Concept 7.1: Catabolic pathways yield energy by oxidizing organic fuels

Vocab

• Catabolic pathways are metabolic pathways that release stored energy by breaking down complex molecules.

• Exergonic reactions are any reaction occurring at constant temperature without input of electrical or photon energy.

• Catabolic pathways are metabolic pathways that release stored energy by breaking down complex molecules.

→ Electron transfer from fuel molecules (e.g., glucose) to other molecules is crucial in this process.

→These processes are central to cellular respiration.

Catabolic Pathways and Production of ATP

• Organic compounds have potential energy due to the arrangement of electrons in their bonds, which can be released in exergonic reactions.

→ Enzymes start the break down of these organize compounds by breaking them down to simpler waste products, releasing energy that can be used for work or dissipated as heat.

• Aerobic respiration is the most efficient catabolic pathway, using oxygen to break down organic fuel, and is common in eukaryotic and many prokaryotic cells.

→ Aerobic respiration is similar to the combustion of gasoline, with food as fuel and carbon dioxide and water as exhaust.

→The overall process of aerobic respiration can be summarized as: 

• Organic compounds +O2→CO2+H2O+Energy.

• Carbohydrates, fats, and proteins can be used as fuel, with glucose being a primary example: C6H12O6+6O2→6CO2+6H2O+Energy (ATP + heat)

• The breakdown of glucose is exergonic, with a free-energy change of -686 kcal/mol (ΔG=-686 kcal/mol), indicating a spontaneous reaction.

• Fermentation is a catabolic process that partially degrades sugars without oxygen.

• Anaerobic respiration occurs in some prokaryotes, using substances other than oxygen to harvest energy.

• Cellular respiration includes both aerobic and anaerobic processes but often refers to aerobic respiration.

• Catabolic pathways release energy but do not directly perform cellular work; instead, they regenerate ATP from ADP and ℗i.

Redox Reactions: Oxidation and Reduction

• Catabolic pathways yield energy through the transfer of electrons during chemical reactions, releasing energy stored in organic molecules and this energy synthesize ATP.

• Reactions usually involve the transfer of one or more electrons from one reactant to another. They are called redox reactions. 

→ oxidation is the loss of electrons and reduction is the gain of electrons

• (These processes always occur together.)

• Adding electrons reduces the positive chargei

→ The electron donor (Xe-) is the reducing agent, and the electron acceptor (Y) is the oxidizing agent.

→Not all redox reactions involve complete electron transfer; some involve changes in electron sharing in covalent bonds

• An example is methane combustion

→ Methane’s covalent electrons are nearly equally shared between its carbon and hydrogen atoms. Methane combustion is an energy-yielding redox reaction where methane interacts with oxygen and forms carbon dioxide, methane is oxidized and oxygen is reduced, releasing energy as electrons move to more electronegative atoms.

→ Energy must be added to pull an electron away from an atom

• The more an atom is electronegative the more energy required

• Electrons loses potential energy that is used for cellular work when it shifts from a less electronegative atom to one that is more electronegative

→A redox reaction that moves electrons closer to oxygen, such as the burning (oxidation) of methane, therefore releases chemical energy that can be put to work.

Oxidation of Organic Fuel Molecules During Cellular Respiration

→ Cellular respiration is a redox process where glucose is oxidized and oxygen is reduced, releasing energy for ATP synthesis.

•  The summary equation is C6H12O6+6O2→6CO2+6H2O+energy

→Organic molecules with an abundance of hydrogen (C-H bonds) are excellent fuels because their electrons release energy when transferred to oxygen. (Hilltop electrons [electrons that haven’t rolled down the hill yet, so it has a lot of energy to be released/loss once it rolls down. When a ball rolls down the hill, it loses potential energy])

→ In the summary equation, hydrogen is transferred to oxygen (oxidation [transfer of energy to a lower emergy state which releases energy])

• The energy state of the electron changes once this happens

• C-H bonds are oxidized into C-O bonds

Stepwise Energy Harvest via NAD++ and the Electron Transport Chain

• Enzymes lower the activation energy barrier, allowing glucose to be oxidized in a series of steps, with electrons transferred to NAD+ forming NADH.

→If glucose is oxidized all at once, or if energy is released all at once, it won’t be very efficient

→each step is catalyzed by an enzyme

• NAD+ acts as an electron carrier, cycling between its oxidized form (NAD+) and reduced form (NADH).

→ Dehydrogenase enzymes facilitate this transfer.

→NADH represents stored energy that can be used to make ATP when electrons move from NADH to oxygen.

• The transfer of electrons from NADH to oxygen is highly exergonic, with a free-energy change of -53 kcal/mol (-222 kJ/mol).

•  Cellular respiration involves the reaction of hydrogen and oxygen to form water, similar to rocket fuel combustion, but with key differences: hydrogen comes from organic molecules, and the process occurs in multiple steps via an electron transport chain.

→The electron transport chain is composed of molecules, mainly proteins, located in the inner mitochondrial membrane in eukaryotes and the plasma membrane in prokaryotes.

→ Electrons from glucose are transferred to NADH, which carries them to the high-energy end of the chain. Oxygen at the low-energy end captures these electrons, forming water.

• In anaerobic respiration, the final electron acceptor is not oxygen.

→ The electron transport chain allows the controlled release of energy in small steps, preventing a single explosive release and enabling ATP production.

→ Electrons move down the chain through a series of redox reactions, each carrier having a higher affinity for electrons than the previous one, culminating in oxygen, which has the highest affinity.

• The overall electron flow in cellular respiration is: glucose → NADH → electron transport chain → oxygen.

The Stages of Cellular Respiration: A Preview

• Cellular respiration consists of three main stages: glycolysis, pyruvate oxidation and the citric acid cycle, and oxidative phosphorylation.

1. Glycolysis occurs in the cytoplasm and breaks down glucose into two molecules of pyruvate, producing a small amount of ATP through substrate-level phosphorylation.

2. In eukaryotes, pyruvate enters the mitochondrion and is oxidized to acetyl CoA, which enters the citric acid cycle. In prokaryotes, these processes occur in the cytosol.

• The citric acid cycle completes the breakdown of glucose to carbon dioxide, generating NADH and FADH2, which carry electrons to the electron transport chain.

3. Oxidative phosphorylation, occurring in the inner mitochondrial membrane (or plasma membrane in prokaryotes), involves the electron transport chain and chemiosmosis, producing the majority of ATP.

• Electron carriers NADH and FADH2 transfer electrons to the electron transport chain, where they combine with oxygen and hydrogen ions to form water.

• The energy released during electron transport is used to synthesize ATP from ADP in a process called oxidative phosphorylation, which accounts for about 90% of the ATP generated by respiration.

• Substrate-level phosphorylation directly transfers a phosphate group from an organic substrate to ADP, forming ATP, and occurs in glycolysis and the citric acid cycle.

• For each glucose molecule, cellular respiration can produce up to 32 molecules of ATP, converting the energy stored in glucose into a more usable form for cellular activities.

Concept Check 7.1

• Aerobic respiration requires oxygen and includes glycolysis, the citric acid cycle, and oxidative phosphorylation. It produces a large amount of ATP.

• Anaerobic respiration does not require oxygen and includes glycolysis followed by fermentation. It produces a smaller amount of ATP.

• ATP is made during cellular respiration through substrate-level phosphorylation and oxidative phosphorylation.

• Substrate-level phosphorylation occurs during glycolysis and the citric acid cycle.

• Oxidative phosphorylation occurs during the electron transport chain and chemiosmosis.

• In the redox reaction C4H6O5+NAD+→C4H4O5+NADH+H+, C4H6O5 is oxidized to C4H4O5, and NAD+ is reduced to NADH.