Page 978 - Fundamentals of anatomy physiology
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Chapter 25 Metabolism and Energetics 965
25-2 Carbohydrate metabolism results: (1) It traps the glucose molecule within the cell, because
involves glycolysis, ATP production, phosphorylated glucose cannot cross the plasma membrane;
and gluconeogenesis
and (2) it prepares the glucose molecule for further biochemical
Learning Outcome Describe the basic steps in glycolysis, the citric acid
cycle, and the electron transport system, and summarize the energy yields reactions.
of glycolysis and cellular respiration.
A second phosphorylation takes place in the cytosol before
Most cells generate ATP and other high-energy compounds by
breaking down carbohydrates, especially glucose. We can sum- the 6-carbon chain is broken into two 3-carbon fragments.
marize the complete reaction as follows:
Then energy benefits begin to appear as these fragments are
converted to pyruvate. Two of the steps release enough energy
to generate ATP from ADP and inorganic phosphate (PO432 or
Pi). In addition, 2 molecules of NAD are converted to NADH.
The net reaction of glycolysis looks like this:
C6H12O6 + 6O2 ¡ 6CO2 + 6H2O Glucose + 2NAD + 2ADP + 2 Pi ¡
glucose oxygen carbon water 2 Pyruvate + 2 NADH + 6ATP
dioxide This reaction sequence is anaerobic (without oxygen) and
provides the cell a net gain of 2 molecules of ATP for each glucose
The breakdown takes place in a series of small steps. Several molecule converted to two pyruvate molecules. A few highly spe-
of these steps release sufficient energy to convert ADP to ATP. cialized cells, such as red blood cells, lack mitochondria and de-
The complete catabolism of one molecule of glucose provides rive all their ATP through glycolysis. Skeletal muscle fibers rely on
a typical body cell a net gain of 36 molecules of ATP. glycolysis for ATP production during periods of active contraction.
Most cells can survive for brief periods using the ATP provided
Most ATP production takes place inside mitochondria, but by glycolysis alone. However, when oxygen is readily available,
the first steps take place in the cytosol. In Chapter 10 we in- mitochondrial activity provides most of the ATP that cells need.
troduced the process of glycolysis, which breaks down glucose
in the cytosol into smaller molecules that mitochondria can Mitochondrial ATP Production
absorb and utilize. p. 343 These reactions are said to be
anaerobic because glycolysis does not require oxygen. The subse- For the cell, glycolysis yields an immediate net gain of 2 ATP mol-
quent reactions take place in mitochondria. These reactions use ecules for each glucose molecule it breaks down. However, a great
oxygen and are considered aerobic. The mitochondrial activity deal of additional energy is still stored in the chemical bonds of
responsible for ATP production is called aerobic metabolism, pyruvate. The cell’s ability to capture that energy depends on the
also known as cellular respiration. availability of oxygen. If oxygen supplies are adequate, mitochon-
dria absorb the pyruvate molecules and break them down. As you
Glycolysis will see in the next section, the hydrogen atoms of each pyruvate
molecule (CH3—CO—COO−) are removed by coenzymes and
Glycolysis (glı.-KOL-i-sis; glykus, sweet 1 lysis, a loosening) are ultimately the source of most of the cell’s energy gain. The car-
is the breakdown of glucose to pyruvic acid. In this pro- bon and oxygen atoms are removed and released as carbon diox-
cess, a series of enzymatic steps breaks the six-carbon glucose ide in a process called decarboxylation (de.-kar-boks-i-LA. -shun).
molecule (C6H12O6) into two 3-carbon molecules of pyruvic
acid (CH3—CO—COOH). At the normal pH inside cells, each 25Recall that a double membrane surrounds each mitochon-
pyruvic acid molecule loses a hydrogen ion and exists as the
negatively charged ion CH3—CO—COO−. This ionized form drion. The outer membrane contains large pores that are perme-
is called pyruvate. able to ions and small organic molecules such as pyruvate. Ions
and molecules easily enter the intermembrane space separating the
Glycolysis requires (1) glucose molecules; (2) appropriate outer membrane from the inner membrane. The inner membrane
cytosolic enzymes; (3) ATP and ADP; (4) inorganic phosphates; contains a carrier protein that moves pyruvate into the mitochon-
and (5) NAD (nicotinamide adenine dinucleotide), a coenzyme drial matrix, where it is broken down through the citric acid cycle.
that removes hydrogen atoms during one of the enzymatic
reactions. Recall from Chapter 2 that coenzymes are organic The Citric Acid Cycle
molecules that are essential to enzyme function. p. 81 If any
of these participants is missing, glycolysis cannot take place. In the mitochondrion, a pyruvate molecule participates in a
complex reaction involving NAD and another coenzyme called
Figure 25–3 provides an overview of the steps in glycolysis. coenzyme A, or CoA. This reaction yields 1 molecule of carbon
Glycolysis begins when an enzyme phosphorylates—that is, dioxide, 1 of NADH, and 1 of acetyl-CoA (a-SE. -til KO. -a. )—
attaches a phosphate group—to the last (sixth) carbon atom a 2-carbon acetyl group (CH3CO) bound to coenzyme A.
of a glucose molecule, creating glucose-6-phosphate. This This sets the stage for a sequence of enzymatic reactions
step “costs” the cell 1 ATP molecule. It has two important
