Question

2. Discuss cellular respiration: Name the four steps of cellular respiration. Give the location of each step. Explain how each step interacts with the other steps. Summarize the net energy gain of each step.

Answer 1

A series of metabolic steps used to extract the energy from the organic molecules like glucose is called cellular respiration

Step1:

Glycolysis: In the cytoplasm of a cell, the glucose is degraded by the glycolysis pathway. One glucose molecule (6 Carbon compound) enters into glycolysis produces two molecules of pyruvate (3 Carbon compound). During the degradation of glucose molecule produces two molecules of NADH and four ATPs but two ATPs are utilized for glycolysis process so the net gain is two ATPs at the end of glycolysis. Finally, only two ATP molecules are produced from degradation of one molecule of glucose by glycolysis pathway.

Energy gain: 2ATP and 2 NADH

Steps:

Glucose -> Glucose 6-phosphate    -> Fructose 6-phosphate -> Fructose 1,6-bisphosphate -> Dihydroxyacetone phosphate & Glyceraldehyde 3-phosphate -> Glyceraldehyde 3-phosphate -> 1,3-Bisphosphoglycerate -> 3-Phosphoglycerate -> 2-Phosphoglycerate -> Phosphoenolpyruvate -> 2 Pyruvate.

Step2:

Formation of acetyl coenzyme A: The two private molecules produced in glycolysis do not directly enter into the Kreb’s cycle for further degradation. First, they are converted to acetyl coenzyme A by the enzyme called pyruvate dehydrogenase which is degraded by Krebs cycle enzymes. During this conversion, two NADH molecules are generated from two pyruvate molecules and two CO_2.

2 pyruvate          ->        2 acetyl-CoA

Energy gain: 2 NADH₂

Step3:

Krebs cycle: also called citric acid cycle or tricarboxylic acid cycle. The acetyl coenzyme A combine with one oxaloacetate molecule produces citrate. The citrate molecule degraded and oxaloacetate molecule is regenerated. During Krebs cycle, two GTP, two FADH₂, six NADH₂ are produced from two molecules of private.

Oxaloacetate + Acetyl CoA -> Citrate -> Isocitrate -> Oxalosuccinate -> α-Ketoglutarate -> Succinyl-CoA -> Succinate   Fumarate -> L-Malate -> Oxaloacetate

Energy gain: two GTP, two FADH₂, six NADH₂

The NADH2 generated in glycolysis & Krebs cycle enters into the electron transport chain oxidized into the NAD+.

Step4:

Electron transport chain: it is located in the inner mitochondrial membrane with four protein complexes which carry electrons from the NADH2 and FADH2 to terminal oxygen and produces the NAD+ and ATP. Generation of ATP by the oxidation of NADH2 and FADH2 by electron transport chain is called oxidative phosphorylation. During the aerobic respiration total, 36 ATPs are produced.

C6H12O6 + 6O2 → 6CO2 +6H2O + 36 ATP

It is the equation for glucose oxidation in aerobic respiration.

Energy gain: total 36 ATPs