Cellular Respiration and Metabolic Pathways: Biochemistry Study Notes

Cellular Respiration

Overview of Cellular Respiration

Cellular respiration is the process by which cells break down organic compounds to produce ATP, the primary energy currency of the cell. This process is essential for maintaining cellular functions and is a central topic in biochemistry.

• Definition: Cellular respiration is the set of metabolic reactions used by cells to convert biochemical energy from nutrients into ATP.

• Importance: Provides energy for cellular activities.

• Types: Aerobic (requires oxygen) and anaerobic (does not require oxygen).

• Major Steps: Glycolysis, Pyruvate Decarboxylation, Citric Acid Cycle, Electron Transport Chain.

Mitochondrial Anatomy

Structure and Function

The mitochondrion is the organelle where most of cellular respiration occurs. Its structure is specialized to maximize ATP production.

• Outer Membrane: Smooth, contains transport proteins.

• Inner Membrane: Highly folded (cristae), contains proteins for the electron transport chain and ATP synthesis.

• Intermembrane Space: Space between outer and inner membranes, important for proton gradient.

• Mitochondrial Matrix: Contains enzymes for the citric acid cycle and pyruvate decarboxylation.

Glycolysis

Pathway and Key Steps

Glycolysis is the first step in cellular respiration, occurring in the cytoplasm. It breaks down glucose into pyruvate, generating ATP and NADH.

• Equation: $\text{Glucose} + 2\text{NAD}^+ + 2\text{ADP} + 2\text{P}_i \rightarrow 2\text{Pyruvate} + 2\text{NADH} + 2\text{ATP} + 2\text{H}_2\text{O}$

• Key Steps: Energy investment phase, cleavage phase, energy payoff phase.

• Regulation: Phosphofructokinase is a major regulatory enzyme.

Pyruvate Decarboxylation

Conversion to Acetyl-CoA

Pyruvate produced from glycolysis is transported into the mitochondria and converted to acetyl-CoA, releasing CO2 and producing NADH.

• Equation: $2\text{Pyruvate} + 2\text{NAD}^+ + 2\text{CoA} \rightarrow 2\text{Acetyl-CoA} + 2\text{NADH} + 2\text{CO}_2$

• Location: Mitochondrial matrix.

Citric Acid Cycle (Krebs Cycle)

Cycle Steps and Products

The citric acid cycle is a series of reactions in the mitochondrial matrix that oxidizes acetyl-CoA to CO2, generating NADH, FADH2, and ATP (or GTP).

• Equation: $\text{Acetyl-CoA} + 3\text{NAD}^+ + \text{FAD} + \text{ADP} + \text{P}_i \rightarrow 2\text{CO}_2 + 3\text{NADH} + \text{FADH}_2 + \text{ATP}$

• Key Steps: Citrate formation, isomerization, oxidative decarboxylation, regeneration of oxaloacetate.

• Regulation: Controlled by substrate availability and feedback inhibition.

Electron Transport Chain (ETC)

Mechanism and ATP Production

The electron transport chain is a series of protein complexes in the inner mitochondrial membrane that transfer electrons from NADH and FADH2 to oxygen, generating a proton gradient used to produce ATP.

• Key Complexes: Complex I (NADH dehydrogenase), Complex II (succinate dehydrogenase), Complex III (cytochrome bc1), Complex IV (cytochrome c oxidase).

• Proton Gradient: Electrons flow through complexes, pumping protons into the intermembrane space.

• ATP Synthesis: ATP synthase uses the proton gradient to synthesize ATP from ADP and Pi.

Oxidative Phosphorylation

ATP Generation

Oxidative phosphorylation is the process by which ATP is formed as electrons are transferred to oxygen in the ETC. The proton motive force drives ATP synthase.

• Equation: $\text{ADP} + \text{P}_i \rightarrow \text{ATP}$ (driven by proton flow)

• Yield: Approximately 34 ATP per glucose molecule (including glycolysis and citric acid cycle).

Anaerobic Respiration and Fermentation

Alternative Pathways

When oxygen is unavailable, cells use anaerobic respiration or fermentation to produce ATP. These pathways are less efficient than aerobic respiration.

• Lactic Acid Fermentation: Pyruvate is reduced to lactate, regenerating NAD+.

• Alcohol Fermentation: Pyruvate is converted to ethanol and CO2 in yeast.

• Yield: Only 2 ATP per glucose (from glycolysis).

Regulation of Carbohydrate Metabolism

Hormonal and Enzymatic Control

Carbohydrate metabolism is tightly regulated by hormones and enzymes to maintain energy balance.

• Insulin: Promotes glucose uptake and storage (glycogenesis).

• Glucagon: Stimulates glucose release (glycogenolysis, gluconeogenesis).

• Enzymatic Regulation: Key enzymes include hexokinase, phosphofructokinase, and pyruvate kinase.

Alternative Energy Sources: Lipids

Triglycerides and Beta-Oxidation

Lipids are important energy sources, especially during fasting. Triglycerides are broken down into fatty acids, which undergo beta-oxidation to produce acetyl-CoA.

• Beta-Oxidation: Sequential removal of two-carbon units from fatty acids, producing acetyl-CoA, NADH, and FADH2.

• Energy Yield: Fatty acids yield more ATP per molecule than carbohydrates.

Alternative Energy Sources: Proteins

Protein Catabolism and Amino Acid Oxidation

Proteins are used as energy sources when carbohydrates and lipids are insufficient. Amino acids are deaminated and their carbon skeletons enter metabolic pathways.

• Oxidative Deamination: Removal of amino group, producing ammonia and keto acids.

• Urea Cycle: Ammonia is converted to urea for excretion.

Summary Table: ATP Yield from Glucose

Comparison of Aerobic and Anaerobic Pathways

The following table summarizes the ATP yield from glucose metabolism under aerobic and anaerobic conditions.

Pathway	ATP Yield per Glucose	Key Products
Aerobic Respiration	~36-38 ATP	CO2, H2O
Anaerobic Respiration	2 ATP	Lactate or Ethanol, CO2

Additional info:

Some diagrams and pathway details were inferred for completeness and clarity. The notes cover core biochemistry topics: glycolysis, citric acid cycle, electron transport chain, fermentation, and regulation of metabolism, all directly relevant to college biochemistry.