Name: glycolysis: how cells break down glucose
Uploaded: 2019-03-11T09:21:10.000Z
Duration: 56 s
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What is Glycolysis?

Glykolyse ist der Stoffwechselprozess, bei dem Zucker abgebaut wird, um Energie zu erzeugen.
 Glucose tritt zuerst in das Zytoplasma der Zelle ein und wird über zwei Hauptphasen transformiert.
 Der erste Teil benötigt Energie in Form des ATP, um das Glukosemolekül im Inneren einzufangen und es durch eine Reihe von Schritten zu modifizieren.
 Die zweite Phase entzieht den Elektronen Energie und nutzt sie mit hochenergetischem NADH und mehr ATP-Molekülen.
 Nach mehreren enzymatischen Schritten werden zwei Dreikohlenstoffzucker, Pyruvat, hergestellt, die für den Transport in die Mitochondrien zum Eintritt in den Zitronensäurekreislauf bereit sind.

was ist die glykolyse?

<h4>Überblick</h4>
 
 Zellen erzeugen durch den Abbau von Makromolekülen Energie. Die Zellatmung ist der biochemische Prozess, in dem die Energie der Nahrung (aus den chemischen Bindungen der Makromoleküle) in chemische Energie in Form von Adenosintriphosphat (ATP) umgesetzt wird. Der erste Schritt dieses streng geregelten und komplexen Prozesses ist die Glykolyse. Das Wort Glykolyse stammt aus dem Lateinischen Glyco (Zucker) und lysis (Abbau). Die Glykolyse erfüllt zwei wesentliche intrazelluläre Funktionen: die Erzeugung von ATP und die Herstellung von Zwischenprodukten, welche in andere Stoffwechselwege eingespeist werden. Der glykolytische Stoffwechselweg führt zur Umwandlung eines Hexose Molekül (sechs Kohlenhydrate, wie Glukose) in zwei Triosemoleküle (drei Kohlenhydrate) wie Pyruvat und zur Nettoproduktion von zwei ATP Molekülen (vier produziert, zwei verbraucht) und zwei Molekülen Nicotinamidadenindinukleotid (NADH).
 
 <h4>Die Entdeckung der Glykolyse</h4>
 
 Wussten Sie, dass die Glykolyse der erste gefundene biochemische Stoffwechselweg war? Mitte der 1800er Jahre stellte Louis Pasteur fest, dass Mikroorganismen den Abbau von Glukose in Abwesenheit von Sauerstoff (Fermentation) bewirken. Eduard Buchner fand 1897 heraus, dass in zellfreien Hefeextrakten noch immer Gärungsreaktionen stattfanden. Das gelang durch das Aufbrechen der Zelle und das Sammelns des Zytoplasmas, in dem die löslichen Moleküle und Organellen enthälen sind. Kurz darauf in 1905 entdeckten Arthur Harden und William Young, dass die Geschwindigkeit der Gärung ohne die Zugabe von anorganischem Phosphat (Pi) abnimmt und dass die Gärung eine hitzeempfindliche Komponente (später wurde gezeigt, dass es bestimmte Enzyme sind) und eine niedermolekulare, hitzestabile Fraktion (anorganische Ionen, ATP, ADP und Coenzyme wie NAD) benötigt. Im Jahr 1940 wurde der vollständige Reaktionsweg der Glykolyse durch Gustav Embden, Otto Meyerhof, Jakub Karol Parnas, und vielen weiteren Wissenschaftler aufgedeckt. Daher ist die Glykolyse auch heute noch als EMP-Weg bekannt.
 
 <h4>Das Schicksal der Glucose</h4>
 
 Glukose kann auf zwei Arten in Zellen gelangen. Zum einen kann sie durch erleichterte Diffusion mit Hilfe einer Gruppe von Integralen Membranproteinen, die als GLUT (Glukose-Transporter) bezeichnet werden, in die Zelle gelangen. Mitglieder der GLUT-Proteinfamilie sind in bestimmten Geweben im gesamten menschlichen Körper vorhanden. Alternativ bewegt der sekundäre aktive Transport die Glukose gegen ihren Konzentrationsgradienten durch einen Transmembran-Symporter-Proteintransporter. Der Symporter nutzt die elektrochemische Energie, die durch das Pumpen eines Ions gewonnen wird. Beispiele sind die Natrium/Glukose-Cotransporter im Dünndarm, Herz, Gehirn und in den Nieren.
 
 Unter sowohl aeroben (O2 reichen) als auch anaeroben (O2 mangelhaften) Bedingungen kann die Glykolyse beginnen, sobald Glukose in das Zytosol einer Zelle gelangt. Es gibt zwei Hauptphasen der Glykolyse. Die erste Phase ist energieaufwendig und gilt als vorbereitender Schritt. Dabei wird Glukose in der Zelle festgehalten und das Sechs-Kohlenstoff-Gerüst so umstrukturiert, dass es effizient gespalten werden kann. In der zweiten Phase wird dann Energie freigesetzt und Pyruvat erzeugt.
 
 <h4>Schicksal des Pyruvats</h4>
 
 Abhängig vom Sauerstoffgehalt und dem Vorhandensein von Mitochondrien kann Pyruvat eines von zwei möglichen Schicksalen ereilen. Unter aeroben Bedingungen tritt das Pyruvat bei Vorhandensein von Mitochondrien in die Mitochondrien ein und durchläuft den Citratzyklus und die Elektronentransportkette (ETK). So wird es zu CO2, H2O und noch mehr ATP oxidiert. Im Gegensatz dazu wird Pyruvat unter anaeroben Bedingungen (beispielsweise bei arbeitenden Muskeln) oder bei fehlenden Mitochondrien (also bei Prokaryoten) einer Milchsäuregärung unterzogen. Dabei wird Pyruvat zu Laktat reduziert. Interessanterweise können Hefe und einige Bakterien unter anaeroben Bedingungen Pyruvat durch einen Prozess, der als Alkoholgärung bekannt ist, in Ethanol umwandeln.
 
 <h4>Regulierung der Glykolyse</h4>
 
 Die genaue Kontrolle und Regulierung enzymvermittelter Stoffwechselwege, wie z.B. der Glykolyse, ist für das ordnungsgemäße Funktionieren eines Organismus von entscheidender Bedeutung. Die Kontrolle wird durch die Begrenzung des Substrats oder die enzymatische Regulierung ausgeübt. Substratlimitierung tritt auf, wenn die Konzentration von Substrat und Produkten in der Zelle nahezu ausgeglichen ist. Folglich bestimmt die Verfügbarkeit des Substrats die Geschwindigkeit der Reaktion. Bei der enzymgebundenen Regulation sind die Konzentrationen von Substrat und Produkten weit vom Gleichgewicht entfernt. Die Aktivität des Enzyms bestimmt die Reaktionsgeschwindigkeit, die den Fluss des Gesamtpfades steuert. Bei der Glykolyse gibt es drei regulatorischen Enzyme: Hexokinase, Phosphofruktokinase und Pyruvatkinase.

Was ist die Glykolyse?

Überblick
Zellen erzeugen Energie, indem sie Makromoleküle abbauen. Die Zellatmung ist der biochemische Prozess, der die "Nahrungsenergie" (aus dem ...

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glycolysis: how cells break down glucose

Glycolysis is the first stage of cellular respiration and a key way cells break down glucose to make energy. Cellular respiration converts food energy from macromolecules into ATP, the cell’s main energy currency. Glycolysis also creates intermediate molecules that can feed into other pathways.

In this pathway, one six-carbon sugar such as glucose is split into two three-carbon molecules, including pyruvate. The process produces a net gain of two ATP molecules, since four ATP are made and two are used. It also generates two molecules of NADH, a carrier used in later energy reactions.

The history of glycolysis began with studies of fermentation. In the mid-1800s, Louis Pasteur showed that microorganisms break down glucose without oxygen. In 1897, Eduard Buchner found that fermentation could still happen in cell-free yeast extracts, which showed that the cytoplasm contains the needed soluble components.

Later, Arthur Harden and William Young found that fermentation slows without inorganic phosphate. They also showed that the process needs both a heat-sensitive part, later linked to enzymes, and a heat-stable part with small molecules such as inorganic ions, ATP, ADP, and coenzymes like NAD. By 1940, Gustav Embden, Otto Meyerhof, Jakub Karol Parnas, and others had established the full pathway, which is now also called the EMP pathway.

Glucose can enter cells in two main ways before glycolysis begins. GLUT proteins move glucose into the cytosol by facilitated diffusion. Transmembrane symporter proteins can also bring glucose in against its concentration gradient by secondary active transport, using energy from ion pumping. Sodium-glucose-linked transporters are examples found in the small intestine, heart, brain, and kidneys.

Glycolysis can begin after glucose enters the cytosol under both aerobic and anaerobic conditions. The pathway has two main phases. The first is an energy-requiring preparatory phase that traps glucose in the cell and rearranges its six-carbon backbone so it can be split efficiently. The second phase releases energy and forms pyruvate.

Pyruvate can follow different paths depending on oxygen and mitochondria. Under aerobic conditions with mitochondria, pyruvate enters the mitochondria and continues through the citric acid cycle and the electron transport chain, where it is oxidized to carbon dioxide, water, and more ATP. Under anaerobic conditions, such as in working muscles, or in cells without mitochondria, such as prokaryotes, pyruvate undergoes lactate fermentation. Yeast and some bacteria can instead convert pyruvate to ethanol through alcohol fermentation.

Glycolysis is tightly regulated because metabolic pathways must be controlled for proper cell function. Regulation can depend on substrate limitation, which occurs when substrate and product levels are near equilibrium and the available substrate sets the reaction rate. It can also depend on enzyme-linked regulation, where the pathway is controlled by key enzymes. Hexokinase, phosphofructokinase, and pyruvate kinase are three enzymes that help determine the overall flux of glycolysis.

Glycolysis: How Cells Break Down Glucose

what is glycolysis?

Glycolysis is the first stage of cellular respiration and a key way cells break down glucose to make energy. Cellular respiration converts food energy from macromolecules into ATP, the cell’s main energy currency. Glycolysis also creates intermediate molecules that can feed into other pathways. In this pathway, one six-carbon sugar such as glucose is split into two three-carbon molecules, including pyruvate. The process produces a net gain of two ATP molecules, since four ATP are made and two...

energy-requiring steps of glycolysis

Glycolysis: Glucose Trapping and Splitting

Glycolysis begins in the cytosol, where glucose is prepared for use as energy. This process has two phases: an energy-requiring phase and an energy-releasing phase. The first phase changes glucose into a more reactive, unstable form before it can move into later steps. In the first step, glucose is phosphorylated, which means a phosphate group is added from ATP. This produces glucose 6-phosphate. The added phosphate is negatively charged, so it cannot cross the hydrophobic cell membrane. As a...

energy-releasing steps of glycolysis

Glycolysis: ATP and NADH Formation

Glycolysis produces ATP and NADH during its energy-releasing phase. These reactions are the last steps of glycolysis and convert glyceraldehyde-3-phosphate, or G3P, into pyruvate. The phase is often described as steps 6 to 10, and it happens twice because two 3-carbon sugars are made earlier in glycolysis. The sixth step begins with two linked events. G3P is oxidized, which means it loses hydrogen, and it is phosphorylated, which means a phosphate group is added. NAD+ acts as the electron...

pyruvate oxidation

How Pyruvate Becomes Acetyl CoA

Pyruvate is prepared for the next stage of cellular respiration inside the mitochondria. After glycolysis, the charged pyruvate molecules enter the mitochondria by active transport. There, they go through three enzymatic reactions so the energy stored in pyruvate can be used by the cell. First, pyruvate dehydrogenase removes the carboxyl group from pyruvate. This step releases carbon dioxide. Next, the stripped molecule is oxidized, and the electrons are picked up by NAD+ to make NADH. The...

the citric acid cycle

Citric Acid Cycle Energy and Products

The citric acid cycle is a key energy-making pathway in cells. It is also called the Krebs cycle or the TCA cycle. In eukaryotic cells, it takes place in the matrix of mitochondria. In prokaryotic cells, it occurs in the cytoplasm. The cycle starts with acetyl CoA, which enters the pathway after pyruvate oxidation. That earlier step makes two acetyl CoA molecules per glucose molecule. For that reason, the citric acid cycle runs twice for each glucose molecule. The pathway includes eight...

electron transport chains

ATP Production by the Electron Transport Chain

ATP production in the electron transport chain is the final stage of cellular respiration. This stage is part of oxidative phosphorylation, which has two steps: the electron transport chain and chemiosmosis. In eukaryotic cells, the electron transport chain sits in the inner mitochondrial membrane. Its main job is to build a proton gradient. That gradient is then used during chemiosmosis to make ATP. The process also regenerates the electron carriers NAD+ and FAD, which are needed again in...

chemiosmosis

ATP Production by Chemiosmosis

ATP production by chemiosmosis is part of oxidative phosphorylation, the efficient stage of cellular respiration that makes most of the cell’s ATP. Oxidative phosphorylation includes two linked processes: the electron transport chain and chemiosmosis. Together, they help generate adenosine triphosphate, or ATP, the main energy molecule used by cells. The electron transport chain is a series of protein complexes in the inner mitochondrial membrane. These complexes pass electrons through a set...

electron carriers

Electron Carriers in Cellular Respiration

Electron carriers help move electrons through cellular respiration. They act like shuttles that can accept electrons, which means they are reduced, or give electrons away, which means they are oxidized. This changing between oxidized and reduced forms is essential for energy production. As glucose breaks down into carbon dioxide and water, it loses electrons at several stages. Electron carriers pick up those electrons and store them temporarily. They then release the electrons into the...

fermentation

Fermentation Pathways Without Oxygen

Fermentation is a metabolic pathway that lets cells make energy without oxygen. It begins after glucose is split into two pyruvate molecules during glycolysis. Glycolysis is the same first step used in aerobic respiration. When oxygen is unavailable, pyruvate does not enter aerobic respiration. Instead, cells convert pyruvate into other organic molecules through pathways that depend on the organism. A key purpose of fermentation is to regenerate NAD+ from NADH so glycolysis can keep running. <br...

dietary connections

How Food Becomes ATP

Food becomes ATP through connected steps in cellular respiration. Carbohydrates, proteins, and lipids all feed into these pathways. The body can use foods such as apples, cheese, and guacamole as sources of energy. Carbohydrate digestion starts with enzymes that break complex carbohydrates into simple sugars. Glucose can enter glycolysis directly. Fructose and galactose must first be converted into sugars that are intermediates of the glycolytic pathway. Proteins are digested into amino acids.

introduction to cellular respiration

Cellular Respiration: ATP Production Steps

Cellular respiration is the process cells use to turn energy from food into ATP. ATP, or adenosine triphosphate, stores energy in chemical bonds that can be released quickly when cells need it. Much of the energy released during respiration becomes heat, but some of it is captured to make ATP. This process depends on oxidation-reduction, or redox, reactions. In these reactions, oxidation means electron loss and reduction means electron gain. The electron carriers NAD+ and FAD, along with their...

products of the citric acid cycle

Citric Acid Cycle Output in Cellular Respiration

The citric acid cycle is a key stage of aerobic respiration in cells. It is also called the Krebs cycle or the tricarboxylic acid (TCA) cycle. Plants and animals use this oxygen-requiring pathway as part of cellular respiration. This cycle happens after glycolysis and pyruvate oxidation. Glycolysis makes two pyruvate molecules from each glucose molecule. During pyruvate oxidation, each pyruvate becomes acetyl-CoA, which is the input for the citric acid cycle. That means one glucose molecule...

outcomes of glycolysis

Glycolysis Products in Cellular Respiration

Glycolysis is the first step of cellular respiration and breaks glucose into smaller molecules. Cells use this process to pull energy from glucose over a series of chemical reactions. Cellular respiration can happen with oxygen or without oxygen. In both aerobic and anaerobic cellular respiration, glycolysis makes a net gain of two pyruvate molecules, two NADH molecules, and two ATP molecules. Four ATP molecules are produced, but two ATP molecules are used earlier in the pathway. Glycolysis...

atp yield

How Cellular Respiration Makes ATP

Cellular respiration makes ATP from glucose in several stages. The process usually produces 30 to 32 ATP per glucose molecule. Most of that ATP comes from oxidative phosphorylation and the electron transport chain, while 4 ATP are made earlier, with 2 from glycolysis and 2 from the citric acid cycle. The electron transport chain, or ETC, sits in the inner mitochondrial membrane. It has four main protein complexes and ATP synthase, the enzyme that makes ATP. NADH and FADH2 deliver electrons to...

key terms and concepts

Learn how glycolysis breaks down glucose, produces ATP and NADH, and connects to pyruvate fate, fermentation, and pathway regulation in cells.

Glycolysis: How Cells Break Down Glucose | JoVE K12

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Glycolysis: How Cells Break Down Glucose

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