CELLULAR RESPIRATION: TRANSFORMING CHEMICAL ENERGY The Principles of Energy Transfer 1. Cellular respiration and fermentation are catabolic, energy- yielding pathways 5.The “fall” of electrons during respiration is stepwise, via NAD + and an electron transport chain 4.Electrons “fall” from organic molecules to oxygen during cellular respiration 3.Redox reactions release energy when electrons move closer to electronegative atoms 2. Cells recycle the ATP they use for work over and over

Living is work. (Ain’t it the truth…) Fig. 9.1 Light energy trapped in organic molecules is available to both photosynthetic organisms and others that eat them. In most ecosystems, energy enters as sunlight. To perform their many tasks, cells require transfusions of energy from outside sources.

Organic molecules store energy in their arrangement of atoms (in the bonds between atoms) Cellular respiration and fermentation are catabolic, energy-yielding pathways Some of the released energy is used to do work and the rest is dissipated as heat. Enzymes catalyze the systematic degradation of large organic molecules (glucose) that are rich in energy to simpler waste products with less energy (CO 2 )

Cellular respiration is similar to the combustion of gasoline in an automobile engine. Some of this energy is used to produce ATP that will perform cellular work. The catabolism of glucose is exergonic, and yields a significant amount of energy. C 6 H 12 O 6 + 6O 2 -> 6CO 2 + 6H 2 O + Energy (ATP + heat) Carbohydrates, fats, and proteins can all be used as the fuel, but it is traditional to start learning with glucose. Organic compounds + O 2 -> CO 2 + H 2 O + Energy The overall process is:

Cells recycle the ATP they use for work An animal cell regenerates ATP from ADP and P i by the catabolism of organic molecules. The price of most cellular work is the conversion of ATP to ADP and inorganic phosphate (P i ). Loss of the end phosphate group “relaxes” the “spring”. The close packing of three negatively charged phosphate groups is an unstable, energy-storing arrangement. It is the chemical equivalent of a loaded spring. ATP, adenosine triphosphate, is the key molecule in cellular energetics.

Fig. 9.2 ATP is used in a wide variety of reactions and work within a cell

Catabolic pathways relocate the electrons stored in food molecules, releasing energy that is used to synthesize ATP. Reactions that result in the transfer of one or more electrons from one reactant to another are oxidation-reduction reactions, or redox reactions. -The loss of electrons is called oxidation. -The addition of electrons is called reduction. Redox reactions release energy when electrons move closer to electronegative atoms Remember OIL RIG

The formation of table salt from sodium and chloride is a redox reaction. -Na + Cl -> Na + + Cl - -Here sodium is oxidized and chlorine is reduced (its charge drops from 0 to -1). More generally: Xe - + Y -> X + Ye - -X, the electron donor, is the reducing agent and reduces Y. -Y, the electron recipient, is the oxidizing agent and oxidizes X. Redox reactions require both a donor and acceptor.

Oxygen is one of the most potent oxidizing agents An electron looses energy as it shifts from a less electronegative atom to a more electronegative one. A redox reaction that relocates electrons closer to oxygen releases chemical energy that can do work. To reverse the process, energy must be added to pull an electron away from an atom.

In cellular respiration, glucose and other fuel molecules are oxidized, releasing energy. In the summary equation of cellular respiration: C 6 H 12 O 6 + 6O 2 -> 6CO 2 + 6H 2 O Glucose is oxidized, oxygen is reduced, and electrons lose potential energy. Molecules that have an abundance of hydrogen are excellent fuels because their bonds are a source of “hilltop” electrons that “fall” closer to oxygen. Electrons “fall” from organic molecules to oxygen during cellular respiration

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

Dehydrogenase enzymes strip two hydrogen atoms from the fuel (e.g., glucose), pass two electrons and one proton to NAD + and release H +. H-C-OH + NAD + -> C=O + NADH + H +

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.

The electrons carried by NADH lose very little of their potential energy in this process. This energy is tapped to synthesize ATP as electrons “fall” from NADH to oxygen.

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.

The electron transport chain, consisting of several molecules (primarily proteins), is built into the inner membrane of a mitochondrion. NADH shuttles electrons from food to the “top” of the chain. At the “bottom,” oxygen captures the electrons and H + to form water. The free energy change from “top” to “bottom” is -53 kcal/mole of NADH. Electrons are passed by increasingly electronegative molecules in the chain until they are caught by oxygen, the most electronegative.