Lecture Notes - Chapter 9
Homework - Review questions 3-7 (p. 182)
Using Chemical Energy to Drive Metabolism
Big Picture - Obtaining energy from the foods we eat is a multi-step process.
First, enzymes active in digestion break the large food molecules down into smaller
molecules. These molecules are taken up by the cells of your body, where other enzymes
harvest energy from the C-H bonds in these molecules and use it to power the production
of ATP.
I.
A. Cellular Respiration –
1) Carbohydrate catabolism –
2) Lipid catabolism –
3) Protein catabolism –
The production of ATP may occur by both anaerobic (doesn’t require oxygen)
and aerobic (requires oxygen) methods. Anaerobic ATP production produces
only 2 ATP molecules for each glucose molecule; aerobic ATP production
(oxidative respiration) produces 36 ATP/glucose.
II. Harvesting Energy By Extracting Electrons
A. A Closer Look at Oxidation/Reduction –
The catabolism of glucose is an oxidation-reduction reaction. Energy is
transferred (or released) when an electron is shifted from a donor molecule to
an acceptor molecule. This energy transfer/release can then be used to make
ATP.
B. NAD+ Harvests the Energy in Stages - cells strip the six hydrogens in the C-H
bonds of glucose in a series of enzyme-catalyzed reactions referred to as
glycolysis and the Krebs cycle. In these reactions, the hydrogen atoms (two at
a time - 2 electrons, 2 protons) are removed by transferring them to a
coenzyme carrier (NAD+ or FAD). The two electrons carried by the coenzyme
carriers are then passed along an electron transport chain, which consists of a
series of molecules embedded within the inner membranes of the
mitochondria. At the end of the chain, the electrons are captured by oxygen,
which joins with hydrogen ions to form water.
III. ATP Production
Substrate-level phosphorylation –
Oxidative phosphorylation –
IV. Stage One: Glycolysis
Glycolysis is a process that involves ten reactions that convert glucose into two
three-carbon molecules of pyruvate. Each molecule of glucose results in the net
production of two ATP molecules by substrate-level phosphorylation.
A. Net Products of Glycolysis –
B. The Regeneration of NAD+ In the process of glycolysis, there will be an accumulation of NADH and a
depletion of NAD+. The NADH can be recycled to NAD+ by passing on the
electrons and the hydrogen atom. This can occur in two ways –
1) oxidative respiration - through a series of electron transfers, the electrons
and H+ atom can be donated to oxygen to form water (aerobic
metabolism)
2) fermentation - when no oxygen is available, the hydrogen can be passed to
an organic molecule (anaerobic metabolism)
V. Stage Two: the Oxidation of Pyruvate
Takes place inside the mitochondria. The oxidation of the pyruvate occurs via a
decarboxylation reaction in which a molecule of CO2 is cleaved off by pyruvate
dehydrogenase enzyme, producing an acetyl group and a pair of electrons associated with
a hydrogen, which is then used to reduce NAD+ to NADH. The acetyl group becomes
complexed to a cofactor called coenzyme A (CoA), forming acetyl-CoA.
A. Net Products of Pyruvate Oxidation –
VI. Stage Three: The Krebs Cycle
In mitochondria. Nine reactions involved in the oxidation of acetyl-CoA. The
process begins when acteyl-CoA combines with a molecule called oxaloacetate to form
citrate.
The succinyl-CoA contains a high-energy bond between the succinyl group and the CoA.
When this bond is cleaved, it drives the formation of GTP from GDP + Pi. GTP can then
drive the production of a molecule of ATP. The final product of one turn of the cycle is
oxaloacetate, which will then react with another molecule of acetyl-CoA.
A. Net Products of the Krebs Cycle –
VII. Stage Four: The Electron Transport Chain
The NADH molecules carry the electrons gained during the oxidation of glucose
to the inner mitochondrial membrane. The FADH2 is already attached to the inner
membrane, and so does not need to diffuse in. There, the NADH and FADH2 are
oxidized by membrane-embedded proteins, releasing the two electrons and one hydrogen
cation for each molecule. The electrons are passed down a series of proteins called
cytochromes, and most of the energy they possess is used to drive several proton pumps,
which serve to pump the released hydrogen cations out of the matrix into the
intermembrane space. The final protein of this electron transport chain is the cytochrome
c oxidase complex, which uses four electrons and four hydrogen cations to reduce a
molecule of oxygen to form two molecules of water. (O2 + 4H+ + 4e-  2 H2O) This is
the final electron acceptor of oxidative respiration.
A. Chemiosmosis –the electrons harvested in oxidative respiration are used to
pump a large number of protons across the inner mitochondrial membrane.
Their re-entry into the mitochondrial matrix drives the synthesis of ATP.
VIII. Summarizing Aerobic Respiration