In most ecosystems, energy enters as sunlight. Light energy trapped in organic molecules is available to both photosynthetic organisms and others that eat them. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 9.1

Cellular respiration and fermentation are catabolic, energy-yielding pathways fermentation, leads to the partial degradation of sugars in the absence of oxygen. A more efficient and widespread catabolic process, cellular respiration, uses oxygen as a reactant to complete the breakdown of a variety of organic molecules. Most of the processes in cellular respiration occur in mitochondria. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Cellular respiration is similar to the combustion of gasoline in an automobile engine. The overall process is: Organic compounds + O 2 -> CO 2 + H 2 O + Energy Carbohydrates, fats, and proteins can all be used as the fuel. C 6 H 12 O 6 + 6O 2 -> 6CO 2 + 6H 2 O + Energy (ATP + heat) The catabolism of glucose is exergonic. Some of this energy is used to produce ATP that will perform cellular work. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Cellular respiration does not oxidize glucose in a single step that transfers all the hydrogen in the fuel to oxygen at one time. Rather, glucose and other fuels are broken down gradually in a series of steps, each catalyzed by a specific enzyme. At key steps, hydrogen atoms are stripped from glucose and passed first to a coenzyme, like NAD + (nicotinamide adenine dinucleotide). The “fall” of electrons during respiration is stepwise, via NAD + and an electron transport chain Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 9.4 This changes the oxidized form, NAD +, to the reduced form NADH. NAD + functions as the oxidizing agent in many of the redox steps during the catabolism of glucose.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 9.5 Unlike the explosive release of heat energy that would occur when H 2 and O 2 combine, cellular respiration uses an electron transport chain to break the fall of electrons to O 2 into several steps.