In cellular respiration where is oxygen consumed

This process releases energy, some of which is transferred to ATP. Next, pyruvate molecules enter the mitochondria to take part in a series of reactions called the Krebs cycle, also known as the citric acid cycle. This completes the breakdown of glucose, harvesting some of the energy into ATP and transferring electrons onto carrier molecules. In the last stage, known as oxidative phosphorylation, electrons pass through an electron transport system in the mitochondrial inner membrane, which maintains a gradient of hydrogen ions.

Cells harness the energy of this proton gradient to generate the majority of the ATP during aerobic respiration. Aerobic respiration requires oxygen, however, there are many organisms that live in places where oxygen is not readily available or where other chemicals overwhelm the environment. Extremophiles are bacteria that can live in places such as deep ocean hydrothermal vents or underwater caves.

Rather than using oxygen to undergo cellular respiration, these organisms use inorganic acceptors such as nitrate or sulfur, which are more easily obtainable in these harsh environments.

This process is called anaerobic respiration. When oxygen is not present and cellular respiration cannot take place, a special anaerobic respiration called fermentation occurs. Fermentation starts with glycolysis to capture some of the energy stored in glucose into ATP. However, since oxidative phosphorylation does not occur, fermentation produces fewer ATP molecules than aerobic respiration.

In humans, fermentation occurs in red blood cells that lack mitochondria, as well in muscles during strenuous activity generating lactic acid as a byproduct, therefore it is named lactic acid fermentation.

Some bacteria carry out lactic acid fermentation and are used to make products such as yogurt. In yeast, a process known as alcoholic fermentation generates ethanol and carbon dioxide as byproducts, and has been used by humans to ferment beverages or leaven dough. Cellular respiration together with photosynthesis is a feature of the transfer of energy and matter, and highlights the interaction of organisms with their environment and other organisms in the community.

Cellular respiration takes place inside individual cells, however, at the scale of ecosystems, the exchange of oxygen and carbon dioxide through photosynthesis and cellular respiration affects atmospheric oxygen and carbon dioxide levels. Interestingly, the processes of cellular respiration and photosynthesis are directly opposite of one another, where the products of one reaction are the reactants of the other.

Photosynthesis produces the glucose that is used in cellular respiration to make ATP. This glucose is then converted back into CO 2 during respiration, which is a reactant used in photosynthesis. More specifically, photosynthesis constructs one glucose molecule from six CO 2 and six H 2 O molecules by capturing energy from sunlight and releases six O 2 molecules as a byproduct.

Cellular respiration uses six O 2 molecules to convert one glucose molecule into six CO 2 and six H 2 O molecules while harnessing energy as ATP and heat. Scientists can measure the rate of cellular respiration using a respirometer by assessing the rate of exchange of oxygen. Understanding the Ideal Gas Law is of fundamental importance for knowing how the respirometer functions.

The Ideal Gas Law states that the number of gas molecules in a container can be determined from the pressure, volume, and temperature. More specifically, the product of the volume and pressure of a gas equals the product of the number of gas molecules, the ideal gas constant and the temperature of the gas. Respirometers contain potassium hydroxide which traps carbon dioxide that is produced by respiration in solid form as potassium carbonate. Small stepwise increases in electron affinity are manifested by small drops in electron free energy along the respiratory electron chain.

Damage produced by reactive oxygen species ROS is an obvious cost of aerobic metabolism, and ROS in the form of hydrogen peroxide H 2 O 2 and phospholipid hydroperoxides are controlled by glutathione reductases and glutathione peroxidases, which depend on NADPH as the reducing agent to reactivate oxidized glutathione.

Protons return through NNT in order to drive this catalytic process in a manner that is directly competitive with production of ATP and heat Fig. See also: Free energy ; Free radical ; Hydrogen peroxide ; Superoxide chemistry.

Respiratory demands vary by type of fuel, by the balance between catabolism and anabolism in which a cell is engaged, and by the degree to which the cell produces cytosolic NADPH anaerobically through processes such as the pentose phosphate pathway in which glucose is metabolized or transformed into NADPH.

See also: Citric acid cycle. In contrast to glucose oxidation, the complete oxidation of triglycerides neutral lipids consisting of three fatty acyl chains esterified to a glycerol backbone is almost entirely aerobic Fig.

The ratio of fatty-acid carbons to glycerol carbons in a triglyceride provides an indication of how aerobically demanding triglyceride oxidation is. Considering that the cytosolic NADH can be effectively reoxidized aerobically via the malate-aspartate shuttle or the glycerolphosphate shuttle and that the glycerol-derived pyruvate can also be oxidized in mitochondria, complete oxidation of a typical triglyceride can demand sufficient oxygen to reoxidize approximately mitochondrial NADH and FADH 2 equivalents.

See also: Lipid ; Lipid metabolism ; Triglyceride triacylglycerol. It should also be pointed out that amino acid oxidation is intermediate in its O 2 requirement between glycolysis and mitochondrial fatty-acid oxidation because some reduced cofactors are produced in the cytosol and others are produced in the mitochondria. See also: Amino acid ; Amino acid metabolism. The other consideration that guides the magnitude of a cellular O 2 requirement is the degree to which a cell is busy with reactions that demand the hydride carried on NADH and NADPH and whether reducing equivalents can be produced cytosolically.

Unlike a fireplace, whose purpose is to combust fuel fully to generate heat Fig. Thus, the logic of life is such that the relatively low energy electrons carried on cytochrome C in the inner mitochondrial membrane have much less power to do meaningful work than the electrons carried on cytosolic NADPH. The former can donate to O 2 to generate water, having already generated a proton gradient in the descent from the high-energy state in NADH to the low-energy state in reduced cytochrome C.

The latter can donate electrons to beta-keto groups and alkenes to perform reductive biosynthesis. Therefore, it would be illogical for cells to let electrons flow downhill too far if they are needed for biosynthetic reactions. One of the best examples of a set of metabolic pathways that minimizes respiration occurs in white adipocytes fat-storing cells , which are specialized to convert glucose to triglycerides Fig.

This begins with import of glucose and conversion to pyruvate in the cytosol. In the mitochondria, pyruvate is converted to oxaloacetate and Ac-CoA by pyruvate carboxykinase and pyruvate dehydrogenase. These products are condensed to form citrate, which is then exported to the cytosol for conversion to cytosolic Ac-CoA and oxaloacetate. The glucose-derived Ac-CoA is not oxidized to CO 2 in the citric acid cycle, but rather is effectively exported to the cytosol to produce fat.

Moreover, because the adipocyte cytoplasm can produce NADPH by running the oxidative and nonoxidative phases of the pentose phosphate pathway and by converting oxaloacetate to malate and then malate to pyruvate, it has a system to capture most of glucose's available electrons into fat synthesis without a high oxygen demand.

Although it is beyond the scope of this article to cover cell replicative and anabolic pathways, it is important to consider that every cell and tissue make everything in the human body from food using metabolic transformations whose biosynthetic complexities greatly exceed the catabolic complexities of breaking down carbohydrates, fats, and proteins. Gluconeogenesis, ketogenesis, amino acid synthesis, nucleic acid synthesis, and steroid synthesis depend on reduced cofactors.

Autotrophs like plants produce glucose during photosynthesis. Heterotrophs like humans ingest other living things to obtain glucose. While the process can seem complex, this page takes you through the key elements of each part of cellular respiration. Cellular respiration is a collection of three unique metabolic pathways: glycolysis, the citric acid cycle, and the electron transport chain. Glycolysis is an anaerobic process, while the other two pathways are aerobic. In order to move from glycolysis to the citric acid cycle, pyruvate molecules the output of glycolysis must be oxidized in a process called pyruvate oxidation.

Glycolysis is the first pathway in cellular respiration. This pathway is anaerobic and takes place in the cytoplasm of the cell. This pathway breaks down 1 glucose molecule and produces 2 pyruvate molecules.

There are two halves of glycolysis, with five steps in each half. This half splits glucose, and uses up 2 ATP. If the concentration of pyruvate kinase is high enough, the second half of glycolysis can proceed. Some cells e. However, most cells undergo pyruvate oxidation and continue to the other pathways of cellular respiration.