Cellular Respiration and Energy Production: A Comprehensive Guide

Posted on May 22, 2024 in Biology

Phosphorylation

Phosphorylation is the synthesis of ATP by linking inorganic phosphate (Pi) to adenosine phosphate (ADP).

Types of Phosphorylation

1. Substrate-level phosphorylation: Glycolysis and Krebs cycle. A phosphate group is transferred from a phosphorylated substrate molecule to ADP, forming ATP.
2. Oxidative phosphorylation: Electron transport chain. The energy released from the electron transfer is used to pump protons across the membrane, creating a proton gradient. The flow of protons back across the membrane through ATP synthase drives the synthesis of ATP from ADP and inorganic phosphate (Pi).
3. Photophosphorylation: Light-dependent reactions of photosynthesis. Light energy is used to generate a proton gradient across a membrane, which drives ATP synthesis through ATP synthase.

Glycolysis

1. Glucose is phosphorylated by two ATP molecules, resulting in fructose-1,6-bisphosphate.
2. It is divided into two three-carbon molecules: dihydroxyacetone phosphate (DHAP) and glyceraldehyde-3-phosphate (G3P).
3. Each G3P molecule is oxidized, and NAD+ is reduced to NADH. This reaction also generates ATP by substrate-level phosphorylation.
4. The end products of glycolysis are 2 pyr, 2 NADH, and a net gain of 2 ATP.

Glycolysis is the initial stage of cellular respiration and occurs in the cytoplasm of cells. It breaks down glucose into smaller molecules, releasing energy in the form of ATP and producing precursor molecules for further energy extraction in later stages of cellular respiration.

Fermentation

Fermentation is an anaerobic metabolic process and serves to regenerate NAD+ from NADH, allowing glycolysis to continue and produce ATP.

Alcoholic Fermentation

Pyruvate is converted into ethanol and carbon dioxide. Commonly observed in yeast and some bacteria. The reaction involves the conversion of pyruvate into acetaldehyde, which is then reduced by NADH to ethanol, regenerating NAD+ in the process.

Lactic Acid Fermentation

Lactic acid fermentation occurs in some bacteria and in certain types of muscle cells during strenuous exercise when oxygen supply is limited. In this process, pyruvate is directly reduced to lactic acid by NADH, regenerating NAD+ in the process.

Oxidative Decarboxylation of Pyruvate

Oxidative decarboxylation of pyruvate is a process that occurs in the mitochondrial matrix during cellular respiration. It links glycolysis to the krebs cycle in the mitochondria.

1. Pyruvate is oxidized by the enzyme pyruvate dehydrogenase (PDC), resulting in the removal of CO2.
2. Acetyl-CoA is generated.
3. NAD+ is reduced to NADH.
4. Acetyl-CoA then enters the Krebs cycle

The Krebs Cycle

The Krebs cycle is a key metabolic pathway occurring in the mitochondrial matrix of eukaryotic cells and the cytoplasm of prokaryotic cells. It’s a central part of cellular respiration, where the breakdown of glucose and other molecules leads to the production of energy in the form of ATP and the generation of electron carriers like NADH and FADH2.

1. The cycle begins when acetyl-CoA, combines with oxaloacetate to form citrate.
2. Citrate is then metabolized through a series of reactions involving decarboxylation and oxidation. These reactions result in the release of energy in the form of NADH and FADH2, which carry high-energy electrons to the electron transport chain for ATP synthesis.
3. At the end of the cycle, malate is oxidized to regenerate oxaloacetate to initiate the next round of the Krebs cycle. This step also produces another molecule of NADH.

Beta-Oxidation

Beta-oxidation is a metabolic pathway that is responsible for the breakdown of fatty acids to produce acetyl-CoA, which enters the Krebs cycle to generate energy in the form of ATP.

1. Before beta-oxidation can occur, fatty acids need to be activated. This involves attaching a CoA molecule to the fatty acid, forming fatty acyl-CoA.
2. The fatty acyl-CoA is then transported into the mitochondrial matrix, where beta-oxidation takes place. The fatty acid undergoes a series of reactions in which two-carbon units (acetyl-CoA) are cleaved off the fatty acid chain at a time. Each cycle of beta-oxidation shortens the fatty acid chain by two carbons.
3. As the fatty acid chain is broken down, acetyl-CoA molecules are generated.
4. Beta-oxidation also generates molecules of NADH and FADH2.
5. The process continues until the entire fatty acid molecule is broken down into acetyl-CoA units.

Light-Dependent Reactions

Occur in the thylakoid membranes of chloroplasts. Chlorophyll and other pigments in the thylakoid membranes absorb light energy. Light energy is used to split water molecules into oxygen, protons (H+ ions), and electrons. This process is called photolysis. The energized electrons from water move

through a series of protein complexes in the thylakoid membrane, releasing energy that is used to pump protons across the membrane into the thylakoid space.The flow of protons back into the stroma through ATP synthase generates ATP through chemiosmosis. As electrons move through the electron transport chain, they are accepted by NADP+ along with protons from the stroma, forming NADPH. The end products of the light-dependent reactions are ATP and NADPH, which are used as energy carriers in the Calvin cycle. Oxygen is also produced.

Chemosynthesis is a process by which certain organisms, typically bacteria and archaea, produce organic molecules from inorganic compounds instead of relying on sunlight. In chemosynthesis, energy is derived from the oxidation of inorganic compounds such as hydrogen sulfide (H2S). Chemosynthetic organisms derive energy from the oxidation of inorganic compounds found in their environment. These compounds serve as electron donors. Using energy derived from the oxidation reactions, chemosynthetic organisms fix carbon dioxide (CO2) to produce organic molecules such as sugars and amino acids.

Calvin Cycle

Takes place in the stroma of chloroplasts. Carbon dioxide molecules are captured from the atmosphere and combined with ribulose bisphosphate (RuBP), by the enzyme rubisco. This forms an unstable six-carbon compound, which quickly splits into two molecules of 3-phosphoglycerate (3-PGA). ATP and NADPH from the light-dependent reactions are used to convert 3-PGA into glyceraldehyde-3-phosphate (G3P). Some G3P molecules are used to regenerate RuBP, while others are used to produce glucose and other carbohydrates. Some of the G3P molecules produced are used to regenerate RuBP through a series of reactions, completing the cycle.

The end products include glucose and other carbohydrates. Additionally, ADP, NADP+, and inorganic phosphate are recycled back to the light-dependent reactions to continue the process.

Gluconeogenesis is a metabolic pathway that allows organisms to synthesize glucose from non-carbohydrate precursors, such as amino acids, glycerol, and lactate. 

Gluconeogenesis involves a series of enzyme-catalyzed reactions that bypass irreversible steps of glycolysis to synthesize glucose from these precursors. Some key steps include the conversion of pyruvate into phosphoenolpyruvate (PEP), the carboxylation of oxaloacetate to form phosphoenolpyruvate, and the reversal of glycolytic reactions to form glucose.

ELECTRON TRANSPORT CHAIN=RESPIRATORY CHAIN

Takes place in the mitochondrial matrix. It consists of a series of protein complexes (oxidoreductases I, II, III, and IV), along with mobile electron carriers. Electrons from NADH2 and FADH2 are passed through these complexes in a series of redox reactions. The inner mitochondrial membrane contains the electron transport chain complexes embedded within its folds, called cristae. NADH2 and FADH2 serve as electron donors, while oxygen (O2) acts as the final electron acceptor in the electron transport chain. As electrons pass through the respiratory chain complexes, protons (H+) are pumped across the inner mitochondrial membrane from the matrix into the intermembrane space. The pumping of protons generates an electrochemical gradient across the inner mitochondrial membrane, with a higher concentration of protons in the intermembrane space compared to the matrix.

ATP synthase is an enzyme complex embedded in the inner mitochondrial membrane. It utilizes the energy from the electrochemical proton gradient to synthesize ATP from ADP and inorganic phosphate through a process called oxidative phosphorylation.

Mitchell Chemiosmotic Theory: Proposed by Peter Mitchell, this theory explains how the electrochemical proton gradient drives ATP synthesis. According to this theory, the proton gradient established across the inner mitochondrial membrane creates a proton motive force that powers ATP synthesis by ATP synthase.