Photosynthesis and Respiration-Respiration

Glycolysis

Reactions of glycolysis:

• Phosphorylation of glucose to form glucose-6-phosphate (G-6-P) by hexokinase.
• Isomerization of G-6-P to fructose-6-phosphate (F-6-P) by phosphoglucose isomerase.
• Phosphorylation of F-6-P to fructose-1,6-bisphosphate (F-1,6-BP) by phosphofructokinase-1 (PFK-1).
• Cleavage of F-1,6-BP into glyceraldehyde-3-phosphate (GAP) and dihydroxyacetone phosphate (DHAP) by aldolase.
• Isomerization of DHAP to GAP by triose phosphate isomerase.
• Oxidation of GAP to 1,3-bisphosphoglycerate (1,3-BPG) by glyceraldehyde-3-phosphate dehydrogenase (GAPDH).
• Phosphorylation of 1,3-BPG to 3-phosphoglycerate (3-PG) by phosphoglycerate kinase.
• Isomerization of 3-PG to 2-phosphoglycerate (2-PG) by phosphoglyceromutase.
• Dehydration of 2-PG to phosphoenolpyruvate (PEP) by enolase.
• Transfer of phosphate from PEP to ADP to form ATP by pyruvate kinase.

Energy yield of glycolysis:

• 2 molecules of ATP are produced for each molecule of glucose undergoing glycolysis.
• 2 molecules of NADH are also produced for each molecule of glucose undergoing glycolysis.

Regulation of glycolysis:

• Glycolysis is regulated at multiple points, including:
  • Phosphorylation of glucose to G-6-P by hexokinase.
  • Phosphorylation of F-6-P to F-1,6-BP by phosphofructokinase-1 (PFK-1).
  • Phosphorylation of PEP to ATP by pyruvate kinase.

Significance of glycolysis:

• Glycolysis is the first stage of respiration and provides the starting point for the Krebs cycle and the electron transport chain.
• Glycolysis also produces ATP and NADH, which are used for energy production.

Krebs Cycle (Citric Acid Cycle)

Reactions of the Krebs cycle:

• Condensation of acetyl-CoA with oxaloacetate to form citrate by citrate synthase.
• Isomerization of citrate to isocitrate by aconitase.
• Oxidation of isocitrate to α-ketoglutarate by isocitrate dehydrogenase.
• Decarboxylation of α-ketoglutarate to succinyl-CoA by α-ketoglutarate dehydrogenase.
• Transfer of CoA from succinyl-CoA to GDP to form GTP by succinyl-CoA synthetase.
• Oxidation of succinate to fumarate by succinate dehydrogenase.
• Hydration of fumarate to malate by fumarase.
• Oxidation of malate to oxaloacetate by malate dehydrogenase.

Energy yield of the Krebs cycle:

• 2 molecules of ATP (or GTP) are produced for each molecule of acetyl-CoA entering the Krebs cycle.
• 3 molecules of NADH and 2 molecules of FADH2 are also produced for each molecule of acetyl-CoA entering the Krebs cycle.

Regulation of the Krebs cycle:

• The Krebs cycle is regulated at multiple points, including:
  • Condensation of acetyl-CoA with oxaloacetate to form citrate by citrate synthase.
  • Oxidation of isocitrate to α-ketoglutarate by isocitrate dehydrogenase.
  • Decarboxylation of α-ketoglutarate to succinyl-CoA by α-ketoglutarate dehydrogenase.

Significance of the Krebs cycle:

• The Krebs cycle is a central metabolic pathway that provides energy and intermediates for many other biochemical processes.
• The Krebs cycle also produces ATP, NADH, and FADH2, which are used for energy production.

Electron Transport Chain (ETC)

Components of the ETC:

• The ETC is composed of a series of protein complexes located in the inner mitochondrial membrane.
• The complexes are:
  • NADH dehydrogenase (Complex I)
  • Succinate dehydrogenase (Complex II)
  • Cytochrome b-c1 complex (Complex III)
  • Cytochrome c oxidase (Complex IV)

Reactions of the ETC:

• Electrons are passed from NADH and FADH2 to oxygen through the ETC.
• The electrons are passed through a series of redox reactions, which release energy.
• The energy released is used to pump protons across the inner mitochondrial membrane, creating a proton gradient.

Energy yield of the ETC:

• The ETC produces ATP by oxidative phosphorylation.
• Oxidative phosphorylation is the process of synthesizing ATP from ADP using the energy released from the proton gradient.

Regulation of the ETC:

• The ETC is regulated at multiple points, including:
  • The availability of NADH and FADH2.
  • The proton gradient across the inner mitochondrial membrane.

Significance of the ETC:

• The ETC is the final stage of respiration and produces the majority of ATP used for energy production.

Oxidative Phosphorylation

Mechanism of oxidative phosphorylation:

• Oxidative phosphorylation is the process of synthesizing ATP from ADP using the energy released from the proton gradient.
• The proton gradient is created by the electron transport chain (ETC).
• The protons flow back down the proton gradient through ATP synthase, an enzyme that synthesizes ATP from ADP.

ATP synthesis during oxidative phosphorylation:

• As protons flow through ATP synthase, they cause a conformational change in the enzyme.
• This conformational change drives the synthesis of ATP from ADP and inorganic phosphate (Pi).

Regulation of oxidative phosphorylation:

• Oxidative phosphorylation is regulated at multiple points, including:
  • The availability of NADH and FADH2.
  • The proton gradient across the inner mitochondrial membrane.
  • The activity of ATP synthase.

Significance of oxidative phosphorylation:

• Oxidative phosphorylation is the final stage of respiration and produces the majority of ATP used for energy production.

Respiratory Quotient (RQ)

Definition of respiratory quotient:

• The respiratory quotient (RQ) is the ratio of the volume of carbon dioxide produced to the volume of oxygen consumed during respiration.

RQ values for different respiratory substrates:

• The RQ varies depending on the respiratory substrate used.
• For example, the RQ for glucose is 1.0, the RQ for fatty acids is 0.7, and the RQ for proteins is 0.8.

Significance of respiratory quotient:

• The RQ can be used to determine the type of respiratory substrate being used.
• The RQ can also be used to calculate the energy yield of respiration.

Anaerobic Respiration

Types of anaerobic respiration:

• There are two main types of anaerobic respiration:
  • Alcoholic fermentation
  • Lactic acid fermentation

Reactions of anaerobic respiration:

• Alcoholic fermentation:
  • Glucose is converted to ethanol and carbon dioxide.
• Lactic acid fermentation:
  • Glucose is converted to lactic acid.

Energy yield of anaerobic respiration:

• The energy yield of anaerobic respiration is lower than the energy yield of aerobic respiration.
• For example, the energy yield of alcoholic fermentation is only 2 ATP molecules per molecule