Cellular respirations • Oxygen atoms, being highly electronegative, pull shared electron pairs toward themselves when bound to carbon. • This electron movement is equivalent to carbon losing electrons. • The attachment of oxygen to carbon places electrons in a more stable configuration, releasing free energy. • Free Energy and Reaction: • Aerobic oxidation of glucose moves valence electrons from high free energy (glucose) to lower free energy (carbon dioxide and water). • Results in: • Decrease in potential energy. • Increase in entropy. • Reaction:  • Standard free energy change (): -2870 kJ/mol glucose under standard lab conditions (25°C, 101.3 kPa). • Cellular conditions yield : ~-3012 kJ/mol glucose. • Standard value of -2870 kJ/mol glucose is used for calculations. • Energy Release: • Burning glucose in a test tube releases energy as heat and light. • In cells: • About 34% of free energy is captured in molecules like ATP. • These molecules serve as energy sources for powering endergonic processes. This breakdown summarizes key points for understanding the aerobic oxidation of glucose and its biological significance.

Notes on Activation Energy and Cellular Respiration

1. Activation Energy in Glucose Combustion:

• Reactants (glucose and oxygen) are stable covalent compounds.

• Oxygen atoms attract electrons in glucose’s H-C bonds but cannot oxidize glucose without activation energy.

• Activation energy prevents spontaneous combustion at room/body temperature (25–37°C).

• If glucose spontaneously reacted with oxygen, life could not exist as organic molecules would quickly oxidize.

2. Role of Enzymes in Cellular Respiration:

• Enzymes catalyze each step of aerobic respiration.

• They lower activation energy, enabling reactions to occur at a rate compatible with cellular needs.

• Energy is transferred to carrier molecules like ATP.

3. Electron Acceptors in Cellular Respiration:

• Oxygen:

• Primary electron acceptor in obligate aerobes (e.g., animals, plants, fungi, and most bacteria).

• Vital for aerobic respiration.

• Alternative Acceptors:

• Used by obligate anaerobes (cannot survive in oxygen).

• Acceptors include NO₃⁻, SO₄²⁻, CO₂, and Fe³⁺.

• Examples:

• Clostridium tetani (tetanus).

• Clostridium botulinum (food poisoning).

• Clostridium perfringens (gas gangrene).

4. Types of Organisms and Oxygen Use:

• Obligate Anaerobes:

• Survive only in oxygen-free environments.

• Die in the presence of oxygen.

• Obligate Aerobes:

• Require oxygen as the final electron acceptor.

• Cannot survive without it.

• Facultative Anaerobes:

• Can switch between aerobic and anaerobic conditions.

• Examples:

• Escherichia coli (dysentery).

• Vibrio cholerae (cholera).

• Salmonella enteritidis (food poisoning).

5. Key Reaction in Aerobic Respiration:

• Reaction:

• Free energy is released as electrons move from high-energy glucose bonds to stable water bonds.

• Activation energy is crucial for controlling the process within cells.

Notes: Energy Transfer in Cellular Respiration

General Overview

1. First Law of Thermodynamics:

• Energy cannot be created or destroyed, only transferred or transformed.

2. Goal of Cellular Respiration:

• Convert chemical potential energy into ATP.

• Capture as much free energy as possible in the form of ATP.

Energy-Transfer Mechanisms

1. Substrate-Level Phosphorylation:

• Definition: ATP is formed directly in an enzyme-catalyzed reaction.

• Mechanism:

• A phosphate group is transferred from a phosphate-containing compound to ADP, forming ATP.

• Approximately 31 kJ/mol of energy is transferred under standard conditions (closer to 50 kJ/mol in living cells).

• Significance in Glycolysis and Krebs Cycle:

• 4 ATP molecules generated in glycolysis.

• 2 ATP molecules generated in the Krebs cycle.

2. Oxidative Phosphorylation:

• Definition: ATP is formed indirectly through enzyme-catalyzed redox reactions.

• Role of Oxygen: Functions as the final electron acceptor in the electron transport chain.

Key Molecules

1. Nicotinamide Adenine Dinucleotide (NAD⁺):

• A coenzyme that shuttles electrons to the electron transport chain in the mitochondrial inner membrane.

2. ADP and PEP (phosphoenolpyruvate):

• Intermediate molecules involved in substrate-level phosphorylation.

Summary of Mechanisms

• Substrate-Level Phosphorylation: Direct ATP production.

• Oxidative Phosphorylation: Indirect ATP production via redox reactions involving oxygen.

Notes: Cellular Respiration and Energy Acquisition

Types of Organisms Based on Energy Source

1. Photoautotrophs:

• Use light energy to synthesize organic compounds from inorganic materials (e.g., plants, photosynthetic microorganisms).

• Transform light energy into chemical potential energy in glucose and other carbohydrates.

• Self-sufficient and dominate Earth’s autotroph population.

2. Heterotrophs:

• Depend on autotrophs or other heterotrophs for energy and building materials.

• Include all animals, fungi, most protists, and many bacteria.

• Obtain energy by consuming organic matter that was once alive.

3. Chemoautotrophs:

• Obtain energy by oxidizing inorganic compounds (e.g., sulfur, iron).

• Do not require light and rely on chemical energy sources.

• Often found in extreme environments (e.g., volcanoes, sulfur springs).

• Represent Earth’s earliest energy-harnessing organisms.

Cellular Respiration Overview

1. Definition:

• Process of harvesting energy from glucose through enzyme-controlled redox reactions.

• Overall reaction:

• Aerobic cellular respiration requires oxygen as the oxidizing agent.

2. Key Characteristics:

• Series of about 20 enzyme-catalyzed reactions.

• Products of one reaction serve as reactants for the next.

• Highly efficient at releasing free energy from glucose.

Energy Conversion in Cellular Respiration

1. Redox Reactions:

• Glucose is oxidized to carbon dioxide.

• Oxygen is reduced to water.

• Process releases energy as covalent bonds are rearranged into more stable configurations.

2. Exergonic Process:

• Similar to the combustion of hydrocarbons like propane or gasoline.

• Both processes release energy, with oxygen acting as the oxidizing agent.

Experimental Focus: Oxygen Consumption in Germinating Seeds

• Hypothesis: Germinating seeds have higher metabolic rates due to increased growth and cell division.

• Experimentation: Measure the relationship between growth and metabolic activity through controlled experiments.

Summary

• Photoautotrophs, heterotrophs, and chemoautotrophs represent the major pathways for energy acquisition in life.

• Cellular respiration, particularly aerobic respiration, involves redox reactions to break down glucose, releasing energy for cellular functions.

• This process mirrors the combustion of hydrocarbons, utilizing oxygen as the primary oxidizing agent.

Summary: Mitochondria

1. Structure and Function:

• Mitochondria are round or sausage-shaped organelles found in the cytoplasm of eukaryotic cells.

• Specialized in producing ATP, the primary energy carrier in cells, through aerobic respiration.

• Requires free oxygen for ATP production.

2. Key Features:

• Double Membrane:

• Outer membrane: Similar to the cell membrane.

• Inner membrane: Highly folded into cristae, contains proteins and enzymes for respiration.

• Compartments:

• Mitochondrial matrix: Protein-rich liquid inside the inner membrane.

• Intermembrane space: Fluid-filled space between the inner and outer membranes.

3. Aerobic Respiration:

• Three stages occur in mitochondria:

• Pyruvate oxidation

• Krebs cycle

• Electron transport and chemiosmosis

• Only eukaryotic cells contain mitochondria; prokaryotic cells perform respiration in the cytoplasm.

4. Genetic Material:

• Mitochondria have their own DNA (mtDNA), RNA, and ribosomes.

• Can reproduce independently, and their DNA resembles prokaryotic DNA, hinting at an evolutionary link to bacteria.

Key Terms

• Cristae: Folds of the inner mitochondrial membrane.

• Matrix: Fluid filling the innermost space of the mitochondrion.

• Intermembrane Space: Space between the inner and outer membranes.

• Eukaryotic Cells: Contain membrane-bound organelles, including mitochondria.

• Prokaryotic Cells: Lack membrane-bound organelles and mitochondria.

Summary: Stage 2 – Pyruvate Oxidation

1. Location:

• Occurs in the mitochondrial matrix after pyruvate is transported through the mitochondrial membranes.

2. Key Steps:

• Decarboxylation:

• A low-energy carboxyl group is removed from pyruvate as CO₂.

• Catalyzed by pyruvate decarboxylase.

• Oxidation:

• The remaining two-carbon compound is oxidized to form acetate.

• NAD⁺ is reduced to NADH by gaining two hydrogen atoms (2 protons, 2 electrons).

• Formation of Acetyl-CoA:

• Coenzyme A (CoA) binds to acetate, forming acetyl-CoA.

• The carbon-sulfur bond in acetyl-CoA is unstable, preparing it for further oxidation in the Krebs cycle.

3. Overall Equation:

4. Fates of Products:

• Acetyl-CoA: Enters the Krebs cycle for further energy transfer.

• NADH: Moves to the electron transport chain for ATP production.

• CO₂: Diffuses out of the mitochondrion and cell as waste.

• H⁺ ions: Remain dissolved in the matrix.

5. Role of Acetyl-CoA:

• Central to energy metabolism.

• Can either:

• Enter the Krebs cycle to produce ATP if ATP levels are low.

• Be used in lipid synthesis if ATP levels are high (energy storage).

6. Significance:

• All nutrients (proteins, lipids, carbohydrates) are ultimately converted into acetyl-CoA.

• Acetyl-CoA acts as a metabolic crossroad, directing energy toward immediate ATP production or fat storage based on the cell’s energy needs.

7. Energy and Fat Accumulation:

• Excess consumption of nutrients leads to fat accumulation, as acetyl-CoA is diverted into lipid synthesis when ATP levels are sufficient.

Summary: The Krebs Cycle

1. Discovery:

• Identified by Sir Hans Krebs in 1937, earning him the 1953 Nobel Prize alongside Fritz Lipmann, who discovered coenzyme A’s role in metabolism.

2. Overview:

• The Krebs cycle (also called the citric acid cycle or tricarboxylic acid (TCA) cycle) is an eight-step cyclic process catalyzed by specific enzymes.

• Occurs in the mitochondrial matrix and processes acetyl-CoA formed during pyruvate oxidation.

• Since two acetyl-CoA molecules are derived from one glucose, the cycle runs twice per glucose molecule.

3. Key Features:

• Oxaloacetate (OAA): A four-carbon compound that combines with acetyl-CoA at step 1 to form citrate (six-carbon). OAA is regenerated at the end of the cycle, making the process cyclic.

• Carbon Release:

• All six carbons of glucose are oxidized to CO₂ by the end of the cycle (two CO₂ per acetyl-CoA).

• CO₂ diffuses out of the mitochondrion and the cell as metabolic waste.

• Energy Harvesting:

• Steps 3, 4, and 8: NAD⁺ reduced to NADH.

• Step 5: ATP formed via substrate-level phosphorylation.

• Step 6: FAD reduced to FADH₂, storing free energy for the electron transport chain.

4. Overall Equation:

5. Energy Production:

• Most free energy from glucose remains stored in NADH and FADH₂.

• These coenzymes transfer their energy to ATP in the electron transport chain (ETC) during oxidative phosphorylation.

6. Significance of Acetyl-CoA:

• Links the metabolism of carbohydrates, fats, and proteins.

• Determines whether energy is used immediately (via ATP production) or stored (via fat synthesis) based on the cell’s energy needs.

7. End of Krebs Cycle:

• By the cycle’s completion, all six carbon atoms from glucose have been oxidized to CO₂.

• The remaining energy from glucose is stored in ATP, NADH, and FADH₂, which proceed to the final stage (electron transport and chemiosmosis).

Summary: Electron Transport and Chemiosmosis

1. Electron Transport Chain (ETC):

• Located in the inner mitochondrial membrane.

• Consists of protein complexes and electron carriers arranged in increasing electronegativity:

• NADH dehydrogenase, ubiquinone (Q), cytochrome b-c₁ complex, cytochrome c, and cytochrome oxidase complex.

• NADH transfers electrons to the first protein complex, while FADH₂ transfers electrons to Q (bypassing the first complex).

• Electrons are passed along the chain, releasing energy at each step.

2. Proton Gradient Formation:

• Free energy from electron movement pumps H⁺ ions (protons) from the matrix into the intermembrane space through three protein complexes.

• Creates an electrochemical gradient (proton motive force), storing energy like a charged battery.

3. Role of Oxygen:

• Oxygen, the final electron acceptor, combines with electrons and protons to form water.

• Without oxygen, the ETC cannot function, halting ATP production.

4. Energy Yield:

• NADH oxidation pumps three protons, leading to the formation of three ATP molecules.

• FADH₂ oxidation pumps two protons, yielding two ATP molecules.

5. NADH Variations:

• Cytosolic NADH (from glycolysis) cannot directly enter the matrix due to the inner membrane’s impermeability.

• Two shuttle systems transfer its electrons:

• Glycerol-phosphate shuttle: Transfers electrons to FAD, producing FADH₂ and two ATP.

• Aspartate shuttle: Transfers electrons to NAD⁺, forming NADH and three ATP (less common).

6. Chemiosmosis:

• Protons flow back into the matrix through ATP synthase, driven by the electrochemical gradient.

• This process synthesizes ATP by oxidative phosphorylation.

7. Recycling NAD⁺ and FAD:

• Once electrons are transferred, oxidized NAD⁺ and FAD are reused in glycolysis, pyruvate oxidation, and the Krebs cycle to continue the process.

8. Efficiency:

• The folds of the inner mitochondrial membrane increase surface area, allowing for multiple ETCs to operate simultaneously, maximizing ATP production.

Overall Significance:

• The ETC and chemiosmosis produce the majority of ATP in cellular respiration, with the final energy yield depending on the type and origin of reduced coenzymes (NADH or FADH₂).

Notes on Cellular Respiration

• Definition: Cellular respiration is the process through which cells convert glucose into ATP energy using redox reactions.

• Stages: Involves aerobic respiration which includes glycolysis, pyruvate oxidation, the Krebs cycle, and the electron transport chain.

• Energy Production: Oxygen acts as the final electron acceptor, allowing the electron transport chain to produce most of the ATP.

• Types of Organisms:

  • Obligate aerobes require oxygen for ATP production.

  • Obligate anaerobes cannot survive in oxygen.

  • Facultative anaerobes can switch between aerobic and anaerobic respiration depending on the availability of oxygen.

• Key Molecules: NADH and FADH₂ are crucial in transporting electrons to the electron transport chain, driving ATP synthesis.

• Goal: The primary aim is to convert stored chemical energy in glucose into usable energy in the form of ATP.