Download Carbohydrate Catabolism in the Presence of Oxygen Releases a

6.2
 Cellular respiration: the set of metabolic
reactions used by cells to harvest energy from
food
 A lot of energy is released when reduced
molecules with many C—C and C—H bonds are
fully oxidized to CO2.
 The oxidation occurs in a series of small steps,
allowing the cell to harvest about 34% of the
energy released.

Catabolism of glucose under aerobic conditions (in the presence
of O2), occurs in three linked biochemical pathways:

Glycolysis—glucose is converted to pyruvate.
Pyruvate oxidation—pyruvate is oxidized to acetyl CoA and CO2.

Citric acid cycle—acetyl CoA is oxidized to CO2.

 Glycolysis
 Ten reactions
 Takes place in the cytosol
 Final products:
 2 molecules of pyruvate (pyruvic acid)
 2 molecules of ATP
 2 molecules of NADH
 Steps 6 and 7 are examples of reactions that occur repeatedly in
metabolic pathways:
 Oxidation–reduction (step 6): exergonic; glyceraldehyde 3-
phosphate is oxidized and energy is trapped via reduction of NAD+
to NADH.
 Substrate-level phosphorylation (step 7): also exergonic; energy
released transfers a phosphate from 1,3-bisphosphoglycerate to
ADP, forming ATP.
 Pyruvate Oxidation
 Occurs in mitochondria in eukaryotes.
 Products: CO2 and acetate; acetate is then bound to coenzyme A (CoA) to form
acetyl CoA.
 NAD+ is reduced to NADH.
 Citric Acid Cycle
 Eight reactions
 Occurs in mitochondria in eukaryotes
 Operates twice for every glucose molecule that enters glycolysis
 Starts with Acetyl CoA; acetyl group is oxidized to two CO2
 Oxaloacetate is regenerated in the last step
 Final reaction of citric acid cycle:
 Cells transfer energy from NADH and FADH2 to ATP by oxidative
phosphorylation:
 NADH oxidation is used to actively transport protons (H+) across the inner
mitochondrial membrane, resulting in a proton gradient.
 Diffusion of protons back across the membrane then drives the synthesis of
ATP.
 When NADH is reoxidized to NAD+, O2 is reduced to H2O:
 NADH + H+ + ½ O2
®
NAD+ + H2O
 This occurs in a series of redox electron carriers, called the respiratory
chain, embedded in the inner membrane of the mitochondrion.
 Electron transport: electrons from the oxidation of NADH and
FADH2 pass from one carrier to the next in the chain.
 The oxidation reactions are exergonic, energy released is used to
actively transport H+ ions across the membrane.
 Oxidation is always coupled with reduction.
 When NADH is oxidized to NAD+, the reduction reaction is the
formation of water from O2.
 2 H+ + 2 e– + ½ O2
®H O
2
 The key role of O2 in cells is to act as an electron acceptor and
become reduced.
 ATP synthase uses the H+ gradient to drive synthesis of ATP by
chemiosmosis:
 Chemiosmosis: Movement of ions across a semipermeable barrier
from a region of higher concentration to a region of lower
concentration.
 ATP synthase converts the potential energy of the proton gradient
into chemical energy in ATP.
 ATP synthase is a molecular motor with two subunits:
 F0 is a transmembrane domain that functions as the H+ channel.
 F1 has six subunits. As protons pass through F0, it rotates, causing part
of the F1 unit to rotate.
 ADP and Pi bind to active sites that become exposed on the F1
unit as it rotates, and ATP is made.
 ATP synthase structure is similar in all organisms.
 In prokaryotes, the proton gradient is set up across the cell
membrane.
 In eukaryotes, chemiosmosis occurs in mitochondria and chloroplasts.
 The mechanism of chemiosmosis is similar in almost all forms of
life.
 Chemiosmosis can be demonstrated experimentally.
 A proton gradient can be introduced artificially in chloroplasts or
mitochondria in a test tube.
 ATP is synthesized if ATP synthase, ADP, and inorganic phosphate
are present.
 About 32 molecules of ATP are produced for each fully oxidized
glucose.
 The role of O2: most of the ATP is formed by oxidative
phosphorylation, which is due to the reoxidation of NADH.
 Some bacteria and archaea use other electron acceptors.
 Geobacter metallireducens can use iron (Fe3+) or uranium, making it
potentially useful in environmental cleanup.