These carriers are involved in oxidation-reduction reactions and transfer 2 electrons.
Oxidation-Reduction Reactions
Definitions
Oxidation: Loss of electrons; does not always include the addition of oxygen.
Reduction: Gain of electrons; often involves the addition of hydrogen.
Electrons in organic redox reactions are typically transferred in the form of a hydride ion (a proton and two electrons), linking to hydrogenation and dehydrogenation reactions.
Reaction Dynamics
Oxidation and reduction occur coupled, as illustrated in the transitions of ADP and similar molecules during these reactions.
Respiation
Aerobic Respiration
Overall reaction for glucose oxidation:
C6H{12}O6 + 6O2
ightarrow 6CO2 + 6H2O + E
The change in free energy: ∆Gext−686extkcal/mol for glucose, with energy released needing to be produced in small steps.
Glycolysis
Overview
Glycolysis is the process that converts glucose (C₆) into two pyruvate (C₃):
A 10-step biochemical pathway.
Takes place in the cytoplasm of both prokaryotic and eukaryotic cells.
Results in:
Formation of 2 pyruvate
Net production of 2 ATP molecules by substrate-level phosphorylation
Production of 2 NADH molecules through reduction of NAD+
Glucose is a fundamental substrate in energy metabolism:
Primary energy source for most cells.
Acquired from dietary carbohydrates (e.g., starch, sucrose, lactose) and stored glucose (glycogen in animals and starch in plants).
Overview of Glycolysis Steps
The glycolytic process can be divided into two phases:
Energy Investment Phase:
Consumes 2 ATP.
Energy Generation Phase:
Produces 4 ATP, resulting in a net gain of 2 ATP.
Involves production of 2 NADH from glyceraldehyde 3-phosphate.
Overall pathway from Glucose (C₆) to Pyruvate (C₃) illustrated through multiple steps:
Glucose → Glucose 6-phosphate → Fructose 6-phosphate → Fructose 1,6-bisphosphate (and so on through the breakdown to pyruvate).
Energy Expenditure and Gain in Glycolysis
Summary of ATP and NADH synthesis in glycolysis and overall energy gain outlined in multiple steps within the pathway leading to pyruvate production.
Fate of Pyruvate
Conditions Affecting Fate
The fate of pyruvate depends on the availability of oxygen:
Aerobic conditions:
Pyruvate is completely oxidized to CO2 and H2O.
Anaerobic conditions:
Fermentation occurs, including lactic acid and alcohol fermentation options.
Pyruvate Oxidation
Overview
In the presence of oxygen, pyruvate undergoes oxidation:
Occurs in the mitochondrial matrix of eukaryotes while happening in the cytosol for prokaryotes.
Process is mediated by pyruvate dehydrogenase, which is a multienzyme complex consisting of over 60 different polypeptide chains, yielding products such as carbon dioxide and acetyl-CoA (2 carbons attached to coenzyme A).
Reaction Catalysis and Transport
The transport of pyruvate into mitochondria is facilitated via active transport due to its charge, while the products resulting from pyruvate oxidation include 1 CO2, 1 NADH, and 1 acetyl-CoA.
Krebs Cycle / TCA Cycle
Overview
The Krebs Cycle is responsible for the complete oxidation of acetyl-CoA:
Takes place in the mitochondrial matrix; consists of 8 total steps.
Overall reaction per turn:
Acetyl-CoA + 3NAD^+ + FAD + GDP + Pi
ightarrow 2CO2 + 3NADH + FADH_2 + GTP + CoA
Reaction Summary
Each turn of the Krebs cycle reduces:
3 NAD+ to NADH,
1 FAD to FADH2,
Produces 1 ATP/GTP,
Regenerates oxaloacetate and releases 2 CO2.
Upon reviewing what has been produced from glycolysis and pyruvate oxidation:
2 CO2, 4 NADH, 2 ATP (produced via substrate-level phosphorylation).
Free-Energy Changes vs. Glucose
Changes relative to glucose quantified and diagrammatic representation along with total energy accounting depicted, summarizing yields (2 NADH, 2 ATP, etc.) from glycolysis to Krebs cycle.