Environmental Impact of Cellular Respiration

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Within natural ecosystems, the interactions among the respiratory pathways ofdifferent organisms occur in characteristic forms. Controlling these interactions is a combination of thermodynamics (free energy yield) and the physiological properties of organisms involved. Also of importance, spatially, are the physical transport of electron-accepting compounds and the reduced end products of respiration (Figures 1 and 5). For instance, while aerobic respiration is the dominant respiratory pathway in many natural systems, there are many environments where oxygen (O2) becomes depleted as a result of restricted physical transport. O2 depletion occurs especially in the face of excess organic matter input and avid biological O2 consumption (see The Significance of O2 for Biology). In such environments, anaerobic respiration becomes the dominant pathway for organic matter decay. These anaerobic respiration pathways result in the production of reduced (electron-rich) inorganic compounds such as ammonium (NH^), reduced manganese (Mn2+) and iron (Fe2+), hydrogen sulfide (H2S), and methane (CH4).

As a consequence of these processes, both aerobic and anaerobic chemoorganotrophic respiration have important impacts on environmental conditions at local as well as global scales. At the local scale, depletion of O2

Anaerobic Respiration

Figure 1 Electron (e~) transport with proton (H+) pumping. The ETS consists of a series of proteins (left to right, as observed in mitochondria, are complexes I, III, and IV) and other molecules (e.g., ubiquinone, which is also described as coenzyme Q, and cytochrome c) found within the inner membrane of certain bacteria and mitochondria, or the plasma (or cytoplasmic membrane) in bacteria lacking an outer membrane. Shown also is electron-transport-driven pumping of protons out of a cell's cytoplasm or mitochondrion's matrix. Complex II, also described in the article as part of the ETS in mitochondria, is not shown.

Figure 1 Electron (e~) transport with proton (H+) pumping. The ETS consists of a series of proteins (left to right, as observed in mitochondria, are complexes I, III, and IV) and other molecules (e.g., ubiquinone, which is also described as coenzyme Q, and cytochrome c) found within the inner membrane of certain bacteria and mitochondria, or the plasma (or cytoplasmic membrane) in bacteria lacking an outer membrane. Shown also is electron-transport-driven pumping of protons out of a cell's cytoplasm or mitochondrion's matrix. Complex II, also described in the article as part of the ETS in mitochondria, is not shown.

across spatial dimensions varying from meters in the water column of aquatic systems, to centimeters or millimeters in aquatic sediments or soil particle aggregates, creates conditions favorable for denitrification (see Denitrification), which has a critical influence on nitrogen cycling in both aquatic and terrestrial ecosystems. For example, conversion of NO^ to N2 gas results in shunting of 'fixed' nitrogen to the atmosphere, which is good for reducing the effects of excess nutrient input to aquatic ecosystems, but bad for agroecosystems because it represents a major pathway for loss of nitrogen fertilizer. Utilization of electron acceptors such as iron and

Cellular Dynamics

Figure 2 Forward- and reverse-acting ATP-dependent proton pumps (ATP synthase). Shown on the left is a forward-running ATP-dependent proton pump. On the right is the reverse-running ATP-dependent proton pump, now an ATP synthase. The dashed line indicates the futility were both processes to operate simultaneously across the same membrane. Overall, this proton-motive force-driven ATP generation is described as chemiosmosis.

Figure 2 Forward- and reverse-acting ATP-dependent proton pumps (ATP synthase). Shown on the left is a forward-running ATP-dependent proton pump. On the right is the reverse-running ATP-dependent proton pump, now an ATP synthase. The dashed line indicates the futility were both processes to operate simultaneously across the same membrane. Overall, this proton-motive force-driven ATP generation is described as chemiosmosis.

manganese oxides and sulfate for anaerobic respiration leads to major changes in local aqueous geochemical conditions, and to the production or destruction of a wide variety of mineral phases. Long-term burial (storage) of reduced iron-sulfur minerals (e.g., pyrite, FeS2) in marine sediments is a key process in maintaining the overall oxidation/reduction balance of the planet on geological timescales. Production of methane (CH4) - a potent greenhouse gas second only to CO2 in contribution to global warming - during anaerobic organic matter decay in natural and agricultural wetlands is responsible for approximately half of total global CH4 emissions.

Chemolithotrophic respiration can also have important effects on local and global biogeochemical conditions. The existence of complex biotic communities supported entirely by chemolithotrophic oxidation of geologically derived inorganic reduced compounds provides a stunning example of how life has evolved to take advantage of Earth's diverse energy sources - in environments ranging from deep-sea hydrothermal vents driven by H2S or CH4 inputs, to kilometer-deep fractured rocks driven by hydrogen production via radiolytic (radiation-catalyzed) splitting of water. Microbially catalyzed oxidation of mineral ore bodies exposed to atmospheric O2 during the mining of precious metals and coal leads to the production of acid mine drainage, one of the most widespread and deleterious environmental impacts of human activity on the planet. In addition, microorganisms that oxidize CH4 during chemolithotrophic metabolism (both aerobic and anaerobic) are recognized as a major controlling factor in the global cycle of this important greenhouse gas. Cellular respiration, in addition to allowing for more efficient conversion of chemical energy into ATP, thus profoundly impacts Earth's ecology.

Phosphate transfer to ADP makes ATP

Phosphorylated substrate

Phosphate transfer to ADP makes ATP

Phosphorylated substrate

Chemical Energy Environmental Impacts

Figure 3 Substrate-level phosphorylation. ATP generation in absence of ETSs is by means of SLP. Note that in the latter a phosphate group chemically attached to a substrate molecule is transferred to ADP to produce ATP.

Figure 3 Substrate-level phosphorylation. ATP generation in absence of ETSs is by means of SLP. Note that in the latter a phosphate group chemically attached to a substrate molecule is transferred to ADP to produce ATP.

ch2oh

Glucose OH

Cellular respiration or fermentation

Figure 4 Glycolysis. Reaction overview: glucose + 2 ATP + 2NAD+ ! 2 pyruvate + 4 ATP + 2NADH). Shown are both ATP priming (2 ATP) and ATP generation by SLP (2 x 2 ATP). Note the conversion of NAD+ to NADH + H+. NAD+ is regenerated via fermentative (see 00272) or cellular respiration pathways (Figure 6).

Cellular respiration or fermentation

Figure 4 Glycolysis. Reaction overview: glucose + 2 ATP + 2NAD+ ! 2 pyruvate + 4 ATP + 2NADH). Shown are both ATP priming (2 ATP) and ATP generation by SLP (2 x 2 ATP). Note the conversion of NAD+ to NADH + H+. NAD+ is regenerated via fermentative (see 00272) or cellular respiration pathways (Figure 6).

Enviornment Respiration Figure

Figure 5 'Train station' diagram of electron flowthrough various respiratory pathways in nature. Electrons typically begin their journey by being fixed into organic matter via photosynthesis at or near the earth's surface. Dead organic matter undergoes oxidation (e.g., in water, soil, or sediment) to CO2 through various chemoorganotrophic respiratory pathways (indicated by solid lines) involving the reduction of major electron acceptors such as oxygen (O2), nitrate (NO3), manganese oxides (MnO2), iron oxides (FeOOH), sulfate (SO43), and carbon dioxide (CO2). These respiratory processes, which are separated in space and/or time as a result of differences in the amount of free energy released during respiration, result in the production of reduced inorganic compounds such as ammonium (NH4), reduced manganese (Mn2+), reduced iron (Fe2+), hydrogen sulfide (H2S), and methane (CH4). These reduced compounds can subsequently undertake a return trip to their oxidized form (indicated by dashed lines) by serving as energy sources for chemolithotrophic respiration. Known pathways of chemolithotrophic respiration are indicated by the various node points. Chemolithotrophic respiration can also be driven by inputs (indicated by dotted lines) of reduced compounds from geological sources (e.g., hydrothermal vents, reduced mineral-rich ore deposits). Reproduced from Benthic respiration in aquatic sediments, 2000, pp. 86-103, Methods in Ecosystem Science, Thamdrup B and Canfield DE, figure(1), with kind permission of Springer Science and Business Media.

Figure 5 'Train station' diagram of electron flowthrough various respiratory pathways in nature. Electrons typically begin their journey by being fixed into organic matter via photosynthesis at or near the earth's surface. Dead organic matter undergoes oxidation (e.g., in water, soil, or sediment) to CO2 through various chemoorganotrophic respiratory pathways (indicated by solid lines) involving the reduction of major electron acceptors such as oxygen (O2), nitrate (NO3), manganese oxides (MnO2), iron oxides (FeOOH), sulfate (SO43), and carbon dioxide (CO2). These respiratory processes, which are separated in space and/or time as a result of differences in the amount of free energy released during respiration, result in the production of reduced inorganic compounds such as ammonium (NH4), reduced manganese (Mn2+), reduced iron (Fe2+), hydrogen sulfide (H2S), and methane (CH4). These reduced compounds can subsequently undertake a return trip to their oxidized form (indicated by dashed lines) by serving as energy sources for chemolithotrophic respiration. Known pathways of chemolithotrophic respiration are indicated by the various node points. Chemolithotrophic respiration can also be driven by inputs (indicated by dotted lines) of reduced compounds from geological sources (e.g., hydrothermal vents, reduced mineral-rich ore deposits). Reproduced from Benthic respiration in aquatic sediments, 2000, pp. 86-103, Methods in Ecosystem Science, Thamdrup B and Canfield DE, figure(1), with kind permission of Springer Science and Business Media.

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Getting Started With Solar

Getting Started With Solar

Do we really want the one thing that gives us its resources unconditionally to suffer even more than it is suffering now? Nature, is a part of our being from the earliest human days. We respect Nature and it gives us its bounty, but in the recent past greedy money hungry corporations have made us all so destructive, so wasteful.

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