Name: glycolysis: how cells break down glucose
Uploaded: 2019-03-11T09:21:10.000Z
Duration: 56 s
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LS.Bio.3.3

What is Glycolysis?

- [Instructeur] La glycolyse est le processus métaboliquede décomposition des sucres pour générer de l'énergie. 
Le glucose pénètre d'abord dans le cytoplasme de la cellule,et il est transformé en deux phases principales. 
La première partie nécessite de l'énergie sous forme d'ATP,pour piéger la molécule de glucose à l'intérieur,et la modifier en une série d'étapes. 
La deuxième phase extrait l'énergie des électrons,et lui atèle de la haute énergie NADH,et plus de molécules ATP. 
Après plusieurs étapes enzymatiques,le pyruvate, deux sucres à trois carbones, est produit,et prêt pour être transporté vers les mitochondries,pour l'entrée dans le cycle de l'acide citrique.

qu'est-ce que la glycolyse ?

<h4>Aperçu</h4>
 
 Les cellules acquièrent de l’énergie en décomposant les macromolécules. La respiration cellulaire est le processus biochimique qui convertit “ l’énergie des aliments ” (à partir des liaisons chimiques des macromolécules) en énergie chimique sous forme d’adénosine triphosphate (ATP). La première étape de ce processus étroitement régulé et complexe est la glycolyse. Le mot glycolyse provient du latin glyco (sucre) et lyse (dégradation). La glycolyse remplit deux fonctions intracellulaires principales : générer de l’ATP et des métabolites intermédiaires pour nourrir d’autres voies. La voie glycolytique convertit un hexose (glucide à six carbone comme le glucose) en deux molécules de triose (glucide à trois carbones), comme le pyruvate, et un réseau de deux molécules d’ATP (quatre produites, deux consommées) et deux molécules de nicotinamide adénine dinucléotide (NADH).
 
 <h4>L’élucidation de la glycolyse</h4>
 
 Saviez-vous que la glycolyse a été la première voie biochimique découverte ? Au milieu des années 1800, Louis Pasteur a déterminé que les micro-organismes causent la dégradation du glucose en l’absence d’oxygène (fermentation). En 1897, Eduard Buchner a constaté que les réactions de fermentation peuvent encore être effectuées dans des extraits de levure sans cellules, réalisés en brisant la cellule et en recueillant le cytoplasme qui contient les molécules solubles et les organites. Peu de temps après, en 1905, Arthur Harden et William Young ont découvert que le taux de fermentation diminue sans l’ajout de phosphate inorganique (Pi) et que la fermentation nécessite la présence à la fois d’un constituant sensible à la chaleur (plus tard identifié comme contenant un certain nombre d’enzymes) et d’une fraction thermostable de faible poids moléculaire (des ions inorganiques, de l’ATP, de l’ADP et des coenzymes comme NAD). En 1940, avec l’effort de nombreux individus, la voie complète de la glycolyse a été établie par Gustav Embden, Otto Meyerhof, Jakub Karol Parnas et al. En effet, la glycolyse est maintenant connue sous le nom de voie EMP.
 
 <h4>Le destin du glucose</h4>
 
 Le glucose peut pénétrer dans les cellules de deux façons : la diffusion facilitée par l’intermédiaire d’un groupe de protéines intégrales appelées protéines GLUT (transporteur de glucose) qui transportent le glucose dans le cytosol. Les membres de la famille des protéines GLUT sont présents dans des tissus spécifiques dans tout le corps humain. Alternativement, le transport actif secondaire déplace le glucose contre son gradient de concentration par l’intermédiaire d’une protéine symporteur transmembranaire. Le symporteur utilise l’énergie électrochimique provenant du pompage d’un ion. Des exemples sont les transporteurs liés au sodium-glucose dans l’intestin grêle, le cœur, le cerveau et les reins.
 
 Dans les conditions aérobies (riches en O2) et anaérobies (carencées en O2), la glycolyse peut commencer une fois que le glucose pénètre dans le cytosol d’une cellule. Il existe deux phases principales de la glycolyse. La première phase a besoin d’énergie et elle est considérée comme une étape préparatoire, piégeant le glucose dans la cellule et restructurant le squelette à six carbones afin qu’il puisse être efficacement clivé. La seconde phase est la phase payante, libérant de l’énergie et générant du pyruvate.
 
 <h4>Le destin du pyruvate</h4>
 
 Selon le niveau d’oxygène et la présence de mitochondries, le pyruvate peut avoir l’un des deux destins possibles. Dans des conditions aérobies, avec des mitochondries présentes, le pyruvate pénètre dans les mitochondries, subissant le cycle de l’acide citrique et la chaîne de transport d’électrons (ETC) pour être oxydé en CO2, H2O et encore plus d’ATP. En revanche, dans des conditions anaérobies (c.-à-d. les muscles en fonctionnement) ou manquant de mitochondries (c.-à-d. les procaryotes), le pyruvate subit une fermentation lactique (c.-à-d. est réduit en lactate dans des conditions anaérobies). Fait intéressant, la levure et certaines bactéries dans des conditions anaérobies peuvent transformer le pyruvate en éthanol par un processus connu sous le nom de fermentation alcoolique.
 
 <h4>La régulation de la glycolyse</h4>
 
 Un contrôle et une régulation étroite des voies métaboliques qui dépendent d’enzymes, comme la glycolyse, sont essentiels au bon fonctionnement d’un organisme. Le contrôle est exercé par la limitation du substrat ou par la régulation liée aux enzymes. La limitation du substrat se produit lorsque la concentration du substrat et des produits dans la cellule est proche de l’équilibre. Par conséquent, la disponibilité du substrat détermine la vitesse de réaction. Dans la régulation liée aux enzymes, la concentration du substrat et des produits est loin de l’équilibre. L’activité de l’enzyme détermine la vitesse de la réaction, qui contrôle le flux de la voie globale. Dans la glycolyse, les trois enzymes régulatrices sont l’hexokinase, la phosphofructokinase et la kinase du pyruvate.

Qu'est-ce que la glycolyse ?

Aperçu
Les cellules produisent de l’énergie en décomposant les macromolécules. La respiration cellulaire est le processus biochimique qui convertit « l’énergie alimentaire » (à partir de l'...

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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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