cellular resp

Cellular Respiration: The process by which cells convert glucose and oxygen into energy (ATP), carbon dioxide, and water. This occurs in three main stages:

1. Glycolysis:

   • Occurs in the cytoplasm.

   • Glucose is broken down into pyruvate, producing a small amount of ATP and NADH.

2. Krebs Cycle (Citric Acid Cycle):

   • Takes place in the mitochondria.

   • Pyruvate is further broken down, releasing carbon dioxide and producing ATP, NADH, and FADH2.

3. Electron Transport Chain:

   • Occurs in the inner mitochondrial membrane.

   • NADH and FADH2 donate electrons, which are passed through a series of proteins, leading to the production of a significant amount of ATP and water (as oxygen is the final electron acceptor).

Cellular respiration can be aerobic (with oxygen) or anaerobic (without oxygen), affecting the amount of ATP produced.

Glycolysis: Glycolysis is the first stage of cellular respiration, occurring in the cytoplasm of cells. It involves a series of ten enzyme-catalyzed reactions that convert one molecule of glucose (a six-carbon sugar) into two molecules of pyruvate (a three-carbon compound). This process can be divided into two main phases:

1. Energy Investment Phase: In the initial reactions, ATP is used to phosphorylate glucose to form glucose-6-phosphate, which is then converted into fructose-1,6-bisphosphate. This step is crucial as it traps glucose within the cell and prepares it for further breakdown. A total of 2 ATP molecules are consumed during this phase.

2. Energy Payoff Phase: In the subsequent reactions, fructose-1,6-bisphosphate is split into two three-carbon molecules, which are further converted into pyruvate. This phase generates a net gain of 4 ATP molecules (2 ATP per pyruvate) and 2 NADH molecules, which are used in the electron transport chain. The overall reaction can be summarized as:

   • Glucose + 2 NAD+ + 2 ATP → 2 pyruvate + 4 ATP + 2 NADH

Glycolysis does not require oxygen and is therefore considered an anaerobic process. It serves as a crucial pathway for both aerobic and anaerobic respiration, providing the initial substrates needed for further energy production.

Krebs Cycle (Citric Acid Cycle): The Krebs cycle is a key metabolic pathway that takes place in the mitochondria and is a crucial part of cellular respiration. It follows glycolysis and plays a vital role in the oxidative degradation of organic substances to produce energy. Here’s a concise breakdown of its key components and processes:

1. Initiation: The cycle begins when Acetyl-CoA (derived from pyruvate) combines with oxaloacetate to form citrate (citric acid), a six-carbon molecule.

2. Rearrangements and Dehydrations: Citrate is then rearranged to form isocitrate, followed by oxidative decarboxylation, during which isocitrate is converted into alpha-ketoglutarate through the release of carbon dioxide and reduction of NAD+ to NADH.

3. Formation of Succinyl-CoA: Alpha-ketoglutarate undergoes another oxidative decarboxylation, forming succinyl-CoA and releasing another molecule of carbon dioxide while reducing another NAD+ to NADH.

4. ATP Production: Succinyl-CoA is converted to succinate, coupled with the phosphorylation of GDP to form GTP (or ATP), which is a direct energy currency.

5. Further Oxidation: Succinate is then oxidized to fumarate, with the reduction of FAD to FADH2.

6. Conversion to Malate: Fumarate is hydrated to form malate.

7. Regeneration of Oxaloacetate: Finally, malate is oxidized to regenerate oxaloacetate and reduce another NAD+ to NADH, thus completing the cycle.

Products of Krebs Cycle: Each turn of the Krebs cycle produces:

• 3 NADH

• 1 FADH2

• 1 GTP (or ATP)

• 2 CO2 (released as waste)

The NADH and FADH2 produced are crucial for the subsequent Electron Transport Chain, where they contribute to ATP synthesis. The Krebs cycle is pivotal for the oxidation of acetyl-CoA and synthesizing precursors for various biosynthetic pathways.

Electron Transport Chain: The electron transport chain (ETC) is the final stage of cellular respiration, occurring in the inner mitochondrial membrane. It plays a crucial role in the production of ATP through oxidative phosphorylation. Here’s a detailed breakdown:

1. Components: The ETC consists of a series of protein complexes (Complex I, II, III, IV) and mobile electron carriers (ubiquinone and cytochrome c).

2. Electron Donation: NADH and FADH2, generated in earlier stages of cellular respiration, donate high-energy electrons to the ETC.

   • NADH donates electrons to Complex I, while FADH2 donates to Complex II.

3. Electron Transport: As electrons move through the complexes (I to IV), they lose energy. This energy is utilized to pump protons (H+) from the mitochondrial matrix into the intermembrane space, creating a proton gradient.

4. Proton Gradient: The accumulation of protons in the intermembrane space establishes an electrochemical gradient, also known as the proton motive force (PMF).

5. ATP Synthesis: Protons flow back into the mitochondrial matrix through ATP synthase, a process called chemiosmosis. This movement drives the phosphorylation of ADP to ATP, producing approximately 26-28 ATP molecules from one molecule of glucose.

6. Oxygen’s Role: Oxygen serves as the final electron acceptor at Complex IV. It combines with the electrons and protons to form water, which is crucial for maintaining the flow of electrons through the chain. Without oxygen, the ETC would halt, and ATP production would significantly decrease.

7. Efficiency: The electron transport chain is highly efficient, generating a majority of the ATP produced during cellular respiration. Optimal conditions lead to about 30-32 ATP molecules being produced from one molecule of glucose through the entire process of cellular respiration, including glycolysis, Krebs cycle, and the electron transport chain.

8. Inhibitors and Poisons: Certain substances can inhibit the ETC, disrupting ATP production. Common examples include cyanide (which inhibits Complex IV) and oligomycin (which blocks ATP synthase).