Energy Transport in the Cell
Energy may be defined as the capacity for doing work, and many of the cell's processes involve doing work. For plant cells, the energy may be obtained from the Sun by photosynthesis but for animal cells the energy is obtained from chemical reactions that release energy - that is, the energy is obtained from foods which contain available energy. The typical general reaction that provides energy is the oxidation of glucose.
C6H12O6 + 6O2 --> 6CO2 + 6H2O
However, even those reactions that release energy (exergonic reactions) usually are not initiated spontaneously but require some activation energy to start them. For the energy-releasing reactions involved in cell metabolism to proceed at satisfactory rates under the conditions inside cells, protein catalysts called enzymes are typically required. These enzymes lower the activation energy required. The energy yield from such fuel reactions may be stored in energy-rich molecules like ATP or may be used to provide energy for energy-requiring reactions (endergonic reactions) like those involved in the synthesis of proteins. Often there are coupled reactions that used the energy from exergonic reactions to fuel specific endergonic synthesis reactions.
Audesirk & Audesirk