Chapter
7
Metabolism
•by
•Norman D. Sossong, MD, PhD
•for NSCC:
•NTR150 – Spring 2008
Metabolism Defined
• = the sum total of all the chemical reactions
within organisms that enable them to maintain
life
Energy Sources for Metabolism
• The chemical reactions of metabolism require energy
• Where does that energy come from?
• It comes from the chemical energy in the molecules that are
within our cells
• Chemical energy is the energy stored in the bonds between
atoms of a molecule
• How did it get there?
• Organisms either ingest the energy-containing molecules or
make the molecules that contain the chemical energy
• Animals generally ingest the energy-rich molecules
• Many plants make the energy-rich molecules through photosynthesis
Photosynthesis
• By way of review:
• Plants containing chlorophyll or similar molecules
absorb light (usually from the sun)
• With the help of the energy contained in light, the
plant cells can take carbon dioxide (CO2) from the
air and water (H2O) from the ground and convert it
into a carbohydrate, usually glucose (C6H12O6)
• The “waste product” of this process is oxygen (O2)
• The chemical bonds in the glucose contain the
energy absorbed from the light
The Laws Governing Energy:
The Laws of Thermodynamics
• The First Law of Thermodynamics
• The “Conservation of Energy”
• Energy is neither created nor destroyed (but it may
change form)
• The Second Law of Thermodynamics
• The movement of energy from one place to another
is never 100% efficient; there is always some loss of
“useful” energy
• The waste energy is called “heat”
Energy:
Fuel for Work
• Energy source
• Chemical energy in
carbohydrates, fat,
protein
• Food energy to cellular
energy
• Stage 1: digestion,
absorption, transport
• Stage 2: breakdown of
molecules
• Stage 3: transfer of
energy to a form cells
can use
What Is Metabolism?
• Catabolism
• Reactions that
breakdown
compounds into
small units
• Anabolism
• Reactions that build
complex molecules
from smaller ones
The Cell is the Metabolic Processing Center
• Nucleus*
• Cytoplasm
• Cytosol + organelles
• The organelles
•
•
•
•
Endoplasmic Reticulum (ER) ± Ribosomes
Golgi Apparatus
Lysosome
Mitochondrion
• * Some would classify the nucleus as an organelle
Energy Currency in
the Body
• ATP is the body’s energy
currency
• ATP = adenosine
triphosphate
• Form of energy cells use
• The useful energy is stored
in the phosphate-phosphate
bonds
• AMP + Pi + E
• → ADP + Pi + E
• → ATP
Energy Currency in the Body
• GTP is another form of the body’s energy
currency where guanine is used instead of
adenine
Other Energy Currencies in the Body
• NADH, FADH2, & NADPH
• Are all high-energy carriers
• These must all be converted to ATP (or GTP) to
be useful for the body’s various chemical
reactions
• Analogy: Consider the usefulness of a hundredor a thousand-dollar bill compared with that of a
twenty-dollar bill
NADH
&
FADH2
as
Energy
Carriers
Breakdown and Release of Energy
• Extracting energy from carbohydrates
•
•
•
•
•
Glycolysis
Pyruvate → Acetyl CoA
Citric Acid Cycle = Krebs Cycle
Electron Transport Chain
End products
• Extracting energy from fats
• Extracting energy from proteins
Extracting Energy from Carbohydrates
• Glycolysis
• Takes place in cytoplasm
• Anaerobic (no oxygen required)
• Pathway splits glucose into 2 pyruvates
• Glucose (6-C) → 2 Pyruvate (3-C)
•
•
•
•
Input: 2 ATP
Output: 2NADH + 4 ATP
Net: Output – Input = 2 NADH + 2 ATP
Applies to Glucose  Fructose  Galactose
• Results are the same
• Note: RBCs use only glycolysis
Glycolysis
• Note:
• Input: 2 ATP
• Output:
2NADH + 4 ATP
• Net: Output – Input
= 2 NADH + 2 ATP
Breakdown and Release of Energy
• Extracting energy from carbohydrate
• Pyruvate (3-C) to Acetyl CoA (2-C)
•
•
•
•
•
•
Aerobic
Releases CO2
Transfers electrons to NAD
→ NADH
Pyruvate can enter mitochondria
but Acetyl CoA cannot leave
• Pyruvate to lactate (3-C)
• Anaerobic
Pyruvate to Acetyl CoA
Breakdown and Release of Energy
• Extracting energy from carbohydrate
• Citric acid cycle = Krebs Cycle
• Releases CO2
• 2 for each Acetyl CoA
• Produces 1 GTP (like ATP)
• Transfers electrons to
• 3 NAD → 3 NADH
and 1 FAD → 1 FADH2
Krebs Cycle or the
Citric Acid Cycle
Breakdown and Release of Energy
• Extracting energy from carbohydrate
• Electron transport chain
• Accepts electrons from NAD
and FAD
• Produces large amounts of ATP
• i.e. converts NADH & FADH2
into ATP
• Produces water
• End products of glucose
breakdown
• ATP, H2O, CO2
Breakdown and Release of Energy
• Revised (1998) estimates of ATP production in
Electron Transport Chain
• Originally:
NADH → 3 ATP
•
FADH2 → 2 ATP
•
Total Glucose Energy = 36-38 ATP
• Revised:
NADH → 2.5 ATP
•
FADH2 → 1.5 ATP
•
Total Glucose Energy = 30-32 ATP
Breakdown and Release of Energy
• Extracting energy from carbohydrates
•
•
•
•
•
Glycolysis
Pyruvate → Acetyl CoA
Citric Acid Cycle = Krebs Cycle
Electron Transport Chain
End products
• Extracting energy from fats
• Extracting energy from proteins
Extracting energy from fat
• Split triglycerides into glycerol and fatty acids
• Note: this is the reverse of the synthesis of
triglycerides
Glycerol
• A 3-carbon molecule
• The cell can convert it to the 3-carbon
pyruvate
• Pyruvate can be handled exactly the same as
it was when the source was glucose
• First, convert it to acetyl CoA
• Then, use it the way acetyl CoA was used when
the source was glucose
Recall that Fatty Acids Have a Chain Length of
• 4-24 carbons
• Always an
even number
Extracting energy from fat
• Beta-oxidation
• Breaks apart fatty acids into 2-carbon fragments
what are converted to the 2-carbon molecule acetyl CoA
• Transfers electrons to NAD and FAD
• The acetyl CoA can be handled just as it was
when the source was glucose
Extracting energy from fat
• Krebs cycle = Citric acid cycle
• Acetyl CoA from beta-oxidation enters cycle
• Electron transport chain
• End products of fat breakdown
• ATP, H2O, CO2
Summary: Extracting energy from fat
• Split triglycerides into glycerol and fatty acids
• Beta-oxidation
• Breaks apart fatty acids into acetyl CoA
• Transfers electrons to NAD and FAD
• Citric acid cycle
• Acetyl CoA from beta-oxidation enters cycle
• Electron transport chain
• End products of fat breakdown
• ATP, H2O, CO2
Breakdown and Release of Energy
• Extracting energy from carbohydrates
•
•
•
•
•
Glycolysis
Pyruvate → Acetyl CoA
Citric Acid Cycle = Krebs Cycle
Electron Transport Chain
End products
• Extracting energy from fats
• Extracting energy from proteins
Extracting Energy from Proteins
• First, split the protein into amino acids
• Note the variety of amino acids available
The Amino
Acids
Extracting Energy from Amino Acids
• Split off amino group
• Converted to urea for
excretion
Extracting Energy from Amino Acids
• Carbon skeleton enters breakdown pathways
• Recall that the skeleton for any amino acid will be
different than the next
• 2-Carbon amino acids can be converted to acetyl CoA
• 3-carbon amino acids can be converted to pyruvate
• 4-carbon amino acids can be converted to one of the
molecules used in the Krebs cyle
• Once in the pathway for glucose, the rest proceeds as
before
• End products
• ATP, H2O, CO2, + urea
Breakdown
and Release
of Energy
Biosynthesis and Storage
• Making carbohydrate (glucose)
• Gluconeogenesis
• Uses pyruvate, lactate, glycerol, certain amino acids
• Storing carbohydrate (glycogen)
• Liver, muscle make glycogen from glucose
• Making fat (fatty acids)
• Lipogenesis
• Uses acetyl CoA from fat, amino acids, glucose
• Storing fat (triglyceride)
• Stored in adipose tissue
Biosynthesis and Storage
• Making ketone bodies (ketogenesis)
• Made from acetyl CoA
• Inadequate glucose in cells
• Making protein (amino acids)
• Amino acid pool supplied from
• Diet, protein breakdown, cell synthesis
Regulation of Metabolism
• May favor either anabolic or catabolic
functions
• Regulating hormones
•
•
•
•
Insulin
Glucagon
Cortisol
Epinephrine
Special States
• Feasting
• Excess energy
intake from
carbohydrate, fat,
protein
• Promotes storage
Special States
• Fasting
• Inadequate
energy intake
• Promotes
breakdown
• Prolonged
fasting
• Protects body
protein as
long as possible