Name: plant carbon fixation in hot climates
Uploaded: 2019-03-11T09:21:13.000Z
Duration: 1 min 27 s
Channel: JoVE

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C4 Pathway and CAM

צמחים מסוימים, כגון תירס, סיגלו לעצמם דרכים אלטרנטיביות לקיבוע פחמן ביעילות רבה בסביבות חמות ויבשות. בדרך אחת, מסלול C4, במהלך שלב קיבוע הפחמן הראשון, פחמן דו-חמצני, CO2, נכנס לתאי מזופיל ובאמצעות האנזים PEP קרבוקסילאז מתאחד עם תרכובת PEP בעלת שלושה פחמנים ליצירת תרכובת אוקסלואצטט בעלת ארבעה פחמנים, שאז מומרת לחומצה אורגנית אחרת, במקרה זה, מאלט. מאלט מועבר פנימה לתוך תאי נדן הצרור עמוק בתוך העלה, שם החמצן מוגבל, ומפורק לשחרור CO2, שיכול אז להמשיך דרך מעגל קלווין ולהתחבר עם רוביסקו כדי לבסוף לייצר סוכרים.צמחים אחרים, כגון אננס, משתמשים במסלול מטבוליזם CAM חומצת הטבוריתיים, או, כדי לקבע פחמן. צמחי CAM פותחים את הפיוניות שלהם רק בלילה, כדי למנוע אובדן מים במהלך היום. בדומה למסלול C4, ה-CO2 קודם מקובע על-ידי PEP קרבוקסילאז לאוקסלואצטט, אשר בתורו מומר למאלט.אך במקום להעביר מאלט לחלק אחר של העלה, צמחי CAM מאחסנים תרכובת זו בבועיות תאי מזופיל ומשחררים אותו במהלך היום, כך שמעגל קלווין יכול להמשיך יחד עם תגובות האור בפוטוסינתזה.

<ol>
	<li>אדוארדס, אי.ג'יי מסלולים אבולוציוניים, נגישות ומטאפורות אחרות: המקרה של פוטוסינתזה C4 ו- CAM. פיטולוג חדש. 223 (4), 1742-1755 (2019).</li>
</ol>

מסלc4 ו-cam

<div style="direction:rtl">רוב הצמחים משתמשים במסלול C3 לקיבוע פחמן. עם זאת, צמחים מסוימים, כגון קני סוכר, תירס וקקטוסים שגדלים בתנאים חמים, משתמשים בדרכים חלופיות כדי לתקן פחמן ולחסוך אובדן אנרגיה עקב photorespiration. Photorespiration הוא התהליך המתרחש כאשר ריכוז החמצן גבוה. בתנאים כאלה, האנזים רוביסקו במחזור קלווין קושר O2 במקום CO2, מה שעוצר את הפוטוסינתזה וצורך אנרגיה. מסלול C4 מסלול C4 משמש צמחים כמו תירס וקנה סוכר. בשלב הראשון, CO2 חודר לתאי המזופיל, והאנזים פוספונולפירובאט קרבוקסילא (קרבוקסילאז PEP ) מוסיף אותו לתרכובת 3-פחמן PEP ליצירת תרכובת 4-פחמן אוקסלואצטט. אז אוקסלואצטט הופך לחומצות אורגניות, בעיקר מלאט. לאחר מכן מועבר מלאט לתוך תאי מעטפת הצרף בעומק העלה, שם ריכוז החמצן נמוך. כאן המלאט מתפרק כדי לשחרר מולקולה של CO2. לאחר מכן, CO2 נכנס למחזור קלווין ומומר לסוכר בעזרת האנזים רוביסקו. בתנאים חמים וצחיחים, ההפרדה הפיזית של קיבוע הפחמן ומעגל קלווין היא יתרון, שכן הצמחים יכולים לסגור את הפיוניות שלהם כדי לחסוך במים. כתוצאה מכך, הם יכולים לשמור על ריכוז החמצן נמוך ולכן, מעדיפים את הקישור של CO2 לרוביסקו ולא ל-O2. מסלול CAM צמחים אחרים, כגון קקטוסים ואננס, משתמשים במסלול המטבוליזם של חומצת הטבוריתיים (CAM) כדי לתקן פחמן. צמחי CAM פותחים את הפיוניות שלהם בלילה כדי למנוע איבוד מים בימים חמים. ברגע שה-CO2 נכנס לתאי המזופיל, הוא מתחבר עם PEP ליצירת אוקסלואצטט ובסופו של דבר מלאט. לאחר מכן מלאט מאוחסן בחלוליות עד למחרת, כאשר הוא משתחרר כדי להיכנס למעגל קלווין. אז, קיבוע הפחמן מתרחש בלילה, ומעגל קלווין, הניזון על ידי התגובות התלויות באור, ממשיך במהלך היום. באופן זה, צמחי CAM מפרידים בין קיבוע פחמן וסינתזה של סוכר בזמנים שונים של היום.</div>

מסלC4 ו-CAM

רוב הצמחים משתמשים במסלול C3 לקיבוע פחמן. עם זאת, צמחים מסוימים, כגון קנה סוכר, תירס, קקטוסים הגדלים בתנאים חמים, להשתמש חלופי ...

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KEY TERMS AND CONCEPTS

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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 action of PEP carboxylase. This forms the 4-carbon compound oxaloacetate, which is then changed into organic acids, mostly malate.

Malate moves into the bundle sheath cells deep in the leaf, where oxygen levels are low. There, malate is broken down and releases CO2. That CO2 enters the Calvin cycle and is turned into sugar with the help of RuBisCo. In hot, dry conditions, this separation of carbon fixing and sugar production helps the plant close its stomata and save water while keeping oxygen low.

Other plants, such as cacti and pineapple, use the CAM pathway, or crassulacean acid metabolism. CAM plants open their stomata at night to limit water loss during the day. CO2 enters the mesophyll cells at night and combines with PEP to form oxaloacetate and then malate.

The malate is stored in vacuoles until the next day. During the day, it is released so the Calvin cycle can begin. In CAM plants, carbon fixation happens at night, while sugar synthesis follows during the day with energy from the light-dependent reactions. This time-based separation helps these plants survive hot, dry environments.

Plant Carbon Fixation in Hot Climates

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.

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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.

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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

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...

c4 pathway and cam

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

Learn how C4 and CAM pathways help plants like corn, sugarcane, cacti, and pineapple reduce photorespiration and save water in hot climates.

Plant Carbon Fixation in Hot Climates | JoVE K12

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Plant Carbon Fixation in Hot Climates

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