Following glycolysis, the mechanism of cellular respiration involves another multi-step process—the Krebs cycle, which is also called the citric acid cycle or the tricarboxylic acid cycle. The Krebs cycle uses the two molecules of pyruvic acid formed in glycolysis and yields high-energy molecules of NADH and flavin adenine dinucleotide (FADH2), as well as some ATP.
The Krebs cycle occurs in
the mitochondrion of a cell (see Figure 6-1). This sausage-shaped organelle
possesses inner and outer membranes and, therefore, inner and outer compartments.
The inner membrane is folded over itself many times; the folds are called cristae. They are somewhat similar to
the thylakoid membranes in chloroplasts (see Chapter 5). Located along the
cristae are the important enzymes necessary for the proton pump and for ATP
Prior to entering the Krebs
cycle, the pyruvic acid molecules are altered. Each three-carbon pyruvic acid
molecule undergoes conversion to a substance called acetyl-coenzyme A, or
acetyl-CoA. During the process, the pyruvic acid molecule is broken down by an
enzyme, one carbon atom is released in the form of carbon dioxide, and the
remaining two carbon atoms are combined with a coenzyme called coenzyme A. This
combination forms acetyl-CoA. In the process, electrons and a hydrogen ion are
transferred to NAD to form high-energy NADH.
Acetyl-CoA enters the Krebs
cycle by combining with a four-carbon acid called oxaloacetic acid. The
combination forms the six-carbon acid called citric acid. Citric acid undergoes
a series of enzyme-catalyzed conversions. The conversions, which involve up to
ten chemical reactions, are all brought about by enzymes. In many of the steps,
high-energy electrons are released to NAD. The NAD molecule also acquires a
hydrogen ion and becomes NADH. In one of the steps, FAD serves as the electron
acceptor, and it acquires two hydrogen ions to become FADH2. Also,
in one of the reactions, enough energy is released to synthesize a molecule of
ATP. Because for each glucose molecule there are two pyruvic acid molecules
entering the system, two ATP molecules are formed.
Also during the Krebs
cycle, the two carbon atoms of acetyl-CoA are released, and each forms a carbon
dioxide molecule. Thus, for each acetyl-CoA entering the cycle, two carbon
dioxide molecules are formed. Two acetyl-CoA molecules enter the cycle, and
each has two carbon atoms, so four carbon dioxide molecules will form. Add
these four molecules to the two carbon dioxide molecules formed in the
conversion of pyruvic acid to acetyl-CoA, and it adds up to six carbon dioxide
molecules. These six CO2 molecules are given off as waste gas in the
Krebs cycle. They represent the six carbons of glucose that originally entered
the process of glycolysis.
At the end of the Krebs
cycle, the final product is oxaloacetic acid. This is identical to the
oxaloacetic acid that begins the cycle. Now the molecule is ready to accept
another acetyl-CoA molecule to begin another turn of the cycle. All told, the
Krebs cycle forms (per two molecules of pyruvic acid) two ATP molecules, ten
NADH molecules, and two FADH2 molecules. The NADH and the FADH2
will be used in the electron transport system.