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

解糖は 糖を分解しエネルギーを生成する代謝の過程です グルコースは最初に細胞の細胞質に入り 2つの主要な段階を経て変質します 第1段階では、ATPの形状でエナジーが必要です グルコース分子を内部に引き込み 一連の手順を通じて変化を加えるためです 第2段階では、電子からエネルギーを抽出し 高エネルギーNADHおよび 新たなATP分子として利用します 複数の酵素工程の後 3炭糖のピルビン酸が2つ生成され ミトコンドリアへ輸送され クエン酸回路に加わる準備ができます

解糖とは？

<h4>概要</h4>
 
 細胞は高分子を分解しエネルギーを作ります。細胞呼吸は、高分子の化学結合から得られる食物エネルギーをアデノシン三リン酸（ATP）の形の化学エネルギーに変換する生化学的プロセスです。この厳密に制御された複雑なプロセスの最初の段階が解糖です。解糖という言葉は、ラテン語のglyco（糖）とlysis（分解）に由来します。解糖は、他の経路に供給するため、ATPと中間代謝物を生成するという細胞内で2つの役割を果たします。解糖経路では、1つのヘキソース（グルコースなどの炭素数6の炭水化物）が、2つのトリオース（ピルビン酸などの炭素数3の炭水化物）に変換され、正味2分子のATP（4つ生成して2つ消費される）と2分子のニコチンアミドアデニンジヌクレオチド（NADH）が生成されます。
 
 <h4>解糖機構の解明</h4>
 
 解糖系は、最初に発見された生化学的経路であることを知っていますか？1800年代半ば、ルイ・パスツールは、微生物が酸素のないところでグルコースを分解する（発酵）ことを発見しました。1897年、エドアルド・ブフナーは、酵母細胞を破壊し、可溶性分子や細胞小器官を含む細胞質を回収することで、細胞でない酵母の抽出物でも発酵反応が行われることを発見しました。その後まもなく、1905年にアーサー・ハーデンとウィリアム・ヤングが、無機リン酸（Pi）を添加しないと発酵速度が低下することを発見し、発酵には熱に弱い成分（後に多くの酵素が含まれていることが判明）と低分子量で熱に強い成分（無機イオン、ATP、ADP、NADなどの補酵素）の両方の存在が必要であることを明らかしました。1940年までに、グスタフ・エルブデン、オットー・マイヤーホフ、ヤクブ・カロル・パーナスら多くの人々の努力により、解糖の完全な経路が確立されました。現在では、解糖系はEMP経路とは呼ばれていません。
 
 <h4>グルコースの運命</h4>
 
 グルコースは2つの方法で細胞に入ります。GLUT (glucose transporter) タンパク質と呼ばれる内在性タンパク質のグループを介した促進拡散により、グルコースは細胞質に取り込まれます。GLUTタンパク質ファミリーのメンバーは、人体の特定の組織に存在します。また、二次能動輸送で、膜貫通型のシンポートタンパク質を介し、濃度勾配に逆らってグルコースは輸送されます。シンポーターは、イオンを駆動する際の電気化学的エネルギーを利用します。例えば、小腸、心臓、脳、腎臓に存在するナトリウム-グルコース共輸送体などです。
 
 有酸素（O2が豊富な状態）および無酸素（O2が不足した状態）の両条件下で、一度グルコースが細胞の細胞質に入ると解糖は始まります。解糖には主に2つの段階があります。第1段階はエネルギーを必要とするもので、準備段階と考えられ、グルコースを細胞内に捕捉し、効率的に切断されるように6つの炭素からなる炭素骨格を再構築します。第2段階は、エネルギーを放出し、ピルビン酸を生成する清算段階です。
 
 <h4>ピルビン酸の運命 </h4>
 
 酸素濃度やミトコンドリアの存在によって、ピルビン酸の運命は2つあります。ミトコンドリアが存在する有酸素条件下では、ピルビン酸はミトコンドリアに入り、クエン酸サイクルや電子伝達系（ETC）を経て、CO2、H2O、さらにはATPへと酸化されます。一方、無酸素状態（例: 働いている筋肉）やミトコンドリアがない状態（例: 原核生物）では、ピルビン酸は乳酸発酵を起こします（つまり、無酸素状態でピルビン酸は乳酸に還元されます）。興味深いことに、嫌気性の酵母や一部の細菌は、アルコール発酵で知られる過程を経て、ピルビン酸をエタノールに変換できます。
 
 <h4>解糖系の制御</h4>
 
 解糖系のように酵素で調節される代謝経路を厳密に制御・調節することは、生体が適切に機能するために重要です。その制御には、基質制限や酵素関連制御があります。基質制限は、細胞内の基質と生成物の濃度がほぼ均衡しているときに生じます。その結果、基質利用能がその反応速度を決定します。酵素関連制御では、基質と生成物の濃度が平衡から離れています。酵素の活性が反応速度を決定し，それが経路全体の流れを制御します。解糖系には、ヘキソキナーゼ、ホスホフルクトキナーゼ、ピルビン酸キナーゼの3つの制御酵素があります。

概要
細胞は高分子を分解することでエネルギーを作ります。細胞呼吸は、「食物エネルギー」(...

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Cellular Processes

細胞プロセス

Cellular Respiration

細胞呼吸

KEY TERMS AND CONCEPTS

主要用語と概念

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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