Respiration in PlantsClass 11 Biology NCERT Solutions

(a) Respiration and Combustion

(b) Glycolysis and Krebs' cycle

(c) Aerobic respiration and Fermentation

Respiratory Substrates: The compounds that are oxidised during the process of respiration to release energy are known as respiratory substrates. These are typically macromolecules from which the cell can derive energy.

Examples of respiratory substrates include carbohydrates, fats, proteins, and even organic acids under certain conditions.

Most Common Respiratory Substrate: The most common and favoured respiratory substrate is glucose, a carbohydrate.

Glycolysis is the process of partial oxidation of glucose to form two molecules of pyruvic acid. It occurs in the cytoplasm and involves a series of ten enzyme-catalysed reactions. The pathway is often referred to as the EMP pathway.

A schematic representation of the steps of glycolysis is as follows (also depicted in Figure 12.1 of the textbook):

Aerobic respiration is the process that leads to the complete oxidation of organic substances in the presence of oxygen, releasing $CO_2$, water, and a large amount of energy.

The Krebs' cycle, also known as the tricarboxylic acid (TCA) cycle or citric acid cycle, is a series of reactions that completely oxidizes acetyl CoA derived from carbohydrates, fats, and proteins into $CO_2$ and water to generate energy. It occurs in the mitochondrial matrix.

A schematic representation of the cycle is as follows (also depicted in Figure 12.3 of the textbook):

The Electron Transport System (ETS) is a metabolic pathway located on the inner mitochondrial membrane. Its primary function is to release and utilise the energy stored in the reduced coenzymes, $NADH+H^+$ and $FADH_2$, which are generated during glycolysis and the Krebs' cycle.

Components and Process: The ETS consists of a series of electron carriers organised into four protein complexes (Complex I to IV), along with mobile carriers.

Coupling with ATP Synthesis (Oxidative Phosphorylation): As electrons pass from one carrier to another through Complexes I, III, and IV, energy is released. This energy is used to pump protons ($H^+$) from the mitochondrial matrix to the intermembrane space, creating a proton gradient. This gradient drives ATP synthase (Complex V) to produce ATP from ADP and inorganic phosphate. The oxidation of one NADH molecule yields 3 ATP molecules, while one $FADH_2$ molecule yields 2 ATP molecules.

(a) Aerobic respiration and Anaerobic respiration

(b) Glycolysis and Fermentation

(c) Glycolysis and Citric Acid Cycle

The calculation of a net gain of 38 ATP molecules for every glucose molecule oxidised is a theoretical exercise. This calculation is based on the following key assumptions, which are not entirely valid in a living system:

The respiratory pathway is traditionally considered a catabolic process because it involves the breakdown of complex substrates like glucose, fats, and proteins to release energy. However, this view is an oversimplification. The respiratory pathway is more accurately described as an amphibolic pathway, meaning it is involved in both catabolism (breakdown) and anabolism (synthesis).

Catabolic Role: The primary function of the respiratory pathway is catabolic. It breaks down complex molecules to release energy, which is then trapped in the form of ATP. For example, glucose is broken down to pyruvate, and fats are broken down to fatty acids and glycerol, which then enter the pathway at different points (e.g., fatty acids as acetyl CoA, glycerol as PGAL) to be oxidized.

Anabolic Role: The intermediates of the respiratory pathway also serve as precursors for the synthesis of various other essential biomolecules. The pathway can be reversed at certain points for anabolic processes.

Because the respiratory pathway serves as a central metabolic route that links both breakdown and synthesis processes, it functions as a bridge between catabolism and anabolism. Therefore, it is more appropriate to call it an amphibolic pathway rather than purely a catabolic one. (This interrelationship is shown in Figure 12.6 of the textbook).

Definition of RQ (Respiratory Quotient): The Respiratory Quotient (RQ), or respiratory ratio, is defined as the ratio of the volume of carbon dioxide ($CO_2$) evolved to the volume of oxygen ($O_2$) consumed during respiration.

Mathematically, it is expressed as: $RQ = \frac{\text{volume of } CO_2 \text{ evolved}}{\text{volume of } O_2 \text{ consumed}}$

The value of RQ depends on the type of respiratory substrate being used.

Value of RQ for Fats: When fats are used as the respiratory substrate, the RQ is less than 1. This is because fats are poorer in oxygen compared to carbohydrates and require more oxygen for their complete oxidation than the amount of $CO_2$ produced.

For example, for the fatty acid tripalmitin: $2(C_{51}H_{98}O_6) + 145O_2 \rightarrow 102CO_2 + 98H_2O + \text{Energy}$

Here, the RQ is calculated as: $RQ = \frac{102 CO_2}{145 O_2} = 0.7$

Oxidative phosphorylation is the process of synthesizing ATP from ADP and inorganic phosphate (Pi) using the energy released during the oxidation of reduced coenzymes ($NADH+H^+$ and $FADH_2$) via the electron transport system (ETS).

It is named 'oxidative phosphorylation' because the phosphorylation of ADP to form ATP is coupled to the oxidation of NADH and $FADH_2$ by the electron transport chain.

The step-wise release of energy during respiration is of great significance for the living cell. If the large amount of energy contained in a glucose molecule were released in a single step, as in combustion, most of it would be liberated as heat and would not be usable by the cell. It could also damage the cell.

The significance of the slow, step-wise process is as follows:

In essence, the step-wise release of energy is a crucial strategy used by living systems to efficiently capture and utilize the chemical energy stored in food molecules.