NCERT Solutions For Class 11 Biology Chapter 14 Respiration in Plants

The complex organic compound that gets oxidized in the cell during respiration to release large amounts of energy is called respiratory substrate. Under normal condition, glucose is the most common respiratory substrate which is a carbohydrate along with six carbon atoms.   

Carbohydrates, proteins, fats and organic acids are used as respiratory substrates and oxidation of these compounds releases energy in the cell. However, the energy released is not dissipated freely in the cell. In other words, it does not occur in one step. Instead, it is released in a series of slow step-wise reactions controlled by enzymes and is trapped in the form of ATP. This prevents the sudden increase in the temperature and avoids wastage of energy. This holds a lot of significance as ATP which stores the energy can be broken down whenever and wherever it is needed in the various energy-requiring processes of the organisms.

Schematic representation of Krebs’ cycle:

Schematic representation of glycolysis:

The metabolic pathway that uses the energy released by the oxidation of nutrients to produce adenosine triphosphate (ATP) is called oxidative phosphorylation. Almost all the forms of life on earth use a range of different nutrients to carry out oxidative phosphorylation to produce the molecule that supplies energy to metabolism i.e. ATP. This is a very efficient process of energy generation.

This process requires the presence of oxygen in the system. Oxygen drives the whole process as it removes hydrogen from the system and acts as the final hydrogen acceptor. During oxidative phosphorylation, electrons are transferred from electron donors like NADH₂ to electron acceptors such as oxygen. These redox reactions release energy, which is used to form ATP. In eukaryotes, these redox reactions are carried out by a series of protein complexes within mitochondria, whereas, in prokaryotes, these proteins are located in the cells' inner membranes. These linked sets of proteins are called electron transport chains. It is the energy of the oxidation-reduction process that is used for the production of proton gradient required for phosphorylation and thus, this process is called oxidative phosphorylation.

Complete oxidation of substrates during aerobic respiration requires oxygen and apart from energy, carbon dioxide is produced as the by-product. The ratio of the volume of CO₂ released to the volume of O₂ consumed during complete oxidation of one molecule of a substrate in a given period of time at standard temperature and pressure is called the respiratory quotient (RQ).

For example, during aerobic respiration of one molecule of glucose, 6 molecules of CO₂ are released and 6 molecules of O₂ are consumed. Thus RQ for glucose is 1.

RQ value for fats: Fats need more oxygen molecule than carbohydrate (glucose) for complete oxidation through aerobic respiration, due to which the value of RQ for fat is always less than 1.

For example, when fatty acid tripalmitin is used as a substrate, 145 molecules of O₂ are consumed whereas 102 molecules of CO₂ are produced, the RQ value is 0.7.

The process of metabolism involves both anabolic and catabolic reactions. Anabolism is the synthesis of complex macromolecules like lipids and proteins from simple molecules like glycerol and amino acid respectively. On the other hand, catabolism includes the breakdown of macromolecules into simple molecules so that they can enter in the respiratory pathway as a substrate for the release of energy. If fatty acids are used as a respiratory substrate they are broken down to glycerol and acetyl CoA. Glycerol gets converted to 3-phosphoglyceraldehyde (PGAL) and enters in glycolysis while Acetyl CoA directly enters in Krebs’ cycle. However, when an organism needs to synthesize fatty acids, acetyl CoA is withdrawn from the above-said pathway and is made available for catabolic reaction. Similarly, when proteins are used as a substrate, they are first broken down to amino acid, which in turn, depending on their structure, gets converted into different intermediates of Krebs’ cycle. At the time of need, the same molecules are withdrawn to synthesize new proteins. Most of these reactions are reversible and depending on the requirement, the cell uses the respiratory substrate in the process of anabolism or catabolism. Thus, the respiratory pathway is known as the amphibolic pathway rather than only a catabolic pathway.

Many assumptions have been made in order to calculate the net gain of ATP from one molecule of glucose. This is required as the cellular system is very complex where numerous biochemical reactions take place simultaneously. The assumptions are as follows: 

1. All the steps of aerobic respiration (glycolysis, TCA cycle, ETS and oxidative phosphorylation) take place in sequential order where the product of the first step enters the subsequent step as a substrate.
2. The NADH synthesized in glycolysis enters in the mitochondria and produces ATP through oxidative phosphorylation.
3. The intermediates formed in various steps of aerobic respiration are not utilized in any other metabolic pathways other than subsequent steps of respiration.
4. The glucose molecule is the only substrate and no other molecule enters as an intermediate substrate in the process.

(a)

(b)

(c)

The electrons removed from the substrates of glycolysis and the Krebs’ cycle are stored in the reduction equivalents, namely NADH₂ and FADH₂. This energy is released when NADH₂ and FADH₂ are oxidized by passing their electrons to a chain of electrons carrier complex called Electron transport system, present in the inner membrane of mitochondria. These complexes transfer the electron through a series of redox reactions with high energy electrons entering the system and low-energy electrons leaving the system. The energy released through this process is utilized to pump out protons which develop a proton gradient (Proton motive force) across the inner membrane. This proton motive force is utilized by ATP synthase to generate high energy ATP molecules at 3 different sites.

Process: The NADH₂ produced during the citric acid cycle are oxidized by an NADH dehydrogenase (complex I), and electrons are then transferred to ubiquinone which gets reduced. Ubiquinone also receives reducing equivalents via FADH₂ (complex II). The reduced ubiquinone (ubiquinol) is then re-oxidized by transferring its electrons to cytochrome c via cytochrome bc1 complex (complex III). The electron is transferred from complex III to complex IV through cytochrome C which is a mobile carrier present in the inner membrane. Complex IV is called cytochrome C oxidase complex and consists of cytochromes a-a3, and two copper centers.

The four main steps of aerobic respiration are as follows:

(a)

(b)

(c)