Metabolism and Valence Electron Energy
Oxygen Has Lower Energy Orbitals Than Carbon or Hydrogen
Oct 22, 2009
The metabolism of cells is driven by rearrangements of carbon, hydrogen and oxygen atoms in carbohydrates. Central to the production of the energy carrier, ATP (adenosine triphosphate) is storing the energy released as electrons in high energy orbitals of carbon and hydrogen atoms move to lower energy bonding orbitals of oxygen.
Electrons are Arranged in Orbitals Around Nuclear Protons and Neutrons
The atomic number of an element indicates the number of protons in the nucleus and the number of electrons arrayed in orbitals of defined energy levels. Thus, carbon, with an atomic number of six, has six protons in the nucleus and orbitals containing a total of six electrons (two in the innermost, low energy orbital and four in outer, higher energy orbitals.) The number of neutrons in the nucleus is usually the same as the number of protons. Protons have a positive charge that is usually balanced by the negative charges of the surrounding electrons.
Electron Orbital Energy is Determined by Distance From the Nuclear Charge
Quantum chemistry determines the electron orbitals available for a particular number of nuclear protons. Lower energy orbitals are closest to the nucleus and are filled first. The innermost orbital of carbon, hydrogen and oxygen holds two electrons. The next shell of orbitals holds a maximum of eight electrons paired into four orbitals. The energy of the electrons in the orbitals reflects the number of protons in the nucleus and the distance of the orbiting electrons from the nucleus. Energy is released as electrons approach a proton, so the lowest energy electrons are those closest to the largest number of protons. Hydrogen has a single proton and its first orbital has only a single, high energy electron. The inner orbital of carbon is filled with two low energy electrons and the next orbitals are further away, making them higher energy. The outer four, valence electrons of carbon atoms have about the same energy as hydrogen electrons. Oxygen has two more protons than carbon, making its inner orbitals lower energy than carbon’s. The higher nuclear proton charge also means that the six valence electrons in outer orbitals in oxygen atoms are lower in energy than the valence electrons of hydrogen or oxygen.
Bond Energy is the Energy Released During Bond Formation
Electrons can be influenced by two nuclei in close proximity. An orbital shared between two different atoms is called a bonding orbital and it is usually lower in energy than either of the valence orbitals. Thus, covalent bond formation can be considered the result of two atoms moving close enough together so that valence orbitals merge to become a bonding orbital with a shared pair of electrons. The energy of the bond is the difference in energies of the valence electrons from each atom compared to the lower energy of the bonding orbital. Bond breaking always requires an input of energy.
Bonding Orbitals With Oxygen are Lower Energy Than Between Carbon and Hydrogen
In cellular metabolism, glycolysis starts with glucose, a six carbon carbohydrate, with each carbon linked to one oxygen and the remaining three bonds to carbons or hydrogens. The valence electrons of carbon and hydrogen, or in covalent bonds with carbon and hydrogen, are high in energy. Valence electron of oxygen or those with covalent bonds with oxygen are low in energy.
Glycolysis results in two molecules of pyruvic acid, two water molecules and two ATPs. ( Two other molecules, NADH, carry high energy electrons for energy production through respiration or for fermentation.) The two molecules of pyruvic acid have three carbons, four hydrogens and three oxygens. Each of the two waters has an oxygen with two hydrogens.
The full accounting for glycolysis in terms of carbon or hydrogen electrons involved in bonds to oxygen are glucose (12) vs. two pyruvic acids (12) + two water (4), i.e. two ATPs worth of energy are derived from four electrons in valence orbitals of carbon or hydrogen in glucose moving to lower energy bonding orbitals with oxygen. (Each of the high energy electrons in NADH can be used in the electron transport chain of respiration to produce another three ATP before being ultimately transferred to oxygen to produce water.)
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