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

تحلل السكر هو عملية التمثيل الغذائي لتحطيم السكريات لتوليد الطاقة. يدخل الجلوكوز أولاًإلى السيتوبلازم في الخلية ويتحول عبر مرحلتين رئيسيتين. يتطلب الجزء الأول طاقة في شكل أدينوسين ثلاثي الفوسفات لاحتجاز جزيء الجلوكوز بالداخل وتعديله من خلال سلسلة من الخطوات.تستخرج المرحلة الثانية الطاقة من الإلكترونات وتُسخّرها كجزيئات ثنائي نوكليوتيد النّيكوتين والأدنين المختزل عالية الطاقة والمزيد من جزيئات بعد خطوات إنزيمية متعددة يتم إنتاج سكريات ثلاثية الكربون البيروفات وتكون جاهزة للنقل إلى الميتوكوندريا للدخول في دورة حمض الستريك.

ما هو تحلل الجلوكوز؟

<h4>نظره عامه</h4>

تنتج الخلايا الطاقة عن طريق تكسير الجزيئات الكبيرة. التنفس الخلوي هو عملية كيميائية حيوية تحول ”الطاقة الغذائية“ (من الروابط الكيميائية للجزيئات الكبيرة) إلى طاقة كيميائية على شكل أدينوسين ثلاثي الفوسفات (ATP). الخطوة الأولى في هذه العملية المعقدة والمنظمة بإحكام هي تحلل السكر. نشأت كلمة تحلل السكر من اللغة اللاتينية glyco (سكر) و lysis (تفكك). يخدم تحلل السكر وظيفتين رئيسيتين داخل الخلايا: توليد أدينوسين ثلاثي الفوسفات ومستقلبات وسيطة لتغذية المسارات الأخرى. يحول مسار التحلل السكري واحداً من الهكسوز (ستة كربوهيدرات مثل الجلوكوز) ، إلى جزيئين ثلاثي (كربوهيدرات ثلاثي الكربون) مثل البيروفات ، وناتج صافي من جزيئين من أدينوسين ثلاثي الفوسفات (أربعة منتجة ، واثنان مستهلكان) وجزيئين من نيكوتيناميد ثنائي النيوكليوتيد الأدينين (NADH).

<h4>توضيح تحلل السكر</h4>

هل تعلم أن تحلل السكر كان أول مسار كيميائي حيوي تم اكتشافه؟ في منتصف القرن التاسع عشر ، قرر لويس باستور أن الكائنات الحية الدقيقة تسبب تكسير الجلوكوز في غياب الأكسجين (التخمير). في عام ١٨٩٧ ، وجد إدوارد بوخنر أنه لا يزال من الممكن إجراء تفاعلات التخمير في مستخلصات الخميرة الخالية من الخلايا ، والتي يتم تحقيقها عن طريق كسر الخلية وتجميع السيتوبلازم الذي يحتوي على الجزيئات والعضيات القابلة للذوبان. بعد ذلك بوقت قصير في عام ١٩٠٥ ، اكتشف آرثر هاردن و ويليام يونغ أن معدل التخمر يتناقص دون إضافة الفوسفات غير العضوي (Pi) وأن التخمير يتطلب وجود مكونين حساسين للحرارة (تم تحديد لاحقاً احتوائهم على عدد من الإنزيمات) و وزن جزيئي منخفض ، وجزء مستقر للحرارة (أيونات غير عضوية ، أدينوسين ثنائي الفوسفات ، أدينوسين ثلاثي الفوسفات وأنزيمات مساعدة مثل نيكوتيناميد ثنائي النيوكليوتيد). بحلول عام ١٩٤٠ ، وبجهود العديد من الأفراد ، تم إنشاء المسار الكامل لتحلل السكر بواسطة جوستاف امبدن و اوتتو مايرهوف و جاكوب كارول بارناس و وآخرون. في الواقع ، يُعرف تحلل السكر الآن باسم مسار امبدن_مايرهوف_بارناس (EMP).


<h4>مصير الجلوكوز</h4>

يمكن أن يدخل الجلوكوز الخلايا بطريقتين: الانتشار الميسر عبر مجموعة من البروتينات المتكاملة تسمى بروتينات ناقل الجلوكوز (GLUT) التي تنقل الجلوكوز إلى العصارة الخلوية. توجد أعضاء عائلة بروتين ناقل الجلوكوز في أنسجة معينة في جميع أنحاء جسم الإنسان. بدلاً من ذلك ، يحرك النقل النشط الثانوي الجلوكوز مقابل تدرج تركيزه بواسطة بروتين مُراحِل عبر الغشاء. يستخدم المُراحِل الطاقة الكهروكيميائية من ضخ أيون. ومن الأمثلة على ذلك ، الناقلات المرتبطة بالجلوكوز والصوديوم في الأمعاء الدقيقة والقلب والدماغ والكلى.

تحت كل من الظروف الهوائية (الغنية بـ O2) و اللاهوائية (نقص الـ O2) ، يمكن أن يبدأ تحلل الجلوكوز بمجرد دخول الجلوكوز إلى العصارة الخلوية للخلية. هناك مرحلتان رئيسيتان من تحلل السكر. تتطلب المرحلة الأولى الطاقة وتعتبر خطوة تمهيدية ، حيث تحبس الجلوكوز في الخلية وتعيد هيكلة العمود الفقري المكون من ستة كربون بحيث يمكن شقها بكفاءة. المرحلة الثانية هي مرحلة السداد ، حيث يتم إطلاق الطاقة وتوليد البيروفات.

<h4>مصير البيروفات</h4>

اعتماداً على مستوى الأكسجين و وجود الميتوكوندريا ، قد يكون للبيروفات أحد مصيرين محتملين. في ظل الظروف الهوائية ، مع وجود الميتوكوندريا ، يدخل البيروفات الميتوكوندريا ، ويخضع لدورة حامض الستريك ، وتتأكسد سلسلة نقل الإلكترون (ETC) إلىCO2 ، H2O و حتى أدينوسين ثلاثي الفوسفات أكثر. و على النقيض من ذلك ، في ظل الظروف اللاهوائية (أي عضلات عاملة) أو عدم و جود الميتوكوندريا (أي بدائيات النوى) ، يخضع بيروفات لتخمير اللاكتات (أي ، يتم اختزاله إلى لاكتات في ظروف لا هوائية). و من المثير للاهتمام ، الخميرة و بعض البكتيريا في ظل الظروف لا هوائية يمكن تحويل بيروفات إلى الإيثانول من خلال عملية تعرف باسم تخمير الكحول.

<h4>تنظيم تحلل السكر</h4>

يعد التحكم الدقيق وتنظيم مسارات التمثيل الغذائي التي تتوسطها الإنزيمات ، مثل تحلل السكر ، أمراً بالغ الأهمية من أجل الأداء السليم للكائن الحي. يتم التحكم عن طريق تقييد الركيزة أو التنظيم المرتبط بالإنزيم. يحدث تقييد الركيزة عندما يكون تركيز الركيزة والمنتجات في الخلية قريبة من التوازن. وبالتالي ، فإن توفر الركيزة يحدد معدل التفاعل. في التنظيم المرتبط بالإنزيم ، يكون تركيز الركيزة والمنتجات بعيداً عن التوازن. يحدد نشاط الإنزيم معدل التفاعل الذي يتحكم في تدفق المسار الكلي. في تحلل السكر ، فإن الإنزيمات التنظيمية الثلاثة هي هيكسوكيناز ، فسفوفركتوكيناز ، وبيروفات كيناز.

نظره عامه
تنتج الخلايا الطاقة عن طريق تكسير الجزيئات الكبيرة. التنفس الخلوي هو العملية الكيميائية الحيوية التي تحول "الطاقة الغذائية" (من ...

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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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