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

Glycolysis is a metabolic process that breaks down sugars to generate energy. This process occurs in two main phases &#8212; an energy requiring phase and an energy releasing phase.
To begin with, the glucose first enters the cell&#39;s cytoplasm. Next, through a series of enzymatic reactions that require energy in the form of ATP,&#160; glucose is modified into two&#160; molecules of three-carbon glyceraldehyde 3-phosphate.
In the second phase, through multiple enzymatic steps, the two molecules of glyceraldehyde 3-phosphate are converted into 2 molecules of pyruvate, 2 NADH and 4 ATP molecules.
Both these pyruvate molecules are then transported to the mitochondria for entry into the citric acid cycle.

what is glycolysis?

<h4>Overview</h4>
Cells make energy by breaking down macromolecules. Cellular respiration is the biochemical process that converts &#34;food energy&#34; (from the chemical bonds of macromolecules) into chemical energy in the form of adenosine triphosphate (ATP). The first step of this tightly regulated and intricate process is glycolysis. The word glycolysis originates from the Latin glyco (sugar) and lysis (breakdown). Glycolysis serves two main intracellular functions: generating ATP and generating intermediate metabolites to feed into other pathways. The glycolytic pathway converts one hexose (a six-carbon carbohydrate such as glucose) into two triose molecules (three-carbon carbohydrate) such as pyruvate, to produce a net total of two molecules of ATP (four produced, two consumed) and two molecules of nicotinamide adenine dinucleotide (NADH).
<h4>Elucidation of Glycolysis</h4>
In the mid-1800s, Louis Pasteur determined that microorganisms cause the breakdown of glucose in the absence of oxygen (fermentation). In 1897, Eduard Buchner found that fermentation reactions can still be carried out in cell-free yeast extracts by breaking open the cell and collecting the cytoplasm containing the soluble molecules and organelles. Shortly after, in 1905, Arthur Harden and William Young discovered that the rate of fermentation decreases without the addition of inorganic phosphate (Pi) and that fermentation requires the presence of both a heat-sensitive component (later identified to contain a number of enzymes) and a low molecular weight, heat-stable fraction (inorganic ions, ATP, ADP and coenzymes like NAD). By 1940, with the effort of many individuals, the complete glycolysis pathway was established by Gustav Embden, Otto Meyerhof, Jakub Karol Parnas, and others. Now, glycolysis is known as the EMP pathway.
<h4>Destiny of Glucose</h4>
There are two ways that glucose can enter the cell. A group of integral GLUT (glucose transporter) proteins shuttles glucose into the cytosol via facilitated diffusion. Members of the GLUT protein family are present in specific tissues throughout the human body. Alternatively, transmembrane symporter proteins move glucose against its concentration gradient via secondary active transport. The symporter uses electrochemical energy from pumping ions. Examples are the sodium-glucose-linked transporters in the small intestine, the heart, the brain, and the kidneys.
Under aerobic (O2-rich) and anaerobic (O2-deficient) conditions, glycolysis can start once glucose enters the cell cytosol. There are two main phases of glycolysis. The first phase is an energy-requiring preparatory step that traps glucose in the cell and restructures the six-carbon backbone so that it can be efficiently cleaved. The second phase releases energy and generates pyruvate.
<h4>Fate of Pyruvate</h4>
Depending on the oxygen level and the presence of mitochondria, pyruvate may have one of two possible fates. Under aerobic conditions with mitochondria present, pyruvate enters the mitochondria, undergoing the citric acid cycle and the electron transport chain (ETC) to be oxidized to CO2, H2O, and even more ATP. In contrast, under anaerobic conditions (i.e., working muscles) or without mitochondria (i.e., prokaryotes), pyruvate undergoes lactate fermentation (i.e., is reduced to lactate in anaerobic conditions). Interestingly, yeast and some bacteria under anaerobic conditions can convert pyruvate to ethanol through a process known as alcohol fermentation.
<h4>Regulation of Glycolysis</h4>
Tight control and regulation of enzyme-mediated metabolic pathways, such as glycolysis, is critical for the proper functioning of an organism. Control is exerted by substrate limitation or enzyme-linked regulation. Substrate limitation occurs when the concentration of the substrates and products in the cell is near equilibrium. Consequently, the availability of the substrate determines the rate of the reaction. In enzyme-linked regulation, the concentration of the substrates and products is far from equilibrium. The activity of three enzymes (hexokinase, phosphofructokinase, and pyruvate kinase) determines the rate of reaction, which controls the flux of the overall pathway.

Overview
Cells make energy by breaking down macromolecules. Cellular respiration is the biochemical process that converts &#34;food energy&#34; (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

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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