Name: carbon fixation in photosynthesis
Uploaded: 2025-08-11T09:30:18.000Z
Duration: 1 min 40 s
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The Calvin Cycle

בצמחים אוטוטרופיים, מעגל קלווין מתחיל כשפחמן דו-חמצני שבאטמוספירה מפעפע בסופו של דבר לתוך הסטרומה של הכלורופלסט. כאן, אטום פחמן אחד מתוך ה‫פחמן הדו-חמצני הינו מתווסף, או מקובע, למולקולת סוכר קולטת בעלת חמישה פחמנים, ריבולוז ביספוספט, או RuBP, בתגובה המזורזת על-ידי האנזים ריבולוז 5, 1-ביספוספט-קרבוקסילאז-אוקסיגנאז, או בקיצור RuBisCo. המולקולה בעלת שישה פחמנים המתקבלת היא מאוד לא יציבה, ומתפצלת לשתי מולקולות בעלות שלושה פחמנים 3PGA של חומצה 3-פוספוגליצרידית.עם ה-ATP שמספק את האנרגיה, וה-NADPH המצרף מימן אחד לכל אחת, שרשראות ה-3PGA מומרות לעוד מרכיב ביניים בעל שלושה פחמנים שנקרא גליצראלדהיד-3-פוספט. G3P אחד יוצא אז מהמעגל, ומחכה לאחד נוסף, כדי לבנות גלוקוז בעל שישה אטומי פחמן. בינתיים, G3P הנותרים חייבים לחכות ארבעה מחזורים נוספים, בעוד הפחמנים נצברים וה-ATP מספק אנרגיה נוספת כדי ליצור את הקולטים RuBP.בסך הכל, שישה סבבים של מעגל קלווין מקבעים שש מולקולות פחמן דו-חמצני מהאטמוספירה, תוך שימוש באנרגיה ובכוח החיזור של 18 ATP, ו-12 NADPH, בהתאמה, כדי ליצור מולקולה אחת של גלוקוז, ולבנות מחדש את RuBP להמשך הלולאה.

Michelet, Laure, Mirko Zaffagnini, Samuel Morisse, Francesca Sparla, María Esther Pérez-Pérez, Francesco Francia, Antoine Danon, et al. "תקנת חיזור של מחזור קלווין-בנסון: משהו ישן, משהו חדש." גבולות במדעי הצמח 4 (2013). [<a href="https://doi.org/10.3389/fpls.2013.00470" target="__blank">מקור</a>]

שרקי, תומאס ד. ושון א. וייס. "שאנט גלוקוז 6-פוספט סביב מחזור קלווין-בנסון." כתב העת לבוטניקה ניסיונית 67, מס '14 (1 ביולי 2016): 4067–77. [<a href="https://doi.org/10.1093/jxb/erv484" target="__blank">מקור</a>]

מעגל קלווין

<div style="direction:rtl"><h4>סקירה כללית</h4> פוטוסינתזה של חמצן ממירה כ-200 מיליארד טון של פחמן דו-חמצני (CO2) מדי שנה לתרכובות אורגניות ומייצרת כ-140 מיליארד טון של חמצן אטמוספרי (O2). פוטוסינתזה היא הבסיס לכל צורכי המזון והחמצן האנושיים. ניתן לחלק את תהליך הפוטוסינתזה לשתי קבוצות של תגובות המתרחשות באזורים שונים של כלורופלסטים צמחיים: התגובה תלוית האור והתגובות הבלתי תלויות באור או “החושך”. התגובה התלויה באור מתרחשת בקרום התילקואיד של הכלורופלסט. הוא ממיר אנרגיית אור לאנרגיה כימית, המאוחסנת כ-ATP ו-NADPH. אנרגיה זו מנוצלת לאחר מכן באזור הסטרומה של הכלורופלסט, כדי להפחית פחמן דו חמצני אטמוספרי לפחמימות מורכבות באמצעות התגובות הבלתי תלויות באור של מעגל קלווין-בנסון. <h4>מעגל קלווין-בנסון</h4> מעגל קלווין-בנסון מייצג את מערך התגובות הפוטוסינתטיות שאינן תלויות באור. הוא משתמש באדנוזין טריפוספט (ATP) ובניקוטינמיד-אדנין דינוקלאוטיד פוספט (NADPH) שנוצרו במהלך התגובות התלויות באור כדי להמיר CO2 אטמוספרי לפחמימות מורכבות. מעגל קלווין-בנסון גם מחדש את אדנוזין דיפוספט (ADP) ו-+^NADP עבור התגובה התלויה באור. בתחילת מעגל קלווין-בנסון, CO2 אטמוספרי חודר לעלה דרך פתחים הנקראים פיוניות. באזור הסטרומה של הכלורופלסט, האנזים ריבולוז-1,5-ביספוספט קרבוקסילאז/אוקסיגנאז (RuBisCO) מוסיף אטום פחמן אחד מ-CO2 למולקולת סוכר מקבלת 5 פחמן ריבולוז-1,5- ביספוספט (RuBP). מולקולת 6C שהתקבלה היא מאוד לא יציבה ומתפצלת לשתי מולקולות של חומצה 3-פוספוגליצרית (3-PGA). האנזים 3- פוספוגליצראט קינאז משתמש ב-ATP כדי לזרחן מולקולות 3-PGA אלה ליצירת 1,3- גליצרלדהיד 3-פוספט דהידרוגנאז. ביספוספוגליצרט משתמש ב-NADPH כדי להפחית מולקולות אלו ליצירת גליצרלדהיד 3-פוספט (G3P), סוכר 3C. מוצר סופי זה מוליד את השם קיבוע פחמן C3 - כינוי למעגל קלווין-בנסון. כדי לתקן שש מולקולות CO2, מעגל קלווין-בנסון מפחית 12 מולקולות NADPH ו-18 ATP. מקורות אנרגיה אלה מתחדשים על ידי התגובות התלויות באור של פוטוסינתזה. שש ה-CO2 מחוברות לשש מולקולות 5C (RuBP) המתפרקות ל-12 מולקולות 3C (G3P). עשר ממולקולות G3P אלו מחדשות שש מולקולות של מקבל ה-RuBP, כדי להמשיך את המחזור. שתי מולקולות של G3P מומרות לגלוקוז אחד. G3P עשוי לשמש גם לסינתזה של פחמימות אחרות, חומצות אמינו ושומנים.</div>

סקירה

פוטוסינתזה חמצן ממירה כ -200 מיליארד טונות של פחמן דו חמצני (CO2) מדי שנה לתרכובות אורגניות ומייצרת כ ...

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KEY TERMS AND CONCEPTS

מונחים ומושגים מרכזיים

carbon fixation in photosynthesis

Photosynthesis uses light energy to turn carbon dioxide into carbohydrates. It also releases oxygen as a by-product. These products help support most food webs and make aerobic life possible.

Photosynthesis has two main stages, and they happen in different parts of the chloroplast. The light-dependent reactions take place in the thylakoid membranes. Chlorophyll absorbs light energy there and helps split water molecules. This releases O2 and makes ATP and NADPH.

The light-independent reactions happen in the stroma, where the Calvin-Benson cycle operates. This cycle uses ATP and NADPH to reduce atmospheric CO2 into carbohydrates. After they are used, ADP and NADP+ return to the thylakoids so the light reactions can recycle them.

The Calvin-Benson cycle has three phases: carbon fixation, reduction, and regeneration of the CO2 acceptor, RuBP. CO2 enters the leaf through stomata. In the stroma, the enzyme RuBisCO attaches each CO2 molecule to ribulose-1,5-bisphosphate, or RuBP. This forms an unstable six-carbon intermediate that quickly splits into two molecules of 3-phosphoglycerate, or 3-PGA.

During reduction, each 3-PGA molecule is phosphorylated by ATP to form 1,3-bisphosphoglycerate. NADPH then reduces it to glyceraldehyde-3-phosphate, or G3P, a three-carbon sugar. For every six CO2 molecules fixed, two G3P molecules leave the cycle. These molecules help form glucose and other organic compounds.

In the final regeneration phase, the remaining G3P molecules are rearranged to rebuild RuBP. This step uses additional ATP. Regenerating RuBP lets the cycle continue fixing CO2.

Carbon Fixation in Photosynthesis

what is photosynthesis?

Photosynthesis in Chloroplasts and Leaves

Photosynthesis happens in plants and in some bacteria. It is the process that captures carbon dioxide and solar energy to make glucose. Glucose stores chemical energy as carbohydrates. The overall biochemical equation is 6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2. Photosynthesis also releases oxygen into the atmosphere and helps maintain Earth’s atmospheric oxygen content. Photosynthetic reactions take place in chloroplasts, which are specialized membrane-enclosed parts of the plant cell.

light as energy

How Plant Pigments Capture Light

Photosynthesis uses light energy from the sun, which is a form of electromagnetic radiation. The full range of possible wavelengths is called the electromagnetic spectrum. Light is made of photons, which are small packets of energy. Photons act like waves and are described by frequency, wavelength, and amplitude. Waves with higher frequencies carry more energy and have shorter wavelengths. Waves with lower frequencies carry less energy and have longer wavelengths. For most organisms,...

anatomy of chloroplasts

Chloroplasts and Photosynthesis in Plant Cells

Chloroplasts are the organelles where photosynthesis takes place in green algae and plants. They are also found in green stems and unripe fruit. In plants, mesophyll cells in the leaf contain the highest density of chloroplasts. Each chloroplast has a double membrane. The outer membrane faces the plant cell cytoplasm on one side and the chloroplast intermembrane space on the other. The inner membrane separates that narrow space from the stroma, which is the watery interior of the chloroplast.

photosystem ii

Photosystem II and Water Splitting

Photosystem II is a multi-protein complex that captures light energy and begins the flow of electrons in photosynthesis. It uses bound pigments to harvest photons and send that energy to its reaction center. From there, the energy continues through the electron transport chain and on to photosystem I. The pigments in photosystem II include chlorophyll a, chlorophyll b, and carotenoids. They are arranged in two parts of the photosystem: the antenna complex and the reaction center. The antenna...

photosystem i

Photosystem I and NADPH Formation

Photosystem I helps convert light energy into NADPH during photosynthesis. It works with photosystem II in the thylakoid membrane of the chloroplast, but it uses a different electron supplier and a different electron acceptor. Electrons excited in photosystem II move to photosystem I through an electron transport chain. That chain includes plastoquinone, the dual-protein cytochrome complex, and plastocyanin. As the electrons travel between the two photosystems, they lose energy and must be...

the calvin cycle

Photosynthesis uses light energy to turn carbon dioxide into carbohydrates. It also releases oxygen as a by-product. These products help support most food webs and make aerobic life possible. Photosynthesis has two main stages, and they happen in different parts of the chloroplast. The light-dependent reactions take place in the thylakoid membranes. Chlorophyll absorbs light energy there and helps split water molecules. This releases O2 and makes ATP and NADPH. The light-independent reactions...

c4 pathway and cam

Plant Carbon Fixation in Hot Climates

Plant carbon fixation in hot climates can follow the C4 pathway or the CAM pathway. These alternative pathways help some plants reduce energy loss from photorespiration. Photorespiration happens when oxygen levels are high and rubisco, the enzyme in the Calvin cycle, binds O2 instead of CO2. When that happens, photosynthesis slows and energy is used up. The C4 pathway is used by plants such as corn and sugarcane. In mesophyll cells, CO2 first joins with PEP, or phosphoenolpyruvate, through the...

key terms and concepts

Explore how the Calvin-Benson cycle fixes carbon dioxide into sugars, using ATP and NADPH in the chloroplast stroma during photosynthesis.

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Carbon Fixation in Photosynthesis

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