Class 11 Biology

Plant Respiration

Aerobic Respiration

Aerobic respiration takes place within the mitochondria. Following are the main steps in aerobic respiration:

Step 1: Stepwise removal of all the hydrogen atoms leads to complete oxidation of pyruvate. This leaves three molecules of CO2. This step takes place in the matrix of mitochondria.

Step 2: Electrons removed from hydrogen atoms are passed on to molecular O2. This happens with simultaneous synthesis of ATP. This step takes place in the inner membrane of mitochondria.

Step 3: Pyruvate enters the mitochondria matrix and undergoes oxidative decarboxylation. This involves a complex set of reactions which are catalysed by pyruvic dehydrogenase.

Pyruvic acid + CoA + NAD+ (in presence pyruvic dehydrogenase) → Acetyl CoA + NADH + H+

During this process, two molecules of NADH are produced from the metabolism of two molecules of pyruvic acid (produced from one glucose molecule during glycolysis).

After this, acetyl CoA enters a cyclic pathway. This pathway is called tricarboxylic acid cycle or Citric Acid Cycle or Krebs’ Cycle. This was first explained by Hans Krebs.

Kreb's Cycle

kreb's cycle

Electron Transport System (ETS) and Oxidative Phosphorylation

The next steps are to release and utilize the energy stored in NADH+H+ and FADH2. This is accomplished when they are oxidised through the electron transport system and the electrons are passed on to O2 resulting in the formation of H2O.

The metabolic pathway through which the electron passes from one carrier to another, is called the electron transport system (ETS). This pathway is present in the inner mitochondrial membrane.

Although the aerobic process of respiration takes place only in the presence of oxygen, the role of oxygen is limited to the terminal stage of the process. But since oxygen drives the whole process by removing hydrogen from the system, the presence of oxygen is vital.

Yet, the presence of oxygen is vital, since it drives the whole process by removing hydrogen from the system. Oxygen acts as the final hydrogen acceptor.

During photophosphorylation, light energy is utilised for the production of proton gradient. But in respiration, the energy of oxidation-reduction is utilised for the production of proton gradient. Hence, this process is called oxidative phosphorylation.

The energy released during the electron transport system is utilised in synthesizing ATP with the help of ATP synthase (Complex V). This complex is composed of two major components, viz. F1 and F0. The F1 headpiece is a peripheral membrane protein complex. It contains the site for synthesis of ATP. F0 is an integral membrane protein complex which forms the channel through which protons cross the inner membrane. The passage of protons through the channel is accompanied by catalytic site of the F1 component for the production of ATP. For each ATP produced, 2H+passed through F0 down the electrochemical proton gradient.

The Respiratory Balance Sheet

The respiratory balance sheet gives theoretical value about net gain of ATP for every glucose molecule oxidized. The calculations for respiratory balance sheet are based on some assumptions which are as follows:

Amphibolic Pathway

Glucose is the most favoured substrate for respiration. Other substrates can also be respired but they do not enter the respiratory pathway at the first step. Respiratory process involves both catabolism and anabolism; because breakdown and synthesis of substrates are involved. Hence, respiratory pathway is considered as an amphibolic pathway rather than a catabolic one.

Respiratory Quotient

The ratio of the volume of CO2 evolved to the volume of O2 consumed during respiration is called the respiratory quotient (RQ) or respiratory ratio. The RQ for carbohydrates is 1. The RQ for fat and protein is less than 1.

Respiratory Quotient = Volume of CO2 ÷ Volume of O2 consumed

Reaction for respiration of fat:

2(C51H98O6 + 145O2 → 102CO2 + 98H2O

RQ of Fat = 102CO2 ÷ 145O2 = 0.7