6 Pathways that Harvest and Store Chemical Energy
Energy is stored in chemical bonds and can be released and transformed by metabolic pathways. Chemical energy available to do work is termed free energy (G).
Five principles governing metabolic pathways: 1. Chemical transformations occur in a series of intermediate reactions that form a metabolic pathway. 2. Each reaction is catalyzed by a specific enzyme. 3. Most metabolic pathways are similar in all organisms.
4. In eukaryotes, many metabolic pathways occur inside specific organelles. 5. Each metabolic pathway is controlled by enzymes that can be inhibited or activated.
In cells, energy-transforming reactions are often coupled: An energy-releasing (exergonic) reaction is coupled to an energy-requiring (endergonic) reaction.
Adenosine triphosphate (ATP) is a kind of energy currency in cells. Energy released by exergonic reactions is stored in the bonds of ATP. When ATP is hydrolyzed, free energy is released to drive endergonic reactions.
Figure 6.1 The Concept of Coupling Reactions
Figure 6.2 ATP
Hydrolysis of ATP is exergonic: ATP H 2 O ADP P i freeenergy ΔG is about 7.3 kcal
Free energy of the bond between phosphate groups is much higher than the energy of the O H bond that forms after hydrolysis. text art pg 102 here (1 st one, in left-hand column)
Phosphate groups are negatively charged, so energy is required to get them near enough to each other to make the covalent bonds in the ATP molecule. ATP can be formed by substrate-level phosphorylation or oxidative phosphorylation.
Energy can also be transferred by the transfer of electrons in oxidation reduction, or redox reactions. Reduction is the gain of one or more electrons. Oxidation is the loss of one or more electrons.
Oxidation and reduction always occur together.
Transfers of hydrogen atoms involve transfers of electrons (H = H + + e ). When a molecule loses a hydrogen atom, it becomes oxidized.
The more reduced a molecule is, the more energy is stored in its bonds. Energy is transferred in a redox reaction. Energy in the reducing agent is transferred to the reduced product.
Figure 6.3 Oxidation, Reduction, and Energy
Coenzyme NAD + is a key electron carrier in redox reactions. NAD + (oxidized form) NADH (reduced form)
Figure 6.4 A NAD + /NADH Is an Electron Carrier in Redox Reactions
Reduction of NAD + is highly endergonic: NAD H 2e NADH Oxidation of NADH is highly exergonic: NADH H 1 O NAD H O 2 2 2
Figure 6.4 B NAD + /NADH Is an Electron Carrier in Redox Reactions
In cells, energy is released in catabolism by oxidation and trapped by reduction of coenzymes such as NADH. Energy for anabolic processes is supplied by ATP. Oxidative phosphorylation transfers energy from NADH to ATP.
Oxidative phosphorylation couples oxidation of NADH: NADH NAD H 2e energy with production of ATP: energy ADP P i ATP
The coupling is chemiosmosis diffusion of protons across a membrane, which drives the synthesis of ATP. Chemiosmosis converts potential energy of a proton gradient across a membrane into the chemical energy in ATP.
Figure 6.5 A Chemiosmosis
ATP synthase membrane protein with two subunits: F 0 is the H + channel; potential energy of the proton gradient drives the H + through. F 1 has active sites for ATP synthesis.
Figure 6.5 B Chemiosmosis
Chemiosmosis can be demonstrated experimentally. A proton gradient can be introduced artificially in chloroplasts or mitochondria in a test tube. ATP is synthesized if ATP synthase, ADP, and inorganic phosphate are present.
Figure 6.6 An Experiment Demonstrates the Chemiosmotic Mechanism
Cellular respiration is a major catabolic pathway. Glucose is oxidized: carbohydra te 6O2 6CO2 6H2O chemical energy Photosynthesis is a major anabolic pathway. Light energy is converted to chemical energy: 6CO 6H2O light energy O2 2 6 carbohydrate
Figure 6.7 ATP, Reduced Coenzymes, and Metabolism