Cellular Respiration Overview

Introduction to Cellular Respiration

Cellular respiration is a fundamental metabolic process by which cells convert biochemical energy from nutrients into adenosine triphosphate (ATP), releasing waste products. It occurs in several stages and is essential for the survival of most organisms.

• Key Pathways: Glycolysis, Oxidation of Pyruvate, Citric Acid Cycle (Krebs Cycle), and Electron Transport Chain.

• Oxygen Availability: The fate of pyruvate, the end product of glycolysis, depends on the presence or absence of oxygen.

Fate of Pyruvate: Aerobic vs. Anaerobic Conditions

• Aerobic Respiration: If O2 is present, pyruvate enters the mitochondria and is further oxidized, leading to the production of large amounts of ATP.

• Fermentation: If O2 is absent, cells perform fermentation, converting pyruvate into lactic acid (in animals) or ethanol and CO2 (in yeast and some plants), yielding much less ATP.

• ATP Yield: Aerobic respiration can produce 34–36 ATP per glucose molecule, while fermentation yields only 2 ATP per glucose.

Example: Muscle cells switch to lactic acid fermentation during intense exercise when oxygen is scarce.

The Mitochondrion: Structure and Function

Mitochondrial Anatomy

The mitochondrion is the organelle where aerobic respiration takes place. Its structure is specialized to maximize ATP production.

• Outer Membrane: Encloses the organelle and separates it from the cytoplasm.

• Inner Membrane: Highly folded into structures called cristae, increasing surface area for chemical reactions.

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

• Matrix: The innermost compartment, containing enzymes for the citric acid cycle and mitochondrial DNA.

Additional info: The inner membrane houses the electron transport chain and ATP synthase, essential for oxidative phosphorylation.

Oxidation of Pyruvate

Conversion of Pyruvate to Acetyl-CoA

After glycolysis, each pyruvate molecule is transported into the mitochondrial matrix, where it is converted into acetyl-CoA, a key substrate for the citric acid cycle.

• Transport: Pyruvate enters the mitochondria via active transport through specific protein channels.

• Decarboxylation: Each pyruvate (3C) loses one carbon as CO2, forming a 2-carbon acetyl group.

• Formation of Acetyl-CoA: The acetyl group combines with coenzyme A to form acetyl-CoA.

• NADH Production: For each pyruvate, one NAD+ is reduced to NADH.

Equation:

Example: For each glucose molecule (which produces two pyruvates), two acetyl-CoA, two NADH, and two CO2 are generated.

The Citric Acid Cycle (Krebs Cycle or TCA Cycle)

Overview and Steps

The citric acid cycle is a series of enzyme-catalyzed reactions in the mitochondrial matrix that completes the oxidation of organic molecules, releasing energy stored in acetyl-CoA.

• Entry: Each acetyl-CoA (2C) combines with oxaloacetate (4C) to form citrate (6C).

• Decarboxylation: Two carbons are released as CO2 per acetyl-CoA.

• Electron Carriers: Three NADH and one FADH2 are produced per acetyl-CoA.

• ATP (or GTP) Production: One ATP (or GTP) is generated per acetyl-CoA via substrate-level phosphorylation.

• Cycle Regeneration: Oxaloacetate is regenerated to continue the cycle.

Equation (per acetyl-CoA):

Example: For each glucose molecule (2 acetyl-CoA), the cycle turns twice, yielding 6 NADH, 2 FADH2, 2 ATP, and 4 CO2.

Summary Table: Products of Glycolysis, Pyruvate Oxidation, and Citric Acid Cycle (per glucose)

Additional info: The majority of ATP is produced later in the electron transport chain via oxidative phosphorylation, using the NADH and FADH2 generated in earlier steps.