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Cell Biology
Biological Energy
Energy and Electrons
Energy and Electrons
All biological energy transactions involve the movement of electrons.

If an electron is moved away from its atomic center (of protons and neutrons), this is a non-spontaneous reaction and an input of energy is required.

If, on the other hand, an electron is moved closer to the atomic center of protons and neutrons, this is a spontaneous reaction and energy is liberated.

Within any biological molecule, such as sucrose, electrons that carry a certain amount of energy are shared by the various constituent atoms. For examples, there are lots of carbon and hydrogen atoms in the sucrose molecule. In each covalent bond holding them together, two electrons are shared.

These electrons contain a relatively high level of energy and are equally shared by both the carbon and hydrogen atoms. Similarly, the electrons in an oxygen molecule also hold a lot of energy and are equally shared by both oxygen atoms.

When, however, the oxygen molecule reacts with the sucrose molecule, the atoms rearrange themselves into two new substances - carbon dioxide and water.

In these molecules, the electrons are shared differently. The electrons are now much closer to the oxygen atom that hey are to either the carbon or hydrogen atoms. They have been moved toward the atomic center of the oxygen atom, and thus to a lower energy state.

The extra energy is given off. The breakdown of sucrose, therefore, is a spontaneous reaction that obeys the second law of thermodynamics. Inside the sugar cane plant, sucrose molecules combine with oxygen in a process called respiration. The basic principles of respiration are quite simple.

Electrons and covalent bonds Food molecules, such as sucrose, contain high levels of organized energy stored in the electrons of their covalent bonds. This energy can be released by moving the electrons closer to an atomic center.

During the chemical reaction of sucrose with oxygen, this is exactly what happens. The electrons end up closer to the atomic center of the oxygen atoms in the water and carbon dioxide molecules.

Excess energy is given off and can be used by the plant cells for doing work or form making non-spontaneous reactions take place.

During synthesis of sucrose, electrons are moved in the opposite direction. Carbon dioxide and water are the raw materials necessary for the construction of this sugar. The electrons are close to the oxygen atom in these molecules; therefore, as they are rearranged into a molecule of sucrose, these electrons must be moved away from the oxygen atomic center.

This is a nonspontaneous process and requires an input of energy before it will take place. Photosynthesis supplies this energy. Plants trap sunlight and use the energy it contains to "pull" electrons away from the oxygen atom. These resulting glucose and fructose molecules can then be joined to form sucrose.

Figure legend: Formation of Water. During this reaction, between molecular oxygen and molecular hydrogen, electrons are moved closer to the oxygen atom. Energy is given off.

© 2001, Professor John Blamire