Question

summarize the main steps of glycolysis, the Krebs cycle, and the electron transport chain

Answer 1

Glycolysis , Krebs cycle and electron transport are together completes the intra-cellular aerobic respiration.

Summarization of Glycolysis :

Glycolysis is the most commonest pathway of glucose breakdown, takes place in cytoplasm of each and every cell. It has 10 steps via which glucose is degraded to pyruvic acid and lactose may be produced from it.

Step 1 : Glucose is phosphorylated to Glucose-6-phosphate with help of Hexokinase enzyme and this step consume an ATP

Step 2 : Glucose-6-phosphate is isomerized to fructose-6-phosphate by phosphohexose isomerase.

Step 3 : Fructose-6-phosphate is further phosphorylated to fructose1,6-bisphosphate . The
enzyme is phosphofructokinase. (consume another ATP)

Step 4 : The 6 carbon fructose-1,6-bisphosphate is cleaved into two 3 carbon units - glyceraldehyde-3-phosphate (G-3-P) and molecule of dihydroxy acetone phosphate (DHAP) , enzyme Aldolase. Enzyme isomerase convert DHAP into G-3-P

Step 5 : G-3-P is dehydrogenated and phosphorylated to 1,3-bisphosphoglycerate (1,3-BPG) with the help of NAD+ The enzyme is G-3-P dehydrogenase.

Step 6 : 1,3-BPG is converted to 3-phosphoglycerate with the help of bisphosphoglycerate kinase. and an ATP molecule is generated from trapped energy.

Step 7 ; 3-phosphoglycerate is isomerized to 2-phosphoglycerate.The enzyme is phospho-glycero-mutase.

Step 8 ;  2-phosphoglycerate is converted to phosphoenol pyruvate (PEP) by the enzyme enolase (require Mg 2+ ion)

Step 9 and 10 : PEP is first converted to the enol intermediate and then to keto pyruvate (the stable form). enzymes are pyruvate carboxylase and phosphoenol pyruvate carboxy kinase. An ATP is generated here.

(You may summarize this more by excluding intermediate steps )

In anaerobic condition pyruvate is converted to Lactate (by lactate dehydrogenase) and in aerobic condition it is converted Acetyl-CoA and enters into mitochondrial matrix to perform Krebs cycle.

Summarization of Krebs cycle : It is final common oxidative pathway that oxidizes Acetyl-CoA to CO2 , H2O and NADH, FADH2 produces as a by-product. It is a cyclical pathway occur in mitochondrial matrix.

Step 1 :  Oxaloacetate ( 4 carbon) condenses with acetyl CoA (2 carbon) to form the Citrate (6 carbon). The enzyme is citrate synthase

Step 2 : Citrate is isomerized to isocitrate (via cis-aconitate) by aconitase enzyme.

Step 3 : isocitrate is converted (via oxalosuccinate) to alpha keto glutarate by enzyme Isocitrate dehydrogenase. NADH generated and CO2 is liberated here.

Step 4 : alpha keto glutarate is oxidatively decarboxylated to form succinyl CoA by the enzyme alpha keto glutarate dehydrogenase. Again NADH generated and CO2 liberated.

Step 5 : Succinate is formed from succinyl CoA by succinate thiokinase enzyme and GTP is formed.

Step 6 : Succinate is dehydrogenated to fumarate by succinate dehydrogenase and FADH2 is formed.

Step 7 : The formation of malate from fumarate is catalyzed by fumarase.

Step 8 : Finally malate is oxidized to oxaloacetate by malate dehydrogenase and NADH is generated.

Summarization of electron transport chain (ETC): The electron flow occurs through successive dehydrogenase enzymes, present in the inner mitocondrial membrane, together known as electron transport chain (ETC). There are four distinct multi-protein complexes;- complex-I, II, III and IV. these are connected by two mobile carriers, co-enzyme Q and cytochrome c.

ETC complex I or NADH Co-Q reductase collect electron pair from NADH and transfer it to CoQ, It act as proton pump

ETC complex II or Succinate Q reductase collects electron from FADH2 (by pass complex I and does not act as proton pump)

CoQ brings the electron to complex III (made up of Cytochrome b, Cytochrome c1 and Fe-S). This complex also act as proton pump.

Cyt.C receives electron from complex III and carry it to complex IV (made up of Cyt a-a3 ). This also act as proton pump and electron are finally taked up by oxygen molecule and H2O is generated.

These all pumps H+ out side the mitochondrial inner membrane and ATP-synthetase pumped in the H+ and uses the energy in ATP synthesis.