Energy production, cellular

FIGURE 49-1. Normal glucose metabolism. Once insulin binds with receptors on the cell membrane, glucose can move into the cell, promoting cellular metabolism and energy production. 

The phosphate group is derived from phosphoric acid (H3 PO4) by replacing an O—H bond by an O—C or O— P bond. Phosphate is an important functional group in biochemistry, being involved in cellular energy production as well as acting as an important monomer in biopolymers, particularly in DNA. Bonds to phosphate groups form or break in the course of a number of important biochemical reactions. 

In addition to energy production, mitochondria play a role in several other cellular activities. For example, mitochondria help regulate the self-destruction of cells (apoptosis). They are also necessary for the production of substances such as cholesterol and heme (a component of hemoglobin, the molecule that carries oxygen in the blood). 

Mitochondria are found in the cell body and all processes of the neuron. They possess a double membrane and their own DNA and they play a role in cellular respiration and energy synthesis. Mitochondria contain enzymes essential for energy production in the form of adenosine triphosphate (ATP). 

Cytotoxicity is the general term used to describe toxicity at the level of the cell. It can be brought about in many ways, usually by a chemical interaction between the toxic agent and one or more components of the cell. Interactions can be permanently damaging or may lead to temporary injury that the cell is capable of repairing. Perhaps the most important sites of intracellular injury are cell membranes, the cell nucleus (home of DNA), mitochondria (home of energy production), and endoplasmic reticulum (home of the biosynthesis of the all-important protein molecules, essential for cell structure and, as enzymes, for the catalysis of all cellular reactions and for the metabolism of foreign chemicals). 

The cyanide ion is called a pseudohalide ion because it behaves like Cl- in forming an insoluble, white silver salt, AgCN. In complex ions such as Fe(CN)63-, CN - acts as a Lewis base (Section 15.16), bonding to transition metals through the lone pair of electrons on carbon. In fact, the toxicity of HCN and other cyanides is due to the strong bonding of CN- to iron(III) in cytochrome oxidase, an important enzyme involved in the oxidation of food molecules. With CN attached to the iron, the enzyme is unable to function. Cellular energy production thus comes to a halt, and rapid death follows. 

Lactic acid fermentation occurs when lactate is the end product of fermentation. Coupled with glycolysis, lactic acid fermentation can generate ATP from ADP and provide energy for cellular processes. The fermentation step in lactic acid fermentation generates NAD+ from NADH + H+, and the NAD+ cycles back to the glycolysis process. The lactic acid fermentation cycle is illustrated in Figure 4.10. 

Mitochondria are the main source of free radicals in the cell and, in turn, ROS can cause inhibition of complex enzymes in the electron transport chain of the mitochondria leading to the shutdown of energy production and amplifying generation of mitochondrial free radicals (Orrenius, 2007). Free radicals can then cause extensive cellular damage by causing oxidation of lipids, proteins, and DNA. 

Ion transport is also often coupled with cellular energy production and with nutrient and product membrane transport. Aside from Papoutsakis work on the influence of methanol transport on growth of methanol-consuming bacteria, the importance of membrane control of nutrient and product fluxes into the cell has been largely ignored by biochemical engineers [25]. Better methods for measuring the pH and electrical potential differences across cell membranes are needed, as is more careful consideration of membrane-mediated processes in cell kinetics models. 

Excitotoxicity Membrane depolarization in response to disrupted cellular oxidative and energy production processes leading to damaging activity of secondary messengers... 

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