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

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9-12

Biology

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From Molecules to Organisms- Structures and Processes

LS.Bio.3.3

What is Glycolysis?

糖酵解是分解糖类 以产生能量的代谢过程。
 葡萄糖首先进入细胞的细胞质中, 并通过两个主要阶段来转化成能量。
 第一阶段需要将葡萄糖分子捕获到内部 并通过一系列步骤转化为 以三磷酸腺苷(ATP)的形式存在的能量。
 第二阶段从电子中提取能量 并将其转化为高能NADH和更多ATP分子， 经过多步酶促反应后，生成两个 三碳糖丙酮酸，并准备将其运至 线粒体进入到柠檬酸循环中。

什么是糖酵解？

细胞通过分解大分子来产生能量。细胞呼吸是转换“食物能量”的生化过程（从大分子的化学键）转化为三磷酸腺苷（ATP）形式的化学能。糖酵解是这个严密而复杂的过程的第一步。糖酵解一词起源于拉丁语glyco（糖）和lysis（分解）。糖酵解具有两种主要的细胞内功能：产生ATP和中间代谢物以进入其它途径。糖酵解途径将一个己糖（六碳碳水化合物，如葡萄糖）转化为两个丙糖分子（三碳碳水化合物），如丙酮酸，两个ATP分子（四个产生，两个消耗）和两分子烟酰胺腺嘌呤二核苷酸（NADH）。

<h4>糖酵解的解释</h4>
 
你知道糖酵解是第一个发现的生化途径吗？19世纪中期，Louis Pasteur确定微生物在缺氧（发酵）的情况下会导致葡萄糖分解。1897年，Eduard Buchner发现，发酵反应仍然可以在无细胞酵母提取物中进行，通过打破细胞并收集含有可溶性分子和细胞器的细胞质来实现。在1905年之后不久，Arthur Harden和William Young发现，不添加无机磷酸盐( Pi )发酵速度会降低，并且发酵需要同时存在一个热敏性组分（后来发现含有许多酶）和一个低分子量、热稳定性组分（无机离子、ATP, ADP和辅酶如NAD)。1940年，在许多人的努力下，Gustav Embden、Otto Meyerhof、Jakub Karol PaRNAs等人建立了完整的糖酵解途径。事实上，糖酵解现在被称为EMP途径。
 
<h4>葡萄糖的命运</h4>
 
葡萄糖可以通过两种方式进入细胞：通过一组称为GLUT（葡萄糖转运蛋白）的整合蛋白促进扩散，GLUT（葡萄糖转运蛋白）将葡萄糖转运到细胞溶胶中。GLUT蛋白家族的成员存在于整个人体的特定组织中。或者，次级活性转运通过跨膜转运体蛋白使葡萄糖逆其浓度梯度移动。转运体利用从离子泵出的电化学能量。例如小肠、心脏、大脑和肾脏中的钠-葡萄糖连接转运蛋白。
 
在有氧( O2 丰富) 和厌氧 (O2缺乏) 条件下，一旦葡萄糖进入细胞胞质，糖酵解就可以开始。糖酵解有两个主要阶段。第一阶段需要能量，被认为是一个准备步骤，将葡萄糖捕获在细胞中，重组六个碳骨架，使其能够有效地被切割。第二阶段是放线阶段，释放能量并产生丙酮酸。
 
<h4>丙酮酸的归宿</h4>
 
根据氧水平和线粒体的存在，丙酮酸可能有两种可能的命运之一。在有氧条件下，随着线粒体的存在，丙酮酸进入线粒体，经历柠檬酸循环和电子传递链（ETC）被氧化成CO2, H2O, 甚至更多ATP。相反，在厌氧条件下（即工作肌肉）或缺乏线粒体（即原核生物），丙酮酸经历乳酸发酵（即在厌氧条件下被还原为乳酸）。有趣的是，在厌氧条件下，酵母和一些细菌可以通过酒精发酵过程将丙酮酸转化为乙醇。
 
<h4>糖酵解调节</h4>
 
严格控制和调节酶介导的代谢途径，如糖酵解，对生物体的正常功能至关重要。通过底物限制或酶联调节来实施控制。当细胞内底物和产物的浓度接近平衡时，就会出现底物限制。因此，底物的可用性决定了反应的速率。在酶联调节中，底物和产物的浓度离平衡点很远。酶的活性决定了反应的速度，它控制着整个路径的流量。在糖酵解过程中，三种调节酶是己糖激酶、磷酸果糖激酶和丙酮酸激酶。

概述  
细胞通过分解大分子来产生能量。细胞呼吸是一种生物化学过程，可将“食物中的能量”（来自……）转化为可用的化学能。

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生物学

Cellular Processes

细胞结构与功能

Cellular Respiration

细胞呼吸

KEY TERMS AND CONCEPTS

关键术语与概念

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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JoVE Core系列通过300多节动画视频课简明易懂地讲解生物学中的关键概念，以及150余段视频展示科学家在现代实验室开展实际研究实验，让生物学变得平易近人。

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Glycolysis: How Cells Break Down Glucose

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