Metabolism
• Metabolism involves two main processes, catabolism and
anabolism
• Catabolic reactions break down large, complex molecules to
provide smaller molecules and energy (ATP)
• Anabolic reactions use ATP energy to build larger molecules
from smaller building blocks
Stages of Catabolism
Catabolic reactions are organized into three stages:
• In Stage 1, digestion breaks down large molecules into
smaller ones that enter the bloodstream
• In Stage 2, molecules enter the cells and are broken down
into two- and three-carbon compounds
• In Stage 3, compounds are oxidized in the citric acid cycle
to provide energy (ATP) for anabolic processes
Stages of Catabolism (Diagram)
Eukaryotic Cell Structure
• Metabolic reactions occur at specific sites within the cell
Components of Eukaryotic Cells
ATP and Energy
• In cells, energy is stored in adenosine triphosphate (ATP)
- there are other cellular energy sources, but ATP is the main
one
Hydrolysis of ATP
• The hydrolysis of ATP to ADP releases 7.3 kcal (31 kJ/mole)
ATP  ADP + Pi + 7.3 kcal (31 kJ/mole)
• The hydrolysis of ADP to AMP releases 7.3 kcal (31 kJ/mole)
ADP  AMP + Pi + 7.3 kcal (31 kJ/mole)
ATP and Muscle Contraction
• Muscle fibers contains
filaments of actin and
myosin
• When a nerve impulse
increases [Ca2+], the
filaments slide closer
together to contract the
muscle
• The hydrolysis of ATP in
muscle provides the
energy for contraction
• As Ca2+ and ATP decrease,
the filaments return to the
relaxed position
Coenzyme NAD+
• When a compound is oxidized by an enzyme, 2H as 2H+ and 2eare removed by a coenzyme, which is reduced
• NAD+ (nicotinamide adenine dinucleotide) participates in
reactions that produce a carbon-oxygen double bond (C=O)
• For example, NAD+ participates in the oxidation of ethanol:
Overall Reaction:
+
OH
O
Alcohol
dehydrogenase
NAD +
+
H
Oxidation:
O
+
OH
2H +
+
2e
-
H
Reduction:
NAD+
+
2H
+
+
2e
-
NADH
+
H
+
NADH
+
H
+
Structure of Coenzyme NAD+
• NAD+ (nicotinamide adenine dinucleotide) contains ADP, ribose,
and nicotinamide (from niacin, B3)
• NAD+ reduces to NADH when the nicotinamide group accepts
H+ and 2e-
Coenzyme FAD
• FAD participates in reactions that produce a carbon-carbon
double bond (C=C)
Oxidation
—CH2—CH2—

—CH=CH—
+ 2H+
Reduction
FAD
+
2H+ +
2e-

FADH2
+ 2e-
Structure of Coenzyme FAD
• FAD (flavin adenine dinucleotide) contains ADP and riboflavin
(vitamin B2)
• FAD reduces to FADH2 when flavin accepts 2H+ and 2e-
Coenzyme A (CoA)
• CoA activates acyl groups, such as the two-carbon acetyl group
for transfer to other compounds
• It consists of pantothenic acid (vitamin B5), phosphorylated ADP
and an aminoethanethiol
Digestion of Carbohydrates (Stage 1)
• In the mouth, salivary amylase hydrolyzes -glycosidic bonds
in polysaccharides to give smaller polysaccharides (dextrins),
maltose, and some glucose
• In the small intestine, pancreatic amylase hydrolyzes dextrins
to maltose and glucose
• The disaccharides maltose, lactose, and sucrose are hydrolyzed
to monosaccharides in the small intestine
• The monosaccharides enter the bloodstream
- fructose and galactose are transported to the liver, where they
are isomerized to glucose
- glucose is transported to cells for metabolism
Overview of Stage 1 of Carbohydrate Catabolism
Glycolysis (Stage 2)
• In Stage 2 of carbohydrate
catabolism, the metabolic
pathway called glycolysis
degrades glucose (6C)
obtained from digestion to
pyruvate (3C)
• Glycolysis is an anaerobic
process that takes place in
the cytoplasm
Energy-Investing Phase of Glycolysis
In reactions 1-5 of glycolysis:
• Energy is used to add phosphate groups to glucose and fructose
• Glucose is converted to two three-carbon molecules
Energy-Producing Phase of Glycolysis
• In reactions 6-10, the hydrolysis of phosphates generates
four ATP molecules
• Two NAD+ coenzymes are also reduced
Glycolysis, Overall Reaction
• Glycolysis generates 2 ATP and 2 NADH
• Two ATP are used in energy-investment to add phosphate groups
to glucose and fructose-6-phosphate
• Four ATP are formed in energy-generation by direct transfers of
phosphate groups to four ADP
Overall Reaction:
Glucose + 2ADP + 2Pi + 2NAD+

2Pyruvate + 2ATP + 2NADH + 4H+
Regulation of Glycolysis
• The amount of glucose that goes through glycolysis is
regulated based on relative levels of ATP, ADP and AMP, as
well as other glycolysis intermediates
• This regulation takes place at three steps:
- Reaction 1: Hexokinase is inhibited by high levels of
glucose-6-phosphate, which prevents the phosphorylation of
glucose
- Reaction 3: Phosphofructokinase, an allosteric enzyme, is
inhibited by high levels of ATP and activated by high levels
of ADP and AMP
- Reaction 10: Pyruvate kinase, another allosteric enzyme
is inhibited by high levels of ATP or acetyl CoA