Respiration

• Cellular respiration


  The process that generates the energy needed for various biological processes in living organisms, including plants, animals, and microorganisms
• Cellular respirationprovides energy for:


  1. Metabolism (anabolic and catabolic reactions)

  2. Active transport

  3. Synthesising large molecules

  4. Replicating DNA

  5. Synthesising organelles

  6. Facilitating movement (e.g., cilia)

  7. Secretion and endocytosis
• Glycolysis occurs in the cytoplasm of cells
• Acetyl CoA


  Two-carbon fragment from pyruvate combined with coenzyme A
• Formation of Acetyl CoA

  Involves the reduction of NAD to NADH
• Link reaction


  • Occurs in the mitochondrial matrix
  • Bridges glycolysis with the Krebs cycle
• Krebs cycle


  • Takes place in the mitochondrial matrix
• Coenzymes


  NAD, FAD, and coenzyme A
• Coenzymes in cellular respiration

  • Facilitate oxidation-reduction reactions
  • Accept and donate electrons during metabolic processes
• NAD


  Accepts hydrogen atoms during glycolysis, the link reaction, and the Krebs cycle, forming NADH
• FAD


  Accepts hydrogen atoms during the Krebs cycle, producing FADH2
• Coenzyme A

  Assists in the formation of Acetyl CoA during the link reaction
• Anaerobic respiration yields less ATP compared to aerobic respiration due to the absence of oxygen, which prevents the complete oxidation of glucose and the efficient functioning of the electron transport chain
• Anaerobic respiration is essential for yeast in anaerobic environments, such as fermenting sugars to produce ethanol in alcoholic beverages
• Carbohydrates, lipids, and proteins as respiratory substrates


  • Carbohydrates provide a quick source of energy and yield approximately 4 kcal/g when oxidized
  • Lipids yield a higher energy value of approximately 9 kcal/g due to their higher energy density
  • Proteins can be utilized as energy substrates but yield the least energy, approximately 4 kcal/g, as they are primarily used for structural and regulatory functions in cells
• Respiratory quotient (RQ)


  • Measure of the ratio of carbon dioxide produced to oxygen consumed during cellular respiration
  • Calculated using the formula: RQ = CO2 Produced / O2 Consumed
  • An RQ value of 1 indicates complete carbohydrate metabolism, while values below 1 suggest lipid or protein metabolism
  • Interpretation of RQ values provides insights into the type of respiratory substrate being utilized by cells
• Practical investigations into the effect of factors on the rate of respiration

  1. Measure oxygen consumption or carbon dioxide production to assess the rate of respiration under various conditions

  2. Use respirometers to explore factors affecting the rate of cellular respiration, such as temperature, substrate concentration, and different respiratory substrates

  3. Analyze data using sensors, data loggers, and statistical techniques like standard deviation and Student's t-test to interpret the effects of factors on the rate of respiration and determine their significance in cellular metabolism
• Anaerobic respiration produces less ATP per molecule of glucose because NAD is not regenerated in oxidative phosphorylation
• Mitochondrion



  • Specialized organelle responsible for energy production through cellular respiration
  • Consists of inner and outer membranes, cristae, matrix, and mitochondrial DNA

  
• Glycolysis



  1. Phosphorylation of glucose to hexose bisphosphate
  2. Splitting of hexose bisphosphate into two triose phosphate molecules
  3. Oxidation of triose phosphate to pyruvate

  
• Glycolysis produces a net gain of two ATP molecules, two molecules of pyruvate, and two molecules of reduced NAD per glucose molecule

  
• The link reaction



  • Pyruvate is oxidised by enzymes to produce acetate, and carbon dioxide, requiring the reduction of NAD to NADH
  • Combination with coenzyme A to form acetyl coenzyme A (acetyl CoA)

  It produces:

  • Acetyl coA
  • Carbon dioxide (CO2)
  • Reduced NAD (NADH)

  
• The Krebs cycle



  1. Acetyl CoA (2C) enters the Krebs Cycle from the link reaction in glucose metabolism or directly from the breakdown of fatty acids and amino acids.
  2. The 4C molecule oxaloacetate accepts the 2C acetyl group from acetyl CoA, forming 6C citrate.
  3. Coenzyme A is released in this reaction.
  4. Citrate is converted back to oxaloacetate through a series of oxidation-reduction (redox) reactions.

  
• Krebs cycle yield


  The total yield from one glucose molecule (2 pyruvate molecules) is six NADH, two FADH2, two ATP and 4 CO2

  
• Oxidative phosphorylation



  1. Electron transport chain (ETC): High-energy electrons from NADH and FADH2 are passed through a series of protein complexes in the inner mitochondrial membrane, releasing energy
  2. Proton pumping: As electrons move through the ETC, protons (H+) are pumped across the inner mitochondrial membrane into the intermembrane space, creating a proton gradient
  3. ATP synthesis: Protons flow back into the mitochondrial matrix through ATP synthase channels embedded in the inner membrane, driving the synthesis of ATP from ADP and inorganic phosphate (Pi)

  
• Site of oxidative phosphorylation



  Inner mitochondrial membrane

  
• Chemiosmosis


  1. Electron transport chain (ETC): Electrons transferred through the ETC create a proton gradient across the inner mitochondrial membrane
  2. Proton gradient: The accumulation of protons in the intermembrane space generates a proton gradient and an electrochemical gradient
  3. ATP synthase: Protons flow back into the mitochondrial matrix through ATP synthase channels, driving the synthesis of ATP from ADP and Pi. This process is known as chemiosmosis.

  
• Anaerobic respiration in mammals

  Lactate fermentation: In mammalian muscles that are deprived of oxygen, pyruvate itself acts as the hydrogen acceptor and is removed by converting it to lactate. Releasing NAD

  
• Anaerobic respiration in yeast

  Alcoholic fermentation: Pyruvate is decarboxylated to acetaldehyde/ethanal

  This accepts hydrogen from reduced NAD and is reduced to ethanol, releasing NAD.

  
• Aerobic Vs Anaerobic

  
• A summary of the reactions of aerobic respiration and the yield of ATP per molecule of glucose. Glycolysis occurs in cell cytoplasm and all other stages within the mitochondria.

  
• Respiratory Quotients


  carbohydrate 1.0

  protein 0.9

  fat 0.7