Chapter 5
Cell Respiration
and Metabolism
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5-1
Metabolism
Is
all reactions in body that involve energy transformations
Divided into 2 categories:
Catabolism breaks down molecules and releases energy
Is primary source of energy for making ATP
Anabolism makes larger molecules and requires energy
Source of body’s large energy-storage compounds
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5-3
Metabolism continued
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5-4
Glycolysis
5-5
Glycolysis
Is
metabolic pathway by which glucose is converted into 2
pyruvates
Does not require oxygen
Overall net equation is:
glucose + 2NAD + 2ADP + 2Pi  2 pyruvates +
2NADH + 2 ATP
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5-6
Glycolysis continued
Glycolysis
is exergonic - produces net of 2ATPs and
2NADHs
However, glucose must be activated with 2ATPs
(phosphorylation) before energy can be obtained
Phosphorylation traps glucose inside cell
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5-7
Lactic Acid Pathway
To
avoid end-product inhibition NADHs produced in
glycolysis need to give Hs away
In absence of O2 NADH gives its Hs to pyruvate creating
lactic acid (anaerobic respiration or fermentation)
Makes muscles feel fatigued
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5-10
Lactic Acid Pathway continued
Only
yields a net gain of 2 ATPs per glucose
RBCs don’t have mitochondria; use only lactic acid pathway
Occurs in skeletal and heart muscle when oxygen supply
falls below critical level
During heavy exercise or vascular blockage
Only in skeletal and heart muscle
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5-11
Glycogenesis and Glycogenolysis
For
osmotic reasons cells can’t store many free glucoses
Instead store glucose as glycogen (glycogenesis)
Skeletal muscle and liver store lots of glycogen
Glycogenolysis clips glucose out of glycogen as glucose
6-phosphate. Breaking down of glycogen
Phosphate groups trap molecules in cells
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5-12
Glycogenesis and Glycogenolysis continued
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5-13
Glycogenesis and Glycogenolysis continued
Skeletal
muscles use
trapped glucose-6phosphate for own
energy needs
Only liver has glucose6-phosphatase that
removes phosphate
groups
So it can secrete
glucose
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5-14
Aerobic Respiration
5-16
Aerobic Respiration
 Begins
when pyruvate formed by glycolysis enters mitochondria
 CO2 is clipped off pyruvate forming acetyl CoA (coenzyme A is a
carrier for acetic acid)
 CO2 goes to lungs
 Energy in acetyl CoA is extracted during aerobic respiration in
mitochondria
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5-17
Krebs Cycle
 Begins
with acetyl
CoA combining with
oxaloacetic acid to
form citric acid
 In a series of reactions
citric acid converted
back to oxaloacetic
acid to complete the
pathway
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5-18
Krebs Cycle continued
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5-20
Fat and Protein Metabolism
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Fats and Proteins as Energy Sources
Fats
can be hydrolyzed to glycerol and fatty acids
These can be modified to run through Kreb’s
Proteins can be broken down to amino acids
Which can be deaminated and run through Kreb’s
These pathways can be used to interconvert carbohydrates,
fats, and proteins
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5-32
Energy Storage
When
more
energy is taken
in than
consumed, ATP
synthesis is
inhibited
Glucose
converted into
glycogen and fat
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5-33
Acetyl CoA
Is
a common substrate for energy and synthetic pathways
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5-34
Fat Synthesis (Lipogenesis)
Acetyl
CoAs can be linked together to form fatty acids
Fatty acids + glycerol = Fat (triglycerides)
Occurs mainly in adipose and liver tissues
Fat is major form of energy storage in body
Yields 9 kilocalories/g
Carbos and proteins yield only 4/g
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5-35
Lipolysis
Is
breakdown of fat into fatty acids and glycerol
Via hydrolysis by lipase
Acetyl CoAs from free fatty acids serve as major energy
source for many tissues
Found in the liver
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5-36
Brown Fat
Amount
of brown fat greatest at time of birth
Is major site for thermogenesis in the newborn
Brown fat produces an uncoupling protein, causing H+ to
leak out of inner mitochondrial membrane
Less ATP is produced, causing electron transport system
to be more active
Heat produced instead of ATP
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5-38
Ketone Bodies
Triglycerides
are continually broken down and
resynthesized
Ensures blood will contain fatty acids for aerobic
respiration
During fasting and diabetes lots of fat is broken down
Causes high levels of ketone bodies
Which are fat metabolites
Gives breath an acetone smell
Cells cannot have big storages of glycogen
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5-39
Amino Acid Metabolism
Nitrogen
(N) is ingested primarily as protein
Which is used in body as amino acids
Excess is excreted mainly as urea
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5-40
Nitrogen (N) Balance
Nitrogen
balance = N ingested minus N excreted
Positive N balance: more Nitrogen ingested than excreted
Negative N balance: less Nitrogen ingested than excreted
In healthy adults amount of N excreted = amount ingested
-if this is not happening, diet is not balanced
Excess amino acids can be converted into carbos and fat
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5-41
Essential and Non-essential Amino Acids
 20
amino acids are used to build proteins
 12 can be produced by body
 8 must come from diet (= essential amino acids)
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5-42
Gluconeogenesis
Occurs
when amino acids or other non-carbos are converted
to Keto acids, then pyruvate, then glucose
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5-46
Uses of Different Energy Sources
Different
cells have different preferred energy substrates
Brain uses glucose as its major source of energy
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5-48
Summary of Metabolic Conversions of
Nutrients
Figure 22-2: Summary of metabolism
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