Metabolism—How do we obtain
energy from foods?
Susan Algert, Ph.D., R.D.
Metabolism
• Human body releases energy from
chemical bonds in nutrients the
body uses for fuel.
• As bonds break they release energy
• During metabolism, energy, water
and carbon dioxide are released
Energy yielding nutrients
• From carbohydrates—glucose
• From lipids (trigylcerides)—
glycerol and fatty acids
• From proteins—amino acids
The Cell
• Cells are work centers of metabolism
• Cells have similar structures
• Two basic parts—nucleus and
cytoplasm
• Mitochondria are power generators that
contain energy generating pathways
Breaking down glucose for
energy--aerobic
• 6-C glucose split in half making two 3Carbon compounds
• Glycolysis means glucose splitting
• -Carbon compounds become 2
pyruvates
• Pyruvates will break down further to
form ATP and heat
Glucose retrieval via the Cori
cycle--anaerobic
• When less oxygen is available, pyruvate is
converted to lactic acid
• Liver can convert lactic acid to glucose in a
recycling process
• Pathway is muscle glycogen to glucose to
pyruvate to lactic acid ( in liver) to glucose
to glycogen
Pyruvate to Acetyl Co-A
• Irreversible step
• Aerobic
• Acetyl Co A to Carbon Dioxide via the TCA
cycle
Electron Transport Chain
• Acetyl Co-A to fat
Pyruvate is pivotal
• ATP levels are low—metabolic pathways
flow toward the production of ATP
• Depending on O-2; ATP routes pyruvate to
acetyl Co-A or lactate
• ATP is abundant; pyruvate converted to
oxaloacetate or amino acid alanine;
oxaloacetate converted to glucose and then
glycogen
Acetyl Co-A at the crossroads
• Breakdown pathways for glucose, fatty
acids and some amino acids converge at
acetyl-CoA.
• Acetyl Co-A cannot return to pyruvate, but
enters energy making pathways
• Acetyl Co-A can also make ketone bodies
and fatty acids
Glycerol and fatty acids
• Glycerol to pyruvate
• Fatty acids to Acetyl Co-A
Beta oxidation
• Glucose not retrievable from fatty acids
• Breakdown of acetyl-Co-A
• Fat burns in flame of carbohydrate
When a person draws on stores
• Fat used to fuel brain
• Acetyl Co-A fragments from fatty acids
combine to produce ketone bodies
• Ketone bodies can provide some fuel for
brain cells
• When ketone bodies contain an acid group
they are called keto acids (COOH)
Energy Compounds
• ATP used to power cellular functions
• NADH and FADH-2 carry energy for
synthesis of ATP
• NADPH delivers energy for
biosynthesis
Amino Acid Catabolism
• Amino acids are deaminated and enter TCA
cycle
• Amino acids used to make pyruvate can
make glucose
• Amino acids that make Acetyl Co-A
provide energy or body fat but not glucose
Energy retrievable from amino
acids• Glucogenic amino acids- a.a. broken down
into pyruvate or intermediate of the TCA
cycle; gluconeogenesis
• Ketogenic amino acids—an a.a.a broken
down into acetyl CoA which can be
converted into ketone bodies
Transamination
• Transfer of amino group from one amino
acid to a keto acid, producing a new non
essential amino acid and a keto acid
Electron Transport Chain
• Series of proteins that serve as electron
carriers
• Mounted in sequence on membrane inside
mitochondria
• Carriers receive electrons, it passes
electrons and gives up energy until end
when any usable energy has captured body’s
ATP molecules
TCA and ETC
• Body’s most efficient means of capturing
the energy from nutrients and transferring it
into the bonds of ATP
• Last step of ETC low energy electrons with
H atoms combine with O2 from the lungs to
make H2O
Which fuels can make glucose
• Parts of protein and fat that can make
pyruvate can provide glucose; parts that
make acetyl Co A cannot, but provide fat
• Glucose is needed to fuel CNS and red
blood cells
• If there is not enough glucose the body will
break down protein
Making glucose on low Carb
diets
• Fat delivers mostly acetyl, so that you need
to break down protein tissue to make
glucose
• High protein diets make your body convert
protein to glucose and convert ammonia to
urea in the liver
• Urea is excreted via the kidneys
• Water is needed to excrete urea
Energy yielding nutrients-fat
provides most kcals per gram
• Nearly all bonds in a fatty acid are between
carbons and hydrogens
• Oxygen can be added to all of them
(making CO2 and H2O)
• Energy in bonds is released as they are
oxidized
• Glucose has less potential for oxidation as
oxygen is already bonded to each C
Feasting
• Surplus protein—Deaminate and
convert to acetyl Co-A and fat
• Surplus carbohydrate--Glycogen
• Surplus fat--Lipogenesis