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Citric Cycle : Energy Flow in Ecosystem

Aug 30, 2022

Key Concepts

  • Introduction
  • Energy flow in ecosystem
  • Citric acid cycle
  • Steps of citric acid cycle
  • Importance of citric acid cycle


Energy flow in ecosystem 

The food chain and food web facilitate the movement of energy. Plants collect sunlight with the aid of chloroplasts during the process of energy flow in the ecosystem, and a portion of it is turned into chemical energy in the process of photosynthesis. When herbivores eat (primary consumers) the plants as food, this energy is stored as various organic products in the plants and passed on to the primary consumers in the food chain. The chemical energy contained in plant products is then converted into kinetic energy, and chemical energy is degraded by heat conversion.  

The flow of energy in the ecosystem is one of the most important variables in the survival of such a large number of creatures. Solar energy is the principal source of energy for practically all species on the Earth. It’s interesting to learn that we only receive around half of the sun’s effective radiation on Earth. When we say effective radiation, we mean radiation that plants can utilize to perform photosynthesis. 

Energy flow


  • The Krebs cycle, also known as the citric acid cycle, is a series of enzyme-catalyzed activities that take place in the mitochondrial matrix and involves the oxidation of acetyl-CoA to produce carbon dioxide and the reduction of coenzymes, both of which produce ATP in the electron transport chain. 
  • The cycle is named after the scientist Hans Krebs, who proposed the detailed cycle. For his work, he received the Nobel Prize in 1953. 
Citric acid cycle 
  • It is an eight-step process that involves oxidizing the acetyl group of acetyl-CoA to create two molecules of CO2 and one molecule of ATP. High energy molecules, NADH and FADH2 are also produced.  
  • It is a frequent route for the full oxidation of carbohydrates, proteins, and lipids as they are metabolized to acetyl coenzyme A or other cycle intermediates.  
  • Acetyl CoA is produced and enters the Tricarboxylic acid cycle or the Citric acid cycle. Glucose is totally oxidized using this approach. Acetyl CoA, a 6-carbon molecule, interacts with oxaloacetate, a 4-carbon chemical, to form 6C citrate. 
 Citric acid cycle 
  • During this process, two molecules of CO2 are released, while oxaloacetate is recycled. Energy is stored in ATP and other high energy molecules such as NADH and FADH2
  • In the first phase of the cycle, acetyl CoA interacts with oxaloacetate, a four-carbon acceptor molecule, to produce citrate, a six-carbon molecule. This six-carbon molecule, after a short rearrangement, releases two of its carbons as carbon dioxide molecules in a pair of identical events, each creating a molecule of NADH. The enzymes that catalyze these processes are important regulators of the citric acid cycle, speeding it up or slowing it down depending on the energy demands of the cell. 
  • The remaining four-carbon molecule proceeds through a sequence of further processes, first producing ATP or, in certain cells, a similar molecule called GTP, then reducing the electron carrier FAD to FADH2 and ultimately producing another NADH. 
  • This sequence of events regenerates the initial molecule, oxaloacetate, allowing the cycle to continue. 


Reaction 1: Formation of Citrate  

The first process in the cycle is the condensation of acetyl-CoA with oxaloacetate to create citrate, which is mediated by citrate synthase. When oxaloacetate is combined with acetyl-CoA, a water molecule attacks the acetyl, causing coenzyme A to be released from the combination. 

Formation of citrate 

Reaction 2: Formation of Isocitrate  


By using the enzyme acontinase, the citrate is rearranged to generate an isomeric form, isocitrate. A water molecule is taken from the citric acid and then replaced in another site in this process. The –OH group is transferred from the 3′ to the 4′ position on the molecule as a result of this conversion. The molecule isocitrate is formed as a result of this reaction.

Formation of isocitrate 

Reaction 3: Oxidation of Isocitrate to α-Ketoglutarate 

Isocitrate dehydrogenase catalyzes the oxidative decarboxylation of isocitrate to produce α-ketoglutarate during this phase. NADH is produced from NAD in the process. The enzyme isocitrate dehydrogenase catalyzes the oxidation of the –OH group at the 4′ position of isocitrate to produce an intermediate, which is then stripped of a carbon dioxide molecule to produce alpha-ketoglutarate. 

Formation of α-Ketoglutarate 

Reaction 4: Oxidation of α-Ketoglutarate to Succinyl-CoA 

To generate the 4-carbon molecule succinyl-CoA, alpha-ketoglutarate is oxidized, carbon dioxide is removed, and coenzyme A is added. NAD+ is oxidized to NADH + H+ during this oxidation. This process is catalyzed by the enzyme alpha-ketoglutarate dehydrogenase. 

Formation of Succinyl-CoA 

Reaction 5: Conversion of Succinyl-CoA to Succinate 

To make succinate, CoA is taken from succinyl-CoA. By substrate-level phosphorylation, the energy liberated is utilized to produce guanosine triphosphate (GTP) from guanosine diphosphate (GDP) and Pi. GTP can then be converted into ATP. This citric acid cycle process is catalyzed by the enzyme succinyl-CoA synthase. 

 Formation of Succinate 

Reaction 6: Oxidation of Succinate to Fumarate 

Succinate is converted to fumarate by oxidation. FAD is converted to FADH2 during this oxidation. Succinate dehydrogenase is an enzyme that catalyzes the removal of two hydrogens from succinate. 

Formation of Fumarate 

Reaction 7: Hydration of Fumarate to Malate 

Fumarate catalyzes the reversible hydration of fumarate to L-malate (fumarate hydratase). Fumarase continues the rearrangement process by reintroducing previously removed Hydrogen and Oxygen into the substrate. 

Formation of malate

Reaction 8: Oxidation of Malate to Oxaloacetate 

Malate dehydrogenase oxidizes malate to create oxaloacetate, the beginning component of the citric acid cycle. NAD+ is oxidized to NADH+ H+ during this oxidation. 

 Formation of oxaloacetate

Significance Of Citric Acid Cycle  

  1. The intermediate chemicals generated during the citric acid cycle are utilised to synthesise biomolecules like amino acids, nucleotides, chlorophyll, cytochromes, and lipids, among others. 
  1. Succinyl CoA, for example, plays a role in the synthesis of chlorophyll. 
  1. Ketoglutaric acid, pyruvic acids, and oxaloacetic acid combine to produce amino acids. 
  1. The citric acid cycle generates a large amount of energy (ATP) that is necessary for the many metabolic functions of the cell. 
  1. This cycle produces carbon skeletons, which are utilised in the process of cell development and maintenance  
Citric acid cycle 


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