How does respiration occur in cells, and what are the two types of respiration based on the presence of oxygen?

Sourav PanNovember 12, 2024

Answered step-by-step

Cellular respiration is the biochemical process by which cells convert nutrients, primarily glucose, into energy in the form of adenosine triphosphate (ATP). This process is essential for powering various cellular functions and maintaining life.

How Respiration Occurs in Cells

Cellular respiration occurs in several stages:

1. Glycolysis: This initial stage takes place in the cytoplasm and involves the breakdown of one glucose molecule into two molecules of pyruvate. During glycolysis, a net gain of 2 ATP molecules and 2 NADH molecules is produced, regardless of whether oxygen is present or not.
2. Krebs Cycle (Citric Acid Cycle): If oxygen is available, the pyruvate enters the mitochondria, where it is converted into acetyl-CoA. Acetyl-CoA then enters the Krebs cycle, which generates ATP, NADH, and FADH₂ while releasing carbon dioxide as a waste product.
3. Electron Transport Chain (ETC): This final stage occurs across the inner mitochondrial membrane and requires oxygen. The NADH and FADH₂ produced in previous stages donate electrons to the ETC, leading to the creation of a proton gradient that drives ATP synthesis through oxidative phosphorylation. The final electron acceptor in this process is oxygen, which combines with protons to form water.

Types of Respiration Based on Oxygen Presence

Cellular respiration can be classified into two main types based on the presence or absence of oxygen:

1. Aerobic Respiration:
   • Definition: This type of respiration occurs in the presence of oxygen.
   • Process: It includes all three stages: glycolysis, Krebs cycle, and electron transport chain.
   • Energy Yield: Aerobic respiration is highly efficient, producing approximately 30-38 ATP molecules per glucose molecule.
   • By-products: The end products are carbon dioxide and water.
2. Anaerobic Respiration:
   • Definition: This type occurs in the absence of oxygen.
   • Process: Only glycolysis takes place; after glycolysis, pyruvate undergoes fermentation instead of entering the Krebs cycle.
     • In animals, pyruvate is converted into lactic acid.
     • In yeast and some bacteria, pyruvate is converted into ethanol and carbon dioxide.
   • Energy Yield: Anaerobic respiration produces significantly less energy, yielding only about 2 ATP molecules per glucose molecule.
   • By-products: The end products vary based on the organism but typically include lactic acid or alcohol along with carbon dioxide.