Name: glycolysis: how cells break down glucose
Uploaded: 2019-03-11T09:21:10.000Z
Duration: 56 s
Channel: JoVE

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Biology

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What is Glycolysis?

- [Istruttore] La glicolisi
è il processo metabolico

di abbattere gli zuccheri
per generare energia.

Il glucosio prima entra nel
citoplasma della cellula

e si trasforma in due fasi principali.

La prima parte richiede
energia sotto forma di ATP,

per intrappolare la molecola
di glucosio all'interno,

e modificarla attraverso
una serie di passaggi.

La seconda fase estrae
energia dagli elettroni,

e la sfrutta con NADH ad alta energia

e altre molecole di ATP.

Dopo varie fasi enzimatiche,

si producono due zuccheri
a tre carboni, i piruvati,

pronti per il trasporto ai mitocondri,

per entrare nel ciclo dell'acido citrico.

cos'è la glicolisi?

<h4>Panoramica</h4>

Le cellule fanno energia scomponendo le macromolecole. La respirazione cellulare è il processo biochimico che converte "energia alimentare" (dai legami chimici delle macromolecole) in energia chimica sotto forma di trifosfato di adenosina (ATP). Il primo passo di questo processo strettamente regolato e intricato è la glicolisi. La parola glicolisi deriva dal glico latino (zucchero) e dalla lisi (rottura). La glicolisi svolge due funzioni intracellulari principali: generare ATP e metaboliti intermedi per nutrirsi in altri percorsi. La via glicolitica converte un esosio (carboidrati a sei carbonio come il glucosio), in due molecole di trioso (carboidrati a tre carbonio) come il piruvato, e una rete di due molecole di ATP (quattro prodotte, due consumate) e due molecole di nicotinamide adenina dinucleotide (NADH).

<h4>Chiarimento della glicolisi</h4>

Sapevi che la glicolisi è stata la prima via biochimica scoperta? A metà del 1800, Louis Pasteur determinò che i microrganismi causano la ripartizione del glucosio in assenza di ossigeno (fermentazione). Nel 1897, Eduard Buchner scoprì che le reazioni di fermentazione possono ancora essere effettuate in estratti di lievito senza cellule, ottenuti rompendo aprire la cellula e raccogliendo il citoplasma che contiene le molecole solubili e gli organelli. Poco dopo, nel 1905, Arthur Harden e William Young scoprirono che il tasso di fermentazione diminuisce senza l'aggiunta di fosfato inorganico (Pi)e che la fermentazione richiede la presenza sia di una componente sensibile al calore (successivamente identificata per contenere un certo numero di enzimi) sia di un basso peso molecolare, frazione stabile al calore (ioni inorganici, ATP, ADP e coenzes). Nel 1940, con lo sforzo di molti individui, il percorso completo della glicolisi è stato stabilito da Gustav Embden, Otto Meyerhof, Jakub Karol Parnas, et al. Infatti, la glicolisi è ora conosciuta come il percorso EMP.

<h4>Destino del glucosio</h4>

Il glucosio può entrare nelle cellule in due modi: diffusione facilitata tramite un gruppo di proteine integrali chiamate proteine GLUT (trasportatore di glucosio) che navetta glucosio nel citosol. I membri della famiglia di proteine GLUT sono presenti in tessuti specifici in tutto il corpo umano. In alternativa, il trasporto attivo secondario sposta il glucosio contro il suo gradiente di concentrazione attraverso una proteina "symporter" (cotrasportatrice) transmembrana. Il symporter utilizza l'energia elettrochimica dal pompaggio di uno ione. Esempi sono i trasportatori collegati al sodio-glucosio nell'intestino tenue, cuore, cervello, e reni.

In condizioni sia aerobiche (o2 ricche) che anaerobiche (o2 carenti), la glicolisi può iniziare una volta che il glucosio entra nel citosol di una cellula. Ci sono due fasi principali della glicolisi. La prima fase richiede energia ed è considerata una fase preparatoria, intrappolando il glucosio nella cellula e ristrutturando la spina dorsale a sei carbonio in modo che possa essere scisto in modo efficiente. La seconda fase è la fase di "pay-off" (pagamento), con rilascio di energia e la generazione di piruvato.

<h4>Il destino del Piruvato</h4>

A seconda del livello di ossigeno e della presenza di mitocondri, il piruvato può avere uno dei due possibili destini. In condizioni aerobiche, con i mitocondri presenti, il pirotto entra nei mitocondri, subendo il ciclo dell'acido citrico e la catena di trasporto degli elettroni (ETC) da ossidare a CO2, H2O e ancora più ATP. Al contrario, in condizioni anaerobiche (per esempio nei muscoli in condizioni di lavoro) o in mancanza di mitocondri (come nei procarioti), il piruvato subisce la fermentazione in lattato (cioè, è ridotto al lattato in condizioni anaerobiche). È interessante notare che, lievito e alcuni batteri in condizioni anaerobiche possono convertire il piruvato in etanolo attraverso un processo noto come fermentazione alcolica.

<h4>Regolazione della glicolisi</h4>

Un controllo rigoroso e la regolazione delle vie metaboliche mediate da enzimi, come la glicolisi, è fondamentale per il corretto funzionamento di un organismo. Il controllo è esercitato dalla limitazione del substrato o dalla regolazione legata all'enzima. La limitazione del substrato si verifica quando la concentrazione di substrato e prodotti nella cellula sono vicini all'equilibrio. Di conseguenza, la disponibilità del substrato determina il tasso della reazione. Nella regolazione legata all'enzima, la concentrazione di substrato e prodotti è lontana dall'equilibrio. L'attività dell'enzima determina il tasso di reazione, che controlla il flusso del percorso complessivo. Nella glicolisi, i tre enzimi regolatori sono esaochinasi, fosfofruttochinasi e chinasi piruvato.

Cos'è la glicolisi?

Overview
Cells make energy by breaking down macromolecules. Cellular respiration is the biochemical process that converts "food energy" (from the ...

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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.

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Glycolysis: How Cells Break Down Glucose

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