Describe the major steps that occur as glucose and its products travel through c

Question

Describe the major steps that occur as glucose and its products travel through cellular respiration. At what points do the carbon atoms from glucose exit cellular respiration and where do these carbon atoms ultimately go? Describe the major steps that occur as glucose and its products travel through cellular respiration. At what points do the carbon atoms from glucose exit cellular respiration and where do these carbon atoms ultimately go? Describe the major steps that occur as glucose and its products travel through cellular respiration. At what points do the carbon atoms from glucose exit cellular respiration and where do these carbon atoms ultimately go?

Explanation / Answer

Aerobic cellular respiration is a specific series of enzymecontrolled chemical reactions in which oxygen is involved in the breakdown of glucose into carbon dioxide and water and the chemical-bond energy from glucose is released to the cell in the form of ATP. Although the actual process of aerobic cellular respiration involves many enzyme-controlled steps, the net result is that a reaction between sugar and oxygen results in the formation of carbon dioxide and water with the release of energy.

The following equation summarizes this process:

glucose (C6H12O6) + oxygen (6 O2) carbon dioxide ( 6 CO2) + water (6 H2O) + energy (ATP + heat)

In cells, these reactions take place in a particular order and in particular places within the cell. In eukaryotic cells, the process of releasing energy from food molecules begins in the cytoplasm and is completed in the mitochondrion. There are three distinct enzymatic pathways involved (figure 6.3): glycolysis, the Krebs cycle, and the electron-transport system.

Glycolysis

Glycolysis is a series of enzyme-controlled reactions that takes place in the cytoplasm. During glycolysis, a 6-carbon sugar molecule (glucose) has energy added to it from two ATP molecules. Adding this energy makes some of the bonds of the glucose molecule unstable, and the glucose molecule is more easily broken down. After passing through several more enzyme-controlled reactions, the 6-carbon glucose is broken down to two 3-carbon molecules known as glyceraldehyde-3- phosphate (also known as phosphoglyceraldehyde2), which undergo additional reactions to form pyruvic acid (CH3COCOOH). Enough energy is released by this series of reactions to produce four ATP molecules. Because two ATP molecules were used to start the reaction and four were produced, there is a net gain of two ATPs from the glycolytic pathway. During the process of glycolysis, some hydrogens and their electrons are removed from the organic molecules being processed and picked up by the electron-transfer molecule NAD to form NADH. Enough hydrogens are released during glycolysis to form 2 NADHs. The NADH with its extra electrons contains a large amount of potential energy, which can be used to make ATP in the electron-transport system. The job of the coenzyme NAD is to transport these energy-containing electrons and protons safely to the electrontransport system. Once they have dropped off their electrons, the oxidized NADs are available to pick up more electrons and repeat the job. The following is a generalized reaction that summarizes the events of glycolysis:

glucose + 2 ATP + 2 NAD+ 4 ATP + 2 NADH + 2 pyruvic acid

The Krebs Cycle

The series of reactions known as the Krebs cycle takes place within the mitochondria of cells. It gets its name from its discoverer, Hans Krebs, and the fact that the series of reactions begins and ends with the same molecule. The Krebs cycle is also known as the citric acid cycle and the TriCarboxylic Acid cycle (TCA). The 3-carbon pyruvic acid molecules released from glycolysis enter the mitochondria, are acted upon by specific enzymes, and are converted to 2-carbon acetyl molecules. At the time the acetyl is produced, 2 hydrogens are attached to NAD+ to form NADH. The carbon atom that was removed is released as carbon dioxide. The acetyl molecule is attached to coenzyme A (CoA) and proceeds through the Krebs cycle. During the Krebs cycle, the acetyl is completely oxidized.

The remaining hydrogens and their electrons are removed. Most of the electrons are picked up by NAD+ to form NADH, but at one point in the process FAD picks up electrons to form FADH2. Regardless of which electron carrier is being used, the electrons are sent to the electron-transport system. The remaining carbon and oxygen atoms are combined to form CO2. As in glycolysis, enough energy is released to generate 2 ATP molecules. At the end of the Krebs cycle, the acetyl has been completely broken down (oxidized) to CO2. The energy in the molecule has been transferred to ATP, NADH, or FADH2. Also, some of the energy has been released as heat. For each of the acetyl molecules that enters the Krebs cycle, 1 ATP, 3 NADHs, and 1 FADH2. If we count the NADH produced during glycolysis, when acetyl was formed, there are a total of 4 NADHs for each pyruvic acid that enters a mitochondrion. The following is a generalized equation that summarizes those reactions: .

pyruvic acid + ADP + 4 NAD+ + FAD = 3 CO2 + 4 NADH + FADH2 + ATP

The Electron-Transport System

Of the three steps of aerobic cellular respiration, (glycolysis, Krebs cycle, and electron-transport system) cells generate the greatest amount of ATP from the electron-transport system (figure 6.6). During this stepwise sequence of oxidationreduction reactions, the energy from the NADH and FADH2 molecules generated in glycolysis and the Krebs cycle is used to produce ATP. Iron-containing cytochrome enzyme molecules are located on the membranes of the mitochondrion. The energy-rich electrons are passed (transported) from one cytochrome to another, and the energy is used to pump protons (hydrogen ions) from one side of the membrane to the other. The result of this is a higher concentration of hydrogen ions on one side of the membrane. As the concentration of hydrogen ions increases on one side, a proton gradient builds up. Because of this concentration gradient, when a membrane channel is opened, the protons flow back to the side from which they were pumped. As they pass through the channels, a phosphorylase enzyme (ATPase) speeds the formation of an ATP molecule by bonding a phosphate to an ADP molecule (phosphorylation). When all the electrons and hydrogen ions are accounted for, a total of 32 ATPs are formed from the electrons and hydrogens removed from the original glucose molecule. The hydrogens are then bonded to oxygen to form water. The following is a generalized reaction that summarizes the events of the electron-transport system:

6 O2 + 8 NADH + 4 FADH2 +32 ADP = 8 NAD+ + 4 FAD + 32 ATP + 12 H2O

Exit of carbon atoms from glucose

All carbon atoms from the original glucose molecule broken down during glycolysis are already released, incorporated in carbon dioxide molecules. That occurs because, for each glucose, two pyruvic acid molecules were made during glycolysis. Each of these two pyruvic acid molecules is then converted into acetyl CoA with the release of one carbon dioxide molecule (two in total). Since each of the two produced acetyl CoA molecules cycles the Krebs cycle once, the initial glucose molecule triggers two rounds of the Krebs cycle and, as a result, four other carbon dioxide molecules are produced.

In short release of carbon atoms occurs at three sites (in the form of CO2)

1. conversion of pyruvate to acetyl coenzyme A (end of glycolysis)

2. conversion of isocitrate to alpha ketoglutarate (krebs cycle)

3. conversion of alpha ketoglutarate to succinyl coenzyme A (krebs cycle)

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