In the earliest days of life on Earth, it had to solve the problems of how to reproduce, how to separate itself from its environment, how to acquire raw materials, and how to make the chemical reactions that it needed. But it was confined to those areas where it could take advantage of briefly available potential energy as . The earliest process of skimming energy from energy gradients to power life is called respiration. That earliest respiration is today called because there was virtually no free oxygen in the atmosphere or ocean in those early days. Respiration was life’s first energy cycle. A biological energy cycle begins by harvesting an energy gradient (usually by a proton crossing a membrane or, in photosynthesis, directly capturing photon energy), and the acquired energy powered chemical reactions. The cycle then proceeds in steps, and the reaction products of each step sequentially use a little more energy from the initial capture until the initial energy has been depleted and the cycle’s molecules are returned to their starting point and ready for a fresh influx of energy to repeat the cycle.
As with other early life processes, the first photosynthetic process was different from today’s, but the important result – capturing sunlight to power biological processes – was the same. The scientific consensus today is that a respiration cycle was modified, and a in a was used for capturing sunlight. Intermediate stages have been hypothesized, including the cytochrome using a pigment to create a shield to absorb ultraviolet light, or that the pigment was part of an infrared sensor (for locating volcanic vents). But whatever the case was, the conversion of a respiration system into a photosynthetic system is considered to have only happened , and all photosynthesizers descended from that original innovation.
I have koi in outside fish ponds. At times during the long summer days there is an occurrence called an algae bloom where the water in the pond becomes filled with very small suspended algae. During the day there is no problem with the respiration of the koi that I have in the pond… but because the algae use up so much available oxygen during the night and do not add any O2 to the water…my koi in the very early morning hours before the sunlight starts photosynthesis of the algae run out of the amount of oxygen they need for respiration and are forced to breathe atmospheric O2 at the surface of the pond! They gasp for O2 out of the water from the atmosphere where there is enough available for them to survive. My point is in water ponds there is a semi closed environment where plants can use up so much oxygen at night that they force the fish to get their oxygen elsewhere. When days become shorter the algae bloom will naturally diminish if I wait it out and do not do massive water changes or resort to killing the floating algae with a chemical plant killer algaecide that will not kill my fish if used in the proper doses. Plants do use O2 at night and do not give off any O2 in darkness!
So this is a very basic definition about the complex process of respiration.
Some things that impact photosynthesis
Intensity of the useful wavelengths of sunlight
Humidity (for terrestrial plants)
Local concentration or partial pressure of carbon dioxide
The Mitochondria and Chloroplast
That action of photosynthesis is going on inside flattened disk shape thylakoids found inside the stroma.
As with enzymes, the molecules used in biological processes are often huge and complex, but ATP energy drives all processes and that energy came from either potential chemical energy in Earth’s interior or sunlight, but even chemosynthetic organisms rely on sunlight to provide their energy. The Sun thus powers all life on Earth. The cycles that capture energy (photosynthesis or chemosynthesis) or produce it (fermentation or respiration) generally have many steps in them, and some cycles can run backwards, such as the . Below is a diagram of the citric acid (Krebs) cycle. (Source: Wikimedia Commons)
The respiration and photosynthesis cycles in complex organisms have been the focus of a great deal of scientific effort, and cyclic diagrams (, ) can provide helpful portrayals of how cycles work. Photosynthesis has several cycles in it, and Nobel Prizes were awarded to the scientists who helped describe the cycles. Chlorophyll molecules , with magnesium in their porphyrin cages, and long tails. Below is a diagram of a chlorophyll molecule. (Source: Wikimedia Commons)
Perhaps a few hundred million years after the first mitochondrion appeared, as the oceanic oxygen content, at least on the surface, increased as a result of oxygenic photosynthesis, those complex cells learned to use oxygen instead of hydrogen. It is difficult to overstate the importance of learning to use oxygen in respiration, called . Before the appearance of aerobic respiration, life generated energy via and . Because oxygen , aerobic respiration generates, on average, about per cycle as fermentation and anaerobic respiration do (although some types of anaerobic respiration can get ). The suite of complex life on Earth today would not have been possible without the energy provided by oxygenic respiration. At minimum, nothing could have flown, and any animal life that might have evolved would have never left the oceans because the atmosphere would not have been breathable. With the advent of aerobic respiration, became possible, as it is several times as efficient as anaerobic respiration and fermentation (about 40% as compared to less than 10%). Today’s food chains of several levels would be constrained to about two in the absence of oxygen. Some scientists have and oxygen and respiration in eukaryote evolution. is controversial.
All animals, , use aerobic respiration today, and early animals (, which are called metazoans today) may have also used aerobic respiration. Before the rise of eukaryotes, the dominant life forms, bacteria and archaea, had many chemical pathways to generate energy as they farmed that potential electron energy from a myriad of substances, such as , and photosynthesizers got their donor electrons from hydrogen sulfide, hydrogen, , , and other chemicals. If there is potential energy in electron bonds, bacteria and archaea will often find ways to harvest it. Many archaean and bacterial species thrive in harsh environments that would quickly kill any complex life, and those hardy organisms are called . In harsh environments, those organisms can go dormant for millennia and , waiting for appropriate conditions (usually related to available energy). In some environments, it can .
Around when Harland first proposed a global ice age, a climate model developed by Russian climatologist concluded that if a Snowball Earth really happened, the runaway positive feedbacks would ensure that the planet would never thaw and become a permanent block of ice. For the next generation, that climate model made a Snowball Earth scenario seem impossible. In 1992, a professor, , that coined the term Snowball Earth. Kirschvink sketched a scenario in which the supercontinent near the equator reflected sunlight, as compared to tropical oceans that absorb it. Once the global temperature decline due to reflected sunlight began to grow polar ice, the ice would reflect even more sunlight and Earth’s surface would become even cooler. This could produce a runaway effect in which the ice sheets grew into the tropics and buried the supercontinent in ice. Kirschvink also proposed that the situation could become unstable. As the sea ice crept toward the equator, it would kill off all photosynthetic life and a buried supercontinent would no longer engage in . Those were two key ways that carbon was removed from the atmosphere in the day's , especially before the rise of land plants. Volcanism would have been the main way that carbon dioxide was introduced to the atmosphere (animal respiration also releases carbon dioxide, but this was before the eon of animals), and with two key dynamics for removing it suppressed by the ice, carbon dioxide would have increased in the atmosphere. The resultant greenhouse effect would have eventually melted the ice and runaway effects would have quickly turned Earth from an icehouse into a greenhouse. Kirschvink proposed the idea that Earth could vacillate between states.
It can be helpful at this juncture to grasp the cumulative impact of , inventing , inventing , inventing that made possible, and inventing . Pound-for-pound, the complex organisms that began to dominate Earth’s ecosphere during the Cambrian Period consumed energy about 100,000 times as fast as the Sun produced it. Life on Earth is an incredibly energy-intensive phenomenon, powered by sunlight. In the end, only so much sunlight reaches Earth, and it has always been life’s primary limiting variable. Photosynthesis became more efficient, aerobic respiration was an order-of-magnitude leap in energy efficiency, the oxygenation of the atmosphere and oceans allowed animals to colonize land and ocean sediments and even fly, and life’s colonization of land allowed for a . Life could exploit new niches and even help create them, but the key innovations and pioneering were achieved long ago. If humanity attains the , new niches will arise, even of the , but all other creatures living on Earth have constraints, primarily energy constraints, which produce very real limits. Life on Earth has largely been a for several hundred million years, but the Cambrian Explosion was one of those halcyonic times when animal life had its greatest expansion, not built on the bones of a mass extinction so much as blazing new trails.