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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Earth’s Systems

HS-ESS2-6

C4 Pathway and CAM

قامت بعض النباتات مثل الذرة بتكييف طرق بديلة لمعالجة الكربون بشكل أكثر كفاءة في البيئات الحارة والجافة. ففي أحدهما،يقوم مسار C4 خلال المرحلة الأولى لتمثيل الكربون،وهي ثاني أكسيد الكربون CO2،بالدخول إلى خلايا الميزوفيل وبواسطة إنزيم PEP كاربوكسيلاس يجتمع مع مركب الكربون الحر،PEP،ليكون مركب أوكسالوآسيتات رباعي الكربون،والذي يتحول بعدها إلى حمض معدني عضوي. ويدعى هنا مالات.يتم نقل المالات إلى خلايا الورقة المتجمعة في أعماق الورقة حيث الأسجين محدود ويتم تكسيره،مطلقاًغاز ثاني اكسيد الكربون،والذي يستمر بعدها خلال دورة كالفين وتتم مباشرته بإنزيم RuBisCO لينتج السكر في النهاية. نباتات أخرى كالأناناس تستخدم مسار استقلاب حمض الكراسولاسيان أو مسار CAM كي تعالج الكربون. تفتح نباتات CAM ثغورها ليلاًفقط لتمنع فقد الماء خلال اليوم.وفي حالة مشابهة لمسار C4 يتم معالجة CO2 أولاًبواسطة إنزيم PEP كاربوكيلاس إلى أوكسالوآسيتات والتي تتحول بدورها إلى مالات. وبدلاًمن نقل المالات إلى جزء أخر من الورقة تخزن نباتات CAM هذا المركب في فجوات خلية الميزوفيل مطلقة إياها خلال اليوم حتى تتمكن دورة كالفين من الإستمرار في عملها بمحازاة تفاعلات الضوء خلال عملية البناء الضوئي.

<ol>
	<li>إدواردز ، إي جيه المسارات التطورية وإمكانية الوصول والاستعارات الأخرى: حالة التمثيل الضوئي C4 و CAM. عالم نباتات جديد. 223 (4) ، 1742-1755 (2019).</li>
</ol>

مسار c4 و cam

<h4>نظره عامه</h4>

بعض النباتات ، مثل قصب السكر و الذرة ، التي تنمو في الظروف الحارة ، و تستخدم عملية بديلة تسمى مسار C4 لإصلاح الكربون. تبدأ الدورة بـ CO2 من الغلاف الجوي الذي يدخل خلايا الميزوفيل حيث يتم استخدامه لتوليد اوكسالاسيتات _ جزيء رباعي الكربون _ من فوسفوينول بيروفات (PEP). ثم يتم تحويل اوكسالاسيتات إلى مالات و نقلها إلى خلايا غلاف الحزمة ، حيث يكون تركيز الأكسجين منخفضاً. هناك ، يتم تحرير ثاني أكسيد الكربون CO2 من مالات ويدخل في دورة كالفن حيث يتم تحويله إلى سكريات. يتم تنفيذ مسار الطبابة البديلة في نباتات مثل الصبار التي تحتاج أيضاً إلى الحفاظ على المياه أثناء النهار. تترك نباتات استقلاب حمض الكراسولاسين ثاني أكسيد الكربون CO2 في الأوراق ليلاً وتنتج مالات يتم تخزينها في فجوات حتى اليوم التالي. ثم يتم تحرير المالات من الفجوات ومعالجتها في دورة كالفين. يفصل مسار C4 بين العمليات المختلفة محلياً ، بينما يفصل مسار استقلاب حمض الكراسولاسين بينها زمنياً.

<h4>مسار C4</h4>

طورت بعض النباتات ، مثل الذرة وقصب السكر ، طرقاً بديلة لإصلاح الكربون الذي يساعد على تجنب فقد الماء في البيئات الحارة والجافة. أحد هذه الطرق هو مسار C4. في الخطوة الأولى ، يدخل ثاني أكسيد الكربون CO2 إلى خلايا الميزوفيل ، ويضيفه إنزيم فوسفوينول بيروفات الكربوكسيلاز إلى المركب ثلاثي الكربون فوسفوينول بيروفات ليشكل مركب اوكسالاسيتات رباعي الكربون. ثم يتم تحويل اوكسالاسيتات إلى حمض عضوي يسمى مالات.

عد ذلك ، يتم نقل مالات إلى خلايا غمد الحزمة في أعماق الورقة حيث يكون تركيز الأكسجين منخفضاً. يتفكك مالات ، ويطلق جزيء CO2 الذي يدخل بعد ذلك دورة كالفن حيث يحولها إنزيم روبيسكو إلى سكر. يوفر مسار C4 ميزة في الظروف الحارة والجافة حيث ستغلق النباتات ثغورها للحفاظ على المياه. نتيجة لذلك ، يمكنهم الحفاظ على تركيز الأكسجين منخفضاً وبالتالي يفضلون ربط CO2 إلى روبيسكو بدلا من O2. عندما يكون تركيز الأكسجين أعلى ، فإن روبيسكو يربط O2 بدلاً من CO2 _ و هي عملية يطلق عليها التنفس الضوئي _ و التي من شأنها أن توقف التمثيل الضوئي و تستهلك الطاقة.

<h4>مسار استقلاب حمض الكراسولاسين (CAM)</h4>

تستخدم نباتات أخرى ، مثل الصبار والأناناس ، مسار استقلاب حمض الكراسولاسين (CAM) لإصلاح الكربون. تفتح نباتات استقلاب حمض الكراسولاسين ثغورها بشكل أساسي ليلاً لمنع فقدان الماء خلال النهار الحار. في الليل يدخل ثاني أكسيد الكربون CO2 الى خلايا الميزوفيل ، حيث يتحد مع فوسفوينول بيروفات لتكوين أوكسالاسيتات وفي النهاية مالات. ثم يتم تخزين مالات في فجوات حتى اليوم التالي عندما يتم تحريره من الفجوات ويدخل في دورة كالفين. تستمر المراحل الأولى من عملية التمثيل الضوئي خلال النهار لأنها تعتمد على الضوء ، بينما تحدث التفاعلات المستقلة عن الضوء لدورة كالفين أثناء الليل. وبهذه الطريقة ، تفصل CO2 تثبيت ثاني أكسيد الكربون وتخليق السكر باستخدام أوقات مختلفة من اليوم.

مسار C4 و CAM

تستخدم معظم النباتات مسار C3 لتثبيت الكربون. ومع ذلك ، فإن بعض النباتات ، مثل قصب السكر والذرة والصبار التي تنمو في الظروف الحارة ، تستخدم بديلا ...

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المصطلحات والمفاهيم الأساسية

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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photosystem i

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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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