Cellular Respiration

Key Concepts

Cellular respiration is the process by which cells extract energy from macromolecules, primarily glucose, using oxygen delivered via the bloodstream. This process involves a series of metabolic pathways that convert biochemical energy from nutrients into adenosine triphosphate (ATP), the cell's energy currency.

• Fuel (macromolecules) + oxygen are required for energy production.

• Glycolysis: Glucose is broken down to pyruvate, producing some ATP and transferring electrons to NAD+ to form NADH.

• Acetyl CoA: Pyruvate is converted to Acetyl CoA before entering the citric acid cycle.

• Citric Acid Cycle: Acetyl CoA is further oxidized, producing electron carriers and ATP.

• Oxidative Phosphorylation: Electrons are transferred through the electron transport chain, driving ATP synthesis.

• H+ turbine (ATP synthase): Uses proton gradient to synthesize ATP.

Energy Production in the Cell

Overview of Pathways

Energy production in eukaryotic cells occurs through a series of interconnected pathways, primarily in the cytosol and mitochondria.

• Glycolysis: Occurs in the cytosol; glucose is split into two pyruvate molecules.

• Pyruvate Oxidation: Pyruvate enters mitochondria and is converted to Acetyl CoA.

• Citric Acid Cycle (Krebs Cycle): Acetyl CoA is oxidized, producing NADH, FADH2, and ATP.

• Oxidative Phosphorylation: Electron transport chain and chemiosmosis produce the majority of ATP.

Glycolysis

Glycolysis is the first step in cellular respiration, breaking down glucose into pyruvate and generating a small amount of ATP and NADH.

• Glucose enters the cell; 2 ATP are invested in the initial steps.

• ATP is produced in later steps (steps 6-10).

• Occurs in the cytosol, outside the mitochondria.

• Net gain: 2 ATP and 2 NADH per glucose molecule.

Pyruvate Oxidation

After glycolysis, pyruvate is transported into the mitochondria, where it is converted to Acetyl CoA, producing NADH and releasing CO2.

• Pyruvate crosses the mitochondrial membrane via transport proteins.

• Requires oxygen and B vitamins (notably thiamine and riboflavin).

• Each pyruvate yields 1 NADH and 1 Acetyl CoA.

Citric Acid Cycle (Krebs Cycle)

The citric acid cycle completes the oxidation of glucose derivatives, generating high-energy electron carriers and ATP.

• Inputs: Acetyl CoA, NAD+, FAD, ADP, and inorganic phosphate.

• Outputs: NADH, FADH2, ATP (or GTP), and CO2.

• Coenzymes required: NAD+ (from niacin), FAD (from riboflavin), and CoA (from pantothenic acid).

Oxidative Phosphorylation

Oxidative phosphorylation is the final stage of cellular respiration, where most ATP is produced via the electron transport chain and chemiosmosis.

• Electrons from NADH and FADH2 are transferred through protein complexes in the inner mitochondrial membrane.

• Proton gradient is established, driving ATP synthesis via ATP synthase.

• Oxygen acts as the final electron acceptor, forming water.

Energy Production - ATP Yield

The complete oxidation of one glucose molecule can yield up to 32 ATP molecules, depending on the efficiency of the system.

• Maximum ATP per glucose: 32

• Each ATP stores about 7.3 kcal of energy.

• Overall efficiency of cellular respiration is about 34% (the rest is lost as heat).

Equation for Cellular Respiration:

Fermentation

Fermentation is an anaerobic process that allows glycolysis to continue in the absence of oxygen by regenerating NAD+. It produces less ATP than aerobic respiration.

• Occurs when oxygen is not available.

• In yeast: produces ethanol and CO2.

• In muscle cells: produces lactate.

• If oxygen becomes available, aerobic respiration resumes, and pyruvate enters the mitochondria.

Energy Efficiency and Alternative Pathways

Cellular respiration is highly efficient compared to fermentation. In the absence of oxygen, cells rely on fermentation, which yields much less ATP.

• Without O2, glycolysis continues, but pyruvate is converted to lactate (muscle) or ethanol (yeast).

• Build-up of lactate can cause muscle fatigue.

• Yeast fermentation is used in baking and alcohol production.

Control Mechanisms in Cellular Respiration

Regulation of Pathways

Cellular respiration is tightly regulated to meet the energy demands of the cell and maintain homeostasis.

• Key regulatory points include glycolysis and the citric acid cycle.

• Allosteric enzymes respond to levels of ATP, ADP, AMP, and other metabolites.

• Feedback inhibition ensures balance between energy supply and demand.

Role of Nutrients

Nutrients provide the substrates and cofactors required for cellular respiration.

• Glucose is the primary fuel, derived from carbohydrates.

• Other sources: glycogen, fats (glycerol), and proteins (via gluconeogenesis).

• Proteins, fatty acids, and vitamins are required for intermediate compounds and coenzymes.

• Nutritional status affects health, reproduction, and production in animals.

Summary Table: Inputs and Outputs of Major Pathways

Key Terms

• ATP (Adenosine Triphosphate): The main energy carrier in cells.

• NADH/FADH2: Electron carriers that transfer high-energy electrons to the electron transport chain.

• Glycolysis: The breakdown of glucose to pyruvate.

• Citric Acid Cycle: A series of reactions that generate electron carriers and ATP from Acetyl CoA.

• Oxidative Phosphorylation: The production of ATP using energy derived from the transfer of electrons in the electron transport chain.

• Fermentation: Anaerobic process that regenerates NAD+ and produces ATP in the absence of oxygen.