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

גליקוליזה היא התהליך המטבולי של פירוק סוכרים להפקת אנרגיה. תחילה, הגלוקוז נכנס לציטופלזמת התא ועובר שינוי בשני שלבים עיקריים. השלב הראשון דורש אנרגיה בצורה של ATP, כדי ללכוד מולקולת הגלוקוז ולשנות אותה באמצעות סדרה של צעדים.השלב השני מפיק אנרגיה מאלקטרונים ורותם אותה כ-NADH בעל אנרגיה גבוהה ומולקולות ATP נוספות. לאחר צעדים אנזימטיים מרובים, נוצרים שני סוכרים בעלי שלושה פחמנים, פירובט, שמוכנים להעברה אל המיטוכונדריה לצורך כניסה למעגל חומצת הלימון.

מהי גליקוליזה?

<div style="direction:rtl"><h4>סקירה כללית</h4> תאים מייצרים אנרגיה על ידי פירוק מקרומולקולות. נשימה תאית היא התהליך הביוכימי הממיר "אנרגיית מזון" (מהקשרים הכימיים של מקרו-מולקולות) לאנרגיה כימית בצורה של אדנוזין טריפוספט (ATP). השלב הראשון בתהליך המווסת והמורכב הזה הוא הגליקוליזה. מקור המילה גליקוליזה בלטינית גליקו (סוכר) וליסיס (פירוק). הגליקוליזה משרתת שתי פונקציות תוך תאיות עיקריות: יצירת ATP ויצירת מטבוליטים ביניים להזנה למסלולים אחרים. המסלול הגליקוליטי הופך הקסוז אחד (פחמימה בעלת שישה פחמנים כגון גלוקוז) לשתי מולקולות טריוז (פחמימה תלת-פחמנית) כגון פירובט, לייצור סה"כ שתי מולקולות של ATP (ארבע מיוצרות, שתיים נצרכות) ושתי מולקולות של ניקוטינאמיד אדנין דינוקלאוטיד (NAD). <h4>הסבר על גליקוליזה</h4> באמצע שנות ה-1800, לואי פסטר קבע שמיקרואורגניזמים גורמים לפירוק הגלוקוז בהיעדר חמצן (תסיסה). בשנת 1897, אדוארד בוכנר מצא שעדיין ניתן לבצע תגובות תסיסה בתמציות שמרים נטולות תאים על ידי פתיחת התא ואיסוף הציטופלזמה המכילה את המולקולות והאברונים המסיסים. זמן קצר לאחר מכן, בשנת 1905, גילו ארתור הארדן וויליאם יאנג שקצב התסיסה יורד ללא תוספת של פוספט אנאורגני (Pi) וכי התסיסה דורשת נוכחות של שני רכיבים רגישים לחום (שזוהה מאוחר יותר כמכיל מספר אנזימים) ושבר משקל מולקולרי נמוך, יציב בחום (יונים לא אורגניים, ATP, ADP וקו-אנזימים כמו NAD). עד 1940, במאמץ של אנשים רבים, מסלול הגליקוליזה המלא הוקם על ידי גוסטב אמבדן, אוטו מאיירהוף, יאקוב קרול פרנס ואחרים. כעת, גליקוליזה ידועה כמסלול EMP. <h4>גורלו של גלוקוז</h4> ישנן שתי דרכים שבהן גלוקוז יכול להיכנס לתא. קבוצה של חלבוני GLUT (טרנספורטר גלוקוז) אינטגרליים מעבירה גלוקוז לתוך הציטוזול באמצעות דיפוזיה קלה. בני משפחת חלבוני GLUT נמצאים ברקמות ספציפיות בכל גוף האדם. לחלופין, חלבונים סימפורטרים טרנסממברניים מזיזים את הגלוקוז כנגד שיפוע הריכוז שלו באמצעות הובלה פעילה משנית. הסימפורטור משתמש באנרגיה אלקטרוכימית משאיבת יונים. דוגמאות לכך הן הטרנספורטרים הקשורים לנתרן-גלוקוז במעי הדק, הלב, המוח והכליות. בתנאים אירוביים (עשירים ב-O2) ואנאירוביים (חסרי O2), הגליקוליזה יכולה להתחיל ברגע שגלוקוז נכנס לציטוזול התא. ישנם שני שלבים עיקריים של גליקוליזה. השלב הראשון הוא שלב הכנה הדורש אנרגיה שלוכד את הגלוקוז בתא ומבנה מחדש את עמוד השדרה של שישה פחמנים כך שניתן יהיה לבקעו ביעילות. השלב השני משחרר אנרגיה ומייצר פירובט. <h4>גורלו של הפירובט</h4> בהתאם לרמת החמצן ולנוכחות המיטוכונדריה, לפירובט עשוי להיות אחד משני גורלות אפשריים. בתנאים אירוביים עם מיטוכונדריה, פירובט חודר למיטוכונדריה, עובר את מחזור חומצת הלימון ואת שרשרת הובלת האלקטרונים (ETC) כדי להתחמצן ל-CO2, H2O ועוד יותר ATP. לעומת זאת, בתנאים אנאירוביים (כלומר, שרירים עובדים) או ללא מיטוכונדריה (כלומר, פרוקריוטים), פירובט עובר תסיסה לקטט (כלומר, מופחת לקטט בתנאים אנאירוביים). מעניין, שמרים וכמה חיידקים בתנאים אנאירוביים יכולים להמיר פירובט לאתנול באמצעות תהליך המכונה תסיסת אלכוהול. <h4>ויסות של גליקוליזה</h4> בקרה וויסות הדוקים של מסלולים מטבוליים בתיווך אנזים, כגון גליקוליזה, הם קריטיים לתפקוד תקין של אורגניזם. השליטה מתבצעת על ידי הגבלת מצע או ויסות מקושר אנזים. הגבלת מצע מתרחשת כאשר ריכוז המצעים והמוצרים בתא קרוב לשיווי משקל. כתוצאה מכך, זמינות המצע קובעת את קצב התגובה. בוויסות מקושר אנזים, ריכוז המצעים והתוצרים רחוק משיווי משקל. פעילותם של שלושה אנזימים (הקסוקינאז, פוספופרוקטוקינאז ופירובאט קינאז) קובעת את קצב התגובה, השולט בשטף המסלול הכולל.</div>

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תאים מייצרים אנרגיה על ידי פירוק מקרומולקולות. נשימה תאית היא התהליך הביוכימי הממיר "אנרגיית מזון" (מ ...

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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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