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| Chem1 Virtual Textbook�-�Chemical Energy Understanding the internal energy of a chemical system is one of the most important steps in mastering science.
HyperPhysics�-�Internal Energy
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| Special points of emphasis
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Work, Energy, and Power Electricity Atomic Theory Periodic Properties The Chemical Bond Intermolecular Forces Thermochemistry Oxidation-Reduction | Often in a chemical process, the transformation leads to charged particles assuming different positions with regard to one another. A transformation might have occurred so that in the final state an electron has been removed from an atom, or maybe a set of electrons in the substance are occupying a new set of molecular orbitals and now they are very tightly held by a powerfully electronegative atom. To judge the internal energy change in a chemical process, try to get a sense of how the arrangement of charges has changed between the initial and the final state. Has the new arrangement allowed opposite charge to move closer together or like charge to spread out? Either of these particular cases corresponds to an internal energy decrease. Internal energy decrease occurs, for example, when a powerful oxidizing agent gains electron control in a redox reaction. Its powerfully positive nucleus draws the new electrons in tightly. The large electrostatic potential energy decrease for the charges contained in the matter means an internal energy decrease for the system. A central road to understanding chemistry is learning to apply this kind of basic analysis to changes occurring at the atomic, molecular, or intermolecular level.
We hope you have really begun to notice how often we are repeating these basic concepts week by week, changing the context and slowly adding ideas as we proceed through our MCAT course. This is a major theme within the spiraling curriculum. Where is it going? To help you have a sense of where we are heading over the next few months, imagine a mitochondrion, the powerhouse of eukaryotic cells, laid out on your desk. In other words, imagine what a mitochondrion would look like if it were as big as the top of your desk. A mitochondrion is a few thousand or tens of thousands of angstroms long, so the mitochondrion can be only a few thousand chemical bonds long. If it were laid out on your desk, you would just be able to make out the individual atoms of the mitochondrion like grains of sand.
Think about what goes on in a mitochondrion. Picture the plasma membranes of its outer and inner membranes. Picture the cytochrome system on the inner membrane. Imagine you can see the contents of the matrix (water, ATP, NADH, the intermediates of the citric acid cycle), and imagine the proton gradient between the inner and outer compartment. What you see within the aqueous solution environment is a very complex system of electric charges with chemical structure.
Imagine step by step, pulling the atoms apart of the molecules of glycolysis or the citric acid cycle and letting them fall together into subsequent forms in the pathways of oxidative metabolism. What is happening to energy? Imagine watching chemiosmosis across the inner membrane as if you were watching water fall through the hydroelectric power station, but instead of gravitational potential energy doing the work of pumping protons, within the mitochondrion, the potential energy decrease is not gravitational but electrostatic. Picture oxygen down at the end of the electron transport chain, electron greedy, with its thin electron cloud barely shielding its powerful nucleus, pulling the electrons towards itself, driving the proton pumps.
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Work, Energy, and Power Electricity Thermochemistry Bioenergetics and Cellular Respiration | Let us take a specific example of an important biological molecule for metabolism to discuss internal energy decrease in chemistry, ATP. With ATP hydrolysis, internal energy decrease corresponds to the separation of like charge.
ATP contains a series of three negatively charged phosphate groups. When hydrolysis of ATP occurs, one of the phosphate groups, bearing two negative charges, is allowed to separate from ADP, containing the other two negative charges. Just as work would be needed to push negative charges together, potential energy decreases when they are separated. (It is not so simple, though, there is also an additional internal energy decrease because of increased resonance stabilization in ADP.)
In summary, defamiliarize your perceptions and think about ATP as a system of charges in molecular form. The three negatively charged phosphate groups are like a coiled spring with the electrostatic potential energy of like charges being held together, constrained by the geometry of the molecule. Imagine pushing it all together and you are imagining the high internal energy and enthalpy of ATP. Combine this idea with a sense of the probability of the phosphate being available, just randomly showing up, and you are getting a sense of the free energy. Many biochemical processes which would be nonspontaneous can occur spontaneously because they are coupled with enzymatic cleavage of ATP.
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Electricity The Chemical Bond Thermochemistry Carbohydrates Lipids Oxidation-Reduction Bioenergetics and Cellular Respiration | Why do triglycerides possesses more food calories per gram than carbohydrates? Let us continue our preview of the thermochemical interpretation of metabolism with this question and review for a moment the overall internal energy changes that nutrient molecules undergo through oxidative metabolism.
In total stoichiometric balance, oxidative metabolism involves breaking relatively weak carbon-hydrogen bonds and forming stronger carbon-oxygen and hydrogen-oxygen bonds.
Remember that bonds with greater electronegativity difference tend to be stronger because they allow a powerful nucleus to draw bonding electrons inwards. Another way to say that a bond is strong is to say that its formation leads to energy decrease (energy that ultimately translates to food calories).
The answer to the question is as simple as this: Because triglycerides have primarily carbon-hydrogen bonds, these molecules have a greater reservoir of electrons which are not already bound to oxygen which can ultimately participate in bonds with oxygen. From a thermochemical perspective, we are breaking more weak bonds with triglycerides and forming strong bonds. From an oxidation-reduction perspective, more oxygen is being reduced per gram of triglyceride.
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