2004-03-09 21:29:00麥斯
Essay #2: Evoulution of Life
1. Describe the origin of the oxygen evolution. How did the oxygen evolution affect the course of evolution of terrestrial organisms? Why should we be concerned about the prospect of a decrease in the ozone concentration in the Earth’s atmosphere?
3.8 billion years ago, the surface temperature of the Earth was much higher than at present. The ultraviolet radiation that reached the surface of the Earth, combined with the effect of electrical storms and thermal activity in the earth’s crust, provided free energy for the formation of complex organic molecule. The composition of the atmosphere was mainly hydrogen, methane, ammonia and water vapour. But there was no oxygen as the atmosphere nowadays. The first photosynthetic organisms, cyanobacteria, which is capable of capturing and storing the solar energy through a special green catalyst called chlorophyll appeared in the ocean. The bacteria used carbon dioxide and water to perform photosynthesis, which manufactures glucose as its food source with free oxygen given off as a by-product. The oxygen evolved dissolved in the sea and some of it escaped to the atmosphere. The free oxygen molecules (O2) reacted with another in the upper atmosphere, the stratosphere, with the energy from the ultraviolet radiation to form an ozone (O3) layer. This layer of gas shields off the ultraviolet radiation from the sun until the present.
The shielding effect of ozone layer in the stratosphere prevents ultraviolet radiation from reaching the surface of the Earth. With the extensive evolution of free oxygen gas given off from the photosynthetic bacteria, another kind of bacteria emerged. This kind of bacteria developed a new and more effect system of cellular aerobic respiration in which oxygen combines with glucose in the cell in a highly effective and regulated way that yield a high amount of energy and gives off carbon dioxide and water as by-products. With a more efficient energy system and free of harmful ultraviolet radiation, the organisms began to form a more complex life form by developing its nuclear membrane and the endosymbiosis. The bacteria captured other bacteria to form its organelles (such as mitochondria) to carry out its metabolism. Consequently, they developed to multicelluar organisms which invaded both the ocean and the land.
By now, the depletion of the ozone layer has become a serious problem among many countries. As the consumption of CFCs (chlorofluorocarbon) in daily uses released these compounds to the stratosphere, the ozone molecules interacted with the Chlorine atom from the compounds are broken down into oxygen. As a result, the ozone layer is being depleted and the shielding effect to the ultraviolet rays is reduced. This brought forth to the problem of global warming which causes the melting of polar ice caps. This in turn increases the sea level and consequently most of the cities at the coastal area will be flooded. Furthermore, an increased level of ultraviolet rays can reach to the surface of the Earth due to the decreasing concentration of ozone, which can cause skin cancer upon human beings and damaged to plants. Needless to say, we must be concerned about such effects of the ozone depletion. Therefore, the use of CFCs and other ozone-depleting chemicals must be ceased to prevent further thinning of the ozone layer. In stead, alternatives for CFCs should be used and solutions for the ozone depletion should be found as soon as possible.
3.8 billion years ago, the surface temperature of the Earth was much higher than at present. The ultraviolet radiation that reached the surface of the Earth, combined with the effect of electrical storms and thermal activity in the earth’s crust, provided free energy for the formation of complex organic molecule. The composition of the atmosphere was mainly hydrogen, methane, ammonia and water vapour. But there was no oxygen as the atmosphere nowadays. The first photosynthetic organisms, cyanobacteria, which is capable of capturing and storing the solar energy through a special green catalyst called chlorophyll appeared in the ocean. The bacteria used carbon dioxide and water to perform photosynthesis, which manufactures glucose as its food source with free oxygen given off as a by-product. The oxygen evolved dissolved in the sea and some of it escaped to the atmosphere. The free oxygen molecules (O2) reacted with another in the upper atmosphere, the stratosphere, with the energy from the ultraviolet radiation to form an ozone (O3) layer. This layer of gas shields off the ultraviolet radiation from the sun until the present.
The shielding effect of ozone layer in the stratosphere prevents ultraviolet radiation from reaching the surface of the Earth. With the extensive evolution of free oxygen gas given off from the photosynthetic bacteria, another kind of bacteria emerged. This kind of bacteria developed a new and more effect system of cellular aerobic respiration in which oxygen combines with glucose in the cell in a highly effective and regulated way that yield a high amount of energy and gives off carbon dioxide and water as by-products. With a more efficient energy system and free of harmful ultraviolet radiation, the organisms began to form a more complex life form by developing its nuclear membrane and the endosymbiosis. The bacteria captured other bacteria to form its organelles (such as mitochondria) to carry out its metabolism. Consequently, they developed to multicelluar organisms which invaded both the ocean and the land.
By now, the depletion of the ozone layer has become a serious problem among many countries. As the consumption of CFCs (chlorofluorocarbon) in daily uses released these compounds to the stratosphere, the ozone molecules interacted with the Chlorine atom from the compounds are broken down into oxygen. As a result, the ozone layer is being depleted and the shielding effect to the ultraviolet rays is reduced. This brought forth to the problem of global warming which causes the melting of polar ice caps. This in turn increases the sea level and consequently most of the cities at the coastal area will be flooded. Furthermore, an increased level of ultraviolet rays can reach to the surface of the Earth due to the decreasing concentration of ozone, which can cause skin cancer upon human beings and damaged to plants. Needless to say, we must be concerned about such effects of the ozone depletion. Therefore, the use of CFCs and other ozone-depleting chemicals must be ceased to prevent further thinning of the ozone layer. In stead, alternatives for CFCs should be used and solutions for the ozone depletion should be found as soon as possible.