- Key People:
- Otto Warburg
cellular respiration
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What is cellular respiration?
Cellular respiration is a process where organisms convert the chemical energy from food into life-sustaining activities and release waste products like carbon dioxide and water.
Cellular respiration consists of three metabolic stages:
* Glycolysis
* The tricarboxylic acid cycle (TCA cycle)
* Oxidative phosphorylationMitochondria transform molecules, such as glucose, into ATP (adenosine triphosphate), which fuels cellular processes.
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Why do living things need cellular respiration?
Living things need cellular respiration to convert the chemical energy found in foodstuff molecules into energy for life-sustaining activities. This process involves combining oxygen with foodstuff molecules and releasing carbon dioxide and water as waste products.
Cellular respiration provides energy for cells to perform tasks such as metabolic reactions, substance transport, and mechanical work. The supply of oxygen to the mitochondria is a critical function of the respiratory system because cells maintain only a limited store of high-energy phosphates and oxygen. Through cellular respiration, cells generate ATP, which fuels various cell processes.
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Where does cellular respiration take place in a cell?
Cellular respiration is how organisms derive energy from food, with the overall process occurring in three steps: glycolysis, the tricarboxylic acid cycle (TCA cycle), and oxidative phosphorylation.
Glycolysis, which breaks down glucose into pyruvate molecules, occurs in the cell's cytoplasm. In eukaryotes, the pyruvate molecules then move into the mitochondria, where the remaining steps of cellular respiration occur. The tricarboxylic acid cycle takes place in the matrix of the mitochondria. Finally, oxidative phosphorylation occurs at the electron-transport chain, located on the inner membrane of the mitochondria.
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What are the main stages of cellular respiration?
Cellular respiration involves a series of metabolic processes where organisms convert chemical energy from food into life-sustaining activities, releasing carbon dioxide and water as waste. While descriptions may vary, it can be simplified into three key stages:
* Glycolysis: In glycolysis, a glucose molecule is broken down into two pyruvate molecules. This process releases energy, which is then stored in ATP, and converts nicotinamide adenine dinucleotide (NAD+) to NADH.
* Tricarboxylic acid cycle (TCA cycle): Also known as the Krebs cycle, this stage occurs in the mitochondrial matrix (in organisms excluding bacteria) and involves eight steps catalyzed by enzymes. Acetyl coenzyme A (acetyl CoA) is converted into carbon dioxide and energy. Energy is captured by nicotinamide adenine dinucleotide (NAD+) and flavin adenine dinucleotide (FAD) and later converted to ATP.
* Oxidative phosphorylation: In this final stage, hydrogen atoms from NADH and FADH2 produce electrons that reduce oxygen, forming water and creating three molecules of ATP. The electron transport chain gradually lowers the energy of the electrons.3 Britannica SourcesCreated from Britannica articles using AI. AI can make mistakes, so verify using Britannica articles. -
How does glucose get broken down during cellular respiration?
During cellular respiration, glucose is broken down through a series of steps that can be distilled into three main stages: glycolysis, the tricarboxylic acid cycle (TCA cycle), and oxidative phosphorylation.
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Glycolysis In glycolysis, a glucose molecule is broken down into two pyruvate molecules, releasing energy that is stored in ATP, along with the conversion of NAD+ to NADH.
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TCA Cycle The pyruvate molecules then enter the mitochondria and are converted into acetyl coenzyme A, which enters the TCA cycle. In the TCA cycle, acetyl CoA is converted into carbon dioxide and energy. Each turn of the cycle results in the regeneration of oxaloacetate and the formation of two molecules of carbon dioxide.
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Oxidative Phosphorylation In oxidative phosphorylation, hydrogen atoms from NADH and FADH2 provide electrons that reduce oxygen to form water and produce ATP.
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What are the main products and byproducts of cellular respiration?
Cellular respiration is a process where organisms utilize oxygen to break down food molecules for energy, releasing carbon dioxide and water as waste products. This process consists of three key stages: glycolysis, the tricarboxylic acid cycle (TCA cycle), and oxidative phosphorylation.
Glycolysis breaks down glucose into pyruvate, capturing energy in ATP and converting NAD+ to NADH. The TCA cycle further processes these molecules, generating more energy carriers and releasing carbon dioxide. Lastly, oxidative phosphorylation uses electrons from NADH and FADH2 to form water and produce a large amount of ATP, which is the main energy currency of the cell.
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cellular respiration, the process by which organisms combine oxygen with foodstuff molecules, diverting the chemical energy in these substances into life-sustaining activities and discarding, as waste products, carbon dioxide and water. Organisms that do not depend on oxygen degrade foodstuffs in a process called fermentation. (For longer treatments of various aspects of cellular respiration, see tricarboxylic acid cycle and metabolism.)
Role of mitochondria
One objective of the degradation of foodstuffs is to convert the energy contained in chemical bonds into the energy-rich compound adenosine triphosphate (ATP), which captures the chemical energy obtained from the breakdown of food molecules and releases it to fuel other cellular processes. In eukaryotic cells (that is, any cells or organisms that possess a clearly defined nucleus and membrane-bound organelles) the enzymes that catalyze the individual steps involved in respiration and energy conservation are located in highly organized rod-shaped compartments called mitochondria. In microorganisms the enzymes occur as components of the cell membrane. A liver cell has about 1,000 mitochondria; large egg cells of some vertebrates have up to 200,000.
Main metabolic processes
Biologists differ somewhat with respect to the names, descriptions, and the number of stages of cellular respiration. The overall process, however, can be distilled into three main metabolic stages or steps: glycolysis, the tricarboxylic acid cycle (TCA cycle), and oxidative phosphorylation (respiratory-chain phosphorylation).
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Glycolysis
Glycolysis (which is also known as the glycolytic pathway or the Embden-Meyerhof-Parnas pathway) is a sequence of 10 chemical reactions taking place in most cells that breaks down a glucose molecule into two pyruvate (pyruvic acid) molecules. Energy released during the breakdown of glucose and other organic fuel molecules from carbohydrates, fats, and proteins during glycolysis is captured and stored in ATP. In addition, the compound nicotinamide adenine dinucleotide (NAD+) is converted to NADH during this step (see below). Pyruvate molecules produced during glycolysis then enter the mitochondria, where they are each converted into a compound known as acetyl coenzyme A, which then enters the TCA cycle. (Some sources consider the conversion of pyruvate into acetyl coenzyme A as a distinct step, called pyruvate oxidation or the transition reaction, in the process of cellular respiration.)
Tricarboxylic acid cycle
The TCA cycle (which is also known as the Krebs, or citric acid, cycle) plays a central role in the breakdown, or catabolism, of organic fuel molecules. The cycle is made up of eight steps catalyzed by eight different enzymes that produce energy at several different stages. Most of the energy obtained from the TCA cycle, however, is captured by the compounds NAD+ and flavin adenine dinucleotide (FAD) and converted later to ATP. The products of a single turn of the TCA cycle consist of three NAD+ molecules, which are reduced (through the process of adding hydrogen, H+) to the same number of NADH molecules, and one FAD molecule, which is similarly reduced to a single FADH2 molecule. These molecules go on to fuel the third stage of cellular respiration, whereas carbon dioxide, which is also produced by the TCA cycle, is released as a waste product.
Oxidative phosphorylation
In the oxidative phosphorylation stage, each pair of hydrogen atoms removed from NADH and FADH2 provides a pair of electrons that—through the action of a series of iron-containing hemoproteins, the cytochromes—eventually reduces one atom of oxygen to form water. In 1951 it was discovered that the transfer of one pair of electrons to oxygen results in the formation of three molecules of ATP.
Oxidative phosphorylation is the major mechanism by which the large amounts of energy in foodstuffs are conserved and made available to the cell. The series of steps by which electrons flow to oxygen permits a gradual lowering of the energy of the electrons. This part of the oxidative phosphorylation stage is sometimes called the electron transport chain. Some descriptions of cellular respiration that focus on the importance of the electron transport chain have changed the name of the oxidative phosphorylation stage to the electron transport chain.
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