Question

(Type your responce) During this unit, you learned about biological, geological, and chemical changes to the Earth through its history. Based on the data and evidence used to develop your model, construct an argument to answer the following prompt: did, photosynthetic life alter, the atmosphere to support life as we know it today? Develop a claim and counterclaim(s), in addition to a, concluding statement or paragraph summarizing your argument.

In your argument, be sure to describe and explain:

• composition of Earth's atmosphere shortly alter its formation
• Current atmospheric composition
• Chemical reactions that support your claim
• Events in Earth's history that support your claim
• Reliability of the data used to support your claim.

Answer 1

Ans. The primitive earth had no oxygen like present time. Oxygen was added to the atmosphere by photosynthetic activity. Hence photosynthetic life has altered the atmosphere to support life. The primitive earth consisted of oxygen less environment such that most organisms were obligate organisms. The primitive life consisted of mainly the unicellular and the prokaryotes of the Kingdom Monera, including some blue green algae. Some of the blue green algae were present in the water bodies. Over time, they underwent evolution and started carrying out photosynthesis to prepare their own food. This led to the formation of oxygen as by products.

$6CO_{2}+6H_{2}O\rightarrow C_{6}H_{12}O_{6}+6O_{2}$

However the primitive earth (back to 4.6 billion years ago) was not like this. After emerging from a mass of hot gases and molten material there was nothing called atmosphere. With time it started to cool and formed the crust of the earth. The atmosphere comprised of the hydrogen, helium, nitrogen as the main components. However, helium escaped from the atmosphere to a great extent as gravitational forces of attraction were not so strong to hold it back as it was a light gas. Since the atmosphere was formed mainly by the volcanic outgassing, so methane, sulfur, carbon dioxide were prevalent. With huge amount of time, the earth further cooled which led to the formation of liquid water.

Clearly life has altered the atmosphere. The photosynthesis of the unicellular blue green algae led to the formation of oxygen, which along with other essential macromolecules led to the evolution of higher forms of life, like green plants. After Monera came up Kingdom Protista which comprised of both unicellular and multicellular organisms, then came Kingdom Fungi, which was followed by Kingdom Plantae. Oxygen was also added by photocatalysis of carbon dioxide and the water vapor molecules in the atmosphere, however its concentration was less. The given equations entail, photocatalysis of water molecule to hydroxyl ion and hydrogen atom, and splitting of carbon dioxide into Carbon monoxide and oxygen atom.

$H_{2}O\rightarrow H+OH$

CO2 CO O

The hydroxyl and the oxygen atoms fuse to form oxygen

$OH+O\rightarrow O_{2}+H$

The current atmosphere has nitrogen (78 %), oxygen (21 %), carbon dioxide (0.4 %) and traces of other gases. Oxygen is abundant since it is essential for every living organisms, right from animals to microorganisms. Animals need oxygen to survive and microorganisms require oxygen to decompose dead organisms and derive their nutrition from them leading to ultimate evolution of carbon dioxide and water.

Сън 1206 + 02 → CO2 + H2O

Counter argument:

The early atmosphere was devoid of oxygen. Since the atmosphere was not so dense as it is now, so the solar radiations were dominant and prevalent. Due to this the ultra violet radiations brought about photocatalysis of carbon dioxide molecule and water molecules which led to the formation of water. The oxygen so produced even by photosynthesis was not free oxygen. The earth's crust was rich in magnesium and lesser silica rich in contrast to the more silica rich crust at present. Tectonic activities have always been dominant, so when water came in contact with these rocks, they took up the oxygen and thus fixed it, thus the free oxygen was less in atmosphere. With time, the silica content kept on increasing due to frequent mixing of the oceanic and continental crust, oxygen could not be any more fixed and was liberated. This is how oxygen came up.