Respiratory Balance Sheet: The balance sheet is the written statement of money earned and paid. The balance sheet of any business gives the idea of the flow of money and profit or loss at the end of the day, month or year. Similarly, every living cell gains and loses energy constantly. We can form a balance sheet for loss and gain of energy for a cell using energy currency or ATP. A cell generates ATP by the oxidation of substrate molecules obtained through food consumption. The process of oxidation of glucose molecules to produce ATP is called respiration. It is a complex biochemical process that occurs at the cellular scale. Read further to learn a brief account of drawing the balance sheet for oxidation of each glucose molecule.

Respiration

The entire process of respiration can be divided into 3 steps: glycolysis, Krebs cycle and electron transport system. These steps generate adenosine triphosphate (ATP) by the process known as phosphorylation. Phosphorylation can be defined as the attachment of a phosphoryl group (Pi) to adenosine diphosphate (ADP). The process of phosphorylation can be substrate level, also known as direct phosphorylation or oxidative.
During substrate-level phosphorylation, direct phosphorylation of ADP with a phosphate group using the energy obtained from a coupled reaction. Oxidative phosphorylation occurs in mitochondria resulting in the synthesis of ATP. Oxidative phosphorylation is linked to electron transport across the mitochondrial membrane, using ATP synthase to produce ATP molecules.

Practically it has not been possible to make the calculations in a living system as it is a complex simultaneous process, and the steps do not occur one after the another. Theoretically, it is possible to make calculations of the net gain of ATP during every step of oxidation of glucose molecules.

Assumptions of ATP Balance Sheet

Theoretical calculation of ATP generation is based upon certain assumptions, which are listed below:

1. The process of respiration is sequential and orderly. The pathway involves steps glycolysis–> TCA cycle –>ETS pathway.
2. Every NADH molecule produced during glycolysis is transferred into the mitochondria for oxidative phosphorylation and ATP generation.
3. Respiration is an isolated pathway, and intermediates formed along the pathway are not utilised to synthesise any other compound.
4. Glucose is the only respiratory substrate being used for ATP generation. Other substrates like protein and fat do not enter the pathway at any stages.

Balance Sheet

Aerobic Respiration

C₆H₁₂O₆ + 6O₂   → 6CO₂ + 6H₂O + 673Kcal

Aerobic respiration involves the complete combustion of glucose. Typically, during the electron transport system in aerobic respiration, oxidation of each NADH molecule produces three ATP and oxidation of each FADH₂ molecule produces two ATP molecules. The numbers of ATP produced vary in prokaryotes and eukaryotes.

\(1\,{\text{NADH}}\, \to \,3\,{\text{ATP}}\)
\(1\,{\text{FADH}}_2 \to \,2\,{\text{ATP}}\)

	Phase	Molecular change	Cost per glucose molecule	Gain per glucose molecule
1.	Glycolysis	Conversion of glucose(6C) to 2 pyruvates (3C)	2ATP	4 ATP 2 NADH
2.	Oxidation of pyruvates	Conversion of pyruvates(3C) to acetyl group (2C)	None	2 NADH
3.	Krebs cycle	Conversion of citric acid(6C) formed by the combination of an acetyl group and oxaloacetate to oxaloacetate(4C)	None	2 ATP 6 NADH 2 FADH2
4.	Electron transport	oxidation of NADH and FADH2 to build ATP and water molecules	2ATP	34 ATP

Pathway	NADH (3 ATP)	FADH2 (2 ATP)	ATP	Total
EMP pathway/ glycolysis	2 x 3 = 6	—	Produced- 4 Utilised- 2 Net gain- 2	8
2 x Pyruvate oxidation to acetyl CoA	2 x 3 = 6	–	–	6
Krebs cycle	6 x 3 = 18	2 x 2 = 4	2	24
Total	10 x 3 = 30	2 x 2 = 4	4	38

The net gain of ATP is 36 in most eukaryotes, while it is 38 in prokaryotes.

Anaerobic Respiration/ Fermentation

Anaerobic respiration or fermentation does not cause the complete combustion of glucose.  Incomplete combustion ends in the formation of ethanol in bacteria and lactic acid in muscle cells.

The difference between fermentation and aerobic respiration is given below.

Aerobic respiration	Fermentation
The complete breakdown of glucose to CO2 and H2O	Partial breakdown of glucose
The net gain is 36-38 ATP	The net gain is only 2 ATP
The rate of NADH oxidation to form NAD+ is rapid.	The rate of NADH oxidation to form NAD+ is very slow.

Efficiency of Respiration

Each glucose molecule has 2870 kJ energy stored. Hydrolysis of ATP generates 30.5 kilojoule, which means during aerobic respiration, the total energy produced is 1159 kJ from 38 ATP molecules.

 It is evident that only 40-45% of energy can be stored and utilised; what happens to the rest? It is believed that the remaining part of the energy is lost in the form of heat.

Significance of Respiratory Balance Sheet

Theoretical calculation suggests a net gain of 38 ATP during the complete oxidation of a glucose molecule. Although the practical calculation is not possible, we do the calculation for the following reasons:

1. To estimate the net gain of ATP
2. To understand the efficiency of the living system in extraction and storing energy

Respiratory Quotient (RQ)

It is the ratio of the volume of carbon dioxide evolved to the volume of oxygen utilised. It is a quantity with no unit. The respiratory quotient is measured to calculate the basal metabolic rate of an individual.

The respiratory quotient is different for each substrate. For carbohydrates, the value of the respiratory quotient is 1, i.e., the volume of carbon dioxide evolved and volume of oxygen utilised are equal. For proteins, the value of the Respiratory quotient ranges from 0.5 to 0.9, depending upon amino acid constituent, which implies the volume of carbon dioxide evolved is lesser than the volume of oxygen utilised. The value of the respiratory quotient for fat is 0.7, which again implies that the volume of carbon dioxide evolved is lesser than the volume of oxygen utilised.

In the case of anaerobic respiration, the value of the respiratory quotient is infinity as no oxygen is utilised. Respirometer is used to measure the value of the respiratory quotient.

Summary

The respiratory balance sheet deals with the gain and loss of energy in the form of an energy currency called ATP. The balance sheet can be drawn only theoretically. There are certain assumptions considered to draw a balance sheet. It is assumed that the process of respiration is a sequential and orderly process. Every NADH molecule produced during glycolysis is transferred into the mitochondria for oxidative phosphorylation and ATP generation.

Respiration is an isolated pathway, and intermediates formed along the pathway are not utilised to synthesise any other compound. And glucose is the only substrate used for ATP generation. During aerobic respiration net gain of ATP is 36 in most eukaryotes while it is 38 in prokaryotes. During anaerobic respiration net gain of ATP is only 8. A respiratory balance sheet is drawn to estimate net energy gain by a cell and understand the efficiency of the living system.

Frequently Asked Questions (FAQs) on Respiratory Balance Sheet

Q.1. What are the three steps of respiration?
Ans: The three steps of respiration are Glycolysis, Krebs cycle and ETS.

Q.2. How many ATPs are generated during the process of glycolysis?
Ans: During glycolysis, 2 ATP are generated by substrate-level phosphorylation and 6 by oxidative phosphorylation of NADH.

Q.3. Why is the number of ATP generated during anaerobic respiration lower?
Ans: During anaerobic respiration, glucose is not completely oxidised; hence the number of ATP produced is significantly lower.

Q.4. Which is the most common substrate of respiration?
Ans: Glucose is the most common substrate of respiration.

Q.5. What is the efficiency of energy during aerobic respiration?
Ans: Efficiency of energy during aerobic respiration is 40-45%.

Study Mechanism of Respiration Here

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