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Prescriptive Activities: Facet Cluster 2.3: PF#84
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Prescriptive #84:
84 The student believes once oxygen combines with something, it can never be oxygen by itself
again.
Materials:
Student handout
The story of oxygen:
We rely on oxygen for our very existence. We take
oxygen for granted despite the fact that it has not
always existed as an abundant atmospheric gas here on
earth. For the first 2 billion years of earth’s 4.6 billion
year history, there was virtually no atmospheric
oxygen. The atmosphere consisted of methane (CH4),
ammonia (NH3), hydrogen (H2) and carbon dioxide
(CO2), collectively referred to as the primordial soup1.
The hydrogen gas, being very light, escaped into space.
Any free oxygen quickly reacted with free iron in the
earth’s oceans creating iron oxides which became buried deep in the earth over hundreds of
millions of years. In the early history of earth, amino acids,
nucleotides, sugars and fatty acids formed from the
primordial soup, and later RNA and DNA2 formed, and
simple chemical processes that form the basis of life began.
Free oxygen is highly toxic to just about every part and
process of living cells, and eventually a symbiotic
relationship between early cells and protective mechanisms
began. Chloroplasts and mitochondria as well as the
mechanisms for shuttling oxygen into and out of the cell are
the only things protected from oxygen’s toxicity. It is
fortunate that there was no atmospheric oxygen in the early
part of the earth’s history because life on earth may never
have begun if there had been.
Light from the Sun broke down the ammonia molecules
released by volcanos, releasing nitrogen into the
atmosphere. Over billions of years, the quantity of nitrogen
built up to the levels we see today.
Although life formed just a few hundred million years after
the earth formed, it wasn’t until the evolution of bacteria 3.3
billion years ago that the early Earth atmosphere really
changed into the one we know today. By 2.7 billion years ago early bacteria, known as
cyanobacteria, used energy from the Sun for photosynthesis, allowing cells to obtain carbon (C)
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Earth’s Early Atmosphere image taken from: http://forces.si.edu/atmosphere/04_00_03.html
DNA and Cell picture taken from: http://www.koshland-science-museum.org/exhibitdna/intro02.jsp
Prescriptive Activities: Facet Cluster 2.3: PF#84
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from carbon dioxide (CO2) and hydrogen (H2) from water (H2O). Much of the bound oxygen was
released as a byproduct, yet it wasn’t for another few hundred million more years that there was a
dramatic increase in atmospheric oxygen levels. Scientists are still trying to figure out the details
of how this increase occurred. One important aspect was that there was a
decrease in the amount of hydrogen gas in the atmosphere, so any free
oxygen would not be immediately taken up by hydrogen. Then, the O2
level increased. This increase resulted in an increase in iron oxides on in
the earth’s oceans. At left is a picture of a “banded iron” rock3 which
shows alternating layers of iron-rich (red) and iron-poor (gray) sediments.
Eventually the increased oxygen levels used up all the free iron, and it
was at this point that the O2 levels in the atmosphere really increased. In
just a few hundred million years, early bacteria completely changed the Earth’s atmosphere
composition, bringing us to our current mixture of 21% oxygen and 78% nitrogen.
Before this all happened, the amount of oxygen in Earth's atmosphere was about one tenquadrillionth of the amount present today. Oxygen now makes up nearly 21 percent of
Earth's atmosphere; most of the rest is nitrogen. The free oxygen gas was produced by
plants when photosynthesis reactions released the oxygen from water.
The history of oxygen on earth shows on the grandest scale that oxygen can be released
from elements to which it is bound in even the most stable compounds such as water and
carbon dioxide. Today photosynthesis is a crucial part of the cycle by which oxygen is
released into the atmosphere, thereby making it available to breathe in for all animals,
including humans.
Questions:
1. In photosynthesis, plants take in carbon dioxide and water, and use these to build sugars
(C6H12O6), releasing oxygen gas as a byproduct. Write the equation for photosynthesis.
2. Write the equation for respiration, in which results in sugar effectively being combined with
oxygen, producing carbon dioxide and water.
3. Which one releases oxygen into the atmosphere? Where did the oxygen come from?
4. When you decompose hydrogen peroxide, it releases oxygen as bubbles. Where did the
oxygen come from?
5. Oxygen is found in nature bonded to many different elements including carbon and hydrogen.
How can this oxygen be released?
6. Provide your own explanation for the concept of the reversibility of chemical reactions?
Resources:
http://www.bettykamen.com/rice/History.html
http://www.universetoday.com/10932/early-atmosphere-looked-very-different-from-today/
http://iridiumminer.blogspot.com/2011/01/banded-iron-formations-clues-to-early.html
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http://iridiumminer.blogspot.com/2011/01/banded-iron-formations-clues-to-early.html