore can proceed in the dark. The simplified equation used to represent this overall process is 6CO+12HO+energy=CHO+6O+6HO. In general, the results of this process are the reverse of those in respiration, in which carbohydrates are oxidized to release energy, with the production of carbon dioxide and water.
The intermediary reactions before glucose is formed involve several enzymes, which react with the coenzyme ATP (see ) to produce various molecules. Studies using radioactive carbon have indicated that among the intermediate products are three-carbon molecules from which acids and amino acids, as well as glucose, are derived. This suggests that fats and proteins are also products of photosynthesis. The main product, glucose, is the fundamental building block of carbohydrates (e.g., sugars, starches, and cellulose). The water-soluble sugars (e.g., sucrose and maltose) are used for immediate energy. The insoluble starches are stored as tiny granules in various parts of the plant—chiefly the leaves, roots (including tubers), and fruits—and can be broken down again when energy is needed. Cellulose is used to build the rigid cell walls that are the principal supporting structure of plants.
Importance of Photosynthesis
Animals and plants both synthesize fats and proteins from carbohydrates; thus glucose is a basic energy source for all living organisms. The oxygen released (with water vapor, in transpiration) as a photosynthetic byproduct, principally of phytoplankton, provides most of the atmospheric oxygen vital to respiration in plants and animals, and animals in turn produce carbon dioxide necessary to plants. Photosynthesis can therefore be considered the ultimate source of life for nearly all plants and animals by providing the source of energy that drives all their metabolic processes.
See I. Asimov, (1969); R. M. Devlin and A. V. Barker, (1972); O. Morton, (2009).