Bring on the S"mores!

This inviting campfire can be used for both heat and light. Heat and light are two forms of energy that are released when a fuel like wood is burned. The cells of living things also get energy by "burning." They "burn" glucose in the process called cellular respiration.

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How much energy does it cost to do your body’s work? A single cell uses about 10 million ATP molecules per second and recycles all of its ATP molecules about every 20-30 seconds.

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Splitting Glucose

The word glycolysis means “glucose splitting,” which is exactly what happens in this stage. Enzymes split a molecule of glucose into two molecules of pyruvate (also known as pyruvic acid). This occurs in several steps, as shown in figure (PageIndex4). Glucose is first split into glyceraldehyde 3-phosphate (a molecule containing 3 carbons and a phosphate group). This process uses 2 ATP. Next, each glyceraldehyde 3-phosphate is converted into pyruvate (a 3-carbon molecule). this produces two 4 ATP and 2 NADH.

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Figure (PageIndex4): In glycolysis, a glucose molecule is converted into two pyruvate molecules.

Results of Glycolysis

Energy is needed at the start of glycolysis to split the glucose molecule into two pyruvate molecules. These two molecules go on to stage II of cellular respiration. The energy to split glucose is provided by two molecules of ATP. As glycolysis proceeds, energy is released, and the energy is used to make four molecules of ATP. As a result, there is a net gain of two ATP molecules during glycolysis. high-energy electrons are also transferred to energy-carrying molecules called electron carriers through the processknown as reduction. The electron carrier of glycolysis is NAD+(nicotinamide adenine diphosphate). Electrons are transferred to 2 NAD+ to produce two molecules of NADH. The energy stored in NADH is used in stage III of cellular respiration to make more ATP. At the end of glycolysis, the following has been produced:• 2 molecules of NADH• 2 net molecules of ATP



Transformation of Pyruvate into Acetyl-CoA

In eukaryotic cells, the pyruvate molecules produced at the end of glycolysis are transported into mitochondria, which are sites of cellular respiration. If oxygen is available, aerobic respiration will go forward. In mitochondria, pyruvate will be transformed into a two-carbon acetyl group (by removing a molecule of carbon dioxide) that will be picked up by a carrier compound called coenzyme A (CoA), which is made from vitamin B5. The resulting compound is called acetyl CoA and its production is frequently called the oxidation or the Transformation of Pyruvate (see Figure (PageIndex5). Acetyl CoA can be used in a variety of ways by the cell, but its major function is to deliver the acetyl group derived from pyruvate to the next pathway step, the Citric Acid Cycle.

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api/deki/files/18010/1280px-Animal_mitochondrion_diagram_en.svg.png?revision=1&size=bestfit&width=412&height=283" />Figure (PageIndex6): The structure of a mitochondrion is defined by an inner and outer membrane. The space inside the inner membrane is full of fluid, enzymes, ribosomes, and mitochondrial DNA. This space is called a matrix. The inner membrane has a larger surface area as compared to the outer membrane. Therefore, it creases. The extensions of the creases are called cristae. The space between the outer and inner membrane is called intermembrane space.

Recall that glycolysis produces two molecules of pyruvate (pyruvic acid). Pyruvate, which has three carbon atoms, is split apart and combined with CoA, which stands for coenzyme A. The product of this reaction is acetyl-CoA. These molecules enter the matrix of a mitochondrion, where they start the Citric Acid Cycle. The third carbon from pyruvate combines with oxygen to form carbon dioxide, which is released as a waste product. High-energy electrons are also released and captured in NADH. The reactions that occur next are shown in Figure (PageIndex7).


Steps of the Citric Acid (Krebs) Cycle

The Citric Acid Cycle begins when acetyl-CoA combines with a four-carbon molecule called OAA (oxaloacetate; see the lower panel of Figure (PageIndex7)). This produces citric acid, which has six carbon atoms. This is why the Krebs cycle is also called the citric acid cycle. After citric acid forms, it goes through a series of reactions that release energy. This energy is captured in molecules of ATP and electron carriers. The Krebs cycle has two types of energy-carrying electron carriers: NAD+ and FAD. The transfer of electrons to FAD during the Kreb’s Cycle produces a molecule of FADH2. Carbon dioxide is also released as a waste product of these reactions. The final step of the Krebs cycle regenerates OAA, the molecule that began the Krebs cycle. This molecule is needed for the next turn through the cycle. Two turns are needed because glycolysis produces two pyruvate molecules when it splits glucose.

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Figure (PageIndex7): In the Citric Acid Cycle, the acetyl group from acetyl CoA is attached to a four-carbon oxaloacetate molecule to form a six-carbon citrate molecule. Through a series of steps, citrate is oxidized, releasing two carbon dioxide molecules for each acetyl group fed into the cycle. In the process, three NAD+ molecules are reduced to NADH, one FAD molecule is reduced to FADH2, and one ATP or GTP (depending on the cell type) is produced (by substrate-level phosphorylation). Because the final product of the citric acid cycle is also the first reactant, the cycle runs continuously in the presence of sufficient reactants.

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Oxidative phosphorylation

Oxidative phosphorylation is the final stage of aerobic cellular respiration. There are two substages of oxidative phosphorylation, Electron transport chain and Chemiosmosis. In these stages, energy from NADH and FADH2, which result from the previous stages of cellular respiration, is used to create ATP.