Cellular respiration is a process that occurs in the cells of all living organisms. Its main purpose is to convert the chemical energy stored in food molecules, like glucose, into a form of energy that cells can use to perform their functions, which is called adenosine triphosphate (ATP). This process involves a series of complex biochemical reactions that take place in different parts of the cell, especially in the mitochondria, often referred to as the cell’s powerhouse. Understanding cellular respiration is essential because it underpins how cells harness energy from nutrients to carry out essential tasks like growth, repair, and maintaining homeostasis.
The process of cellular respiration can be broken down into three major stages: glycolysis, the citric acid cycle (or Krebs cycle), and oxidative phosphorylation, which includes the electron transport chain and chemiosmosis. Each of these stages plays a specific role in the overall conversion of glucose into ATP, while also producing other byproducts like carbon dioxide and water.
The first stage of cellular respiration is glycolysis, which occurs in the cytoplasm of the cell. In this step, one molecule of glucose, a six-carbon sugar, is broken down into two molecules of pyruvate, a three-carbon compound. This breakdown process does not require oxygen, making glycolysis an anaerobic process. During glycolysis, a small amount of ATP is produced, as well as high-energy molecules called NADH. Specifically, glycolysis produces a net gain of two ATP molecules per glucose molecule, as well as two NADH molecules that carry high-energy electrons to later stages of respiration. In summary, glycolysis is the initial step where glucose is split to produce some energy and prepare molecules for further breakdown in the next stages of cellular respiration.
Once glycolysis is complete, the two pyruvate molecules enter the mitochondria, where they undergo further transformation. Before entering the citric acid cycle, each pyruvate molecule is converted into a molecule called acetyl-CoA. This conversion process also releases one molecule of carbon dioxide for each pyruvate and produces another molecule of NADH. Acetyl-CoA then enters the citric acid cycle, which occurs in the mitochondrial matrix, the innermost compartment of the mitochondria.
The citric acid cycle, also known as the Krebs cycle or the tricarboxylic acid (TCA) cycle, is a series of reactions that generate high-energy molecules by further breaking down acetyl-CoA. During this cycle, two molecules of carbon dioxide are released for each acetyl-CoA molecule that enters the cycle. Additionally, the citric acid cycle produces two more molecules of ATP (one for each acetyl-CoA) and generates more high-energy molecules in the form of NADH and FADH2. These high-energy molecules are crucial because they carry electrons to the final stage of cellular respiration, oxidative phosphorylation.
Oxidative phosphorylation, which takes place along the inner mitochondrial membrane, consists of two connected processes: the electron transport chain and chemiosmosis. The electron transport chain is a series of protein complexes and other molecules embedded in the inner membrane that pass electrons from NADH and FADH2 down the chain. As the electrons move through the chain, energy is released, which is used to pump protons (H+ ions) from the mitochondrial matrix into the intermembrane space, creating an electrochemical gradient known as the proton gradient.
At the end of the electron transport chain, the electrons are transferred to oxygen molecules, which combine with protons to form water. Oxygen is essential for this process because it acts as the final electron acceptor. Without oxygen, the electron transport chain cannot function, and ATP production would halt. This is why cellular respiration is often called an aerobic process, as it requires oxygen to proceed efficiently.
Once the proton gradient is established, chemiosmosis occurs. This process involves the movement of protons back into the mitochondrial matrix through a protein complex called ATP synthase, which acts like a molecular turbine. As protons flow through ATP synthase, the enzyme uses the energy from this flow to synthesize ATP from adenosine diphosphate (ADP) and inorganic phosphate. This step is where the bulk of ATP is produced during cellular respiration, with approximately 34 molecules of ATP being generated for each molecule of glucose.
In total, cellular respiration produces up to 38 molecules of ATP from a single glucose molecule: two from glycolysis, two from the citric acid cycle, and about 34 from oxidative phosphorylation. However, the actual yield may vary slightly depending on the efficiency of the process in different cells and organisms.
While the main purpose of cellular respiration is to generate ATP, it also produces other byproducts that are essential for the cell and the organism. Carbon dioxide, produced during the citric acid cycle, is a waste product that is eventually exhaled by animals during respiration. Water is another byproduct formed during the electron transport chain when oxygen accepts electrons and combines with protons. The generation of both carbon dioxide and water is crucial for maintaining metabolic balance and allowing cells to continue functioning properly.
It’s important to note that cellular respiration is not unique to animals. Plants, fungi, and many types of microorganisms also perform cellular respiration to obtain energy. While plants can produce their own glucose through photosynthesis, they still rely on cellular respiration to break down that glucose and produce ATP for cellular processes. The cycle of energy production and consumption is a fundamental aspect of life on Earth.
Additionally, cellular respiration can occur without oxygen in a process known as anaerobic respiration or fermentation. In anaerobic conditions, cells can still generate ATP through glycolysis, but since the electron transport chain cannot function without oxygen, the citric acid cycle and oxidative phosphorylation are halted. Instead, cells convert pyruvate into other compounds like lactic acid (in animals) or ethanol (in yeast) to regenerate NAD+ and allow glycolysis to continue. While anaerobic respiration produces far less ATP than aerobic respiration—only about two ATP molecules per glucose molecule—it enables cells to survive in low-oxygen environments temporarily.