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Lecture 5

5. Lecture Five - September 22.docx

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Jessica Hawthorn

DIVERSITY OF LIFE – EVOLUTION ONE Thus far, we haven’t spoken a lot about metamorphic rock. Because it is rock that has been altered by temperature and pressure, it is not good for building a geologic time scale. It is composed of rock that often times has minerals that have partially or completely re-crystallized. Things like the magnetic signature have been erased. Minerals are reformed. Radioactive decay patterns are disrupted. This rock really cannot help. Within the geologic time scale, we have eons, eras, periods, and epochs. However, these are not fixed. We’re constantly tweaking ages and dates as we discover and study more rocks. We’ll talk about biogeochemical cycles and the origin of life. Earth is a closed system. That is to say, within the envelope of Earth, within the atmosphere, the lithosphere, the hydrosphere, and the biosphere, the elements that compose Earth, the actual atoms, do not leave Earth, with very rare exceptions. Nothing achieves the velocity necessary to escape the planet. We have a total budget of elements, of carbon, hydrogen, oxygen, and all the way down to the rare Earth elements. This budget of elements, their total amount, has been there since Earth formed as a full planetoid. The total budget does not change. All the carbon will be the same amount as it always has been. Where the carbon is located and stored does change over time, and changes dramatically. The same thing is true for oxygen, nitrogen, and the rest of the elements. It’s the movement of these elements, the movement of carbon through atmosphere, lithosphere, hydrosphere, and biosphere that is called the biogeochemical cycle. So, we understand that where elements are stored changes. In this picture (see professor’s notes), we have three planets: Venus, Mars, and Earth. Venus’s atmosphere is composed almost entirely of CO . Th2 same is true for Mars. Now, Earth’s atmospheric makeup is quite different. It is 78.1 Nitrogen, 21% Oxygen, and less than 1% Carbon Dioxide. There is a huge difference in the concentration of CO . Th2 total amount of carbon, nitrogen, and oxygen, the total elemental budget is about the same in all three planets. However, where they reside is very different. Earth has a totally different atmosphere compared to Venus and Mars. Why? The carbon that goes into CO is loc2ed up in different portions of the planet. Where is it locked up? A lot of it is locked up in this room. On Earth, the primary reservoirs of carbon are in the lithosphere (i.e., the rock record) and the biosphere (i.e., living organisms). We cycle CO through the lithosphere (i.e., the rock record), the biosphere (i.e., ‘the 2 living organism record’), and the atmosphere. How is it cycled? Well, through photosynthesis and respiration. Photosynthesis and respiration are two metabolic processes. They’re chemical processes that both generate energy to power cells. When you breathe, that is respiration. You take in oxygen. This oxygen participates in burning (or chemically breaking down) food to generate special energy molecules that power your cells. Photosynthesis is a process whereby plants take CO (from 2he atmosphere) and water and, with the use of some light energy from the sun, generate sugar and energy. In the process of creating sugars, solid sugars, which go then to form the physical constitution of the plant, photosynthesis generates a waste gas: Oxygen. Oxygen is a waste by-product of photosynthesis. Respiration: we inhale oxygen, and along with sugar, convert it into CO and wate2, and generate energy in the process to power ourselves. It is a cycle. We have a balance in the atmosphere and the biosphere between photosynthesis and respiration. We have photosynthesis occurring in plants and oceans (i.e., microorganisms). Then, there is the respiration function, which is found primarily in animals. It is a cycle. Carbon and oxygen are cycling between photosynthesis and respiration. Every single atom in your body was fairly recently atmospheric CO ,2and it will go back to being atmospheric CO . 2 How does this cycling of Oxygen and CO take pl2ce? Gaseous CO is pulled ou2 of the atmosphere by plants and consumed. So, what happens once the CO leaves the atmosphere as 2 gas (i.e., once it’s pulled into the plant and converted into a solid)? Well, if there was no respiration, or no animals breathing, what would happen to the total concentration of CO ? It 2 would go down (as more and more would be consumed by plants). The more photosynthesis increases, the more atmospheric CO is drawn out of the atmosphere and turned into a solid. If 2 photosynthesis slows down, and respiration increases, the CO concent2ation in the atmosphere will go up. Keep in mind, however, the total budget of CO on Earth will not change. Its storage 2 places will change. When you have low levels of photosynthesis, CO increasin2ly resides in the atmosphere. When you have high levels of photosynthesis, more and more CO resides in p2ants. If there was no rock record involved, we’d just have a cycle going back and forth. CO moves 2 between the atmosphere and biosphere through photosynthesis and respiration. It doesn’t just go between those two however. What can happen is when CO is locked u2 as a solid (i.e., within plants), that material instead of simply being respired back into the atmosphere, can get buried into the ground (i.e., the lithosphere). What do I mean? Okay, so you live your life. You eat your plants. You will eventually die. What happens to your organic remains? They eventually become oxidized. Eventually, we will become oxidized. Our carbon will go back into atmospheric CO . 2 What happens, however, is that instead of simply being drawn out of the atmosphere by photosynthesis, then going into the biosphere, and being oxidized back into the atmosphere, carbon can be locked up and buried. If this happens, what would happen to the total amount of CO ? It would decrease. If you increase the rate of burial of organic material (which contains 2 CO )2 and you don’t allow it to decompose back into organic CO gas, ove2time you lower the amount of atmospheric CO . You lock the carbon up for a period of time. How though? How can 2 you bury enough carbon to affect a change in the amount of CO ? Well, 2O can be bur2ed through sedimentary processes like mountain building. Also, remember the white cliffs of Dover? What are they made up of? Calcium carbonate (i.e., chalk). The more chalk you build, the more carbon you pull out of the air. So, you can lock carbon as chemical sediment rock. When you do that, you can lower the atmospheric concentration of CO . Now, when you bury all 2 that CO m2terial, or lock it up as rock, you lower the rate of respiration. The CO materi2l is not being oxidized. Keep in mind that we consume oxygen. If you lower the rate of respiration by lowering the amount of carbon that is available to burn up, you increase the amount of atmospheric oxygen. If we bury an entire forest, we reduce the amount of atmospheric CO , 2 since it is now not being oxidized. The amount of atmospheric concentration of CO goes down, 2 and the amount of oxygen goes up! So, we learn that the atmospheric concentration of CO 2 and oxygen are inversely related to each other. So, let’s say we bury a bunch of organic carbon and we lower the atmospheric concentration of CO . What wo2ld happen if we took the buried carbon and we burned it up? What do we create? We’d create CO gas. What h2ppens to the atmospheric concentration of CO as a result of this? It would increase. Remember, the Earth 2 is a closed system. There’s no new carbon or oxygen coming in from outer space. It just depends on how it is partitioned. So, you have CO cycling through living organisms. It will either cycle 2 bac
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