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To provide you with the best study resources, we have come up with the CSIR NET Study Notes - Carbohydrate Metabolism along with the direct link to watch the Live Video, Attempt Quiz on the same topic, all on the same platform. Candidates will also get access to download the PDF and save it for revision purposes. So, here we are offering the best study resources that will definitely make your ongoing preparation more precise and easier.
Study Notes on Carbohydrate Metabolism
Catabolism/breakdown of Glucose:
Glycolysis: Glycolysis occurs in the cytoplasm of all the living cells. It is also called the EMP (Embden, Mayerhoff, Parnas) pathway. Glycolysis is a common step between aerobic and anaerobic respiration. It is a 10 step (reactions) pathway in which glucose is partially oxidized into 2 molecules of pyruvic acid. The ten reactions could be divided into two phases: (i) Activation phase and (ii) energy synthesis phase, each phase has five reactions.
Few Important Points About Glycolysis:
- Steps catalysed by hexokinase, phosphofructokinase-I (PFK-I) and pyruvate kinase (1st, 3rd and 10th steps) are three irreversible steps of glycolysis, rest all are reversible.
- Step catalysed by PFK-I is the slowest and rate-limiting step of glycolysis.
- Fructose-2,6-bisphosphate and AMP molecules are allosteric activators of PFK-I enzyme, while ATP is the allosteric inhibitor of PFK-I enzyme.
- In last five reactions, a total of 4 ATP molecules (in two different steps) are synthesised via substrate level phosphorylation i.e., ATP synthesis directly without any electron transport process.
- Net gain from glycolysis is 2 ATP, 2 NADH and 2 pyruvate molecules.
Fate of Pyruvate Molecules Synthesized in Glycolysis:
Pyruvate Molecules Synthesized in Glycolysis, Can Either Enter in mitochondria for aerobic respiration or those pyruvate molecules can remain in cytoplasm to undergo fermentation or anaerobic respiration.
Few Important Points About Anaerobic Respiration or Fermentation:
- Alcoholic Fermentation Usually Takes Place in yeast (Saccharomyces cerevisiae) while lactic acid fermentation takes place in muscle cells & Lactobacillus bacteria.
- In fermentation, pyruvate molecules act as external electron acceptor molecules i.ePyruvate molecules undergo reduction.
- In the end of fermentation, along with the end products of incomplete oxidation (like CO2 and ethanol in alcoholic fermentation & lactate in lactic acid fermentation) 2 ATP as the net gain are obtained, which were synthesized in the glycolysis.
Aerobic respiration of pyruvate molecules: After entering the mitochondria, pyruvate molecules undergo two steps for complete (aerobic) oxidation.
- Link reaction
- Krebs cycle (TCA or citric acid cycle)
It is the oxidative-decarboxylation reaction, occurring in mitochondria, where each pyruvate molecule produces a CO2 and an acetyl CoA molecule.
- Link reaction is the link between glycolysis and Krabs cycle in aerobic respiration.
- In Link reaction, each pyruvate molecule produces one NADH molecule as the by-product of the redox reaction.
- Link reaction is catalysed by pyruvate dehydrogenase complex enzyme, which is a combination of three enzymes: Dihydrolipoyl dehydrogenase (NAD+ as the cofactor), Dihydrolipoyl transacetylase (lipoic acid and coenzyme A cofactors), pyruvate dehydrogenase (Mg2+ and thiamine pyrophosphate; TPP as cofactor).
Krebs cycle: It is an enzymatic cyclic process, operating in the matrix of mitochondria. Krebs cycle was discovered by Hans Krebs as the major step in the aerobic respiration of glucose or other respiratory substrates (energy providing biomolecule).
Few Important Points About Krebs Cycle:
- Krebs cycle is also called TCA cycle or citric acid cycle, because citrate, the first product of this cycle is a tricarboxylic acid (TCA; having three COOH groups).
- By the end of a Krebs cycle, from one pyruvate molecule, there is formation 3 NADH, 1 FADH2 and 1 GTP molecule via substrate level phosphorylation. So, a total of 6 NADH, 2 FADH2 and 2 GTP molecules will be produced in the TCA cycle from a single glucose molecule (i.e., from 2 pyruvate molecules).
- All the NADH molecules (produced in glycolysis, link reaction and TCA cycle) and FADH2 molecules (produced in TCA cycle), enter into mitochondrial electron transport system (ETS) to produce ATP molecules. Each NADH produces 3 ATPs via ETS, while each FADH2 produces 2 ATPs via ETS.
Pentose phosphate pathway (PPP) or Hexose Monophosphate (HMP) Shunt:
HMP shunt is a metabolic pathway parallel to glycolysis, which operates in the cytoplasm. It generates 12 NADPH and ribose 5-phosphate, a precursor for the synthesis of nucleotides. While the pentose phosphate pathway does involve oxidation of glucose, its primary role is anabolic rather than catabolic. During catabolism of glucose HMP shunt operates usually when mitochondria are engaged or temporarily inhibited.
Calvin Cycle or C3 cycle:
The Calvin cycle is the light-independent or biosynthetic phase of photosynthesis. This enzymatic cycle operates in the stroma of chloroplast, in which CO2 molecules via different intermediates eventually get assimilated into glucose. Melvin Calvin discovered C3 cycle in unicellular green alga, Chlorella using 14C radioactive isotope.
Few Important Points About Calvin Cycle:
- Calvin Cycle Has three important steps: (i) Carboxylation of RUBP (a 5C-ketose sugar), (ii) Reduction of 3-Phosphoglyceric acid (3-PGA) and (iii) Regeneration of RUBP.
- The first stable intermediate of the Calvin cycle is a 3-carbon PGA molecule; hence the cycle is also known as C3 cycle.
- The most important step of the Calvin cycle is carboxylation of RUBP, catalysed by RuBisCO (RUBP carboxylase-oxygenase) enzyme. RuBisCO is the most abundant protein/enzyme present on this planet.
All glucogenic amino acids (except leucine and lysine) can form into TCA cycle intermediates or pyruvate. Pyruvate then via oxaloacetate, forms PEP (phosphoenol pyruvate), which in a series of reactions that are opposite to glycolysis in terms of direction, eventually can synthesize glucose molecules. This pathway is called gluconeogenesis. Gluconeogenesis can operate in the cytoplasm of liver or kidney cells.
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