Cellular Respiration and Fermentation (Catabolic pathways yield energy by…
Cellular Respiration and Fermentation
Catabolic pathways yield energy by oxidizing organic fuels
One type of catabolic process, fermentation, leads to the partial degradation of sugars without the use of oxygen
A more efficient and widespread catabolic process, aerobic respiration, consumes oxygen as a reactant to complete the breakdown of a variety of organic molecules
Although cellular respiration technically includes both aerobic and anaerobic processes, the term is commonly used to refer only to the aerobic process
Reactions that result in the transfer of one or more electrons from one reactant to another are oxidation-reduction reactions, or redox reactions.
The loss of electrons from a substance is called oxidation. The addition of electrons to another substance is called reduction.
X, the electron donor, is the reducing agent and reduces Y by donating an electron to it. Y, the electron recipient, is the oxidizing agent and oxidizes X by removing its electron.
The hydrogen atoms are not transferred directly to oxygen but are passed first to a coenzyme called NAD+
(nicotinamide adenine dinucleotide).
Glycolysis occurs in the cytosol. It begins catabolism by breaking glucose into two molecules of pyruvate
Glycolysis harvests chemical energy by oxidizing glucose to pyruvate
During glycolysis, glucose, a six-carbon sugar, is split into two three-carbon sugars
The net yield from glycolysis is 2 ATP and 2 NADH per glucose.
Glycolysis can occur whether or not O2 is present.
After pyruvate is oxidized, the citric acid cycle completes the energyyielding oxidation of organic molecules
After pyruvate enters the mitochondrion via active transport, it is converted to a compound called acetyl coenzyme A, or acetyl CoA.
More than three-quarters of the original energy in glucose is still present in the two molecules of pyruvate
This step, linking glycolysis and the citric acid cycle, is carried out by a multienzyme complex that catalyzes three reactions
Due to the chemical nature of the CoA group, a sulfur-containing compound derived from a B vitamin, acetyl CoA has a high potential energy
During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis
The inner mitochondrial membrane couples electron transport to ATP synthesis
Most of the remaining electron carriers between ubiquinone and oxygen are proteins called cytochromes
A protein complex in the cristae, ATP synthase, actually makes ATP from ADP and inorganic phosphate
This process, in which energy stored in the form of a hydrogen ion gradient across a membrane is used to drive cellular work such as the synthesis of ATP, is called chemiosmosis
The H+ gradient that results is the proton-motive force, a gradient with the capacity to do work
Fermentation and anaerobic respiration enable cells to produce ATP without the use of oxygen
However, glycolysis generates 2 ATP whether oxygen is present (aerobic) or not (anaerobic)
In alcohol fermentation, pyruvate is converted to ethanol in two steps.
During lactic acid fermentation, pyruvate is reduced directly by NADH to form lactate (the ionized form of lactic acid) without the release of CO2
Obligate anaerobes carry out only fermentation or anaerobic respiration and cannot survive in the presence of oxygen
Yeast and many bacteria are facultative anaerobes that can survive using either fermentation or respiration
6 Glycolysis and the citric acid cycle connect to many other metabolic pathways
The rich energy of fatty acids is accessed as fatty acids are split into two-carbon fragments via beta oxidation
Feedback mechanisms control cellular respiration
The metabolic pathways of respiration also play a role in anabolic pathways of the cell.