In Chapter 12 we described the energy currencies NADH and ATP. In this chapter we will describe the chemical pathways that regenerate these currencies when their levels are depleted. We will then consider some other important chemical pathways, some that operate in all cells, others that are found only in certain types of organism or in specialized biochemical centers like the liver. Figure 13.1 is an overview of the main metabolic pathways within a cell.

All the processes that occur within a living cell are ultimately driven by energy taken from the outside world. Green plants and some bacteria take energy directly from sunlight. Other organisms take compounds made using sunlight and break them down to release energy, a process called catabolism. The most common way of breaking down these food compounds is to oxidize them, that is, to burn them but in a controlled way. The energy trapped in energy currencies can then be used for the building, repair, and homeostatic processes termed anabolism. The collective term for all of the reactions going on inside a cell is metabolism. All metabolic reactions share some general features:

• They are catalyzed by enzymes.

• They are universal in that all organisms show remarkable similarity in the main pathways.

• They involve relatively few types of chemical reaction.

Cell Biology: A Short Course, Second Edition, by Stephen R. Bolsover, Jeremy S. Hyams, Elizabeth A. Shephard, Hugh A. White, Claudia G. Wiedemann ISBN 0-471-26393-1 Copyright © 2004 by John Wiley & Sons, Inc.


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