Chapters 7 & 8
Metabolism &
Energy Balance
METABOLISM
Metabolism – the sum total of all chemical
reactions that take place in living cells
Metabolic Organs
• Digestive System – to ingest, digest
& absorb nutrients
• Liver – the “control panel” and most
active processing center in the body
1. Receives absorbed nutrients first
2. Manufactures important compounds
3. Detoxifies drugs & metabolic wastes
METABOLISM
• Pancreas
1. Secretes digestive enzymes
2. Produces insulin & other hormones
involved in glucose regulation
• Heart & Blood Vessels
1. Carry nutrients & O2 to cells
2. Return cell wastes
• Kidneys
1. Filter wastes
2. Produce important body compounds
ENERGY METABOLISM
• After absorption, 2 processes occur:
1. Anabolism – “building reactions”
to create larger molecules
Example: Glucose>>>Glycogen
2. Catabolism – “breakdown
reactions” to release energy
Example: Glycogen>>>Glucose
Chemical Reactions
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ENERGY METABOLISM
• Energy Metabolism – how the body
obtains & uses energy from foods or
body stores
• Nutrients in the body that generate energy
1. CHO>>>Glucose (liver converts
fructose & galactose to glucose)
2. Fat (Triglycerides)>>>Glycerol &
Fatty Acids
3. Protein>>>Amino Acids
ENERGY PATHWAYS
• Glucose breakdown pathway =
glycolysis
1. The 6- carbon glucose is split
in half forming two 3-carbon
compunds that are converted
to glucose
2. The net yield of one glucose
molecule is two pyruvate
molecules
3. This process does not require
oxygen and is reversible
Glycolysis:
Glucose-toPyruvate
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Glucose
• The fate of pyruvate
– Anaerobic vs. aerobic pathways
 Anaerobic
-Pyruvate converted to lactic acid
in the muscles
-Lactic acid converted to glucose
in the liver (Cori cycle)
-The glucose can travel back to the
muscles
-Used when the body needs
energy quickly
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Glucose
 Aerobic
- Pyruvate converted to acetyl
CoA
- This process is irreversible
- Acetyl CoA may be used to
synthesize fat or to generate
ATP (the TCA cycle)
- Slower energy expenditure
but can be sustained longer
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The TCA Cycle
The Final Pathway - Electron Transport Chain
and ATP Synthesis
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ENERGY PATHWAYS
Fat (triglycerides) breakdown
pathway>>>Glycerol & Fatty
acids
1. Glycerol can be used to
make glucose or to form
pyruvate and then Acetyl
CoA
FATTY ACIDS
2. Fatty Acids can be oxidized to
form Acetyl CoA and then
proceed through TCA cycle
-Fatty acids cannot form
pyruvate so they cannot be
converted to glucose
Fats Enter the Energy Pathway
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ENERGY PATHWAYS
• Protein breakdown pathway:
1. Amino acids must be deaminated
to lose their nitrogen-containing
amino group
2. Then ~50% of amino acids are
converted to Pyruvate
3. Others are converted to Acetyl
CoA or enter the TCA cycle
directly
Amino Acids
• Amino acids-to-acetyl CoA
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ENERGY PATHWAYS
• Amino acids are only used for
energy if:
-Consumed in excess
-Inadequate energy is
available from carbohydrate or
the body is given only fat
forcing the breakdown of
protein tissue to make glucose
The Paths of Pyruvate & Acetyl CoA
Copyright 2005 Wadsworth Group, a division of Thomson Learning
Copyright 2005 Wadsworth Group, a division of Thomson Learning
ENERGY BUDGET
• Feasting – when a person eats more
than he/she needs, excess energy
will be stored as:
1. Glycogen – excess carbohydrate
2. Body fat – excess carbohydrate,
fat, protein, or alcohol
3. Metabolism favors fat formation
especially when the excess
energy is derived from dietary fat
Economics of Feasting
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Energy Budget
• Fasting – when a person doesn’t eat
enough, the body draws on its
energy stores (glycogen & fat) to
meet its constant demand
• If a person chooses not to eat,
he/she is fasting; if not by choice,
he/she is starving
• The body makes no such distinction
ENERGY BUDGET
• During a fast or starvation, glucose will be
released from stored glycogen to fuel the
brain & central nervous system
1. Glycogen stores will only last ~1/2
day
2. Other cells in the body can use
stored fat, but the brain requires
glucose; thus, body proteins begin
rapid breakdown to supply glucose
to brain
REMEMBER: Fatty acids cannot form glucose
ENERGY BUDGET
Eventually, the nervous system adapts to using
stored body fat.
1. AcetlyCoAs combine to form ketones
(acidic compounds that arise from the
incomplete breakdown of fat)
2. High blood ketone levels (ketonemia)
+ ketones in urine (ketonuria) = ketosis
3. Protein continues to breakdown, but at a
slower rate to conserve vital protein
tissues as long as possible
Economics of Fasting
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ENERGY BUDGET
• As fasting continues:
1. Metabolism slows
2. Appetite is suppressed
3. Wasting of lean body tissue
occurs
4. Susceptibility to disease
increases
5. Body temperature drops