Cellular respiration is the process by which cells extract energy from nutrients in the form of ATP (adenosine triphosphate) molecules. ATP is the primary energy currency in living cells, and it provides energy for various cellular processes, such as muscle contraction, protein synthesis, and active transport. The process of cellular respiration occurs in all living cells, including both eukaryotic and prokaryotic cells. It involves a series of chemical reactions that are catalyzed by specific enzymes and occur in different cellular compartments. In this essay, we will discuss the major chemical reactions involved in cellular respiration.
There are three main stages of cellular respiration: glycolysis, the Krebs cycle (also called the citric acid cycle or the TCA cycle), and oxidative phosphorylation (also called the electron transport chain or ETC). Each of these stages involves a series of chemical reactions that convert nutrients, such as glucose or fatty acids, into ATP molecules.
Glycolysis is the first stage of cellular respiration, and it occurs in the cytoplasm of cells. The process begins with the breakdown of glucose, a six-carbon sugar molecule, into two three-carbon molecules of pyruvate. This process is facilitated by a series of enzymatic reactions, which involve the investment of two ATP molecules and the production of four ATP molecules, resulting in a net gain of two ATP molecules. In addition to ATP, glycolysis also produces two molecules of NADH (nicotinamide adenine dinucleotide), a molecule that carries electrons to the next stage of cellular respiration.
The Krebs cycle is the second stage of cellular respiration, and it occurs in the mitochondria of cells. The process begins with the oxidation of pyruvate, which is converted into acetyl-CoA (acetyl coenzyme A) by the enzyme pyruvate dehydrogenase. Acetyl-CoA then enters the Krebs cycle and reacts with oxaloacetate, a four-carbon molecule, to form citrate, a six-carbon molecule. Citrate is then converted into isocitrate by the enzyme aconitase. Isocitrate is further converted into alpha-ketoglutarate by the enzyme isocitrate dehydrogenase. Alpha-ketoglutarate is then converted into succinyl-CoA by the enzyme alpha-ketoglutarate dehydrogenase. Succinyl-CoA is then converted into succinate by the enzyme succinyl-CoA synthetase. Succinate is further converted into fumarate by the enzyme succinate dehydrogenase. Fumarate is then converted into malate by the enzyme fumarase. Malate is then converted into oxaloacetate by the enzyme malate dehydrogenase, completing the cycle. During the Krebs cycle, two ATP molecules, six NADH molecules, and two FADH2 (flavin adenine dinucleotide) molecules are produced per glucose molecule.
Oxidative phosphorylation is the third and final stage of cellular respiration, and it occurs in the inner membrane of mitochondria in eukaryotic cells or in the plasma membrane of prokaryotic cells. The process involves the transfer of electrons from NADH and FADH2 to a series of electron carriers, including ubiquinone and cytochrome c, ultimately leading to the generation of a proton gradient across the membrane. This proton gradient is used by ATP synthase to synthesize ATP molecules from ADP (adenosine diphosphate) and inorganic phosphate. The process of oxidative phosphorylation generates about 32-34 ATP molecules per glucose molecule, depending on the cell type and the efficiency of the electron transport chain.
The process of oxidative phosphorylation can be divided into two main components: electron transport and chemiosmosis. During the electron transport, the electrons from NADH and FADH2 are passed through a series of electron carriers, such as ubiquinone, cytochromes, and iron-sulfur proteins. As the electrons move through the carriers, some of the energy is released and used to pump protons (H+) from the mitochondrial matrix to the intermembrane space, creating a proton gradient.
The proton gradient generated by the electron transport chain is then used to power ATP synthase, which is a transmembrane enzyme that catalyzes the synthesis of ATP from ADP and inorganic phosphate. This process is called chemiosmosis because it involves the flow of ions (protons) across a membrane, which drives the synthesis of ATP.
In addition to ATP, the process of oxidative phosphorylation also produces water molecules as a byproduct. At the end of the electron transport chain, the electrons are combined with oxygen molecules and protons to form water molecules, a process known as the final electron acceptor.
In summary, cellular respiration is a complex process that involves a series of chemical reactions that convert nutrients, such as glucose or fatty acids, into ATP molecules. The process can be divided into three main stages: glycolysis, the Krebs cycle, and oxidative phosphorylation. Glycolysis occurs in the cytoplasm of cells and involves the breakdown of glucose into two molecules of pyruvate, which are then used in the Krebs cycle. The Krebs cycle occurs in the mitochondria of cells and involves the oxidation of pyruvate to produce ATP, NADH, and FADH2. Oxidative phosphorylation occurs in the inner membrane of mitochondria and involves the transfer of electrons from NADH and FADH2 to generate a proton gradient, which is then used by ATP synthase to synthesize ATP molecules from ADP and inorganic phosphate. Overall, cellular respiration is a critical process for the survival of cells and organisms, as it provides the energy necessary for various cellular processes.