UNIT 9 Energy transfer by charge carriers

Energy transfer by charge carriers is the most common reaction in metabolic processes. It is a process which can take very different courses. The redox process is a classical example of this. It consists basically in the transfer of one or two electrons from the donor to the acceptor molecule. In this way, the donor becomes oxidized and the acceptor reduced. This apparently simple scheme however conceals a number of complicated sub-routines which have not yet been completely resolved.

For this process of electron transfer, donor and acceptor molecules must be in exactly denned position to each other and at a minimum distance, so that overlapping of respective electron orbitals can occur. In the first place, donor and acceptor will form a complex of highly specific stearic configuration, a so-called charge transfer complex. This process of complex formation which occasionally requires stearic transformations of both molecules, causes the actual transfer. It takes place at lower rates than energy transfer by induction as discussed earlier. Hence, the charge-transfer complex is an activated transition state which enables redox processes to take place between highly specific reaction partners in the enzyme systems of the cellular metabolism. Because of the oscillating nature of electron transfer, this coupling of two molecules is strengthened by additional electrostatic forces sometimes called charge-transfer forces.

In the process of energy transfer, differences between energetic potentials of donor and acceptors play an important role. An uphill transfer of electrons is only possible through an input of external radiation energy. Actually, these differences are slight, when compared with the absolute values of the ionization energy. They are only in the region of about 1.5 eV.

What is the scale of these energy gradients? Szent-Gyorgyi in his remarkable "Study in Cellular Regulations, Defense, and Cancer" in 1968 proposed to introduce a scale of so-called biopotentials. This scale is based on the ionization energy of water (12.56 eV), the basic molecule of life, and has the opposite direction to the scale of ionization energy. This scale has never really come to the fore, probably because the name is somewhat confusing as the same term had already been used for a completely different parameter, namely for the electrical transmembrane potential. On the other hand, it is quite illustrative.

Define the following words using a dictionary

To reduce, to conceal, denned, stearic, a sub-routine, oscillating, to come to the fore, illustrative

Complete the sentences

1. The redox process consists basically…

2. This apparently simple scheme however conceals…

3. Hence, the charge-transfer complex is an activated transition state which…

4. Because of the oscillating nature of electron transfer…

5. An uphill transfer of electrons is only possible…

6. For this process of electron transfer, donor and acceptor molecules…

7. Actually, these differences are slight,…

8. This scale has never really come to the fore, probably because…