Why do we need oxygen?

The human body's reliance on oxygen is fundamental to our survival, as it provides the energy necessary for virtually every bodily function. Oxygen is essential for cellular respiration at a microscopic level, allowing cells to break down nutrients like glucose and produce adenosine triphosphate (ATP), the energy currency of cells. This process, known as cellular respiration, is crucial for normal bodily functions and is most efficient in the presence of oxygen, referred to as aerobic respiration. Without oxygen, cells cannot generate the energy required for essential actions and functions, ultimately leading to collapse and potential death within a short timeframe of 10 to 15 minutes.

During aerobic respiration, glucose is oxidized in a series of steps that occur in the mitochondria of cells. Oxygen acts as the final electron acceptor in the electron transport chain, which is a key component of aerobic respiration. Here's how oxygen dependence is involved in cellular respiration:

1. Glycolysis: The first stage of cellular respiration is glycolysis, which occurs in the cytoplasm and doesn't directly require oxygen. In glycolysis, glucose is broken down into pyruvate, producing a small amount of ATP and NADH.

2. Pyruvate Oxidation: In the presence of oxygen, pyruvate molecules from glycolysis enter the mitochondria. Pyruvate is then converted into acetyl-CoA, which enters the citric acid cycle (also known as the Krebs cycle). This step links glycolysis to the subsequent stages of aerobic respiration.

3. Citric Acid Cycle: The citric acid cycle takes place in the mitochondrial matrix and generates ATP, NADH, and FADH2. This cycle completely oxidizes acetyl-CoA, releasing electrons and protons that are carried by NADH and FADH2 to the electron transport chain.

4. Electron Transport Chain (ETC): The electron transport chain is embedded in the inner mitochondrial membrane. Electrons carried by NADH and FADH2 are passed through a series of protein complexes in the ETC. As electrons move through the chain, they release energy, which is used to pump protons (H+) across the membrane, creating a proton gradient.

5. ATP Synthesis: The proton gradient created by the ETC drives ATP synthesis through a process called oxidative phosphorylation. Protons flow back into the mitochondrial matrix through ATP synthase, and this flow of protons is coupled with the synthesis of ATP from adenosine diphosphate (ADP) and inorganic phosphate (Pi).

6. Oxygen as the Final Electron Acceptor: In aerobic respiration, oxygen is the final electron acceptor in the electron transport chain. Oxygen combines with electrons and protons to form water (H2O). This step is essential because it ensures the continuous flow of electrons through the ETC and allows ATP production to continue efficiently.

Overall, the dependence of cellular respiration on oxygen is fundamental to the efficient production of ATP in aerobic organisms. Without oxygen, cells cannot carry out aerobic respiration optimally, leading to a decrease in ATP production and cellular energy availability.

What happens if cells do not get enough oxygen?

When cells are deprived of oxygen, they cannot undergo aerobic respiration, which is the most efficient way to produce energy in the form of adenosine triphosphate (ATP). Without oxygen, cells switch to anaerobic respiration, a less efficient process that produces ATP but also generates lactic acid as a byproduct. This buildup of lactic acid can lower the pH level inside the cell, leading to cellular damage and dysfunction.

In the absence of oxygen, cells also experience a decrease in their ability to generate energy, which can impair various cellular functions. Over time, prolonged oxygen deprivation can result in cellular stress, dysfunction, and even cell death. This is why oxygen is crucial for maintaining the normal functioning and survival of cells in the human body.