Fermentation occurs if oxygen is absent. The following is a summary word equation for aerobic respiration. In reality the process of respiration consists of a series of reactions that can be divided into two stages. Respiration releases energy - it is an exothermic process. This means it releases heat energy. Photosynthesis is the process by which plants, and some bacteria, synthesise food molecules which they then use, in addition to other things, for respiration.
The process of photosynthesis requires energy — it is endothermic. In contrast, when oxygen is available, the pyruvates produced by glycolysis become the input for the next portion of the eukaryotic energy pathway.
During this stage, each pyruvate molecule in the cytoplasm enters the mitochondrion, where it is converted into acetyl CoA , a two-carbon energy carrier, and its third carbon combines with oxygen and is released as carbon dioxide. At the same time, an NADH carrier is also generated. Acetyl CoA then enters a pathway called the citric acid cycle , which is the second major energy process used by cells. Figure 6: Metabolism in a eukaryotic cell: Glycolysis, the citric acid cycle, and oxidative phosphorylation Glycolysis takes place in the cytoplasm.
Within the mitochondrion, the citric acid cycle occurs in the mitochondrial matrix, and oxidative metabolism occurs at the internal folded mitochondrial membranes cristae. The third major process in the eukaryotic energy pathway involves an electron transport chain , catalyzed by several protein complexes located in the mitochondrional inner membrane.
This process, called oxidative phosphorylation, transfers electrons from NADH and FADH 2 through the membrane protein complexes, and ultimately to oxygen, where they combine to form water. As electrons travel through the protein complexes in the chain, a gradient of hydrogen ions, or protons, forms across the mitochondrial membrane. Cells harness the energy of this proton gradient to create three additional ATP molecules for every electron that travels along the chain.
Overall, the combination of the citric acid cycle and oxidative phosphorylation yields much more energy than fermentation - 15 times as much energy per glucose molecule! Together, these processes that occur inside the mitochondion, the citric acid cycle and oxidative phosphorylation, are referred to as respiration , a term used for processes that couple the uptake of oxygen and the production of carbon dioxide Figure 6.
The electron transport chain in the mitochondrial membrane is not the only one that generates energy in living cells. In plant and other photosynthetic cells, chloroplasts also have an electron transport chain that harvests solar energy.
Even though they do not contain mithcondria or chloroplatss, prokaryotes have other kinds of energy-yielding electron transport chains within their plasma membranes that also generate energy.
When energy is abundant, eukaryotic cells make larger, energy-rich molecules to store their excess energy. The resulting sugars and fats — in other words, polysaccharides and lipids — are then held in reservoirs within the cells, some of which are large enough to be visible in electron micrographs. Animal cells can also synthesize branched polymers of glucose known as glycogen , which in turn aggregate into particles that are observable via electron microscopy. A cell can rapidly mobilize these particles whenever it needs quick energy.
Athletes who "carbo-load" by eating pasta the night before a competition are trying to increase their glycogen reserves. Under normal circumstances, though, humans store just enough glycogen to provide a day's worth of energy. Plant cells don't produce glycogen but instead make different glucose polymers known as starches , which they store in granules. In addition, both plant and animal cells store energy by shunting glucose into fat synthesis pathways.
One gram of fat contains nearly six times the energy of the same amount of glycogen, but the energy from fat is less readily available than that from glycogen. Still, each storage mechanism is important because cells need both quick and long-term energy depots. Fats are stored in droplets in the cytoplasm; adipose cells are specialized for this type of storage because they contain unusually large fat droplets.
Humans generally store enough fat to supply their cells with several weeks' worth of energy Figure 7. Figure 7: Examples of energy storage within cells. A In this cross section of a rat kidney cell, the cytoplasm is filled with glycogen granules, shown here labeled with a black dye, and spread throughout the cell G , surrounding the nucleus N. B In this cross-section of a plant cell, starch granules st are present inside a chloroplast, near the thylakoid membranes striped pattern.
C In this amoeba, a single celled organism, there is both starch storage compartments S , lipid storage L inside the cell, near the nucleus N. Qian H. Letcher P. When the cell uses oxygen to break down sugar, oxygen is used, carbon dioxide is produced, and energy is released.
What are the products of respiration? Cellular respiration is this process in which oxygen and glucose are used to create ATP, carbon dioxide, and water. ATP, carbon dioxide, and water are all products of this process because they are what is created.
Carbon dioxide is released as a gas when you exhale. Where do most reactions of respiration occur? The enzymatic reactions of cellular respiration begin in the cytoplasm, but most of the reactions occur in the mitochondria.
Cellular respiration occurs in the double-membrane organelle called the mitochondrion. Where does respiration take place in the body? The cells take in glucose and produce ethanol alcohol and carbon dioxide. Most aerobic respiration happens in the mitochondria, but anaerobic respiration takes place in the fluid portion of the cytoplasm.
What is the main source of energy of the human body? Carbohydrates, such as sugar and starch, for example, are readily broken down into glucose, the body's principal energy source. Glucose can be used immediately as fuel, or can be sent to the liver and muscles and stored as glycogen. What type of energy is food? Chemical energies.
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