The Cellular Respiration

The Cellular Respiration truly explains that "Energy and life go hand in hand!”

Now what!!  “Oxygen we breathe is transmitted to fifteen trillion cells in our body, whereby glucose gets consumed by different biochemical pathways.” As Humans, we are an incredibly vivacious machine!

By – Manjula Banerjee

 

Cellular respiration is a biochemical process that utilizes oxygen within mitochondria to chemically disintegrate organic molecules like glucose to extract the energy stored in the mitochondria. ATP behaves as the energy currency of cells. 

Cellular respiration represents aerobic mechanisms through which glucose is broken down by cells to release energy and shape ATP molecules. Eventually, a three-stage cycle includes glucose and oxygen, contributing to the formation of carbon dioxide and water. The glycolysis process is universal and is different from species to species based on the availability of oxygen. If oxygen is not available, pyruvate can be transformed to lactic acid or ethanol as well as carbon dioxide to regenerate NAD++, named anaerobic respiration. In the presence of oxygen, pyruvate enters the mitochondria for further breakdown, releases much more energy to produce numerous additional ATP molecules in the latter two stages of aerobic respiration-the Krebs cycle and the electron transport chain.

Biochemical reactions throughout cellular respiration:

The entire cycle of cellular respiration can be described in a simple chemical equation:

C₆H₁₂O₆ + 6O₂                         6CO₂ + 6H2O + energy as ATP

(Sugar)   (Oxygen)               (Carbon  (water)

                                             Dioxide)

Reactant                                                      product

 Cellular respiration reactions are catabolic, which break down larger molecules into smaller ones, energy is released because weak high-energy bonds, especially molecular oxygen, are substituted by stronger product bonds.

To order for the cells to stay intact, they must be able to control essential machinery, including pumps, within their cell membranes that sustain the internal cell atmosphere in a manner that is suitable to human life.

Where does cellular respiration happen?

Cellular respiration continues in the cell cytoplasm and ends up in mitochondria. Mitochondrial is a membrane-enclosed organelle within the cytoplasm. It is also called the “powerhouse of the cell”. Aerobic respiration happens in mitochondria that have an internal folded membrane offering a broad surface region for enzyme systems throughout aerobic respiration.

Types of cellular respiration:

• Cellular respiration progresses with glycolysis, which may occur in either the absence or in the presence of oxygen.

• Cellular respiration that occurs in the lack of oxygen, named as anaerobic respiration.

• Cellular respiration, which takes place in the presence of oxygen, is aerobic.

• Anaerobic respiration progress before aerobic respiration.

v Aerobic Processes of cellular respiration:

Aerobic respiration typically happens as the two glycolysis pyruvate molecules are transformed and diffused in the mitochondria after that, the next two processes occur.

·         stages of aerobic cellular respiration:

• glycolysis,
• the Krebs Cycle, and
• The electron transport chain.

1^st stage: Glycolysis

·         In glycolysis, glucose is converted into pyruvate. Enzymes are found in the cytosol. Glycolysis reaction:

C₆H₁₂O₆ + 2 NAD⁺ + 2 ADP + 2 P —–> Two Pyruvic Acid + (CH₃(C=O) COOH + 2 ATP + 2 NADH + 2 H⁺

·         Glycolysis is a 10 step biochemical enzymatic pathway. These are as followed:

 

·         1^st step: Hexokinase

Glycolysis begins with the conversion of D-glucose to glucose-6-phosphate. The enzyme catalyzes the reaction named hexokinase.

·         2^nd step:  Phosphoglucose Isomerase

 Glycolysis second step involve reordering of glucose 6-phosphate (G6P) into fructose 6-phosphate (F6P) using glucose phosphate isomerase (Phosphoglucose Isomerase).

·         3^rd step: Phosphofructokinase

Phosphofructokinase, including magnesium as a cofactor, transforms 6-phosphate fructose to 1,6-bisphosphate fructose.

·         4^th step: Aldolase

The enzyme Aldolase breaks fructose 1, 6-bisphosphate into sugar molecules that are isomers of one another.  Two sugars are dihydroxyacetone phosphate (DHAP) as well as glyceraldehyde 3-phosphate (GAP).

·         5^th step: Trisphosphate isomerase

This step generates dihydroxyacetone phosphate (DHAP) and glyceraldehyde 3-phosphate (GAP) by using Triosephosphate isomerase enzyme. Glyceraldehyde phosphate is removed/used in the next level of glycolysis.

·         6^th step: Glyceraldehyde-3-phosphate Dehydrogenase

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) dehydrogenates, thereby adding an inorganic phosphate to glyceraldehyde 3-phosphate generated by 1,3-bisphosphoglycerate.

·          7^th step: Phosphoglycerate Kinase

Phosphoglycerate kinase shifts the phosphate group from 1,3-bisphosphoglycerate over ADP to ATP and 3-phosphoglycerate.

·         8^th step: Phosphoglycerate Mutase

The enzyme phosphoglycero mutase transmits P from 3-phosphoglycerate to 2-phosphoglycerate via 3-phosphoglycerate carbon.

·         9^th step: Enolase

The enzyme enolase extracts a water molecule from 2-phosphoglycerate to produce phosphoenolpyruvic acid (PEP).

·         10^th step: Pyruvate Kinase

The enzyme pyruvate kinase shifts P from phosphoenolpyruvate (PEP) over ADP to form pyruvate acid that results in  ATP formation at the 10^th step.

2^nd stage: The Krebs Cycle

The Krebs cycle exists within the matrix of the mitochondria and thereby generates a chemical energy pool of ATP, NADH, and FADH2 from pyruvate oxidation, the final product of glycolysis.

 1st step:   Krebs cycle begins with a condensation step, merging a two-carbon acetyl group (from acetyl CoA) along with a four-carbon oxaloacetate molecule to create a six-carbon citrate molecule.

2^nd step: In this step, citrate gets transformed into its isomer named as isocitrate.

3^rd step: In this step, isocitrate gets oxidized, thereby releases a carbon dioxide molecule. The end product is a five-carbon molecule—α-ketoglutarate.

4^th step: The fourth step is close to the third step.  Here, the α-ketoglutarate oxidation happens. The enzyme catalyzes named α-ketoglutarate dehydrogenase that plays a vital role in the regulation of the citric acid cycle.

5^th step: Here, a new four-carbon molecule named succinate produced. 

6^th step: In stage six, succinate oxidized to shape another four-carbon molecule called fumarate.

7th step: water molecule added to the four-carbon fumarate molecule, thereby converting it into a four-carbon unit named malate.

8^th step: The citric acid cycle ends with, oxaloacetate, the first four-carbon compound is regenerated by malate oxidation.

 

3^rd stage: Electron transport chain & oxidative phosphorylation

 

·         The final stage of aerobic respiration begins with the electron transport chain, which is situated at the mitochondrial inner membrane.

·         The inner membrane is folded structured increases (cristae) that increases surface area accessible for the transportation chain. The electron transport chain unlocks the energy contained inside the reduced hydrogen carrier.

·         It refers to oxidative phosphorylation, as ATP synthesizing energy is obtained from the oxidation of hydrogen carriers.

• There are four structural proteins complex (from I-IV complex) throughout the transport chain of electrons involved in the procurement of electrons from NADH and FADH2 to oxygen molecules.

• Complex I established the hydrogen ion gradient via pumping four hydrogen ions out of the cell from the matrix to the intermembrane domain.

• Complex II receives FADH2, which bypasses Complex I, as well as delivers electrons directly to the ETC.

• Ubiquinone (Q) receives and transfers electrons from Complex I and Complex II to Complex III.

• Complex III pumps out protons from the membrane, thereby passes its electrons to cytochrome c to transport the fourth protein and enzymes complex.

• Complex IV reduces oxygen; afterword, the reduced oxygen take two hydrogen ions from its surrounding medium to produce water.

 

v  Anaerobic process of cellular respiration - The fermentation process is an anaerobic operation. These are lactic acid fermentation as well as alcoholic fermentation. 

·         Lactic acid fermentation: Tends to occur in animal cells where oxygen is absent. Pyruvic acid gets converted to a waste product named lactic acid. 

·         Alcoholic fermentation: It happens in individual plants as well as in unicellular organisms, like yeast and bacteria. This cycle transforms pyruvic acid to ethyl alcohol. 

 

Cellular respiration energy yield comparison:

Aerobic respiration (with oxygen) generates 36 ATP molecules per glucose molecule. Anaerobic respiration (in the absence of oxygen) only enables the production of 2 ATP molecules out of each glucose molecule. Thus the aerobic respiration is way more successful than anaerobic respiration.

One glucose molecule oxidized by aerobic respiration throughout prokaryotes that that contains:

 

·         Glycolysis: net gain of 2 ATP from substrate-level phosphorylation.
 And 2 NADH molecules generate 6 ATP (that means 3 ATP form each NADH) via oxidative phosphorylation.

·         Transition Reaction: 2 NADH generates net 6 ATP (consider 3 ATP per NADH) from oxidative phosphorylation.

·         Citric Acid Cycle: Net 2 ATP release at substrate-level phosphorylation.
However, 6 NADH yields 18 ATP (taking 3 ATP per NADH) from oxidative phosphorylation. Then, 2 FADH₂ produces 4 ATP (assuming 2 ATP per FADH₂) by oxidative phosphorylation.

 

Conclusion:

The Cell Respiration Concept Map is a smarter way to analyze biological processes within a cell. Aspirants can quickly determine such directions by putting each word in a correct position upon this map. It is an excellent tool to render a range of metabolism processes more usable and appealing to students.

 

FAQs:

1.       How does the organelle have such a distinctive internal structure?

 

Several internal folds within the inner mitochondrial membrane enable it to be useful in the formation of ATP. It is incredibly effective in the presence of oxygen, although the cells that require a lot of energy possess thousands of such organelles.

 

2.       Name a few molecules other than glucose that takes part in cell respiration?

 

Carbohydrates, fats, as well as proteins, are used as food for cell respiration. However, glucose is more widely used as an indicator to analyze various reactions and biochemical pathways.

3.       How will you measure the total theoretical ATP yield by a single Glucose molecule throughout Prokaryotes?

Net gain of 38 ATP out of which 4 comes from substrate phosphorylation and 34 from oxidative phosphorylation. Within eukaryotic cells, the maximum theoretical yield of ATP produced per glucose is 36 to 38, based on how the 2 NADH produced in the cytoplasm enters the mitochondria during glycolysis or whether the resulting yield is 2 or 3 ATP per NADH.