Introduction
Cellular respiration is the process by which plants and animals’ cells break down sugar and convert it to energy. Cellular respiration has a simple goal: it provides energy to cells so they can function.
Organisms mix oxygen with food molecules, channelling the chemical energy in these molecules to life-sustaining processes while discarding carbon dioxide and water as waste products. Fermentation is also part of cellular respiration in yeast,bacteria etc.Fermentation is the decomposition of food by microbes that do not require oxygen.
Sugar, amino acids and fatty acids are frequent nutrients used by animal and plant cells in respiration and molecular oxygen is the most common oxidising agent that provides the majority of the chemical energy (O2). The chemical energy stored in ATP can then be used to power energy-intensive processes like biosynthesis, locomotion or molecule transport across cell membranes.
Glycolysis is a non-oxygen-dependent metabolic process. Glycolysis’ widespread prevalence in various species suggests that it is an old metabolic mechanism. Indeed, the events that make up glycolysis and its counterpart system, the pentose phosphate pathway, take place in the Archean oceans’ oxygen-free environment in the absence of enzymes.
The glycolysis pathway is divided into two sections:
- Preparatory phase-ATP is utilised during the investment phase.
- Payoff phase-the yield phase occurs when more ATP is created than is consumed.
Products of Glycolysis
Glycolysis or the aerobic catabolic breakdown of glucose, creates energy in the form of ATP, NADH and pyruvate, which then enters the citric acid cycle to generate additional energy. In anaerobic respiration breakdown of glucose into products like alcohol, lactic acid and ATP.
Equation
The following is the net equation for glycolysis:
C6H12O6 + 2 ADP + 2 [P]i + 2 NAD → 2 pyruvates + 2 ATP + 2 NADH
The total outcomes of glucose metabolism are astounding; your cell may produce 38 molecules of ATP for every molecule of glucose. Your cell successfully retains 40% of the energy generated by breaking down glucose since it takes 30.5 kilojoules per mole to synthesise ATP(adenosine triphosphate). The remaining 60% is lost as heat, which serves to keep your body temperature stable. While 40% may seem little, it is far more efficient than many human-designed technologies. Even the most advanced automobiles, for example, can only transform a percent of the energy contained in the fuel into motion.
Process of glycolysis
The steps of glycolysis are described below.
- Phosphorylation: Glucose phosphorylated to glucose-6-phosphate by ATP in the presence of enzyme hexokinase and Mg2+.
- Isomerisation: Glucose-6-phosphate is changed into its isomers, fructose-6-phosphate with the help of enzyme phosphohexose isomers.
- Phosphorylation: Fructose-6-phosphate is phosphorylated by ATP to form fructose 1, 6-diphosphate in presence of enzyme phosphofructokinase and Mg2+.
- Phosphorylation of glucose to fructose 1, 6-diphosphate activates the sugar and prevents it from getting out of the cell.
- Splitting: Fructose 1, 6-diphosphate is then broken down into two molecules of triose phosphate (3 carbon sugars), viz. glyceraldehyde-3 phosphate (PGAL) and dihydroxyacetone phosphate (DiHAP). The reaction is catalysed by enzyme aldolase. Dihydroxyacetone phosphate is further converted into glyceraldehyde 3 phosphate with the help of enzyme phosphor triose isomerase.
- Dehydrogenation and Phosphorylation: Each glyceraldehyde 3-phosphate molecule loses hydrogen to NAD+ to form NADH+H+ and accept inorganic phosphate from phosphoric acid (H3PO4) to form 1, 3-diphosphoglycerate in the presence of enzyme triose- phosphate dehydrogenase.
- Dephosphorylation (ATP Formation): One of the two phosphates of 1, 3-diphosphoglycerate is linked by a high energy bond. In the presence of enzyme phosphoglycerate kinase, 1, 3-diphosphoglycerate is converted to 3-phosphoglycerate. One molecule of ATP is phosphorylated to ATP in the reaction. Formation of ATP directly from metabolites is known as substrate level phosphorylation.
- Isomerization: 3-phosphoglycerate is charged to its isomer 2-phosphoglycerate by the enzyme phosphoglyceromutase.
- Dehydration: 2-Phosphoglycerate loses a molecule of water in the presence of enzyme enclose and Mg2+, and changes into phosphoenol pyruvate.
- Dephosphorylation (ATP formation): High energy phosphate group of phosphoenolpyruvate is transferred to a molecule of ADP with the help of the enzyme pyruvate kinase in the presence of Mg2+ and K+. This produces pyruvate and a molecule of ATP by substrate level phosphorylation.
Aerobic Respiration
To produce ATP, aerobic respiration requires oxygen (O2). Even though the carbs, lipids, and proteins are eaten as reactants, aerobic respiration is the preferred route of pyruvate breakdown in glycolysis and pyruvate must be transported to the mitochondria for the citric acid cycle to fully oxidise it. Through substrate-level phosphorylation, carbon dioxide and water are created and the energy imparted is used to break bonds in ADP and add a third phosphate group to form ATP (adenosine triphosphate), NADH and FADH2. Through an electron transport chain with oxygen and protons as the “terminal electron acceptors,” the potential of NADH and FADH2 is transformed to additional ATP. Oxidative phosphorylation produces the majority of the ATP generated by aerobic cellular respiration. By pumping protons over a membrane, the energy released by the O2 is utilised to produce a chemiosmotic potential. This potential is then utilised to activate ATP synthase, which converts ADP and phosphate groups into ATP. During cellular respiration, 38 ATP molecules can be produced peroxidised glucose molecules, according to biology textbooks. However, due to leaky membrane losses and the expense of transporting pyruvate and ADP into the mitochondrial matrix, this maximal yield is never fully attained, and current estimates vary from 29 to 30 ATP per glucose.
Aerobic metabolism is up to 15 times more efficient than anaerobic metabolism because the double bond in O2 contains more energy than other double bonds or pairs of single bonds in other common compounds in the biosphere (which creates 2 molecules of ATP for 1 molecule of glucose). Some anaerobic organisms, such as methanogens, may maintain anaerobic respiration while producing more ATP by utilising high-energy inorganic molecules (not oxygen) as final electron acceptors in the electron transport chain. Although they both begin with glycolysis, aerobic metabolism continues with the Krebs cycle and oxidative phosphorylation. Post-glycolytic reactions occur in the mitochondria of eukaryotic cells, while they occur in the cytoplasm of prokaryotic cells.
Anaerobic Glycolysis
When just a little amount of oxygen (O2) is available, anaerobic glycolysis converts glucose to lactate. Anaerobic glycolysis is only useful for producing energy during brief bouts of high-intensity activity, lasting anywhere from 10 seconds to 2 minutes. This occurs far more quickly than aerobic metabolism. During maximum exertion, ATP is created at a rate of around 100 times that of oxidative phosphorylation.
Anaerobic glycolysis is assumed to have been the predominant source of energy in previous creatures before oxygen levels in the environment were high, and hence represents a more ancient mode of energy generation in cells.
Pyruvate metabolism under anaerobic conditions:
In lactic acid fermentation, pyruvate is the last electron acceptor. When there isn’t enough oxygen in the muscle cells for the pyruvate and NADH created in glycolysis to be further oxidised, NAD+ is replenished from NADH by reducing pyruvate to lactate. Lactate dehydrogenase is an enzyme that converts lactate to pyruvate. The reaction’s standard free energy change is -25.1 kJ/mol.
Fermentation of ethanol
Fermentation
Fermentation is named after products like alcoholic fermentation and lactic acid fermentation. Buchner (1897) found that fermentation could be caused by mixing sugar solution with yeast extract, instead of living yeast cells. The enzyme complex present in the extract was named zymase. Because of the latter, fermentation is also called zymosis.
In anaerobic respiration, electrons are removed from the substrate during oxidation but are not finally transferred to molecular oxygen. The final electron acceptors are frequently compounds such as pyruvic acid or acetaldehyde, which form a part of the anaerobic respiratory pathway. The end products are lactic acid or ethyl alcohol and not water.
The mechanism of anaerobic respiration or fermentation resembles that of respiration up to glycolysis. Pyruvate formed at the end of glycolysis is anaerobically broken down to yield various products depending upon the organism and the type of tissue. The two common products are ethyl alcohol and lactic acid.
Conclusion
Respiration involves catabolic processes, which break big molecules into smaller ones, releasing energy when weak high-energy connections, such as those found in molecular oxygen, are replaced by stronger bonds in the products. Respiration is one of the most essential ways a cell releases chemical energy to power cellular activities. The overall process is made up of a series of biochemical phases, some of which contain redox reactions. Although cellular respiration is technically a combustion process, the progressive and regulated release of energy from the chain of processes separates it from combustion when it occurs in a living cell.