Chapter 25
Metabolism and Nutrition
• The food we eat is our only source of energy for performing
biological work.
• There are three major metabolic destinations for the
principle nutrients. They will be used for energy for active
processes, synthesized into structural or functional
molecules, or synthesized as fat or glycogen for later use as
energy.
Glucose
Catabolism
Coenzymes
• Two coenzymes are commonly used by living cells to carry
hydrogen atoms: nicotinamide adenine dinucleotide (NAD)
and flavin adenine dinucleotide (FAD).
• An important point to remember about oxidation-reduction
reactions is that oxidation is usually an energy-releasing
reaction.
Mechanisms of ATP Generation
• Phosphorylation is
– bond attaching 3rd phosphate group contains
stored energy
• Mechanisms of phosphorylation
– within animals
• substrate-level phosphorylation in cytosol
• oxidative phosphorylation in mitochondria
– in chlorophyll-containing plants or bacteria
• photophosphorylation.
Phosphorylation in Animal Cells
• In cytoplasm (1)
• In mitochondria (2, 3 & 4)
Carbohydrate Review
• In GI tract
– polysaccharides broken down into simple sugars
– absorption of simple sugars (glucose, fructose &
galactose)
• In liver
– fructose & galactose transformed into glucose
– storage of glycogen (also in muscle)
• In body cells --functions of glucose
– oxidized to produce energy
– conversion into something else
– storage energy as triglyceride in fat
Fate of Glucose
• Glucose can be used to form amino acids, which then can
be incorporated into proteins.
• Excess glucose can be stored by the liver and skeletal
muscles as glycogen, a process called glycogenesis.
• If glycogen storage areas are filled up, liver cells and fat
cells can convert glucose to glycerol and fatty acids that can
be used for synthesis of triglycerides (neutral fats) in the
process of lipogenesis.
Glucose Movement
into Cells
• In GI tract and kidney tubules
– Na+/glucose symporters
• Most other cells
– GluT facilitated diffusion
transporters
– insulin increases the insertion
of GluT transporters in the
membrane of most cells
– in liver & brain, always lots of
GluT transporters
• Glucose 6-phosphate forms
immediately inside cell (requires
ATP) thus, glucose is “hidden”
when it is in the cell.
– Concentration gradient
remains favorable for more
glucose to enter.
Glucose
Catabolism
Glucose
Oxidation
• Cellular respiration
– 4 steps are involved
– glucose + O2 produces
H2O + energy + CO2
• Anaerobic respiration
– called glycolysis (1)
– formation of acetyl CoA (2)
is transitional step to Krebs cycle
• Aerobic respiration
– Krebs cycle (3) and electron transport chain (4)
Glycolysis
• Glycolysis refers to the
breakdown of the six-carbon
molecule, glucose, into two
three-carbon molecules of
pyruvic acid.
– 10 step process occurring in
cell cytosol
– use two ATP molecules, but
produce four, a net gain of
two (Figure 25.3).
Glycolysis of Glucose & Fate of Pyruvic Acid
• Breakdown of six-carbon
glucose molecule into 2
three-carbon molecules of
pyruvic acid
– Pyruvic acid is
converted to acetylCoA,
which enters the Kreb’s
Cycle.
– The Kreb’s Cycle will
require NAD+
• NAD+ will be
reduced to the highenergy intermediate
NADH.
Glycolysis of Glucose &
Fate of Pyruvic Acid
When O2 falls short in a cell
– pyruvic acid is reduced
to lactic acid
• coupled to oxidation
of NADH to NAD+
• NAD+ is then
available for further
glycolysis
– lactic acid rapidly
diffuses out of cell to
blood
– liver cells remove lactic
acid from blood &
convert it back to
pyruvic acid
Pyruvic Acid
• The fate of pyruvic acid depends on the availability of O2.
Formation of Acetyl
Coenzyme A
• Pyruvic acid enters the
mitochondria with help
of transporter protein
• Decarboxylation
– pyruvate
dehydrogenase
converts 3 carbon
pyruvic acid to 2
carbon fragment
acetyl group plus
CO2.
Formation of Acetyl Coenzyme A
• 2 carbon fragment (acetyl
group) is attached to
Coenzyme A to form Acetyl
coenzyme A, which enter Krebs
cycle
– coenzyme A is derived from
pantothenic acid (B vitamin).
Krebs Cycle
• The Krebs cycle is also called
the citric acid cycle, or the
tricarboxylic acid (TCA) cycle.
It is a series of biochemical
reactions that occur in the
matrix of mitochondria (Figure
25.6).
Krebs Cycle
Krebs Cycle
• The large amount of chemical potential energy stored in
intermediate substances derived from pyruvic acid is
released step by step.
• The Krebs cycle involves decarboxylations and oxidations
and reductions of various organic acids.
• For every two molecules of acetyl CoA that enter the Krebs
cycle, 6 NADH, 6 H+, and 2 FADH2 are produced by
oxidation-reduction reactions, and two molecules of ATP are
generated by substrate-level phosphorylation (Figure 25.6).
• The energy originally in glucose and then pyruvic acid is
primarily in the reduced coenzymes NADH + H+ and FADH2.
Krebs Cycle (Citric Acid Cycle)
• The oxidation-reduction
& decarboxylation
reactions occur in matrix
of mitochondria.
– acetyl CoA (2C)
enters at top &
combines with a 4C
compound
– 2 decarboxylation
reactions peel 2
carbons off again
when CO2 is formed
• Potential energy (of chemical bonds) is released step by
step to reduce the coenzymes (NAD+NADH &
FADFADH2) that store the energy
Review:
Krebs Cycle
• Glucose 2 acetyl CoA molecules
• each Acetyl CoA
molecule that enters the Krebs
cycle produces
– 2 molecules of C02
– 3 molecules of NADH + H+
– one molecule of ATP
– one molecule of FADH2
Electron Transport Chain
• The electron transport chain involves a sequence of electron
carrier molecules on the inner mitochondrial membrane,
capable of a series of oxidation-reduction reactions.
• As electrons are passed through the chain, there is a
stepwise release of energy from the electrons for the
generation of ATP.
• In aerobic cellular respiration, the last electron receptor of
the chain is molecular oxygen (O2). This final oxidation is
irreversible.
• The process involves a series of oxidation-reduction
reactions in which the energy in NADH + H+ and FADH2 is
liberated and transferred to ATP for storage.
Electron
Transport Chain
• Pumping of
hydrogen is linked
to the movement of
electrons passage
along the electron
transport chain.
• It is called
chemiosmosis
(Figure 25.8.)
• Note location.
Chemiosmosis
• H+ ions are
pumped from matrix
into space between
inner & outer
membrane
• High concentration
of H+ is maintained
outside of inner
membrane
• ATP synthesis
occurs as H+
diffuses through a
special H+ channels
in the inner
membrane
Electron Transport Chain
Steps in Electron Transport
• Carriers of electron transport chain are clustered into 3 complexes that
each act as a proton pump (expelling H+)
• Mobile shuttles (CoQ and Cyt c) pass electrons between complexes.
• The last complex passes its electrons (2H+) to oxygen to form a water
molecule (H2O)
Proton Motive Force & Chemiosmosis
• Buildup of H+ outside the inner membrane creates + charge
– The potential energy of the electrochemical gradient is called the proton
motive force.
• ATP synthase enzymes within H+ channels use the proton motive force to
synthesize ATP from ADP and P
Summary of Aerobic Cellular Respiration
• The complete oxidation of glucose can be represented as
follows:
• C6H12O6 + 6O2 => 36 or 38ATP + 6CO2 +6H2O
• During aerobic respiration, 36 or 38 ATPs can be generated
from one molecule of glucose.
– Two of those ATPs come from substrate-level
phosphorylation in glycolysis.
– Two come from substrate-level phosphorylation in the
Krebs cycle.
Review
• Table 25.1 summarizes the ATP
yield during aerobic respiration.
• Figure 25.8 summarizes the sites
of the principal events of the
various stages of cellular
respiration.
Glycogenesis &
Glycogenolysis
• Glycogenesis
– glucose storage as
glycogen
– 4 steps to glycogen
formation in liver or
skeletal muscle
– stimulated by insulin
• Glycogenolysis
– glucose release
Glycogenesis &
Glycogenolysis
• Glycogenesis
– glucose storage as glycogen
• Glycogenolysis
– glucose release
– not a simple reversal of
steps
– Phosphorylase enzyme is
activated by glucagon
(pancreas) & epinephrine
(adrenal gland)
– Glucose-6-phosphatase
enzyme is only in
hepatocytes so muscle can
not release glucose into the
serum.
Gluconeogenesis
• Gluconeogenesis is the conversion of protein or fat
molecules into glucose (Figure 25.12).
Gluconeogenesis
• Glycerol (from fats) may be converted to glyceraldehyde-3phosphate and some amino acids may be converted to
pyruvic acid. Both of these compounds may enter the Krebs
cycle to provide energy.
• Gluconeogenesis is stimulated by cortisol, thyroid hormone,
epinephrine, glucagon, and human growth hormone.