8.3: Kutumia Nishati ya Mwanga kufanya Molekuli za kikaboni

Ujuzi wa Kuendeleza

• Eleza mzunguko wa Calvin
• Eleza fixation kaboni
• Eleza jinsi photosynthesis inavyofanya kazi katika mzunguko wa nishati ya viumbe hai vyote

Baada ya nishati kutoka jua kugeuzwa kuwa nishati ya kemikali na kuhifadhiwa kwa muda katika molekuli za ATP na NADPH, kiini kina mafuta yanayotakiwa kujenga molekuli za kabohaidreti kwa ajili ya kuhifadhi nishati ya muda mrefu. Bidhaa za athari za kutegemea mwanga, ATP na NADPH, zina maisha katika millionths ya sekunde, ambapo bidhaa za athari za kujitegemea za mwanga (wanga na aina nyingine za kaboni iliyopunguzwa) zinaweza kuishi kwa mamia ya mamilioni ya miaka. Molekuli za kabohaidreti zilizofanywa zitakuwa na mgongo wa atomi za kaboni. Je, kaboni hutoka wapi? Inatoka kwa dioksidi kaboni, gesi ambayo ni bidhaa taka ya kupumua katika microbes, fungi, mimea, na wanyama.

Calvin Cycle

Katika mimea, dioksidi kaboni (CO ₂) inaingia majani kupitia stomata, ambapo inatofautiana juu ya umbali mfupi kupitia nafasi za seli hadi kufikia seli za mesophyll. Mara moja katika seli za mesophyll, CO ₂ huenea katika stroma ya kloroplasti-tovuti ya athari za kujitegemea za usanisinuru. Athari hizi kwa kweli zina majina kadhaa yanayohusiana nao. Neno lingine, mzunguko wa Calvin, ni jina la mtu aliyeigundua, na kwa sababu athari hizi hufanya kazi kama mzunguko. Wengine huiita mzunguko wa Calvin-Benson kuingiza jina la mwanasayansi mwingine aliyehusika katika ugunduzi wake. Jina la muda mrefu zaidi ni athari za giza, kwa sababu mwanga hauhitajiki moja kwa moja (Kielelezo\(\PageIndex{1}\)). Hata hivyo, neno majibu ya giza linaweza kupotosha kwa sababu linamaanisha vibaya kwamba mmenyuko hutokea tu usiku au ni huru na mwanga, ndiyo sababu wanasayansi wengi na waalimu hawatumii tena.

Kielelezo\(\PageIndex{1}\): Mwanga athari kuunganisha nishati kutoka jua kuzalisha vifungo kemikali, ATP, na NADPH. Molekuli hizi za kubeba nishati zinafanywa katika stroma ambako fixation kaboni hufanyika.

Athari za kujitegemea za mzunguko wa Calvin zinaweza kupangwa katika hatua tatu za msingi: fixation, kupunguza, na kuzaliwa upya.

Hatua ya 1: Kurekebisha

Katika stroma, pamoja na CO ₂, vipengele vingine viwili vilipo kuanzisha athari za kujitegemea: enzyme inayoitwa ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), na molekuli tatu za bisphosphate ya ribulose (RubP), kama inavyoonekana kwenye Kielelezo\(\PageIndex{2}\). RubP ina atomi tano za kaboni, iliyozunguka na phosphates mbili.

Sanaa Connection

Kielelezo\(\PageIndex{2}\): Mzunguko wa Calvin una hatua tatu. Katika hatua ya 1, Rubisco ya enzyme inashirikisha dioksidi kaboni katika molekuli ya kikaboni, 3-PGA. Katika hatua ya 2, molekuli ya kikaboni imepunguzwa kwa kutumia elektroni zinazotolewa na NADPH. Katika hatua ya 3, RubP, molekuli inayoanza mzunguko, imerejeshwa ili mzunguko uweze kuendelea. Moja tu kaboni dioksidi molekuli ni kuingizwa kwa wakati, hivyo mzunguko lazima kukamilika mara tatu kuzalisha moja tatu-carbon GA3P molekuli, na mara sita kuzalisha sita carbon glucose molekuli.

Ni ipi kati ya kauli zifuatazo ni kweli?

1. Katika usanisinuru, oksijeni, dioksidi kaboni, ATP, na NADPH ni reactants. GA3P na maji ni bidhaa.
2. Katika photosynthesis, chlorophyll, maji, na dioksidi kaboni ni reactants. GA3P na oksijeni ni bidhaa.
3. Katika usanisinuru, maji, dioksidi kaboni, ATP, na NADPH ni reactants. RubP na oksijeni ni bidhaa.
4. Katika photosynthesis, maji na dioksidi kaboni ni reactants. GA3P na oksijeni ni bidhaa.

RubiSCO huchochea mmenyuko kati ya CO ₂ na RubP. Kwa kila molekuli ya CO ₂ ambayo humenyuka na RubP moja, molekuli mbili za kiwanja kingine (3-PGA) fomu. PGA ina kaboni tatu na phosphate moja. Kila upande wa mzunguko unahusisha RubP moja tu na dioksidi kaboni moja na huunda molekuli mbili za 3-PGA. Idadi ya atomi za kaboni inabakia sawa, kwani atomi zinahamia kuunda vifungo vipya wakati wa athari (atomi 3 kutoka atomi 3CO ₂ + 15 kutoka 3RubP = atomi 18 katika atomi 3 za 3-PGA). Utaratibu huu unaitwa fixation kaboni, kwa sababu CO ₂ ni “fasta” kutoka fomu isokaboni ndani ya molekuli za kikaboni.

Hatua ya 2: Kupunguza

ATP na NADPH hutumiwa kubadili molekuli sita za 3-PGA ndani ya molekuli sita za kemikali inayoitwa glyceraldehyde 3-phosphate (G3P). Hiyo ni mmenyuko wa kupunguza kwa sababu inahusisha faida ya elektroni kwa 3-PGA. Kumbuka kwamba kupunguza ni faida ya elektroni kwa atomu au molekuli. Molekuli sita za ATP na NADPH hutumiwa. Kwa ATP, nishati hutolewa na kupoteza atomi ya phosphate ya terminal, kuibadilisha kuwa ADP; kwa NADPH, nishati zote na atomi ya hidrojeni zinapotea, kuzibadilisha kuwa NADP ⁺. Wote wa molekuli hizi kurudi karibu athari mwanga tegemezi kutumika tena na reenergized.

Hatua ya 3: Urejesho

Kushangaza, katika hatua hii, moja tu ya molekuli ya G3P huacha mzunguko wa Calvin na hutumwa kwa cytoplasm ili kuchangia kuundwa kwa misombo mingine inayohitajika na mmea. Kwa sababu G3P iliyosafirishwa kutoka kloroplast ina atomi tatu za kaboni, inachukua “zamu” tatu za mzunguko wa Calvin kurekebisha kaboni wavu wa kutosha ili kuuza nje G3P moja. Lakini kila upande hufanya G3Ps mbili, hivyo zamu tatu kufanya G3Ps sita. Moja ni nje wakati iliyobaki G3P molekuli tano kubaki katika mzunguko na ni kutumika regenerate RubP, ambayo inawezesha mfumo kujiandaa kwa ajili ya zaidi CO ₂ kuwa fasta. Molekuli tatu zaidi za ATP hutumiwa katika athari hizi za kuzaliwa upya.

Unganisha na Kujifunza

Kiungo hiki kinasababisha uhuishaji wa mzunguko wa Calvin. Bonyeza hatua ya 1, hatua ya 2, na kisha hatua ya 3 kuona G3P na ATP regenerate kuunda RubP.

Evolution uhusiano: usanisinuru

Wakati wa mageuzi ya usanisinuru, mabadiliko makubwa yalitokea kutokana na aina ya bakteria ya usanisinuru inayohusisha photosystem moja tu na kwa kawaida ni anoxygenic (haina kuzalisha oksijeni) katika oksijeni ya kisasa (haina kuzalisha oksijeni) usanisinuru, kuajiri photosystems mbili. Photosynthesis hii ya kisasa ya oksijeni hutumiwa na viumbehai wengi—kutoka majani makubwa ya kitropiki katika msitu wa mvua hadi seli ndogo za sainobakteria-na mchakato na vipengele vya usanisinuru huu hubakia kwa kiasi kikubwa sawa. Photosystems kunyonya mwanga na kutumia minyororo ya usafiri wa elektroni kubadilisha nishati kuwa nishati ya kemikali ya ATP na NADH. Athari za kujitegemea za mwanga zinazofuata kisha hukusanyika molekuli za kabohaidreti

Photosynthesis katika mimea ya jangwa imebadilika marekebisho yanayohifadhi maji. Katika joto kali kavu, kila tone la maji linapaswa kutumika kuishi. Kwa sababu stomata lazima ifunguliwe ili kuruhusu matumizi ya CO ₂, maji hupuka kutoka kwenye jani wakati wa usanisinuru wa kazi. Mimea ya jangwa imebadilika michakato ya kuhifadhi maji na kukabiliana na hali ngumu. Matumizi bora zaidi ya CO ₂ inaruhusu mimea kukabiliana na kuishi na maji kidogo. Baadhi ya mimea kama vile cacti (Kielelezo\(\PageIndex{3}\)) can prepare materials for photosynthesis during the night by a temporary carbon fixation/storage process, because opening the stomata at this time conserves water due to cooler temperatures. In addition, cacti have evolved the ability to carry out low levels of photosynthesis without opening stomata at all, an extreme mechanism to face extremely dry periods.

Figure \(\PageIndex{3}\): The harsh conditions of the desert have led plants like these cacti to evolve variations of the light-independent reactions of photosynthesis. These variations increase the efficiency of water usage, helping to conserve water and energy. (credit: Piotr Wojtkowski)

The Energy Cycle

Whether the organism is a bacterium, plant, or animal, all living things access energy by breaking down carbohydrate molecules. But if plants make carbohydrate molecules, why would they need to break them down, especially when it has been shown that the gas organisms release as a “waste product” (CO₂) acts as a substrate for the formation of more food in photosynthesis? Remember, living things need energy to perform life functions. In addition, an organism can either make its own food or eat another organism—either way, the food still needs to be broken down. Finally, in the process of breaking down food, called cellular respiration, heterotrophs release needed energy and produce “waste” in the form of CO₂ gas.

In nature, there is no such thing as waste. Every single atom of matter and energy is conserved, recycling over and over infinitely. Substances change form or move from one type of molecule to another, but their constituent atoms never disappear (Figure \(\PageIndex{4}\)).

CO₂ is no more a form of waste than oxygen is wasteful to photosynthesis. Both are byproducts of reactions that move on to other reactions. Photosynthesis absorbs light energy to build carbohydrates in chloroplasts, and aerobic cellular respiration releases energy by using oxygen to metabolize carbohydrates in the cytoplasm and mitochondria. Both processes use electron transport chains to capture the energy necessary to drive other reactions. These two powerhouse processes, photosynthesis and cellular respiration, function in biological, cyclical harmony to allow organisms to access life-sustaining energy that originates millions of miles away in a burning star humans call the sun.

Figure \(\PageIndex{4}\): Photosynthesis consumes carbon dioxide and produces oxygen. Aerobic respiration consumes oxygen and produces carbon dioxide. These two processes play an important role in the carbon cycle. (credit: modification of work by Stuart Bassil)

Summary

Using the energy carriers formed in the first steps of photosynthesis, the light-independent reactions, or the Calvin cycle, take in CO₂ from the environment. An enzyme, RuBisCO, catalyzes a reaction with CO₂ and another molecule, RuBP. After three cycles, a three-carbon molecule of G3P leaves the cycle to become part of a carbohydrate molecule. The remaining G3P molecules stay in the cycle to be regenerated into RuBP, which is then ready to react with more CO₂. Photosynthesis forms an energy cycle with the process of cellular respiration. Plants need both photosynthesis and respiration for their ability to function in both the light and dark, and to be able to interconvert essential metabolites. Therefore, plants contain both chloroplasts and mitochondria.

Art Connections

Figure \(\PageIndex{2}\): Which of the following statements is true?

1. In photosynthesis, oxygen, carbon dioxide, ATP, and NADPH are reactants. G3P and water are products.
2. In photosynthesis, chlorophyll, water, and carbon dioxide are reactants. G3P and oxygen are products.
3. In photosynthesis, water, carbon dioxide, ATP, and NADPH are reactants. RuBP and oxygen are products.
4. In photosynthesis, water and carbon dioxide are reactants. G3P and oxygen are products.

Answer

D

Glossary

Calvin cycle

light-independent reactions of photosynthesis that convert carbon dioxide from the atmosphere into carbohydrates using the energy and reducing power of ATP and NADPH

carbon fixation

process of converting inorganic CO₂ gas into organic compounds

reduction

gain of electron(s) by an atom or molecule