All Jharkhand Competitive Exam JSSC, JPSC, Current Affairs, SSC CGL, K-12, NEET-Medical (Botany+Zoology), CSIR-NET(Life Science)

Wednesday, February 3, 2021

Respiration & Energy Transfer (Aerobic Respiration) - NEET


Respiration & Energy Transfer (Aerobic Respiration) 

  • Occurs: in Mitochondria.

  • Aerobic respiration is a biological process in which glucose is complete breakdown into energy in the presence of oxygen (O2).

  • The energy is released by splitting of a glucose molecule with the help of oxygen gas (O2).

  • However, in anaerobic respiration, the breakdown of glucose is incomplete. The end product of anaerobic respiration is lactic acid instead of carbon dioxide & water. This process occurs in oxygen debt. Hence, the amount of oxygen required to oxide lactic acid to carbon dioxide & water is not present. 

  • So, at the end of the chemical reaction, energy, water molecules, and carbon dioxide gas are released as the by-products of the reactions.

  • The 2900 kJ of energy is released during the process of breaking the glucose molecule and in turn, this energy is used to produce ATP- Adenosine triphosphate molecules, which are used by the system for various purposes.

  • The aerobic reaction process takes place in all multicellular organisms and plants.

  • Aerobic respiration produces 38 ATPs whereas, anaerobic respiration produces only 2 ATP molecules.


  • During the respiration process in plants, the oxygen (O2) gas enters the plant cells through the stomata, which is found in the epidermis of the leaves and stem of a plant. 

  • With the help of the photosynthesis process, all green plant synthesize their food and thus releases energy.

Aerobic Respiration:

  • It involves molecular oxygen as the final electron acceptor which is liberated during the oxidation of glucose.

  • The glucose is completely oxidized in this process which is operated through steps like a.) Glycolysis, b.) Production of acetyl CoA (connecting link reaction), c.) Kreb's cycle, d.) Electron transfer chain (ETC), e.) Terminal oxidation.

Conversion of  Pyruvic Acid to Acetyl CoA:

  • It is an oxidative decarboxylation reaction. This process occurs in the cytoplasm in the case of prokaryotes and in mitochondria in the case of eukaryotes. 

  • The glycolytic product i.e. pyruvic acid is converted into acetyl CoA. It is catalyzed by a multienzyme complex- pyruvate dehydrogenase complex (PDH).

  • Pyruvate dehydrogenase complex needs thiamin (vitamin B1) as a co-enzyme. It can not function in the absence of Vit. B1. Hence, thiamine deficiency causes pyruvic acidosis and lactic acidosis, life-threatening conditions.

  • Acetyl CoA is an important intermediate in lipid metabolism, cholesterol biosynthesis.

  • This enzyme is present in the mitochondria of eukaryotes and cytosol of prokaryotes.

  • This reaction is called the 'connecting link' reaction between Glycolysis & Krebs cycle.


  • It is a series of enzymatic reactions that occurs in all aerobic organisms to generate energy through the oxidization of acetate derived from carbohydrates, fats, and proteins into carbon dioxide (CO2).

  • It involves the oxidative metabolism of acetyl units and serves as the main source of cellular energy.

  • Some intermediates of the TCA cycle are used in synthesizing important biomolecules such as glutamate & aspartate.

Steps involved in Citric Acid Cycle (Krebs cycle):

Step I: Condensation

  • The first step is the condensation step combining the 2-carbon acetyl group (from acetyl CoA) with a 4-carbon oxaloacetate molecule to form a six-carbon molecule of citrate.

  • CoA is bound to a sulfhydryl (-SH) and diffuses away to eventually combine with another acetyl group.

  • This step is irreversible because it is highly exergonic.

  • The rate of this reaction is controlled by negative feedback and the amount of ATP available.

  • If the ATP level increase, the rate of this reaction decreases.

  • If the ATP is short supply, the rate increases.

Step II: Dehydration

  • Citrate loses 1-H2O (one-water molecule) and gains another as citrate is converted into its isomer, isocitrate.

Step III & IV: Oxidative Decarboxylation

  • In step III, isocitrate is oxidized, producing a 5-carbon molecule, alpha-ketoglutarate, together with a molecule of CO2 and 2-electrons, which reduces NAD+ to NADH.

  • This step is also regulated by negative feedback from ATP and NADH and by a positive effect of ADP.

  • Step III product: Alpha-ketoglutarate.

  • Step IV product: Succinyl CoA.

  • The enzymes that catalyze step IV is regulated by feedback inhibition of ATP, succinyl CoA, and NADH.

Step V: Substrate-level phosphorylation

  • A substrate group is substituted for CoA and a high-energy bond is formed.

  • This energy is used in the substrate-level phosphorylation (during the conversion of the succinyl group into succinate) to form either GTP (guanine triphosphate) or ATP.

  • There are two forms of enzymes, called isoenzymes, depending upon the types of animal tissue in which they are found.

  • One form is found in tissues that use large amounts of ATP, such as the heart and skeletal muscle.

  • The second form of the enzyme is found in tissues that have a large number of anabolic pathways, such as the liver. This form produces GTP.

  • GTP is energetically equivalent to ATP; however, its use is more restricted. In particular, protein synthesis primarily uses GTP. 

Step VI: Dehydrogenation

  • It converts succinate into fumarate.

  • 2-H atoms are transferred to FAD, producing FADH2.

  • The energy contained in the electrons of these atoms is insufficient to reduce NAD+ but adequate to reduce FAD.

  • Unlike NADH, this carrier remains attached to the enzymes and transfers the electrons to the electron transport chain directly.

  • This process is made possible by the localization of the enzyme catalyzing this step inside the inner membrane of the mitochondrion.

Step VII: Hydration

  • Water (H2O) is added to fumarate and malate is formed.

  • The last step of the citric acid cycle regenerates oxaloacetate by oxidizing malate.

  • Another molecule of NADH is produced.

Products of Krebs cycle:

  • 2-Carbon atoms come into the citric acid cycle from each acetyl group, representing 4 out of 6-carbons of one glucose-molecule.

  • 2- Carbon dioxide molecules are released at each turn of the cycle; however, these do not necessarily contain the most recently-added carbon atoms.

  • The 2- acetyl carbon atoms will eventually be released on later turns of the cycle; thus, all six- carbon atoms from the original glucose molecule are eventually incorporated carbon dioxide.

  • These carriers will connect with the last portion of aerobic respiration to produce ATP molecules.

  • 1- GTP/ ATP is also made in each cycle.

  • Several of the intermediate compounds in the citric acid cycle can be used in synthesizing non-essential amino acids; like alpha-ketoglutarate, oxaloacetate is used as precursors for the synthesis of fatty acids, glutamic acid, and aspartic acid respectively. Therefore, the cycle is referred to as an 'Amphibolic pathway' i.e. involving catabolism as well as anabolism.



Post a Comment

Unordered List

Search This Blog

Powered by Blogger.

About Me

My photo
Education marks proper humanity.

Text Widget

Featured Posts

Popular Posts

Blog Archive