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compare and contrast photosynthesis and cellular respiration
An additional pathway for CO2 uptake involves photosynthesis by phytoplankton. The organic carbon produced by phytoplankton moves up the food chain and about 90% is converted eventually to CO2(aq) by respiration and decay within the oceanic mixed layer. The 10% fraction that precipitates (fecal pellets, dead organisms) represents a biological pump transferring carbon to the deep ocean. The biological productivity of the surface ocean is limited in part by upwelling of nutrients such as nitrogen from the deep (), so that the efficiency of the biological pump is again highly dependent on the vertical circulation of the ocean water. It is estimated that the biological pump transfers 7 Pg C yr-1 to the deep ocean, as compared to 40 Pg C yr-1 for CO2(aq) transported by deep water formation.
What, then, controls atmospheric oxygen? The next place to look is the lithosphere. Rock material brought to the surface in a reduced state is weathered (oxidized) by atmospheric O2. Of most importance is sedimentary organic carbon, which gets oxidized to CO2, and FeS2 (pyrite), which gets oxidized to Fe2O3 and H2SO4. The total amounts of organic carbon and pyrite in sedimentary rocks are estimated to be 1.2x107 Pg C and 5x106 Pg S, respectively. These amounts are sufficiently large that weathering of rocks would eventually deplete atmospheric O2 if not compensated by an oxygen source. The turnover time of sedimentary rock, that is the time required for sedimentary rock formed at the bottom of the ocean to be brought up to the surface, is of the order of 100 million years. The corresponding weathering rates are 0.12 Pg C yr-1 for rock organic carbon and 0.05 Pg S yr-1 for pyrite. Each atom of carbon consumes one O2 molecule, while each atom of sulfur as FeS2 consumes 19/8 O2 molecules. The resulting loss of O2 is 0.4 Pg O yr-1, which yields a lifetime for O2 of 3 million years. On a time scale of several million years, changes in the rate of sediment uplift could conceivably alter the levels of O2 in the atmosphere.
What is the difference between photosynthesis and respiration?
Simple comparison of these inventories tells us that cycling with the biosphere cannot control the abundance of O2 in the atmosphere, because the inventory of O2 is considerably larger than that of either CO2 or organic carbon. If photosynthesis were for some reason to stop, oxidation of the entire organic carbon reservoir would consume less than 1% of O2 presently in the atmosphere and there would be no further O2 loss (since there would be no organic carbon left to be oxidized). Conversely, if respiration and decay were to stop, conversion of all atmospheric CO2 to O2 by photosynthesis would increase O2 levels by only 0.2%.
It is instructive to compare the evolution of the Earth's atmosphere to that of its neighbor planets Venus and Mars. All three planets presumably formed with similar assemblages of elements but their present-day atmospheric compositions are vastly different (). Venus has an atmosphere ~100 times thicker than that of Earth and consisting mostly of CO2. Because of the greater proximity of Venus to the Sun, the temperature of the early Venus was too high for the outgassed water to condense and form oceans (see for further discussion). As a result CO2 remained in the atmosphere. Water vapor in Venus's upper atmosphere photolyzed to produce H atoms that escaped the planet's gravitational field, and the O atoms left behind were removed by oxidation of rocks on the surface of the planet. This mechanism is thought to explain the low H2O concentrations in the Venusian atmosphere. On Earth, by contrast, the atmosphere contains only 10-5 of all water in the surface reservoirs (the bulk is in the oceans) so that loss of water to outer space is extremely slow and is compensated by evaporation from the oceans.
Energy Transformation: Photosynthesis vs. Cellular Respiration
Cycling of atmospheric CO2 with the biosphere involves processes of photosynthesis, respiration, and microbial decay, as discussed in and illustrated in . It is difficult to distinguish experimentally between photosynthesis and respiration by plants, nor is this distinction very useful for our purpose. Ecologists define the net primary productivity (NPP) as the yearly average rate of photosynthesis minus the rate of respiration by all plants in an ecosystem. The NPP can be determined experimentally either by long-term measurement of the CO2 flux to the ecosystem from a tower () or more crudely by monitoring the growth of vegetation in a selected plot. From these data, quantitative models can be developed that express the dependence of the NPP on environmental variables including ecosystem type, solar radiation, temperature, and water availability. Using such models one estimates a global terrestrial NPP of about 60 Pg C yr-1.
which implies that atmospheric CO2 responds quickly, on a time scale of a decade, to changes in NPP or in decay rates. It is now thought that increased NPP at middle and high latitudes of the northern hemisphere over the past century may be responsible for the 20% missing sink of CO2 emitted by fossil fuel combustion (). Part of this increase in NPP could be due to conversion of agricultural land to forest at northern midlatitudes, and part could be due to greater photosynthetic activity of boreal forests as a result of climate warming. The organic carbon added to the biosphere by the increased NPP would then accumulate in the soil. An unresolved issue is the degree to which fossil fuel CO2 fertilizes the biosphere. Experiments done in chambers and outdoors under controlled conditions show that increasing CO2 does stimulate plant growth. There are however other factors limiting NPP, including solar radiation and the supply of water and nutrients, which prevent a first-order dependence of NPP on CO2.
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Cellular Respiration and Photosynthesis
Supply of elements to the surface reservoirs was set in the earliest stages of Earth's history when the crust formed and volcanic outgassing transferred material from the deep Earth to the atmosphere. The early Earth was a highly volcanic place due to energy released in its interior by radioactive decay and gravitational accretion. Present-day observations of volcanic plumes offer an indication of the composition of the outgassed material. It was mostly H2O (~95%), CO2, N2, and sulfur gases. There was no O2; volcanic plumes contain only trace amounts of O2, and examination of the oldest rocks on Earth show that they formed in a reducing atmosphere devoid of O2. The outgassed water precipitated to form the oceans. Carbon dioxide and the sulfur gases then dissolved in the oceans, leaving N2 as the dominant gas in the atmosphere. The presence of liquid water allowed the development of living organisms (self-replicating organic molecules). About 3.5 billion years ago, some organisms developed the capacity to convert CO2 to organic carbon by photosynthesis. This process released O2 which gradually accumulated in the atmosphere, reaching its current atmospheric concentration about 400 million years ago.
7th Grade Science Skills - Internet4Classrooms
Often many "hands-on" advanced aquarium keepers will utilize the most advanced pressurized CO2 system (as well as complicated fertilizer delivery). While this might be the way to go for certain high end hobbyists, from my experience (as well as other experienced pros) the expense of time and money these methods require are not always justified if all you desire is a nice but basic planted aquarium.
In fact, as a generalization, just utilizing good lighting, filtration, and basic fertilizers, as well as simple, natural CO2 generation methods (e.g. fish respiration, buffers, and plant material decomposition) can still produce good results. Albeit not to the level of advanced methods (think Walstad Method or German method)!
This of course is not to knock advanced methods for those who want optimal plant growth.
Plant Energy Transformations-Photosynthesis - …
What plants utilize nitrates/nitrogen for is leaf growth, which in turn maximizes surface area for essential photosynthesis.
Although I have not performed controlled tests, my observations are that nitrate levels that are too low will stunt plant growth and possibly even encourage certain algae (such as green spot), while higher nitrate levels will encourage algae to outperform plants and take over an aquarium.
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