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Functions of food source of energy essential nutrients stored for future use
Metabolism is all the chemical reactions of the body some reactions produce the energy which
is stored in ATP that other reactions consume
all molecules will eventually be broken down and recycled or excreted from the body
26-1
Catabolic reactions breakdown complex organic compounds providing energy (exergonic) glycolysis, Krebs cycle and electron
transport Anabolic reactions synthesize complex
molecules from small molecules requiring energy (endergonic)
Exchange of energy requires use of ATP (adenosine triphosphate) molecule.
26-2
Each cell has about 1 billion ATP molecules that last for less than one minute
Over half of the energy released from ATP is converted to heat
26-3
Energy is found in the bonds between atoms
Oxidation is a decrease in the energy content of a molecule
Reduction is the increase in the energy content of a molecule
Oxidation-reduction reactions are always coupled within the body whenever a substance is oxidized,
another is almost simultaneously reduced.26-4
Biological oxidation involves the loss of electron and a proton (hydrogen atom)dehydrogenation reactions require
coenzymes to transfer hydrogen atoms to another compound
common coenzymes of living cells that carry H+
NAD (nicotinamide adenine dinucleotide ) NADP (nicotinamide adenine dinucleotide phosphate ) FAD (flavin adenine dinucleotide )
NAD+ + 2 H NADH + H+
Biological reduction is the addition of electron and a proton (hydrogen atom) to a moleculeincrease in potential energy of the molecule
26-5
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Dietary carbohydrate burned as fuel within hours of absorption
All oxidative carbohydrate consumption is essentially a matter of glucose catabolism
C6H12O6 + 6O2 6CO2 + 6H2O+
energy
Function of this reaction is to transfers energy from glucose to ATP not to produce carbon dioxide and water 26-7
Glucose catabolism – a series of small steps, each controlled by a separate enzyme, in which energy is released in small manageable amounts, and as much as possible, is transferred to ATP and the rest is released as heat
Three major pathways of glucose catabolism glycolysis
glucose (6C) split into 2 pyruvic acid molecules (3C)
anaerobic fermentation occurs in the absence of oxygen reduces pyruvic acid to lactic acid
aerobic respiration occurs in the presence of oxygen completely oxidizes pyruvic acid to CO2 and H2O
26-8
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.26-9
26-10
Glucose
Glucose 6-phosphate
Glycogen Fat
Fructose 6-phosphate
Fructose 1,6-diphosphate
2 PGAL
22 NAD+
2 NADH + 2 H+
2
2 H2O
2
2
2 pyruvic acid
2 NADH + 2 H+
2 NAD+
O 2 la
ckingO
2 present
22 lactic acid
2
2
Aerobic respirationAnaerobic fermentation
5 Dephosphorylation
1 Phosphorylation
2 Priming
3 Cleavage
4 Oxidation
Pi
ATP
ATP
ATP
2 ATP
ADP
ADP
ADP
2 ADP
KeyCarbon atoms
Phosphategroups
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Phosphorylation glucose enters cell has
phosphate added - ATP used
maintains favorable concentration gradient, prevents glucose from leaving cell
Priming isomerization occurs phosphorylation further
activates molecule - ATP used
Cleavage molecule split into 2
three-carbon molecules
26-11
Oxidation removes H+
NAD+ + H NADHDephosphorylation
transfers phosphate groups to ADP to form ATP
4 ATPs produced (2 ATP used) for a net gain of 2 ATP2 ATP
produces 2 pyruvic acid Animation
26-12
4 ATP are produced but 2 ATP were consumed to initiate glycolysis, so net gain is 2 ATP per glucose molecule
Some energy originally in the glucose is contained in the ATP, some in the NADH, some is lost as heat, but most of the energy remains in the pyruvic acid
End-products of glycolysis are: 2 pyruvic acid + 2 NADH + 2 ATP
26-13
Fate of pyruvic acid depends on oxygen availability
In an exercising muscle, demand for ATP > oxygen supply; ATP produced by glycolysis glycolysis can not continue without supply of
NAD+
NADH reduces pyruvic acid to lactic acid, restoring NAD+
Lactic acid travels to liver to be oxidized back to pyruvic when O2 is available (oxygen debt) then stored as glycogen or released as
glucose Fermentation is inefficient, not favored by
brain or heart26-14
Lactic acid leaves the cells that generate it enter bloodstream and transported to the liver when oxygen becomes available the liver oxidized it back to
pyruvic acid oxygen is part of the oxygen debt created by exercising
muscle
Liver can also convert lactic acid back to G6P and can: polymerize it to form glycogen for storage remove phosphate group and release free glucose into the
blood
Drawbacks of anaerobic fermentation wasteful, because most of the energy of glucose is still in the
lactic acid and has contributed no useful work lactic acid is toxic and contributes to muscle fatigue
Skeletal muscle is relatively tolerant of anaerobic fermentation, cardiac muscle less so the brain employs no anaerobic fermentation 26-15
Most ATP generated in mitochondria, which requires oxygen as final electron acceptor
In the presence of oxygen, pyruvic acid enters the mitochondria and is oxidized by aerobic respiration
Occurs in two principal steps: matrix reactions – their controlling
enzymes are in the fluid of the mitochondrial matrix
membrane reactions - whose controlling enzymes are bound to the membranes of the mitochondrial cristae
26-16
26-17
10
7
6
Pyruvic acid (C3)
CO2
NAD+
NADH + H+
Acetyl group (C2)
Acetyl-Co A
Coenzyme A
H2O
Citric acid (C6)
Oxaloacetic acid (C4) H2O
(C6)
CO2
FAD
FADH2
H2O
NADH + H+
NAD+
11
14
15
17
18
GTP GDP
12
13
16
Occurs inmitochondrialmatrix
ADP
9
8
Pi
Citricacidcycle H2O
NAD+
NADH + H+
(C4)
(C5)
NAD+
NADH + H+
CO2
(C4)
(C4)
(C4)
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
ATP
Pyruvic acid oxidation
Citric acid (Krebs) Cycle
Three steps prepare pyruvic acid to enter citric acid cycledecarboxylation so that a 3-
carbon compound becomes a 2-carbon compound
CO2 removed from pyruvic acid
convert that to an acetyl group (acetic acid)
NAD+ removes hydrogen atoms from the C2 compound
acetyl group binds to coenzyme A
results in acetyl-coenzyme A (acetyl-CoA)
26-18
Citric Acid Cycle acetyl-Co A (a C2 compound)
combines with a C4 to form a C6 compound (citric acid)-- start of cycle
hydrogen atoms are removed and accepted by NAD+
another CO2 is removed and the substrate becomes a five-carbon chain
previous step repeated removing another free CO2 leaving a four-carbon chain
ATP two hydrogen atoms are removed
and accepted by the coenzyme FAD two final hydrogen atoms are
removed and transferred to NAD+ reaction generates oxaloacetic acid,
which starts the cycle again 26-19
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2 pyruvate + 6H2O 6CO2
2 ADP + 2 Pi 2 ATP
8 NAD+ + 8 H2 8 NADH + 8 H+
2 FAD + 2 H2 2 FADH2
Carbon atoms of glucose have all been carried away as CO2 and exhaled
Energy lost as heat, stored in 2 ATP, 8 reduced NADH, 2 FADH2 molecules of the matrix reactions and 2 NADH from glycolysis
Citric acid cycle is a source of substances for synthesis of fats and nonessential amino acids
26-21
Membrane reactions have two purposes:to further oxidize NADH and
FADH2 and transfer their energy to ATP
to regenerate NAD+ and FAD and make them available again to earlier reaction steps
Mitochondrial electron-transport chain – series of compounds that carry out this series of membrane reactionsmost bound to the inner
mitochondrial membranearranged in a precise order that
enables each one to receive a pair of electrons from the member on the left side of it.
pass electrons to member on the other side 26-22
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50
40
30
20
10
Enzyme complex 1
Rel
ati
ve
free
en
erg
y (k
cal/
mo
le)
0
NADH + H+
NAD+
FADH2FAD
FMNFe-S
CoQ
Cyt b
Fe-SCyt c1
Cyt c
CuCyt a
Enzyme complex 2
Reaction progress
Enzyme complex 3
½ O2 + 2 H+
H2O
1
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
26-24
Figure 26.5
Cyt a3
Electron transport chain energy fuels respiratory enzyme complexes pump protons from matrix into space
between inner and outer mitochondrial membranes
creates steep electrochemical gradient for H+ across inner mitochondrial membrane
Inner membrane is permeable to H+ at channel proteins called ATP synthase
Chemiosmotic mechanism - H+ current rushing back through these ATP synthase channels drives ATP synthesis (ANIMATION)
26-25
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NADH releases an electron pair to electron transport system and H+ to prime pumps enough energy to synthesize 3 ATP
FADH2 releases its electron pairs further along electron-transport system enough energy to synthesize 2 ATP
Complete aerobic oxidation of glucose to CO2 and H2O produces 36-38 ATP efficiency rating of 40% - 60% is lost as heat
26-27
26-28
26-29
2 NADH + 2 H+ 2 pyruvate Cytosol
Mitochondria
Glucose
2 NADH + 2 H+
6 NADH + 6 H+
Citric acidcycle
2 FADH2
Electron-transportchain
H2OO2
Glycolysis
Total 36–38
ATP2
ATP2
4
(net)
28–30
CO2
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
ATP
ATP
ATP
ATP is quickly used after it is formed it is an energy transfer molecule, not an energy storage
molecule converts the extra glucose to other compounds better
suited for energy storage (glycogen and fat)
Glycogenesis - synthesis of glycogen stimulated by insulin chains glucose monomers together
Glycogenolysis – hydrolysis of glycogen releases glucose between meals stimulated by glucagon and epinephrine only liver cells can release glucose back into blood
Gluconeogenesis - synthesis of glucose from noncarbohydrates, such as glycerol and amino acids occurs chiefly in the liver and later, kidneys if necessary
26-30
26-31Figure 26.8
Extracellular
Intracellular
Glucose 6-phosphate
Glycolysis
Key
Glycogenesis
Glycogenolysis
Glycogensynthase
Glycogenphosphorylase
Pi
Glycogen
Glucose6-phosphatase(in liver, kidney,and intestinal cells)
Bloodglucose
Hexokinase (in all cells)
Glucose1-phosphate
Pi
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Triglycerides are stored in body’s adipocytes constant turnover of lipid molecules every
2 - 3 weeks released into blood, transported and either
oxidized or redeposited in other fat cells
Lipogenesis - synthesis of fat from other types of molecules amino acids and sugars used to make
fatty acids and glycerol PGAL can be converted to glycerol
26-32
Lipolysis – breaking down fat for fuel begins with the hydrolysis of a triglyceride
to glycerol and fatty acids stimulated by epinephrine, norepinephrine,
glucocorticoids, thyroid hormone, and growth hormone
glycerol easily converted to PGAL and enters the pathway of glycolysis
generates only half as much ATP as glucose
beta oxidation in the mitochondrial matrix catabolizes the fatty acid components
removes two carbon atoms at a time which bonds to coenzyme A
forms acetyl-CoA, the entry point for the citric acid cycle
a fatty acid with 16 carbons can yield 129 molecules of ATP
richer source of energy than the glucose molecule26-33
26-34
Glucose
PGAL
Glucose 6-phosphate
Key
LipogenesisLipolysis
Glycerol
Fatty acidsGlycerol
Beta oxidation
Acetyl-Co A
Storedtriglycerides
Ketone bodies β-hydroxybutyric acid Acetoacetic acid Acetone
Acetyl groups
Citricacidcycle
Pyruvicacid
Fattyacids
Newtriglycerides
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fatty acids catabolized into acetyl groups (by beta-oxidation in mitochondrial matrix) may: enter citric acid cycle as acetyl-CoA undergo ketogenesis
metabolized by liver to produce ketone bodies acetoacetic acid -hydroxybutyric acid acetone
rapid or incomplete oxidization of fats raises blood ketone levels (ketosis) and may lead to a pH imbalance (ketoacidosis)
26-35
Amino acids in the pool can be converted to others
Free amino acids also can be converted to glucose and fat or directly used as fuel
Conversions involve three processes: deamination – removal of an amino group (-NH2) amination – addition of -NH2
transamination – transfer of -NH2 from one molecule to another
As fuel - first must be deaminated (removal of -NH2) what remains is keto acid and may be converted to pyruvic
acid, acetyl-CoA, or one of the acids of the citric acid cycle during shortage of amino acids, citric acid cycle
intermediates can be aminated and converted to amino acids
in gluconeogenesis, keto acids are used to synthesis glucose26-36
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