Cellular Respiration: Overview

Introduction to Cellular Respiration

Cellular respiration is a set of metabolic processes that break down organic molecules to release energy, which is then used to synthesize ATP. This process occurs in both plant and animal cells, primarily within the mitochondria, and is essential for powering cellular activities.

• ATP (Adenosine Triphosphate): The main energy currency of the cell, produced during cellular respiration.

• Energy Transformation: Chemical energy in food is converted to chemical energy in ATP; some energy is lost as heat.

• Photosynthesis vs. Cellular Respiration: Photosynthesis stores energy in organic molecules; cellular respiration releases that energy for cellular work.

Catabolic Pathways and ATP Production

Catabolic Pathways

Catabolic pathways break down complex molecules into simpler ones, releasing stored energy. Electron transfer is central to these pathways, especially in cellular respiration.

• Fermentation: Partial degradation of sugars without oxygen.

• Aerobic Respiration: Consumes organic molecules and oxygen, yielding ATP.

• Anaerobic Respiration: Similar to aerobic, but uses electron acceptors other than oxygen.

• General Equation for Aerobic Respiration:

• ATP Regeneration: Cells regenerate ATP from ADP and phosphate using energy from catabolic pathways.

Redox Reactions in Cellular Respiration

Oxidation-Reduction (Redox) Reactions

Redox reactions involve the transfer of electrons between molecules, playing a key role in energy release during cellular respiration.

• Oxidation: Loss of electrons from a substance.

• Reduction: Gain of electrons by a substance (reduces positive charge).

• Reducing Agent: Electron donor.

• Oxidizing Agent: Electron acceptor.

• Electron Transfer: Electrons move from less electronegative atoms to more electronegative atoms (like oxygen), releasing energy.

Example: In the oxidation of glucose, electrons are transferred from glucose to oxygen, releasing energy used to make ATP.

Electron Carriers and the Electron Transport Chain

NAD+ and the Electron Transport Chain

Electrons from organic molecules are usually transferred to electron carriers like NAD+ before reaching oxygen.

• NAD+ (Nicotinamide Adenine Dinucleotide): Functions as an electron carrier and oxidizing agent.

• NADH: The reduced form, stores energy for ATP synthesis.

• Dehydrogenases: Enzymes that remove hydrogen atoms from substrates, transferring electrons to NAD+.

• Electron Transport Chain (ETC): A series of molecules in the inner mitochondrial membrane that transfer electrons in a stepwise fashion to oxygen, releasing energy gradually.

Key Point: The ETC allows controlled release of energy, preventing explosive reactions.

Stages of Cellular Respiration

Three Main Stages

• Glycolysis: Occurs in the cytoplasm; breaks glucose into two pyruvate molecules.

• Pyruvate Oxidation and Citric Acid Cycle: Completes glucose breakdown to CO2 in the mitochondria.

• Oxidative Phosphorylation: Electron transport and chemiosmosis produce most ATP.

ATP Production: Most ATP is generated by oxidative phosphorylation; some is produced by substrate-level phosphorylation during glycolysis and the citric acid cycle.

Overall ATP Yield: Up to 32 ATP per glucose molecule.

Glycolysis

Process and Phases

Glycolysis is the first step in cellular respiration, occurring in the cytoplasm and not requiring oxygen.

• Energy Investment Phase: 2 ATP are used to split glucose into two three-carbon sugars.

• Energy Payoff Phase: 4 ATP are produced (net gain of 2 ATP), 2 NAD+ are reduced to NADH, and 2 pyruvate and 2 H2O are formed.

• Key Point: All carbon from glucose ends up in pyruvate; no CO2 is released.

Pyruvate Oxidation and the Citric Acid Cycle

Pyruvate Oxidation

Pyruvate is transported into the mitochondrion (in eukaryotes) and converted to acetyl CoA before entering the citric acid cycle.

• Carboxyl group is oxidized, releasing CO2.

• NAD+ is reduced to NADH.

• Remaining two-carbon fragment combines with coenzyme A to form acetyl CoA.

The Citric Acid Cycle (Krebs Cycle)

The citric acid cycle completes the breakdown of glucose, generating electron carriers and ATP.

• Each turn produces 1 ATP, 3 NADH, 1 FADH2, and 2 CO2 (per acetyl CoA).

• Since 2 pyruvate are produced per glucose, the cycle runs twice per glucose molecule.

• NADH and FADH2 carry electrons to the electron transport chain.

Oxidative Phosphorylation and Chemiosmosis

Electron Transport Chain (ETC)

The ETC is a series of protein complexes in the inner mitochondrial membrane that transfer electrons from NADH and FADH2 to oxygen.

• Electrons are passed through carrier molecules, including cytochromes.

• Electrons drop in free energy as they move down the chain, finally reducing O2 to H2O.

• No ATP is produced directly by the ETC.

Chemiosmosis and ATP Synthase

Energy released by the ETC is used to pump H+ ions across the inner mitochondrial membrane, creating a proton gradient (proton-motive force).

• H+ flows back into the matrix through ATP synthase, driving ATP synthesis.

• This process is called chemiosmosis.

Energy Flow Sequence: glucose → NADH → ETC → proton-motive force → ATP

Efficiency: About 34% of glucose energy is transferred to ATP; the rest is lost as heat.

Fermentation and Anaerobic Respiration

ATP Production Without Oxygen

When oxygen is unavailable, cells can produce ATP via fermentation or anaerobic respiration.

• Anaerobic Respiration: Uses an ETC with a final electron acceptor other than oxygen (e.g., sulfate).

• Fermentation: Glycolysis followed by reactions that regenerate NAD+ by transferring electrons to pyruvate or its derivatives.

Types of Fermentation

• Alcohol Fermentation: Pyruvate is converted to ethanol; CO2 is released.

• Lactic Acid Fermentation: Pyruvate is reduced to lactate; no CO2 is released.

Applications: Alcohol fermentation is used in brewing and baking; lactic acid fermentation is used in making cheese and yogurt.

Comparing Fermentation, Anaerobic, and Aerobic Respiration

Metabolic Pathways and Regulation

Catabolic Versatility

Cellular respiration can use carbohydrates, fats, and proteins as fuel.

• Carbohydrates: Enter glycolysis as glucose or other sugars.

• Proteins: Digested to amino acids; amino groups are removed (deamination) and excreted as nitrogenous waste.

• Fats: Broken down to glycerol (enters glycolysis) and fatty acids (converted to acetyl CoA via beta oxidation).

• Energy Yield: Fats produce more than twice as much ATP per gram as carbohydrates.

Biosynthesis (Anabolic Pathways)

Cells use small molecules from food or from glycolysis/citric acid cycle to build macromolecules (e.g., proteins, nucleic acids).

Regulation of Cellular Respiration

Cellular respiration is regulated by feedback mechanisms to match ATP supply with demand.

• Feedback Inhibition: High ATP levels inhibit key enzymes, slowing respiration; low ATP levels speed it up.

• Enzyme Regulation: Key enzymes are regulated at strategic points in the pathway (e.g., phosphofructokinase in glycolysis).

Evolutionary Significance of Glycolysis

Glycolysis is an ancient metabolic pathway, present in nearly all organisms and functioning with or without oxygen. It likely evolved before oxygen was abundant in Earth's atmosphere.

Summary Table: Key Steps and Products of Cellular Respiration

Additional info: The exact ATP yield per glucose varies due to differences in shuttle mechanisms, proton leak, and use of the proton-motive force for other cellular work.