ENERGY FLOW IN METABOLISM
Energy in metabolism often flows in terms of electrons. If electrons are lost, this is called oxidation. If electrons are gained, this is called reduction. Oxidation is coupled to reduction; that is, if something gets oxidized, then something else gets reduced.
In most of the oxidations and reductions which we will study, electrons will be moved with protons (H+). Watching the hydrogens therefore provides a convenient way to tell if a molecule has been oxidized or reduced.
Also, in many of the oxidation-reduction reactions we will look at, the molecule nicotinamide adenine dinucleotide (NAD) will serve as an electron-shuttle. NAD can become reduced to NADH2, and then carry the electrons to some other reaction and become oxidized back to NAD. In other words, NAD can pick up electrons from one reaction and carry them to another.
Note that when a molecule gets oxidized it loses energy. Also, the more reduced a molecule is the more energy it contains. (See pgs 110 - 111, figs. 5.9 and 5.10 for descriptions of NAD and oxidation-reduction reactions.)
The ultimate goal in many instances of catabolism will be to take energy from a (food source) molecule, trap the energy and store it as ATP.
There are three ways to make ATP:
1.) substrate level phosphorylation - where a high energy phosphate on the (food source) molecule gets transferred directly onto ADP converting it to ATP.
2.) oxidative phosphorylation - where a (food source) molecule is oxidized and the energy is extracted from the electrons by an electron transport chain. The extracted energy is then used to make ATP by a process known as chemiosmosis.
3.) photophosphorylation - where light energy is used to generate electrons and then the energy is extracted from the electrons by an electron transport chain. As in oxidative phosphorylation, the extracted energy is used to make ATP by chemiosmosis.
THE CATABOLISM OF GLUCOSE: AN OVERVIEW
(1.) GLYCOLYSIS - Embden Meyerhof Pathway
(See pg. 115 for details of the pathway.)
a. the partial oxidation of a glucose molecule (a 6- carbon molecule) into 2 pyruvic acid molecules (3-carbon molecules).
b. uses 2 ATP's and makes 4 ATP's
c. makes 2 NADH2
(2.) FERMENTATION (See pg. 122-125 for (3.) RESPIRATION (See pg. 117 for
discussion and diagrams of discussion of the Kreb's cycle. See
fermentation, pg. 6 for a description pgs. 118-120 for discussion of the
of sourdough bread.) electron transport chain.)
a. Organic molecule is the final a. the final electron acceptor is
electron acceptor. These organic inorganic and the process represents
substances are things like alcohol and the complete oxidation of glucose to
organic acids. This represents the CO2 and water
incomplete oxidation of glucose.
b. Anaerobic process. Doesn't require b. The process is usually aerobic
oxygen or any other inorganic electron wherein oxygen serves as the inorganic
acceptor. final electron acceptor. Some
organisms are capable of anaerobic
respiration using other inorganic
molecules as terminal electron
acceptors.
c. No electron transport. No citric c. Both the electron transport chain
acid cycle. and the citric acid cycle operate in
respiration.
d. Small amount of ATP produced - the d. Alot of ATP and NADH2 is produced.
ATP is made during glycolysis.
e. Regenerates NAD from NADH2 so that e. NADH2 is used to make ATP. It is
more glycolysis can take place but no by this means that NAD is regenerated.
ATP is made from the NADH2.
What is the electron transport chain?
It is a series of enzymes imbedded in a membrane. These enzymes use the membrane to set up a chemiosmotic gradient of hydrogen ions. This gradient of hydrogen ions is called a proton motive force and this force supplies the energy for an ATP synthetase.
The electron transport chain enzymes are a series of oxidation-reduction electron carrier molecules and proton pumps. These enzymes use the energy in the electrons to move protons against a concentration gradient to form the proton motive force.
CLASSIFICATION OF ORGANISMS BY NUTRITIONAL PATTERN:
ENERGY SOURCE
light -- phototroph
oxidation reduction reactions -- chemotroph
CARBON SOURCE
carbon dioxide -- autotroph
organic compounds -- heterotroph