Summary 9–1 Chemical Pathways

Cellular Respiration
Food is the energy source for cells. The energy in food is measured
in calories. A calorie is the amount of energy needed to raise the
temperature of 1 gram of water 1 degree Celsius. The Calorie
(capital C) used on food labels is equal to 1000 calories.
Cells do not burn glucose or other food compounds. They
gradually release the energy. The process begins with a pathway
called glycolysis.

Glycolysis is the process in which a glucose molecule is split
in half. This forms two molecules of pyruvic acid, a 3-carbon
compound. Glycolysis takes place in the cytoplasm of a cell.
Through glycolysis, the cell gains 2 ATP molecules. In addition,
the electron carrier NAD
accepts a pair of high-energy electrons,
producing NADH. By doing this, NAD
helps pass energy from
glucose to other pathways in the cell.

When oxygen is not present, fermentation follows glycolysis.
Fermentation releases energy from food molecules by forming
ATP. Fermentation does not need oxygen, so it is said to be
anaerobic. During fermentation, cells convert NADH back into
the electron carrier NAD
that is needed for glycolysis. This lets
glycolysis continue to make a steady supply of ATP. The two
types of fermentation are alcoholic fermentation and lactic
acid fermentation.

• Yeasts and a few other microorganisms carry out alcoholic
fermentation. The equation for alcoholic fermentation after
glycolysis is:
pyruvic acid
NADH ➝ alcohol
CO2
NAD

• Lactic acid fermentation occurs in muscles during rapid
exercise. The equation for lactic acid fermentation after
glycolysis is:
pyruvic acid
NADH ➝ lactic acid
NAD

If oxygen is present, the Krebs cycle and electron transport
chain follow glycolysis. Together, these pathways make up
cellular respiration. Cellular respiration is the process that
releases energy by breaking down glucose and other food
molecules in the presence of oxygen. Cellular respiration takes
place in mitochondria. The equation for cellular respiration is:
6O2
C6H12O6 ➝ 6CO2
6H2O
Energy
oxygen
glucose ➝ carbon dioxide
water
Energy

9–2 The Krebs Cycle and Electron Transport
Cellular respiration requires oxygen, so it is said to be aerobic.
The Krebs cycle is the second stage of cellular respiration.
During the Krebs cycle, pyruvic acid is broken down into
carbon dioxide in a series of energy-extracting reactions.
The Krebs cycle is also known as the citric acid cycle, because
citric acid is one of its first products.
Here are the stages of the Krebs cycle.

• The Krebs cycle starts when pyruvic acid formed by
glycolysis enters the mitochondrion.
• The pyruvic acid is broken down into carbon dioxide and
a 2-carbon acetyl group.
• The two carbons of the acetyl group join a 4-carbon compound
to produce citric acid. The Krebs cycle continues in
a series of reactions. In these reactions, two energy carriers
accept high-energy electrons. NAD
is changed to NADH,
and FAD is changed to FADH2. These molecules carry the
high-energy electrons to the electron transport chain. The
carbon dioxide is released as a waste product.

The electron transport chain uses the high-energy electrons
to change ADP into ATP. In the electron transport chain, highenergy
electrons move from one carrier protein to the next. At
the end of the chain, oxygen pulls electrons from the final carrier
molecule. These electrons join with hydrogen ions, forming water.
Each transfer along the chain releases a small amount of energy.
ATP synthase uses the energy to produce ATP.

Glycolysis produces 2 ATP molecules from one molecule
of glucose. The Krebs cycle and the electron transport chain let the
cell form 34 ATP molecules per glucose molecule. The total, then,
for cellular respiration is 36 ATP molecules per glucose molecule.
The energy flows in photosynthesis and cellular respiration
occur in opposite directions. On a global level, photosynthesis and
cellular respiration are also opposites. Photosynthesis removes
carbon dioxide from the atmosphere and puts back oxygen.
Cellular respiration removes oxygen from the atmosphere and
puts back carbon dioxide.