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The process of respiration is how the respiratory substrate is decomposed to release energy. The two basic mechanisms of cellular respiration are aerobic and anaerobic respiration.
- Aerobic respiration requires the presence of oxygen while anaerobic respiration does not.
- The ratio of oxygen consumed to carbon dioxide evolved during respiration at constant pressure and temperature is known as the respiratory quotient.
- Glycolysis, the TCA cycle, the electron transport chain, and oxidative phosphorylation are the metabolic pathways that break down the substrate.
- Through these processes, cells are able to make and store ATP, and carbon dioxide is produced as a byproduct.
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Key Terms: respiration, respiratory quotient, dimensionless, glycolysis, indirect calorimetry, insulin.
What is Respiratory Quotient?
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Respiratory Quotient is the ratio of the amount of carbon dioxide exhaled to the amount of oxygen received during breathing. It is also known as the respiratory ratio (RQ).
- The respiratory quotient depends on the substrate that has been digested.
- This is so because the ratio is calculated as the sum of the O2 molecules consumed and the CO2 molecules emitted.
Respiratory Ratio
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The respiratory ratio is denoted as follows –
RQ = Volume of CO2 released/Volume of O2 absorbed
- It is determined for a specific substrate, including carbohydrates, organic acids, fats, and proteins.
- Through aerobic respiration, the oxidation of carbohydrates produces an equal amount of CO2 emission and oxygen consumption.
Protein, fat, and anaerobe all have RQs of 0.7, 0.8, and 0 respectively.
Characteristics of Respiratory Quotient
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Any substance or living thing has a respiratory quotient that ranges from 0.7 to 1.0.
- The higher the degree of RQ in the molecule, the less oxygen it consumes or the more carbon dioxide it produces.
- A respiratory quotient is a dimensionless number.
- The basal metabolic rate is calculated using the respiratory quotient.
- Since more oxygen is required when oxidising carbohydrates than when oxidising fatty substances, the respiratory quotient of carbohydrate oxidation often stays low.
As insulin enhances the lipid storage of substances during respiration, the presence of insulin in substances causes a higher level of the respiratory quotient.
Terms Related to Respiratory Quotient
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Cellular Respiration: Cellular respiration is the process by which cells convert organic molecules, such as carbohydrates, fats, and proteins, into usable energy in the form of ATP (adenosine triphosphate). It involves a series of biochemical reactions that take place in the mitochondria of cells.
Aerobic Respiration: Aerobic respiration is a type of cellular respiration that occurs in the presence of oxygen. It is the most efficient way of generating ATP and involves the complete oxidation of respiratory substrates, such as glucose, to produce carbon dioxide, water, and a large amount of ATP.
Anaerobic Respiration: Anaerobic respiration is a type of cellular respiration that occurs in the absence of oxygen. It is less efficient compared to aerobic respiration and involves the partial oxidation of respiratory substrates. Different types of anaerobic respiration exist, such as lactic acid fermentation and alcoholic fermentation, which result in the production of lactic acid or ethanol, respectively.
Clinical Significance
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The clinical significance of the respiratory quotient is as follows -
Chronic Obstructive Pulmonary Disease: Airflow obstruction from the lungs is a symptom of chronic obstructive pulmonary disease (COPD), a chronic inflammatory lung disease.
The bronchioles become distorted and narrowed as a result of mucus production and chronic inflammation of the bronchioles, which restricts airflow during exhalation.
Non-Insulin Dependent Weight Gain: In non-insulin-dependent diabetic individuals, weight gain can be predicted using the respiratory quotient. Normal insulin-resistant receptors are present in diabetic patients, which causes hyperglycemia. This increases lipolysis by preventing glycolysis as a means of metabolization.
Nutrition Guide for Sick Patients: When determining caloric needs is difficult, there is an insufficient clinical response to a projected equation, or there are clinical signs of overeating or undereating, using indirect calorimetry and respiratory energy expenditure can help determine the appropriate number of calories patients should consume each day.
Applications of Respiratory Quotient
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- The nature of the persistent condition - liver cirrhosis can be determined using it.
- It is used to gauge feeding rate (i.e., overfeeding or underfeeding).
- When a diabetic patient is not dependent on insulin, it is used to predict the patient's weight gain.
- It is used to examine how the liver is working.
RQ for Different Respiratory Substrates
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RQ for different respiratory substrates are as follows -
Carbohydrates: RQ = 1.0
Carbohydrates, such as glucose, are the most common respiratory substrates. When glucose is completely oxidized in aerobic respiration, the ratio of CO2 produced to O2 consumed is 1.0. This means that for every molecule of glucose metabolized, one molecule of CO2 is produced and one molecule of O2 is consumed.
Fats/Lipids: RQ ≈ 0.7
Fats or lipids are energy-rich molecules and are also utilized as respiratory substrates. The oxidation of fatty acids during cellular respiration produces more reduced electron carriers (NADH and FADH2) compared to carbohydrates. As a result, more oxygen is consumed relative to the amount of CO2 produced, leading to an RQ value of approximately 0.7.
Things to Remember
- The kind of respiratory substrate that is dependent during respiration has an impact on the respiratory quotient.
- The respiratory quotient equals one when the substrate of carbohydrates is entirely oxidised.
- Adenosine triphosphate (ATP) molecules provide a variety of functions when a cell needs energy.
- Anaerobic and aerobic respiration are the two different forms of respiration.
- The respiratory quotient measures the proportion of oxygen absorbed to carbon dioxide generated during food metabolism.
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Sample Questions
Ques. What is the significance of the respiratory quotient? (1 mark)
Ans: The utilisation of fat and carbohydrates can be calculated using the "Respiratory Quotient," which is the proportion of oxygen consumption to carbon dioxide production. The oxidation of carbohydrates produces one molecule of carbon dioxide for every oxygen molecule ingested.
Ques. Why is aerobic respiration important? (5 marks)
Ans: Aerobic respiration is a crucial metabolic process that occurs in the cells of organisms, including humans. It plays a fundamental role in generating energy required for various biological functions. Here are some reasons why aerobic respiration is important:
Energy production: Aerobic respiration is highly efficient in producing adenosine triphosphate (ATP), the energy currency of cells. Through a series of chemical reactions, glucose is broken down in the presence of oxygen, yielding a large amount of ATP. This energy is utilized by cells for activities such as muscle contraction, cell division, and maintaining vital bodily functions.
Sustaining life: Oxygen is an essential component of aerobic respiration. By combining with glucose, it facilitates the release of energy required to support life processes. Without aerobic respiration, organisms would rely solely on anaerobic processes, which are less efficient and can lead to the accumulation of toxic byproducts.
Efficient energy utilization: Compared to anaerobic processes like fermentation, aerobic respiration produces significantly more ATP per glucose molecule. This allows organisms to extract more energy from the same amount of resources, enabling them to carry out more complex tasks and adapt to diverse environments.
Waste elimination: Aerobic respiration also plays a role in eliminating waste products. Carbon dioxide, a byproduct of this process, is expelled through the respiratory system. The removal of carbon dioxide helps maintain the pH balance in the body and prevents the accumulation of toxic levels of this waste product.
Ques. What are the steps involved in aerobic respiration? (5 marks)
Ans: Aerobic respiration is a complex process that occurs in the cells of organisms, involving several steps. Here are the main steps involved in aerobic respiration:
Glycolysis: Aerobic respiration begins with glycolysis, which occurs in the cytoplasm. In this step, one molecule of glucose (a six-carbon sugar) is broken down into two molecules of pyruvate (a three-carbon compound). This process requires an investment of two ATP molecules but produces four ATP molecules and two molecules of NADH (nicotinamide adenine dinucleotide).
Pyruvate Decarboxylation: The two molecules of pyruvate produced in glycolysis move into the mitochondria. Before entering the next stage, each pyruvate molecule is converted into Acetyl Coenzyme A (Acetyl CoA). This step involves the removal of one carbon in the form of carbon dioxide and the transfer of electrons to NAD+ to form NADH.
Citric Acid Cycle (Krebs cycle): Acetyl CoA enters the citric acid cycle, which takes place in the mitochondrial matrix. Here, Acetyl CoA combines with oxaloacetate (a four-carbon compound) to form citrate (a six-carbon molecule). Through a series of chemical reactions, citrate is gradually broken down, releasing carbon dioxide and transferring high-energy electrons to carrier molecules such as NADH and FADH2. These carriers will play a role in the next step.
Electron Transport Chain (ETC): The high-energy electrons from NADH and FADH2 produced in the previous steps enter the electron transport chain, located in the inner mitochondrial membrane. This is where the majority of ATP production occurs. The electrons pass through a series of protein complexes, which pump protons (H+) across the membrane, creating an electrochemical gradient. The final electron acceptor is oxygen, which combines with protons to form water. This process releases a large amount of energy used to generate ATP through oxidative phosphorylation.
ATP Synthesis: The electrochemical gradient established during the electron transport chain powers ATP synthesis. Protons flow back across the membrane through an enzyme called ATP synthase. This movement drives the synthesis of ATP from adenosine diphosphate (ADP) and inorganic phosphate (Pi).
Ques. Define anaerobic respiration. (2 marks)
Ans: Anaerobic (cellular) respiration, a respiratory process in which cells break down sugar molecules to produce energy in the absence of oxygen, is a respiratory mechanism used by both prokaryotes and eukaryotes. While fermentation only involves the glycolysis phase, other anaerobic respiration processes use the electron transport chain system to move electrons from the source to the final electron acceptor.
Ques. What is indirect calorimetry? (2 marks)
Ans: The study of metabolic processes is known as indirect calorimetry. While indirect calorimetry measures breathing gases like oxygen (O2) and carbon dioxide (CO2) that are influenced by metabolism to meet energy needs, direct calorimetry measures total body heat produced directly, such as through a thermally sealed container.
Ques. Why a respiratory quotient is a dimensionless number? (2 marks)
Ans: The ratio of the amount of carbon dioxide exhaled during respiration to the amount of oxygen received is known as the respiratory quotient (RQ), also known as the respiratory ratio (RQ). When estimated from carbon dioxide production to oxygen absorption, it is a dimensionless quantity that is employed in computation for basal metabolic rate.
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