Cellular Energy: Glucose Oxidation

Cells Harvest Chemical Energy from Glucose Oxidation

• Cells derive energy from glucose through a sequence of metabolic pathways.
• Complex transformations occur through separate reactions.
• Each reaction is facilitated by a specific enzyme.
• Metabolic pathways exhibit similarity across various organisms.
• In eukaryotes, metabolic pathways are organized within specific organelles.
• Enzymes can be either inhibited or activated, thus controlling the pathway's speed.
• The metabolism or combustion of glucose is represented by: C6H12O6 + 6 O2 → 6 CO2 + 6 H2O + free energy.
• The change in Gibbs free energy (ΔG) for the reaction is -686 kcal/mol.
• The reaction that metabolizes or combusts glucose is highly exergonic, and drives the endergonic formation of ATP molecules.
• Glycolysis is the anaerobic catabolic process.
• Cellular respiration is the aerobic catabolic process.
• Fermentation is the anaerobic catabolic process.
• Oxidation-Reduction (Redox) reactions entail electron transfer between substances.
• Reduction is the gaining of electrons.
• Oxidation is the losing of electrons.
• Oxidation and reduction reactions always happen in tandem.
• Not all redox reactions involve a complete transfer of electrons.
• Electrons are not lost or gained, instead the sharing of electrons by an atom changes as a result of polar bonds.
• Carbon-carbon bonds are shared equally in glucose, while Carbon-oxygen bonds are polar in carbon dioxide.
• Glucose acts as the reducing agent while oxygen (O2) acts as the oxidizing agent during glucose metabolism.
• The more a molecule is reduced, the more energy it holds.
• Some energy is transferred from the reducing agent, glucose, to the reduced product in a redox reaction.
• Transfer of electrons often occurs alongside the transfer of hydrogen ions.
• When a molecule loses hydrogen atoms, it is oxidized.
• Coenzyme NAD+ serves as a critical electron carrier in redox reactions.
• Reduction with NAD+: NAD+ + H+ + 2e- → NADH
• Oxidation with NADH: NADH + H+ + 1/2 O2 → NAD+ + H2O, with a ΔG = -52.4 kcal/mol.
• Oxygen (O2) is the ultimate electron acceptor under aerobic conditions, driving four metabolic pathways.
• Pyruvate, which is created through glycolysis, is fermented under anaerobic conditions.

Glucose Is Fully Oxidized in the Presence of Oxygen

• Glycolysis occurs in the cytoplasm.
• In glycolysis, glucose turns into 2 pyruvate molecules.
• 2 ATP and 2 NADH molecules are produced by glycolysis.
• The glycolisis process happens in 10 steps.
• Steps 1-5 in glycolysis require ATP inputs, energy-investing reactions.
• Steps 6-10 in glycolysis results in ATP and NADH generation (energy-harvesting reactions).
• Oxidation-reduction is a type of reaction where energy released through glucose oxidation is captured via reduction of NAD+ to NADH.
• Substrate-level phosphorylation is a type of reaction that converts ADP to ATP by transferring a phosphate from the substrate.
• As a material becomes oxidized, it loses electrons which are then transferred to another material which becomes reduced.
• Redox reactions results in the transfer of large amounts of energy.
• Glucose is oxidized when it donates electrons in highly exergonic reactions.
• Coenzymes NAD and FAD store and transport energy in biological redox reactions, and exist in oxidized (NAD+) and reduced (NADH) forms.
• Glycolysis functions whether oxygen (O2) is present or not.
• Pyruvate is transported to the mitochondria via active transport during pyruvate oxidation.
• Pyruvate oxidation occurs within the mitochondrial matrix.
• Pyruvate is transformed into acetate and carbon dioxide (CO2) during pyruvate oxidation.
• The resulting acetate binds with coenzyme A, leading to the formation of acetyl CoA.
• Pyruvate oxidation is exergonic, resulting in NAD+ converting to NADH.
• The pyruvate dehydrogenase complex, consisting of three enzymes, catalyzes pyruvate oxidation.
• Acetyl CoA initiates the citric acid cycle by donating its acetyl group to oxaloacetate, forming citrate.
• The citric acid cycle starts with Acetyl CoA.
• The citric acid cycle has eight reactions that completly oxidize the acetyl group to 2 molecules of CO2.
• GDP, NAD+, and FAD capture the energy that is released during the citric acid cycle.
• Oxaloacetate gets regenerated in the citric acid cycle's last step.
• GTP can transfer its high-energy phosphate to form ATP.
• Complete oxidation of a glucose molecule produces 6 CO2, 10 NADH, 2 FADH2, and 4 ATP.
• Replenishment of acetyl CoA and oxidized electron carriers is essential for the citric acid cycle to keep running.
• Electron carriers are reduced, and they must be reoxidized.
• Oxygen (O2) will accept electrons and produce H2O if present, electrons are not passed directly to oxygen.

Oxidative Phosphorylation Forms ATP

• ATP is synthesized through reoxidation of electron carriers when oxygen is present through oxidative phosphorylation.
• Electron transport is a component of oxidative phosphorylation.
• Chemiosmosis is a component of oxidative phosphorylation.
• Electrons from NADH and FADH2 travel through the respiratory chain.
• The respiratory chain is made of membrane-associated carriers.
• An electron flow generates a proton concentration gradient across the inner mitochondrial membrane.
• Electrons return through the membrane via ATP synthase (a channel protein) during chemiosmosis.
• ATP synthase links diffusion with ATP synthesis.
• The electron transport chain uses many steps to prevent too much free energy being realeased at once.
• Reactions in series releases a small amount of energy which is captured by an endergonic reaction.
• The folded inner mitochondrial membrane is where the respiratory chain is located.
• Electron movement between carriers is how energy is released.
• The inner mitochondrial membrane is where oxidation of NADH and FADH2 happens in the respiratory chain.
• Four protein complexes include I, II, III, IV within the mitochondrial membrane.
• Ubiquinone (Q) is one lipid within the mitochondrial membrane.
• Cytochrome c is one peripheral protein within the mitochondrial membrane.
• Electrons transfer from NADH to Protein complex I and FADH to Protein complex II.
• Complex I and II go to Q and Q to complex III.
• Cytochrome c follows, leading to complex IV.
• Finally, electrons are passed to Oxygen, forming H2O.
• Protons (H+) are transported into the intermembrane space during electron transport.
• Concentration gradient and charge difference creates potential energy called the proton-motive force.
• Diffusion of protons back across the membrane gets coupled to ATP synthesis through chemiosmosis.
• ATP synthase may act as ATPase which results in the hydrolyzing of ATP to ADP and Pi.
• Synthesis of ATP is favored because ATP leaves the matrix quickly, which lowers the concentration of ATP within the matrix.
• Maintained by active transport, a Hydrogen gradient.
• Early proof of chemiosmosis was found from research on thylakoid membranes isolated from chloroplasts.
• UPC1 causes protons to be permeable, which inserts into the mitochondrial membrane of brown fat cells.
• Chemiosmosis and electron transport are uncoupled by protein UPC1, thus causes energy to be dissipated as heat.
• While the ATP synthase remains consistent across all living organisms, it comprises a molecular motor divided into two components.
• The Fo unit serves as a transmembrane channel for hydrogen ions (H+).
• The F1 unit extends into the matrix and rotates to expose active sites for ATP synthesis.
• When hydrogen ions (H+) travel through the channel, potential energy transforms into kinetic energy, which then causes the central polypeptide to turn.
• Transmission of energy to F1's catalytic subunits facilitates the synthesis of ATP.
• Instead of oxygen, bacteria and archaea often uses alternative electron acceptors such as SO4-2, Fe+3, and CO2, for anaerobic respiration.
• This helps these organisms to live in environments where oxygen 02, is little or not present.

In the Absence of Oxygen, Some Energy Is Harvested from Glucose

• Without the presence of oxygen, ATP can be made by glycolysis and fermentation.
• Glycolysis and fermentation occurs in the cytoplasm.
• Only partial oxidization of glucose occurs.
• Substrate-level phosphorylation releases 2 ATP per glucose.
• NAD+ is regenerated, helping glycolysis go on.
• Lactic acid fermentation uses pyruvate as the electron acceptor with lactate produced.
• Microorganisms and complex organisms use lactic acid fermentation.
• The enzyme, Lactate dehydrogenase promotes fermentation, but can oxidize lactate to pyruvate if there is oxygen.
• Insufficient oxygen (O2) during intense exercise in vertebrate muscle cells leads to lactic acid fermentation after which muscle cells break down glycogen.
• Muscle pain results from increased lactate levels leading to a lower pH.
• Alcoholic fermentation is found in yeasts and some plant cells.
• Two enzymes are required to metabolize pyruvate to ethanol.
• The reactions are reversible.
• Alcoholic fermentation is used to make alcholic beverages.
• More enery is collected during cellular respiration than fermentation.
• Glycolysis and fermentation products generate 2 ATP.
• Glycolysis and cellular respiration generates 32 ATP.
• Partial oxidization of glucose yields produces that retain more energy than CO2; which occurs during fermentation only.