Biology Krebs Cycle and Glycolysis Quiz

The Krebs Cycle

• The Krebs Cycle is a series of reactions that occurs in the matrix of mitochondria.
• It uses acetyl CoA from the link reaction and regenerates oxaloacetate.
• It produces reduced NAD, reduced FAD, and ATP.
• Oxaloacetate is regenerated through redox reactions, decarboxylation, oxidation (dehydrogenation), and substrate-linked phosphorylation.
• Decarboxylation of citrate releases two CO2 molecules as waste gas.
• Oxidation (dehydrogenation) of citrate releases H atoms that reduce coenzymes NAD and FAD, which are later used in oxidative phosphorylation.
• Substrate-linked phosphorylation transfers a phosphate from an intermediate to ADP, forming 1 ATP.
• Two cycles are required per glucose molecule because two acetyl-CoA are produced.
• Products of two cycles are:
  • Two ATP
  • Six NADH (reduced NAD)
  • Two FADH2 (reduced FAD)
  • Four CO2

Glycolysis

• It is the first stage of respiration.
• It doesn't require oxygen and is therefore the first step for both aerobic and anaerobic respiration.
• Glucose is only partially oxidized.
• It takes place in the cytoplasm of the cell.
• It involves trapping glucose by phosphorylation and oxidizing triose phosphate.
• It produces:
  • Two pyruvate molecules which move to the matrix of mitochondria for the link reaction.
  • Net gain of 2 ATP.
  • Two reduced NAD, which are used during oxidative phosphorylation.
• Under anaerobic conditions, it produces lactic acid or lactate instead of pyruvate.
• Steps of glycolysis involve:
  • Phosphorylation of glucose: Two ATP molecules are used to phosphorylate glucose, producing two molecules of triose phosphate and two ADP.
  • Oxidation of triose phosphate: Triose phosphate loses hydrogen, forming two pyruvate molecules. The hydrogen ions are collected by NAD, reducing the coenzyme to two reduced NAD or NADH. A net gain of two ATP molecules occurs.

The Electron Transport Chain

• It is the last stage of aerobic respiration.
• It takes place at the inner mitochondrial membrane.
• It results in the production of many ATP molecules and the production of water from oxygen.
• The chemiosmotic theory explains how energy is used:
  • Energy from electrons is passed through a chain of proteins in the membrane (electron transport chain).
  • This energy pumps protons (hydrogen ions) against their concentration gradient into the intermembrane space.
  • Protons flow by facilitated diffusion through ATP synthase into the matrix.
  • The energy of protons flowing down their concentration gradient is harnessed to phosphorylate ADP into ATP by ATP synthase.
• Outline of oxidative phosphorylation:
  • Reduced NAD (NADH) and reduced FAD (FADH2) from the Krebs Cycle donate hydrogen atoms.
  • Hydrogen atoms split into protons (H+ ions) and electrons.
  • High-energy electrons enter the electron transport chain and release energy as they move through the chain.
  • The energy released is used to transport protons across the inner mitochondrial membrane from the matrix into the intermembrane space.
  • A concentration gradient of protons is established.
  • Protons return to the matrix via ATP synthase, providing energy for ATP synthesis.
  • Oxygen acts as the final electron acceptor, combining with protons and electrons to form water.
• The electron transport chain is made up of a series of membrane proteins/electron carriers.
  • They are positioned close together to allow electrons to pass from carrier to carrier.
  • They pump protons across the membrane to establish the concentration gradient.

Respiratory Quotient (RQ)

• The RQ is the ratio of carbon dioxide molecules produced to oxygen molecules taken in during respiration.
• Formula: RQ = CO2 ÷ O2.
• Carbohydrates, lipids, and proteins have different RQ values due to the number of carbon-hydrogen bonds in each molecule:
  • More carbon-hydrogen bonds mean more hydrogen atoms for proton gradient formation, leading to more ATP production and higher oxygen requirement.
• When glucose is aerobically respired, equal amounts of carbon dioxide and oxygen are produced, resulting in an RQ value of 1.
• RQ can be calculated using a respirometer, which measures the volume of oxygen consumed (x) and the change in gas volume (y) after removing soda-lime.
• Equation for calculating RQ values: RQ = (x + y) ÷ x.