Question

Explain the process of aerobic cellular respiration. List all steps, main reactants, and main products. Why would an organism perform go through the Citric Acid Cycle and Electron Transport Chain/Chemiosmosis instead of solely performing Glycolysis?

Answer 1

The goal of cellular respiration is to produce ATP for use by the body to power physiological processes.

A glucose molecule will get modified to two pyruvate molecules In glycolysis. When oxygen is available, the pyruvate molecules will be converted to acetyl CoA which enters in the citric acid cycle. Both glycolysis and the citric acid cycle produce a small amount of ATP (2 ATP per pathway), but the majority of the ATP produced by aerobic metabolism is achieved when the products of glyolysis and the citric acid, NADH and FADH₂ , carry their electrons to the electron transport chain. The electron transport chain transfers electrons through electron carriers, ultimately to oxygen in a process called oxidative phosphorylaton. This final process of cellular respiration harnesses the energy delivered by NADH and FADH₂ to produce 34 ATP per glucose.

Glycolysis-

It commences with the phosphorylation of glucose by hexokinase enzyme to form glucose-6-phosphate. This step uses one ATP, which is the donor of the phosphate group. Under the action of phosphofructokinase, glucose-6-phosphate is converted into fructose-6-phosphate. At this point, a second ATP donates its phosphate group, forming fructose-1,6-bisphosphate. This six-carbon sugar is split to form two phosphorylated three-carbon molecules, glyceraldehyde-3-phosphate and dihydroxyacetone phosphate, which are both converted into glyceraldehyde-3-phosphate. The glyceraldehyde-3-phosphate is further phosphorylated with groups donated by dihydrogen phosphate present in the cell to form the three-carbon molecule 1,3-bisphosphoglycerate. The energy of this reaction comes from the oxidation of glyceraldehyde-3-phosphate. These series of reaction led to form pyruvate, the two phosphate groups are then transferred to two ADPs to form two ATPs. Thus, glycolysis uses two ATPs but generates four ATPs, yielding a net gain of two ATPs and two molecules of pyruvate.

Glycolysis can be expressed as =

Glucose + 2NAD⁺ +2ATP + 4ADP + 2P_i → 4ATP + 2NADH + 2H⁺ + 2 Pyruvate

Enzymes used –

Enzyme hexokinase adds a phosphate to convert it into glucose-6-phosphate.

Enzyme glucose-6-phosphate isomerase converts glucose-6-phosphate into fructose-6-phosphate.

Enzyme phosphofructokinase-1 then adds one more phosphate to convert fructose-6-phosphate into fructose-1-6-bisphosphate, another six-carbon sugar, using another ATP molecule.

Enzyme aldolase breaks down this fructose-1-6-bisphosphate into two three-carbon molecules, glyceraldehyde-3-phosphate and dihydroxyacetone phosphate.

Enzyme triosephosphate isomerase converts dihydroxyacetone phosphate into a second glyceraldehyde-3-phosphate molecule.

Enzyme Glyceraldehyde-3-phosphate dehydrogenase converts each three-carbon glyceraldehyde-3-phosphate into 1,3-bisphosphoglycerate.

Enzyme phosphoglycerate kinase dephosphorylate 1,3-bisphosphoglycerate into 3-phosphoglycerate.

Enzyme phosphoglycerate mutase then converts the 3-phosphoglycerate molecules into 2-phosphoglycerate.

Enzyme enolase acts upon the 2-phosphoglycerate molecules to convert them into phosphoenolpyruvate molecules.

Enzyme pyruvate kinase dephosphorylate two phosphoenolpyruvate molecules to create two pyruvate molecules and two ATP molecules.

Kreb cycle-

The three-carbon pyruvate molecule generated during glycolysis moves from the cytoplasm into the mitochondrial matrix, where it is converted by the enzyme pyruvate dehydrogenase into a two-carbon acetyl coenzyme A (acetyl CoA) molecule.

Citrate synthase combines acetyl CoA and oxaloacetate to form a six-carbon citrate molecule. CoA is subsequently released and combine with another pyruvate molecule to begin the cycle again.

The aconitase enzyme converts citrate into isocitrate. In two successive steps of oxidative decarboxylation, two molecules of CO₂ and two NADH molecules are produced when isocitrate dehydrogenase converts isocitrate into the five-carbon α-ketoglutarate, which is then catalyzed and converted into the four-carbon succinyl CoA by α-ketoglutarate dehydrogenase.

The enzyme succinyl CoA dehydrogenase converts succinyl CoA into succinate and forms the high-energy molecule GTP, which transfers its energy to ADP to produce ATP.

Succinate dehydrogenase converts succinate into fumarate, forming a molecule of FADH₂.

Fumarase converts fumarate into malate, which malate dehydrogenase then converts back into oxaloacetate while reducing NAD⁺ to NADH.

Oxaloacetate is then ready to combine with the next acetyl CoA to start the Krebs cycle again. For each turn of the cycle, three NADH, one ATP, and one FADH₂ are created. Each carbon of pyruvate is converted into CO₂, which is released as a byproduct of aerobic respiration.

Electron Transport chain-

ETC uses the NADH and FADH₂ produced by the Krebs cycle to generate ATP. The ETC couples the transfer of electrons between a donor like NADH and an electron acceptor O₂ with the transfer of protons across the inner mitochondrial membrane, enabling the process of oxidative phosphorylation. In the presence of oxygen, energy is passed, stepwise, through the electron carriers to collect gradually the energy needed to attach a phosphate to ADP and produce ATP.

For every glucose molecule that enters aerobic respiration, a net total of 36 ATPs are produced

Organism go through Kreb cycle and ETC apart from glycolysis to produces more energy as glycolysis produces only net 2 ATP while Kreb cycle and ETC altogether produces net 36 ATP.