Photosynthesis and Cellular Respiration ..

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.

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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.

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These two subunits enable eukaryotic cells that contain them to conduct respiration and photosynthesis, respectively.

Oxygenic photosynthesis uses two systems for capturing photons. The first one (called ) uses . The second one (called because it was discovered before Photosystem II) uses captured photon energy to add an electron to captured carbon dioxide to help transform it into a sugar. That “” is accomplished by the , and an enzyme called Rubisco, , catalyzes that fixation. Below is a diagram of the Calvin cycle. (Source: Wikimedia Commons)

Respiration and photosynthesis summary

Plants can turn the glucose produced in photosynthesis into starch for storage, and turn it back into glucose when it is needed for respiration. for storage.

Summary equation for photosynthesis

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)