Cellular Respiration Definition
Cellular respiration is a metabolic process that occurs in cells, where glucose is broken down to produce energy in the form of ATP (adenosine triphosphate).
It is the process by which organisms combine oxygen with food molecules, diverting the chemical energy in these substances into life-sustaining activities and discarding, waste products, carbon dioxide, and water.
Cellular respiration can also be defined as a series of metabolic processes that take place within a cell. Biochemical energy is harvested from organic compounds (e.g., glucose) to produce ATP molecules.
Cellular Respiration Overview
Cellular respiration is a metabolic process that occurs in the cells of organisms to convert biochemical energy from nutrients into ATP, which can be used as energy to power many reactions throughout the body.
The process involves breaking down glucose and other molecules from food in a complex series of chemical reactions. There are three main steps of cellular respiration: glycolysis, the citric acid cycle, and oxidative phosphorylation.
Glycolysis is the first step of cellular respiration. It takes place in the cytoplasm and breaks down glucose into two pyruvate molecules, producing a small amount of ATP and NADH. The second step is the citric acid cycle, which takes place in the mitochondria.
In this step, pyruvate is broken down into carbon dioxide and water, producing more ATP and NADH. The final step is oxidative phosphorylation, which also takes place in the mitochondria. This step uses oxygen to produce a large amount of ATP through a process called chemiosmosis.
Aerobic respiration requires oxygen while anaerobic respiration does not require oxygen. Aerobic respiration releases a large amount of energy from glucose that can be stored as ATP.
Anaerobic cellular respiration occurs when there is no oxygen present. This type of respiration produces less ATP than aerobic respiration but still allows for some energy production.
After oxidative phosphorylation, the ATP created is located in the mitochondrial matrix. To be used as energy by other parts of the cell, it must leave the mitochondrion. This happens through transport proteins that move ADP (adenosine diphosphate) into the mitochondrion and move ATP out of it.
Cellular Respiration Equation
Aerobic Respiration Equation
The equation for aerobic respiration is
C6H12O6 + 6O2 → 6CO2 + 6H2O + ATP Energy
This equation represents the process of breaking down glucose using oxygen to produce carbon dioxide, water, and energy in the form of ATP.
The process of cellular respiration involves many different steps or reactions that occur in three stages: glycolysis, the Krebs cycle, and oxidative phosphorylation.
Glycolysis occurs in the cytoplasm and breaks down glucose into two molecules of pyruvate. The Krebs cycle occurs in the mitochondria and produces NADH and FADH2. Oxidative phosphorylation also occurs in the mitochondria and uses NADH and FADH2 to produce ATP.
Aerobic respiration is a vital process for all living organisms that require oxygen to survive. It is used by eukaryotic cells to generate energy from food molecules such as glucose.
The equation for aerobic respiration shows how glucose reacts with oxygen to produce carbon dioxide, water, and energy in the form of ATP. This process is essential for providing cells with the energy they need to carry out their functions.
Lactic Acid Fermentation Equation
The equation for lactic acid fermentation is
C6H12O6 → 2C3H6O3 + 2ATP
During glycolysis, one glucose molecule is converted to two pyruvate molecules, producing two net ATP and two NADH.
In lactic acid fermentation, the pyruvate made in glycolysis serves as the prerequisite for the lactate made in the latter process. Lactic acid fermentation is an anaerobic process that occurs in animal tissues with low metabolic requirements and little to no mitochondria.
Fermentation is a widespread pathway, but it is not the only way to get energy from fuels anaerobically (in the absence of oxygen). Some living systems instead use an inorganic molecule other than oxygen, such as sulfate, as a final electron acceptor for an electron transport chain.
Another familiar fermentation process is alcohol fermentation, in which pyruvate donates its electrons to a derivative of pyruvate, producing ethanol. Going from pyruvate to ethanol is a two-step process.
In the first step, a carboxyl group is removed from pyruvate and released as carbon dioxide, producing a two-carbon molecule called acetaldehyde. In the second step, NADH passes its electrons to acetaldehyde, regenerating NAD+ and forming ethanol.
Alcoholic Fermentation Equation
Alcoholic fermentation is a process that converts glucose into ethanol and carbon dioxide. The equation for alcoholic fermentation is
C6H12O6 → 2C2H5OH + 2CO2
During glycolysis, one glucose molecule is converted to two pyruvate molecules, producing two net ATP and two NADH.
In the first step of alcoholic fermentation, a carboxyl group is removed from pyruvate and released in the form of carbon dioxide, producing a two-carbon molecule called acetaldehyde. In the second step, NADH passes its electrons to acetaldehyde, regenerating NAD+ and forming ethanol.
Alcoholic fermentation is used by yeast to produce alcohol. It is also used in the production of alcoholic beverages such as beer and wine. The process begins with the conversion of sugar into alcohol by yeast cells through alcoholic fermentation.
Cellular Respiration Steps
Cellular respiration occurs in three main steps: glycolysis, the citric acid cycle (also known as the Krebs cycle), and oxidative phosphorylation. Glycolysis is the first step of cellular respiration and occurs in the cytoplasm.
It breaks down glucose into two molecules of pyruvate, producing a small amount of ATP and NADH. The citric acid cycle occurs in the mitochondria and produces NADH, FADH2, and ATP. It also releases carbon dioxide as a waste product.
Oxidative phosphorylation is the final step of cellular respiration and occurs in the inner mitochondrial membrane. It uses NADH and FADH2 to produce ATP through a process called chemiosmosis. This process requires oxygen to function properly and is therefore known as aerobic respiration.
Step 1
Glycolysis is the first step in cellular respiration, which is the process that all living things use to convert glucose into energy. Glycolysis converts glucose into two 3-carbon pyruvate molecules.
The process of glycolysis involves ten enzymatic reactions that are catalyzed by their own enzyme, with phosphofructokinase being the most essential for regulation as it controls the speed of glycolysis. During glycolysis, glucose ultimately breaks down into pyruvate and energy; a total of 2 ATP is derived in the process.
Glycolysis can occur in both aerobic and anaerobic conditions. In aerobic conditions, pyruvate enters the citric acid cycle and undergoes oxidative phosphorylation leading to the net production of 32 ATP molecules.
In anaerobic conditions, pyruvate converts to lactate through anaerobic glycolysis. When oxygen is present, NADH passes its electrons into the electron transport chain, regenerating NAD+ for use in glycolysis.
When oxygen is absent, cells may use other simpler pathways to regenerate NAD+. In these pathways, NADH donates its electrons to an acceptor molecule in a reaction that doesn’t make ATP but does regenerate NAD+ so that glycolysis can continue. This process is called fermentation.
Step 2
The citric acid cycle, also known as the Krebs cycle, is the second step of cellular respiration and occurs in the mitochondria.
It is a series of chemical reactions that converts acetyl-CoA into carbon dioxide, NADH, FADH2, and ATP or GTP. The citric acid cycle begins when acetyl-CoA combines with oxaloacetate to form citrate.
Citrate is then converted into a series of intermediate molecules through a series of enzyme-catalyzed reactions. During this process, carbon dioxide is released as a waste product and NADH and FADH2 are produced as energy carriers.
The final product of the citric acid cycle is oxaloacetate, which can be used to start the cycle again. The citric acid cycle generates most of the ATP produced in cellular respiration.
Step 3
he final step of cellular respiration is oxidative phosphorylation, which involves the electron transport chain. During oxidative phosphorylation, NADH and FADH2 produced during glycolysis and the citric acid cycle are used to generate ATP via chemiosmosis.
The electron transport chain (ETC) is a series of electron transporters embedded in the inner mitochondrial membrane that shuttles electrons from NADH and FADH2 to oxygen molecules. This process creates a proton gradient across the inner mitochondrial membrane, which drives ATP synthesis via ATP synthase.
ATP synthase contains two subunits: F0 and F1. The F0 subunit is hydrophobic and embedded in the inner mitochondrial membrane, while the F1 subunit protrudes into the mitochondrial matrix. The flow of hydrogen ions through ATP synthase powers its catalytic action, which phosphorylates ADP to produce ATP.
Products of Cellular Respiration
ATP
The product of cellular respiration is ATP and H2O. Cellular respiration is the process by which glucose and oxygen react to form ATP, water, and carbon dioxide. Glycolysis produces two pyruvate molecules, four ATPs (a net of two ATP), two NADH, and two H2O. The Krebs cycle produces 2 ATP molecules.
Most of the ATP produced by aerobic cellular respiration is made by oxidative phosphorylation. The energy released is used to create a chemiosmotic potential by pumping protons across a membrane.
The potential energy stored in the electrochemical gradient generated by pumping protons across a membrane drives the synthesis of ATP from ADP and inorganic phosphate through a process called chemiosmosis.
According to some newer sources, the ATP yield during aerobic respiration is not 36–38 but only about 30–32 ATP molecules per molecule of glucose because the stoichiometry here is difficult to establish.
Carbon Dioxide
Carbon dioxide is a waste product of cellular respiration. During cellular respiration, glucose, and oxygen react to form carbon dioxide and water, releasing energy in the process. Carbon dioxide is produced by the tricarboxylic acid cycle (TCA cycle) and released as a waste product.
The citric acid or Krebs cycle also produces carbon dioxide as a byproduct. Cellular respiration is an oxidative process whereby an electron donor is oxidized and oxygen is reduced to produce carbon dioxide, water, and energy.
Other Products
Apart from ATP and carbon dioxide, water is also a product of cellular respiration. During the electron transport chain, spent electrons combine with oxygen to form water.
In photosynthesis, water is broken down to form oxygen while in cellular respiration, oxygen combines with hydrogen to form water. The oxygen consumed during cellular respiration is involved directly in accepting electrons at the end of the electron transport chain.
Purpose of Cellular Respiration
Cellular respiration is the process by which cells in plants and animals break down sugar and turn it into energy, which is then used to perform various functions within the cell. The main function of cellular respiration is to synthesize biochemical energy.
This process occurs in the mitochondria of organisms (animals and plants) and releases energy in the form of ATP. Cellular respiration produces about 36 to 38 ATP molecules for every glucose molecule.
Cellular respiration is essential to both eukaryotic and prokaryotic cells. It takes place in all living organisms, including plants, and is vital because the energy produced is used to maintain life. The process can be carried out either in the presence or absence of oxygen.
However, essentially, the process is called cellular respiration because the cell seems to “respire” by taking in molecular oxygen (as an electron acceptor) and releasing carbon dioxide (as an end product).
Types of Cellular Respiration
Aerobic Respiration
Aerobic respiration is a process of cellular respiration that takes place in the presence of oxygen to produce energy from food. It is the process of cellular respiration that takes place in the presence of oxygen gas to produce energy from food.
Aerobic respiration is used by all cells to burn fuel, such as fats and sugars, into chemical energy. The product of aerobic respiration is a molecule called adenosine triphosphate (ATP), which uses the energy stored in its phosphate bonds to power chemical reactions.
The complete process of aerobic respiration occurs in four different stages: glycolysis, pyruvate oxidation, Krebs cycle, and electron transport chain. During glycolysis, glucose is broken down into two molecules of pyruvate.
Pyruvate oxidation converts pyruvate into acetyl-CoA. The Krebs cycle oxidizes acetyl-CoA to produce ATP and carbon dioxide. Finally, the electron transport chain produces ATP using the energy released by electrons flowing through a series of proteins.
Aerobic respiration takes place in all multicellular organisms including animals, plants, and other living organisms. It is often referred to as oxidative metabolism or cell respiration. In contrast, anaerobic respiration does not use oxygen.
Aerobic respiration produces more ATP than anaerobic respiration because it can fully break down glucose molecules into carbon dioxide and water.
Fermentation
Fermentation is a process that breaks down glucose in the absence of oxygen, producing energy and other substances. Fermentation follows glycolysis, which is the first step in cellular respiration.
Glycolysis breaks down glucose into two pyruvate molecules, which are then used in fermentation. There are two types of fermentation: alcoholic fermentation and lactic acid fermentation.
Alcoholic fermentation produces ethanol, carbon dioxide, and NAD+. This type of fermentation is used by yeast to ferment barley malt into beer. Alcoholic fermentation requires electrons from NADH and results in the production of ethanol.
Lactic acid fermentation produces lactic acid (lactate) and NAD+. This type of fermentation occurs in our muscles when they are low on oxygen during exercise.
Fermentation is a widespread pathway for breaking down glucose anaerobically. It is performed by many types of organisms and cells. Fermentation can also be used to produce substances that can be used in making chemical energy. The process has been used for at least 10,000 years to manufacture wine and beer.
Methanogenesis
Methanogenesis is the process of anaerobic respiration that generates methane as the final product of metabolism. Methanogens are microbes that are capable of producing methane and use carbon as an electron acceptor in their metabolism.
Methanogens do not use oxygen to respire, and oxygen inhibits their growth. The two best-described pathways for methanogenesis involve the use of acetic acid or carbon dioxide and hydrogen gas.
(CO2 + 4H2 → CH4 + 2H2O)
Methane is a major greenhouse gas, and its production is an important form of microbial metabolism. In most environments, it is the final step in the decomposition of biomas.
Methane is produced by methanogens in anaerobic digesters during anaerobic digestion. Anaerobic digestion is a process through which bacteria break down organic matter such as animal manure, wastewater biosolids, and food wastes in the absence of oxygen.
The biochemistry of methanogenesis involves several coenzymes and cofactors such as F420, coenzyme B, coenzyme M, methanofuran, and methanopterin. The unique biochemistry of methanogenesis has been studied extensively to understand how these microbes produce methane.
Methane production by methanogens has been shown to use carbon from other small organic compounds such as formic acid (formate), methanol, methylamines, tetramethylammonium, dimethyl sulfide, and methanethiol depending on pH and temperature.
Methane production by methanogens plays a significant role in natural gas accumulations. However, it also contributes to global warming since methane is a potent greenhouse gas.
The average cow emits around 250 liters of methane per day as a result of the breakdown of cellulose by methanogens. Therefore, the large-scale raising of cattle for meat contributes significantly to global warming.
