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NBL 355: Module 3

Terms in this set (25)

In a noncovalent bond, the atoms do not share electrons. Instead, the atoms are attracted to each other. There are several types of noncovalent bonds/interactions. Noncovalent bonds are weaker bonds than covalent bonds and in aqueous solutions an individual noncovalent bonds can be broken apart (dissociate) fairly easily. The four types of noncovalent bonds are ionic, hydrogen, Van deer Waals, and hydrophobic.

In an ionic bond, a positively charged ion (a cation) is attracted to a negatively charged ion (an anion). Hydrogen bonds form between a hydrogen atom in one polar molecule that is attracted to a more electronegative atom such as oxygen or nitrogen, in another polar molecule. Hydrogen bonds form between water molecules and are found in lots of other polymers in cells, including proteins and DNA. Van der Waals attractions occur for a very short time period when electrons from two atoms are positioned so that the slightly more negative side of one atom attracts the slightly more positive side of another atom. Hydrophobic bonds form between nonpolar molecules, and as we will discuss in the next modules, are in fact driven to form by the nonpolar molecules being repulsed by water.

Noncovalent bonds are much weaker than covalent bonds. However, noncovalent bonds are important because they give water many of its characteristics. And importantly, they allow for the secondary and tertiary structures of proteins and DNA to ensure the packing and three dimensional structure and function of these molecules. Even though individual non-covalent bonds are fairly weak, since a protein (or DNA) contains tens to hundreds to thousands of noncovalent bonds, its three dimensional structure is stable in aqueous solution. Hydrophobic bonds are essential in forming the lipid bilayers in biological membranes, which are essential for cells and signaling.
Glycolysis occurs in the cytoplasm and converts one glucose molecule into two pyruvates, with the net production of two molecules of ATP and two molecules of NADH. Glycolysis does not require oxygen.

Mitochondria involve three processes, called the Krebs cycle (also called the citric acid cycle or TCA cycle), electron transport chain, and oxidative phosphorylation, to produce ATP from pyruvates and NADH. (This is sometimes called cellular respiration or aerobic respiration and depends on oxygen.)

For each glucose, during glycolysis in the cytoplasm, there is initially a net of two ATP molecules, two NADH and two pyruvates produced. From glycolysis, the two pyruvates and two NADH are shuttled into mitochondria where an additional net 30 molecules of ATP are produced. (You don't need to memorize the numbers for this course.) This means that typically, for one molecule of glucose, through glycolysis (anaerobic) and aerobic metabolism in mitochondria the net ATP production is ~32 molecules. Note that although there is a theoretical yield of 38 ATP molecules per glucose during cellular respiration, which is often shown in Biology texts, such conditions are generally not realized because of the cost (use of ATP/energy) of moving pyruvate (from glycolysis), phosphate, and ADP (substrates for ATP synthesis) into the mitochondria.

The glucose-lactate shuttle occurs in astrocytes. Astrocytes (specifically their end feet) take up (transport) glucose (and other nutrients) from the extracellular fluid around the vascular endothelial cells, into the astrocyte cytoplasm. Astrocytes can then release the glucose and other nutrients into the extracellular fluid (ECF). In addition, in the cytoplasm, through the process of glycolysis, in astrocytes the glucose is converted to pyruvate. Then, the pyruvate is converted to lactate by the enzyme lactate dehydrogenase (LDH). That lactate is transported out of the astrocyte and into the ECF around the astrocyte. Lactate in the ECF is taken up by neurons, converted to pyruvate by LDH in the neuron, and then the pyruvate is shuttled into mitochondria where it is used to produce ATP by the Kreb's cycle, electron transport chain and oxidative phosphorylation (aerobic respiration). Note that the interconversion of pyruvate and lactate is an unusual reaction in cells in that the delta G is close to 0, meaning that the reactant and product have very similar energies, and so it is a reversible reaction. The enzyme LDH catalyzes both reactions: pyruvate to lactate and lactate to pyruvate. The neurons, their axons and other glial cells such as oligodendrocytes can take up the glucose and lactate that the astrocytes provide to the ECF.