Cellular Respiration

This is a biochemical process in which the cells of an organism obtain energy by combining oxygen and glucose, resulting in the release of carbon dioxide, water, and ATP (the currency of energy in cells). Sugar is broken down with the help of oxygen to produce ATP molecules. The ATP molecules are required in all the processes of the body.
The cellular respiration involves 4 stages: Glycolysis, link reaction, Kreb cycle and oxidative phosphorylation.

Glycolysis

This is the first stage of cellular respiration in which 6-carbon glucose is split into two 3-carbon pyruvate molecules. This process takes place in the absence or absence of oxygen.
This takes place in the cytoplasm.  The first half of glycolysis is the energy investment phase:
Step 1: Glucose is phosphorylated with the help of enzyme hexokinase and an ATP molecule. This results in the formation of glucose 6-phosphate.
Step 2: Glucose 6-phosphate is converted too fructose-6-phosphate by enzyme isomerase.
Step 3: Fructose-6-phosphate is phosphorylated by enzyme phosphofructokinase with the help of ATP to form fructose-1,6 bisphosphate. It is the rate-limiting enzyme and thus, it is a key step for the regulation of glycolysis. When the amount of ATP is sufficient, the activity of phosphofructokinase reduces.
Step 4: Fructose 1,6-bisphosphate is broken down into dihydroxyacetone phosphate and glyceraldehyde-3-phosphate by enzyme aldolase. These are 3-carbon molecules.
Step 5: All the dihydroxyacetone phosphate molecules are converted into glyceraldehyde-3-phosphate by enzyme isomerase. Thus, 2 glyceraldehyde-3-phosphate molecules are formed by one glucose molecule (Fig. 1).

Total two (2) ATPs are used per molecule of glucose.

The second half of the glycolysis is the energy payoff phase:
Step 6: Glyceraldehyde 3-phosphate molecules are oxidised with the help of NAD+ which forms NADH molecules. Then glyceraldehyde-3-phosphate is phosphorylated by enzyme glyceraldehyde-3-phosphate dehydrogenase to form 1,3-bisphosphoglycerate molecules.
Step 7: From 1,3-bisphosphoglycerate molecules, the phosphate group is transferred to ADP to form ATP molecules. 1,3-bisphosphoglycerate molecules are converted into 3-phosphoglycerate molecules with the help of enzyme phosphoglycerate kinase. Thus, 2 ATP molecules are formed here from two molecules of 1,3-bisphosphoglycerate.
Step 8: 3-phosphoglycerate molecules are converted into 2-phosphoglycerate molecules by shifting the phosphate group with the help of enzyme phosphoglycerate mutase.
Step 9: 2-phosphoglycerate molecules lose water molecules to form Phosphoenol pyruvate. This reaction is catalysed by enzyme enolase.
Step 10: In the last step, the phosphoenol pyruvate (PEP) molecules are converted into pyruvate molecules by enzyme pyruvate kinase. Here, each PEP molecule produces 1 ATP molecule. Thus, two PEP molecules produce 2 ATP molecules. 2 pyruvate molecules are formed. Pyruvate kinase is also a rate-limiting enzyme for glycolysis (Fig. 2).

Total four (4) ATPs are produced per molecule of glucose.

Link reaction:

This is a reaction that links glycolysis with the Krebs cycle. In this, oxidation of pyruvate occurs to form acetyl CoA. It takes place after pyruvate enters the mitochondria.
Step 1: A Carboxyl group gets removed from the pyruvate that is release as a carbon dioxide molecule. This takes place by enzyme pyruvate dehydrogenase. The two-carbon hydroxyethyl group left after carbon dioxide release bind to the enzyme.
Step 2:  Oxidation of hydroxyethyl group to acetyl group occurs and the electrons released are taken by NAD+ to form NADH.
Step 3: The acetyl group is released by the enzyme and is transferred to the CoA group to form acetyl CoA molecules.
Remember: As each glucose molecule forms 2 pyruvate molecules, 2 acetyl CoA molecules are formed from each glucose molecules with the release of 2 carbon dioxide molecules and 2 NADH molecules.

Kreb cycle:
Other names of this cycle are tricarboxylic acid cycle or citric acid cycle. It takes place in the mitochondrial matrix.
Step 1: The acetyl group (2 carbon) from the acetyl CoA binds with the oxaloacetate molecule (4 carbon) to form a molecule of citrate (6 carbon).
Step 2: Citrate is converted to isocitrate which is its isomer.
Step 3: Isocitrate (6 carbon) is converted into α ketoglutarate molecule (5 carbon) with the release of a carbon dioxide molecule. 2 electrons are released that converts NAD+ to NADH.
Step 4: α ketoglutarate (5 carbon) is oxidised to Succinyl CoA (4 carbon) with the release of another carbon dioxide molecule. Electrons are released that converted NAD+ to NADH. Succinyl group is bound to CoA group here.
Step 5: The CoA group in the Succinyl CoA is replaced by a phosphate group. Succinyl CoA is converted into succinate and phosphate is transferred to GDP to form GTP molecule. GTP can be converted to ATP.
Step 6: Succinate is dehydrated to form fumarate. The hydrogen atoms released bid with FAD to form FADH2.
Step 7: Hydrolysis of fumarate occurs to form malate.
Step 8: This is the last step of the cycle in which malate is converted into oxaloacetate which can be reused. One molecule of NAD+ is converted to NADH (Fig. 3).

Products: 3 NADH molecules, 1 GTP molecule and 1 FADH2 molecule per molecule of pyruvate. 2 carbon dioxide molecules are released.

Oxidative phosphorylation-

It is the process that involves the electron transport chain and chemiosmosis.

A. Electron transport chain:
It takes place in the inner membrane of the mitochondria in which electron acceptors are arranged in the form of a sequence. Due to the presence of cristae in the inner membrane, the surface area is increased to help in more ATP production. It is an aerobic process. 
Four components of electron carriers are available and the electrons move from one component to the other. These carriers undergo redox reactions, while accepting and transferring electrons.

Complex I: Electrons are transferred to it from NADH molecules. It consists of Flavin mononucleotide (FMN or flavoprotein) and Iron Sulphur protein (Fe-S protein). The complex has enzyme dehydrogenase that can transfer four hydrogen molecules into the intermembrane space from the matrix.

Complex II: It receives electrons from FADH2 molecules. It is not capable of transferring hydrogen into intermembrane space.

Ubiquinone or Q: It receives electrons from complex I and complex II. It transfers the electrons to the next acceptor complex III.

Complex III: It is formed of many types of cytochromes like cytochrome b and c1 and is known as cytochrome oxidoreductase. It transfers an electron to the complex IV and also transfers hydrogen into the intermembrane space.

Complex IV: It consists of cytochrome c, a and a3. Cytochrome a3 is the last acceptor. They have two heme molecules and three copper ions that hold the oxygen molecule until it receives two electrons. It then forms a water molecule and is removed (Fig. 4).

Net ATP in cellular respiration from one glucose:

Net ATP in glycolysis is 2 ATP.
In glycolysis, 2 NADH are formed which will form 6 ATP by oxidative phosphorylation.
6 NADH and 2 FADH2 are formed from each glucose in the citric acid cycle. Each NADH produces 3 ATP while each FADH2 produced 2 ATP. So, 18 ATP from NADH and 4 ATP from FADH2, total 22 ATP formed from 1 glucose molecule in oxidative phosphorylation. 2 GTPs formed from each glucose molecule in Krebs is converted into 2 ATP.

So, the total is 22+6+2+2= 32 ATP from one glucose molecule.

QUIZ: TEST YOURSELF