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.

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Types of Respiration

Respiration is of two main types:

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.

Diagram of aerobic respiration

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.

Pathway of Anaerobic Respiration

Cellular Respiration

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.

Mitochondria: The Powerhouse of the Cell

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.

Structure of Mitochondria

ATP (Adenosine Triphosphate): The Energy Currency

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 in Organisms

Phases of Respiration in Organisms In prokaryotic cells, 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.

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

Net Gain of ATP

Glycolysis is impressive in that it gains 2 ATP molecules against a cost of 1 ATP in this process. While cycling through both these reactions 4 ATP molecules are synthesized, and 2 ATP molecules are used up in early steps thus giving a net ATP of 2. Also, glycolysis generates 2 molecules of NADH and 2 molecules of pyruvate that are used in the later steps of cellular respiration.

Krebs Cycle (Citric Acid Cycle)

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.

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.

Net Gain of ATP and Other High-Energy Molecules

For each acetyl-CoA that enters the Krebs cycle, the net gain includes: For each acetyl-CoA that enters the Krebs cycle, the net gain includes:

• 1 ATP (or GTP): Synthesised through this process since they are dependent on the substrate level for their phosphorylation.

• 3 NADH: Aformed during the oxidative decarboxylation steps and the regeneration of oxaloacetate.

• 1 FADH₂: Present during the process of oxidation of succinate to fumarate.

Since each glucose molecule produces two acetyl-CoA molecules, the overall yield per glucose molecule is: Since each glucose molecule produces two acetyl-CoA molecules, the overall yield per glucose molecule is:

The transport cost is equivalent to 2 ATP (or GTP) per cycle of rotation of one of the subunits.

• 6 NADH

• 2 FADH₂

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).

Role of Oxygen in the ETC

Oxygen serves as the last of the pass on the ETC. It reacted with electrons and protons from water which is very critical in the flow of electrons in the chain. If oxygen is lacking, the mentioned ETC would be ceased and, thus, no ATP would be synthesised resulting in cellular energy depletion.

Net Gain of ATP

Oxidative phosphorylation the final stage in ATP manufacture yields 26 to 28 ATP’s for every molecule of glucose through the electron transport chain. The following estimate takes into consideration, ATP produced from the electrons transported by NADH and FADH₂ generated during glycolysis, the Krebs cycle and other pathways. Therefore, the ETC is vital for the highest level of energy generation given the energy demand by the cell through cellular respiration.

Factors Affecting Respiration

The respiration process is affected by the following factors:

Temperature

• Respiration is affected by temperature since it has an impact on the activity of enzymes.

• Temperature relations are related to enhanced respiration which can rise to an optimum level, to which enzymes may deteriorate.

• Temperature affects the rate of respiration this is true as colder temperatures slow down respiration in enzymes.

Oxygen Concentration

• Oxygen is needed in aerobic respiration prevalent in most organisms including animals and fungs.

• More oxygen enables the rate of respiration to increase due to the increased formation of ATP while in low oxygen. The cells are forced to undergo and rely on fermentation.

Carbon Dioxide Concentration

• Higher levels of CO₂ causes the blood pH to decrease and this triggers a faster rate of respiration to get rid of the excess CO₂.

• High CO₂ levels also represent high metabolic activity. Thus, they influence the general rates of respiration.

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