Operation of a Cell

Whereas DNA has a single role as the storehouse of genetic information, RNA plays many roles in the operation of a cell. There are several different types of RNA, each having its own function. The principal job of RNA is to provide the information needed to synthesize proteins. Protein synthesis requires several steps, each assisted by RNA. One type of RNA copies the genetic information from DNA and carries this blueprint out of the nucleus and into the cytoplasm, where construction of the protein takes place. The protein is assembled on the surface of a ribosome, a cell component that contains a second type of RNA. The protein is consfructed by sequential addition of amino acids in the order specified by the DNA. The individual amino acids are carried to the growing protein chain by yet a third type of RNA. The details of protein synthesis are well understood, but the process is much too complex to be described in an introductoiy course in chemistry. 

Why doesn t operation of a cell change the concentrations in the cell Cell voltage is measured under conditions of negligible current flow. The resistance of a high-quality pH meter is 1013 ft. If you use this meter to measure a potential of 1 V, the current is... 

Fig. 4.6.1. Schematic diagram showing the direction of the emf, electrochemical potential gradient, electrostatic potential gradient, and electric field during spontaneous operation of a cell operating in conformity with Conventions 1 and 2, described below.

To reconcile the operation of an order-producing cell with the second law of thermodynamics, we must remember that A5 u iv, not A5 sys, must be positive for a process to be spontaneous. A process for which A5 sys is negative can be spontaneous if the associated AS surr is both larger and positive. The operation of a cell is such a process. 

If all the substances participating in the reversible operation of a cell are in their standard states (unit activities), the potential of an individual electrode is simply its respective standard potential, E°. The free energy change for the cell reaction is therefore... 

Unless special measures are taken, the distribution of local current over the surface of a fuel cell is nonuniform. Inhomogeneity in local current is detrimental for stable long-term operation of a cell. A large local current is accompanied by an increase in local polarization voltage and heating. These processes accelerate the ageing of cell components thereby reducing cell lifetime. 

Figure 1.3. Operation of a cell Junctioning as a generator (discharge ofan accumulator) and as a receiver (charge of an accumulator)...

The operation of a cell during discharge is also shown schematically in Fig. 1.1. When the cell is connected to an external load, electrons flow from the anode, which is oxidized, through the external load to the cathode, where the electrons are accepted and the cathode material is reduced. The electric circuit is completed in the electrolyte by the flow of anions (negative ions) and cations (positive ions) to the anode and cathode, respectively. 

End-point voltage Voltage below which connected equipment will not operate, or below which operation of a cell is not recommended frequently interchangeable with cut-off voltage. ... 

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