NEET Biology Important Chapter- Respiration in Plants

This document on Respiration in Plants is specially designed for the students who are in search of concise content as per the NEET point of view. It carries all the important topics of the chapter along with the solved sample questions and practice questions, added at the end of the article. 

Topics such as how do plants respire, what all are the processes included, Glycolysis and its steps, aerobic and anaerobic respiration and other important information have been added in this piece article. So, by going through the article, the student can revise the chapter in one go.  

Important Topics of Respiration in Plants

• Glycolysis

• Fermentation

• Aerobic Respiration

• Electron Transport System (ETS)

• Oxidative Phosphorylation

• Krebs Cycle

• Amphibolic Pathway

• Respiratory Quotient

Important Concepts of the Chapter

Plants, just like most species on the planet, need oxygen to survive, and as a result, they breathe. Plants, on the other hand, do not have organs specialised in the process of gaseous exchange, therefore they do not breathe like most mammals. Instead, they use stomata and lenticels, which are organelles that let them breathe.

The oxidative breakdown of food materials within the cell to liberate energy and the storage of this energy for the production of ATP is referred to as cellular respiration.

How Do Plants Respire?

Plants undergo the process of photosynthesis to make sugars (such as glucose), which are combined with oxygen to produce energy for the plant's growth. The process of respiration is similar to that of photosynthesis, however, it occurs in the opposite direction.

$\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}+6 \mathrm{O}_{2} \rightarrow 6 \mathrm{CO}_{2}+6 \mathrm{H}_{2} \mathrm{O}+32 \text { ATP (energy) }$

Glycolysis

The EMP pathway is a term used to describe the glycolysis system. In anaerobic species, glycolysis is the only method of respiration. This is a mechanism in which glucose is partially oxidised and two molecules of pyruvic acid are formed in the cytoplasm of the cell and are found in all living things.

Fermentation

In this mechanism, under anaerobic conditions, incomplete oxidation of glucose is achieved through a series of processes that culminate in the conversion of carbon dioxide to ethanol, reactions catalysed by enzymes such as alcohol dehydrogenase and pyruvic acid decarboxylase. 

The below-given diagram represents the steps involved in fermentation:

Aerobic Respiration

The complete oxidation of organic compounds in the presence of oxygen occurs in this process, releasing water, carbon dioxide, and a tremendous quantity of energy from the substrate. Higher creatures are the ones who use aerobic respiration the most. Pyruvate is transferred from the cytoplasm into the mitochondria in order for it to happen inside the mitochondria, the final product of glycolysis. 

The following are some significant events that are taking place:

• Complete pyruvate oxidation, leaving 3 molecules of $CO_{2}$ 

• Transferring electrons lost as part of hydrogen atoms to molecular oxygen, while simultaneously synthesising ATP.

Tricarboxylic Acid Cycle/Krebs Cycle

The Tricarboxylic Acid (TCA) Cycle, also known as the Krebs Cycle, and the Electron Transport System play a pivotal role in aerobic respiration.

The TCA cycle is a series of chemical reactions that occur in the mitochondria, where acetyl-CoA is oxidized to release carbon dioxide and generate NADH and FADH2. These high-energy molecules then fuel the Electron Transport System.

The Electron Transport System is a chain of protein complexes in the inner mitochondrial membrane. It harnesses the energy from NADH and FADH2, generating a proton gradient. This gradient drives ATP synthesis through chemiosmotic coupling, resulting in the production of a maximum of 34 ATP molecules in aerobic respiration. Understanding this process is crucial in comprehending how plants extract energy from organic molecules.

The summary equation for this phase of respiration may be written as follows:

Electron Transport System (ETS) and Oxidative Phosphorylation

Cellular respiration ends with the electron transport chain, which is the most critical phase. While Glycolysis and the Citric Acid Cycle provide the essential precursors, the electron transport chain is responsible for the majority of ATP production. Both photosynthesis and cellular respiration rely heavily on it.

It is the metabolic process by which electrons travel from one carrier to another and is found in the inner mitochondrial membrane.

More information can be found in the diagram below.

Below Given Table Represents The Differences Between Oxidative Phosphorylation and Phosphorylation.

Comparison Between Aerobic Respiration and Fermentation

Amphibolic Pathway

• For respiration, glucose is the preferred substrate. Before being used for respiration, all carbs are normally transformed into glucose.

• Fats must first be broken down into glycerol and fatty acid, which must then be transformed into Acetyl CoA before entering the respiratory system.

• Proteins are broken down into amino acids, which then go through the Krebs cycle.

• Catabolism is the breakdown process within a living thing, while anabolism is the synthesis process. As a result, respiration is an Amphibolic process.

Respiratory Quotient

The Respiratory Quotient (RQ) is defined as the ratio of carbon dioxide emitted to oxygen consumed during breathing. The respiratory ratio is another name for it. The type of respiratory substrate used during respiration affects this ratio. When carbohydrates are used as substrates and totally oxidised, the RQ is 1. This is because carbon dioxide and oxygen are both produced and consumed in equal amounts.

Solved Examples from the Chapter

1. The cofactor required for the activity of pyruvate dehydrogenase is

1. Zinc

2. Magnesium

3. Manganese

4. Copper

Ans: b. Magnesium 

Key point to remember: 

2. In the absence of oxygen, the primary purpose of fermentation is to

1. Produce amino acids for protein synthesis

2. Generate a proton gradient for ATP synthesis

3. Oxidise glucose to generate reduced electron carriers

4. Regenerate $NAD^{+}$ from $NADH$allowing glycolysis to continue.

Ans: d. Regenerate $NAD^{+}$ from $NADH$ allowing glycolysis to continue.

By replenishing $NAD^{+}$ from $NADH$, fermentation permits glycolysis to proceed. $NAD^{+}$ is a limiting molecule that must be rebuilt in order to keep the glycolytic breakdown of glucose creating ATP going.

Key point to remember: In fermentation, $NADH$ is oxidised to $NAD^{+}$ slowly.

3. When proteins are used as respiratory substrates, the respiratory quotient would be about:

1. 1.2

2. 1.0

3. 0.9

4. 0.7

Ans: c. 0.9

$O_{2}$ is consumed and $CO_{2}$ is released during aerobic respiration. The respiratory quotient (RQ) or respiratory ratio is the ratio of the volume of $CO_{2}$ evolved to the volume of $O_{2}$ consumed in respiration. The type of respiratory substrate used during respiration affects the respiratory quotient. 

Key Point to Remember: The RQ will be 1 when carbohydrates are utilised as a substrate and are totally oxidised because equal amounts of $CO_{2}$ and $O_{2}$ are produced and consumed. The RQ is less than 1 when fats are employed in respiration. The ratio would be around 0.9 if proteins were respiratory substrates.

4. Oxidative Phosphorylation is

1. Formation of ATP by transfer of 

2. Oxidation of phosphate group in ATP

3. Addition of phosphate group to ATP

4. Formation of ATP by energy release from electrons removed during substrate oxidation.

Ans: d. Formation of ATP by energy release from electrons removed during substrate oxidation.

The energy released by the oxidation of reducing powers $NADH$ and $FADH$ by the transfer of their electrons through the electron transport chain to oxygen drives the synthesis of ATP by ADP and Pi. The inner mitochondrial membrane contains an electron transport chain that transports electrons through a succession of carriers and pumps protons into the intermembrane gap. The final electron acceptor is oxygen, and the ensuing proton gradient provides energy for the ATP synthase enzyme to produce ATP.

Solved Problems of Previous Year Question from the Chapter

1. Which of the following statements is incorrect?

1. ATP is synthesised through complex V.

2. Oxidation-reduction reactions produce proton gradients in respiration. 

3. During aerobic respiration, the role of oxygen is limited to the terminal stage.

4. In ETC, one molecule of $NADH + H^{+}$ gives rise to 2ATP molecules, and one $FADH_{2}$ gives rise to 3 ATP molecules.

Ans: d.  In ETC, one molecule of $NADH + H^{+}$ gives rise to 2ATP molecules, and one $FADH_{2}$ gives rise to 3 ATP molecules.

Oxidation of one molecule of $NADH$ gives rise to 3 molecules of ATP, while that of one

a molecule of $FADH_{2}$ produces 2 molecules of ATP.

Key point to remember: The electrons are connected to ATP synthase (complex V) for the generation of ATP from ADP and inorganic phosphate as they move from one carrier to another in the electron transport chain via complex I to IV. The number of ATP molecules produced is determined by the electron donor.

2. The number of substrate levels of phosphorylations in one turn of the citric acid cycle is 

1. One

2. Two

3. Three 

4. Four

Ans: a. One

There occurs the conversion of succinyl-CoA to succinate during the Krebs' or citric acid cycle. This happens in the presence of the enzyme named succinyl-CoA synthetase (a 4C compound). By substrate-level phosphorylation, the reaction releases enough energy to generate ATP (in plants) or GTP (in animals). As a result, in one round of the citric acid cycle, there is only one substrate-level phosphorylation.

Key point to remember: In one turn of the citric acid cycle (Kreb’s cycle), substrate level phosphorylation occurs at one step when succinyl $CoA$ is converted into succinic acid.

3. Conversion of glucose to glucose-6-phosphate, the first irreversible reaction of glycolysis, is catalysed by:

1. Phosphofructokinase

2. Aldolase

3. Hexokinase

4. Enolase

Ans: c. Hexokinase

Hexokinase is responsible for catalysing the conversion of glucose to glucose-6-phosphate. ATP is utilised in this reaction.

Key Point to Remember: 

• Phosphofructokinase is responsible for converting Fructose-6-Phosphate to Fructose-1,6-Bisphosphate.

• Aldolase is responsible for converting fructose 1,6-bisphosphate to glyceraldehyde-3-phosphate and dihydroxy-acetone phosphate.

• Enolase is involved in the conversion of 2-phosphoglycerate (2-PG) to phosphoenolpyruvate (PEP).

Practice Questions

1. Pyruvate dehydrogenase complex is used in converting:

1. Pyruvate to glucose

2. Glucose to pyruvate

3. Pyruvic acid to lactic acid

4. Pyruvate to acetyl CoA

Ans: d. Pyruvate to acetyl CoA

The enzyme complex Pyruvate Dehydrogenase transforms pyruvic acid to acetyl CoA. One molecule of carbon dioxide is liberated and one molecule of reduced NADH is generated in this oxidative decarboxylation reaction.

2. What is the function of molecular oxygen in cellular respiration?

1. It causes the breakdown of citric acid.

2. To combine with glucose to produce carbon dioxide.

3. To combine with carbon from organic molecules to produce carbon dioxide.

4. To combine with hydrogen from organic molecules to produce water. 

Ans: d. To combine with hydrogen from organic molecules to produce water

The oxygen obtained from cellular respiration combines with the hydrogen obtained from the oxidation of organic molecules to form water.

3. Excess ATP inhibits the enzyme:

1. Phosphofructokinase

2. Hexokinase

3. Aldolase (Lyase)

4. Pyruvate decarboxylase

Ans: a. Phosphofructokinase

Phosphofructokinase is an essential glycolysis regulating enzyme. It converts fructose-6-phosphate to fructose-1, 6-bisphosphate by phosphorylating it. It's an allosteric enzyme, which means it works in both directions. This enzyme has a significant allosteric inhibitor in the form of ATP. A high level of ATP implies an energy surplus and the need to stop or slow down glucose lysis. Inhibition of phosphofructokinase, as well as hexokinase and pyruvate kinase, is used to achieve this. 

Conclusion

Respiration in plants has many important topics which are repetitively asked in the examination. The summary of the chapter has been made in such a way that the last-minute goes through and can bring clarity to the topics. Also, there are certain sample questions and practice questions added at the end of the chapter. The student must go through them to have an understanding of the type of questions asked in NEET.