Pathways that Harvest and Store Chemical Energy

Chemical energy available to do work is termed free energy (G). 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). Exergonic reaction = -∆G Endergonic reactions = +∆G

Five Principles Governing Metabolic Pathways Chemical transformation occurs in a series of intermediate reactions that form a metabolic pathway. Each reaction is catalyzed by a specific enzyme. Most metabolic pathways are similar in all organisms. In eukaryotes, many metabolic pathways occur inside specific organelles. Each metabolic pathway is controlled by enzymes that can be inhibited or activated.

Role of ATP in Energy Transfer Energy released by exergonic reactions is stored in the bonds of ATP When ATP is hydrolyzed, free energy is released to drive endergonic reactions.

Role of Oxidation and Reduction Energy can also be transferred by the transfer of electrons in oxidation–reduction, or redox reactions. Transfers of hydrogen atoms involve transfers of electrons (H = H+ + e-) When a molecule loses a hydrogen atom, it becomes oxidized.

Role of Coenzymes Coenzyme NAD+ is a key electron carrier in redox reactions. Reduction of NAD+ is highly endergonic: Oxidation of NADH is highly exergonic:

Oxidative Phosphorylation Oxidative phosphorylation couples the oxidation of NADH with the production of ATP: This coupling is achieved by the process of chemiosmosis.

Chemiosmosis Chemiosmosis is the diffusion of protons across a membrane, which drives the synthesis of ATP.

Cellular Respiration and Photosynthesis Cellular respiration is a major catabolic pathway: Photosynthesis is a major anabolic pathway:

Oxidation occurs in a series of small steps in three pathways: Concept 6.2 Carbohydrate Catabolism in the Presence of Oxygen Releases a Large Amount of Energy A lot of energy is released when reduced molecules with many C—C and C—H bonds are fully oxidized to CO2. Oxidation occurs in a series of small steps in three pathways: 1. glycolysis 2. pyruvate oxidation 3. citric acid cycle

Oxidation of Carbohydrates Glycolysis 10 reactions Occurs in the cytosol Final Products 2 molecules of pyruvate 2 molecules of ATP (substrate-level phosphorylation) 2 molecules of NADH

Oxidation of Carbohydrates Pyruvate Oxidation Final Products 2 molecules of CO2 2 molecules of Acetate- which is bound to coenzyme A (Acetyl CoA)

Oxidation of Carbohydrates Citric Acid Cycle 8 reactions- turns twice for each glucose molecule Final Products 4 molecules of CO2 (2 per turn) 2 molecules of ATP (by substrate-level phosphorylation) 6 molecules of NADH 2 molecules of FADH2

Oxidation of Carbohydrates Electron Transport and ATP Synthesis NADH is reoxidized to NAD+ and O2 is reduced to H2O in a series of steps. Final Products 32 molecules of ATP 6 molecules of H2O

Electron Transport/ ATP Synthesis Respiratory chain- series of redox carrier proteins imbedded in the cristae of the mitochondria oxidize NADH and FADH2 by passing the electrons from one carrier to the next. Energy is used to transport H+ ions out of the mitochondrial matrix so ATP synthase can synthesize ATP by chemiosmosis.

Under anaerobic conditions, NADH is reoxidized by fermentation. Concept 6.3 Carbohydrate Catabolism in the Absence of Oxygen Releases a Small Amount of Energy Under anaerobic conditions, NADH is reoxidized by fermentation. The overall yield of ATP is only two—the ATP made in glycolysis. Lactic acid fermentation NADH reduces pyruvate to lactic acid. Alcoholic fermentation NADH reduces acetaldehyde to ethanol

Concept 6.4 Catabolic and Anabolic Pathways are Integrated Metabolic pathways are linked. Catabolism Anabolism Amounts maintain a constant level- the metabolic pool.

Photosynthesis involves two pathways: Concept 6.5 During Photosynthesis, Light Energy Is Converted to Chemical Energy Photosynthesis involves two pathways: Light reactions convert light energy into chemical energy. Carbon-fixation reactions use the ATP and NADPH, along with CO2, to produce carbohydrates.

Pigments Pigment molecules absorb wavelengths in the visible spectrum. Chlorophylls a and b provide most of the photosynthesis in plants Absorption spectrum Action spectrum

Light-Harvesting Reaction Pigments are arranged into light-harvesting complexes, or antenna systems. A photosystem, consisting of antenna systems and a reaction center, spans the thylakoid membrane in the chloroplast

Chlorophyll When chlorophyll (Chl) absorbs light, it enters an excited state (Chl*), then rapidly returns to ground state, releasing an excited electron. Final electron acceptor is NADP+, which gets reduced: ATP is produced chemiosmotically during electron transport (photophosphorylation).

Noncyclic Electron Pathway (noncyclic photophosphorylation)

Cyclic Photophosphorylation Cyclic electron transport uses only photosystem I and produces ATP; an electron is passed from an excited chlorophyll and recycles back to the same chlorophyll.

Each reaction is catalyzed by a specific enzyme. Concept 6.6 Photosynthetic Organisms Use Chemical Energy to Convert CO2 to Carbohydrates The Carbon-fixation reactions occur in the stroma in the metabolic pathway known as the Calvin Cycle. Each reaction is catalyzed by a specific enzyme.

The Calvin Cycle 1. Fixation of CO2: CO2 is added to ribulose 1,5-bisphosphate (RuBP). Ribulose bisphosphate carboxylase/oxygenase (rubisco) catalyzes the reaction.

The Calvin Cycle 2. 3PG is reduced to form glyceraldehyde 3-phosphate (G3P).

The Calvin Cycle 3. The CO2 acceptor, RuBP, is regenerated from G3P. Some of the extra G3P is exported to the cytosol and is converted to hexoses (glucose and fructose).

REFERENCES Campbell, Neil A. and Jane B. Reece; Biology, 6th Ed. Benjamin Cummings, 2002 Campbell, Neil A. and Jane B. Reece; Biology, 8th Ed. Benjamin Cummings, 2008 Hillis, David M. et. al., Principles of Life, 1st Ed. W.H. Freeman and Company, 2012