Respiration: Definition, Types, Diagram and Examples

What is Respiration?

Respiration is a life cycle in which living organisms use oxygen to change it into energy while on the other end expelling carbon dioxide as a waste product. Contrary to the respiratory process which is the process of breathing in and out of air, cellular respiration is a process that takes place in the cell, which involves multiple chemical reactions to generate ATP, which is the energy for cells. This process is crucial for the effective running of cellular processes such as metabolism, generation of energy and even growth, repair and survival of living organisms.

Types of Respiration

There are two types of respiration:

Aerobic Respiration

• Aerobic respiration is a process by which cells use glucose and oxygen to produce energy (in the form of ATP), carbon dioxide and water. It occurs in the mitochondria and is commonly the major mode of power generation in most eukaryote organisms.

• Aerobic respiration serves as the final electron receptor in the electron transport chain which makes the process go on and thus yield the maximum energy in the form of ATP.

Anaerobic Respiration

• Anaerobic respiration is a process of cellular respiration that takes place without using oxygen. It is the process by which glucose is partially fermented to generate energy resulting in by-products like lactic acid in animal cells and ethanol and carbon dioxide in yeast cells.

• The first consists of glycolysis, in which glucose is turned into pyruvic acid, and then fermentation which turns pyruvic acid into lactic acid or ethanol with the assistance of CO2.

Cellular Respiration – Overview

The catabolic process is the breaking down of substances in the body which involves glucose and oxygen to produce ATP, CO2, and water. This involves both glycolysis, the Krebs or citric acid cycle, as well as the electron transport system. It’s required for making the energy that various kinds of cells require to perform particular tasks, which include growth, repair, and maintenance.

Structure & Role of Mitochondria

Mitochondria are of course the energetic organelles localised in the cytoplasm of eukaryotic cells and which are surrounded by membranes. They are called the ‘powerhouses’ of the cell since they produce the majority of ATP for the cell, through a process called cell respiration. In the structure of mitochondria there are two membranes, the inner membrane has cristae which are folds to increase the surface area, hence the ability to produce ATP is boosted.

ATP – Energy Currency of the Cell

In living cells, ATP or adenosine triphosphate is widely known as the energy currency of the cells. It is a molecule formed by the joining of an adenosine part with three phosphate groups. ATP when it is hydrolyzed gives out energy in the form of ADP (adenosine diphosphate) and an inorganic phosphate and this energy is utilised by the cell to carry out things like muscle contraction and cell division among other responsibilities.

Phases of Respiration

Respiration occurs within the cytosol and near the plasma membrane. In contrast, eukaryotic cells carry out respiration in the mitochondria, often referred to as the powerhouse of the cell due to its role in energy production. This process is comparable to how an internal combustion engine works in a car.

Organic molecules and oxygen serve as inputs, while water and carbon dioxide are released as outputs. The energy generated during this process powers cellular activities, much like how energy drives a car. Respiration can be divided into three main phases:

Glycolysis

Glycolysis takes place in the cytosol of the cell and not within the mitochondria organelle. This process is the first step of cellular respiration by which glucose is split to release energy.

The steps involved in glycolysis:

• Phosphorylation: Glucose is thereby converted to glucose-6-phosphate through phosphorylation with the use of ATP. The last step in the process is facilitated by the hexokinase enzyme.

• Isomerisation: Glucose-6-phosphate is isomerised to fructose-6-phosphate with the help of the enzyme phosphoglucose isomerase.

• Second Phosphorylation: It is phosphorylated to fructose-6-phosphate by ATP creating fructose-1,6-bisphosphate with the help of the phosphofructokinase enzyme.

• Cleavage: Dihydroxyacetone phosphate and glyceraldehyde-3-phosphate are two three-carbon molecules which are formed from fructose-1,6-bisphosphate with the help of aldolase.

• Isomerisation (Second): Another enzyme called triose phosphate isomerase turns dihydroxyacetone phosphate into another glyceraldehyde-3-phosphate.

• Oxidation and ATP Formation: Each of the glyceraldehyde-3-phosphate is oxidized to produce 1, 3-bisphosphoglycerate which in turn produces ATP and NADH. This step is catalysed by the enzyme which is glyceraldehyde-3-phosphate dehydrogenase.

• Phosphorylation to ATP: 1,3-bisphosphoglycerate is reduced into 3-phosphoglycerate coupled with the generation of ATP. This step is coupled with the help of phosphoglycerate kinase.

• Rearrangement: Phosphoglycerate mutase then converts 3-phosphoglycerate into 2-phosphoglycerate.

• Dehydration: Enolase is another enzyme that acts on the 2-phosphoglycerate to deprive it of its water-producing phosphoenolpyruvate.

• Final ATP Formation: Thus, phosphoenolpyruvate is converted into pyruvate, and the final ATP molecule in the glycolysis process is produced. This step is catalyzed by the pyruvate kinase enzyme

Krebs Cycle (Mitochondrial matrix)

The Krebs cycle, also called the citric acid cycle, takes place in the mitochondrial matrix, the semisolid material enclosed within the inner membrane of mitochondria. This place can be considered vitally important for the cycle in the process of energy production and the processes of intermediary metabolism.

The steps involved in the Krebs cycle:

• Formation of Citrate: The first step in this cycle is the condensation of the acetyl-CoA group obtained from pyruvate with the oxaloacetate to form citrate with the help of citrate synthase enzyme.

• Isomerisation to Isocitrate: Aconitase is an enzyme that is used in converting citrate into isocitrate through the process of isomerisation and this takes two processes.

• Oxidative Decarboxylation: Thus, isocitrate is oxidatively decarboxylated to α-ketoglutarate which eliminates CO₂ and in doing so generates NADH and is catalysed by isocitrate dehydrogenase.

• Formation of Succinyl-CoA: α-Ketoglutarate is decarboxylated to succinyl-CoA; another molecule of CO₂ is released and NADH is reduced and another one is produced by α-ketoglutarate dehydrogenase.

• Conversion to Succinate: Succinyl-CoA is then oxidatively demanded to succinate and with every turn passed through there is a net gain of one molecule of GTP ( or ATP) through substrate-level phosphorylation with the help of enzyme succinyl-CoA synthetase.

• Oxidation to Fumarate: Succinate is oxidised from fumarate and generates FADH₂ with the help of succinate dehydrogenase enzyme.

• Hydration to Malate: Malate is synthesised from fumarate through the hydration process with the help of the enzyme fumarase.

• Regeneration of Oxaloacetate: Therefore, malate oxidises to form oxaloacetate by generating NADH, in a reaction for which is used the enzyme called malate dehydrogenase. After this CO2 is ready to begin a new turn with the next molecule of Acetyl-CoA.

Electron Transport Chain (ETC)

The Electron Transport Chain (ETC) is anchored on the inner mitochondrial membrane where the process of oxidative phosphorylation occurs to generate ATP. This membrane has a large surface area which is further enlarged by its folds known as cristae. This feature enables good ET and ATP manufacture.

Steps Involved in the ETC

• Electron Transfer: The electrons from NADH and FADH₂ formed in the earlier steps of cellular respiration are donated to the ETC complexes, Complex I & Complex II, present in the inner membrane of the mitochondria.

• Proton Pumping: While transferring the electrons through the ETC complexes (Complex I, III, and IV), the release of energy is utilised for pumping the protons (H⁺ ions) from the mitochondrial matrix to the intermembrane space covering the generation of an electromechanical gradient.

• Formation of Water: Finally at the complex IV electrons are transferred to molecular oxygen (O₂). Oxygen ties with electrons and protons to give water (H₂O) which helps reduce the likelihood of electrons piling in the chain.

• ATP Synthesis: Protein complex ATP synthase causes the protons to flow back into the mitochondria’s matrix through an electrochemical gradient developed by the pumping of protons. This flow of protons continues to rotate ATP synthase to generate ATP from ADP and inorganic phosphate (Pi).

Factors Affecting Respiration

The rate of respiration is is affected by different internal and external factors. Understanding these factors will help in getting to know the health of a plant and its responses to environmental stress. Several factors can influence the process of respiration in green plants:

Respiration NEET MCQs (With Answers & Explanations)

Important topics for NEET are:

• Types of respiration

• Respiration in different plant parts

• Cellular respiration in plants

Practice Questions for NEET

Q1. The structures present in plants for the purpose of gaseous exchange are

1. Lenticels

2. Stomata

3. Nostrils

4. Both a and b

Correct answer: 4) Both a and b

Explanation:

Unlike animals, plants do not have specialized organs for gaseous exchange. Instead, they rely on structures like stomata, tiny openings found on the surfaces of leaves, and lenticels, small pores present on stems and woody parts. These structures allow the exchange of oxygen, carbon dioxide, and water vapour, enabling plants to carry out processes like photosynthesis and respiration effectively.

Hence, the correct option is 4) Both a and b

Q2. Respiratory pathway is called as amphibolic pathway because

1. It includes catabolic reactions

2. It includes anabolic reactions

3. It includes catabolic and anabolic reactions both

4. None of these

Correct answer: 4) None of these

Explanation:

Catabolic Role: In the respiration context, catabolism is the breakdown of glucose and other substrates for energy (ATP) through glycolysis, citric acid cycle, or oxidative phosphorylation. That energy would play an important role in lots of cell functions.
Anabolic Role: The respiratory pathway supplies intermediates for anabolic processes, such as amino acid synthesis, fatty acid synthesis, and nucleotide synthesis. These can be diverted out of the ways of catabolic reactions to be used in support of the synthesis of complex molecules needed for cell growth and repair.
It will then be possible to consider that as an amphibolic pathway, therefore, the respiratory pathway is an essential part of maintaining cellular metabolism through the production of energy and biosynthesis processes. This further provides a holistic view of metabolic flexibility within biochemical pathways and their interrelations.

Hence, the correct answer is option 4) None of these.

Q3. Which of the following exhibits the highest rate of respiration?

1. Growing shoot apex

2. Germinating seed

3. Root tip

4. Leaf bud

Correct answer: 2) Germinating seed

Explanation:

Germination is the process that has the maximum growth rate as compared to other options and has the highest respiration rate. During germination, the seed undergoes rapid metabolic activity to support cell division and elongation, resulting in a high growth rate. The increased respiration rate provides the necessary energy (ATP) for these intense developmental processes.

Hence, the correct answer is option 2) Germinating seed.

Also Read:

• MCQs on Respiration in Plants

• TCA Cycle

• Amphibolic Pathway

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