Lactic Acid Pathway
•
(continued)
Some tissues better adapt to anaerobic conditions:
– RBCs do not contain mitochondria and only use the lactic
acid pathway.
– Occurs in skeletal muscles and heart when ratio of oxygen
supply to oxygen need falls below critical level.
™ Skeletal muscle:
¾ Normal daily occurrence.
¾ Does not harm muscle tissue.
™ Cardiac muscle normally respires aerobically:
¾ Myocardial ischemia occurs under anaerobic
conditions.
22
Cori Cycle
•
•
•
Lactic acid produced by anaerobic respiration
delivered to the liver.
LDH converts lactic acid to pyruvic acid.
Pyruvic acid converted to glucose-6phosphate:
– An intermediate for glycogen synthesis.
– Can be converted to free glucose.
23
Cori Cycle
24
8
Glycogenesis and Glycogenolysis
•
•
•
Cells cannot store many glucose molecules.
Increased intracellular [glucose] increases
osmotic pressure, drawing H20 into the cell.
Many organs must store carbohydrates in
form of glycogen.
– Glycogenesis: formation of glycogen from
glucose.
– Glycogenolysis: conversion of glycogen to
glucose-6-phosphate.
25
Glycogenesis and Glycogenolysis
•
•
•
(continued)
Glucose-6-phosphate
cannot leak out of the
cell.
Skeletal muscles
generate glucose-6phosphate for own
glycolytic needs.
Liver contains the
enzyme glucose-6phosphatase that can
remove the phosphate
group and produce free
glucose.
26
Lipid Metabolism
•
•
When more
energy is
taken in than
consumed,
ATP
production is
inhibited.
Glucose
converted
into glycogen
and fat.
27
9
Lipogenesis
•
•
•
Formation of fat.
Acetic acid
subunits from
acetyl CoA
converted into
various lipids.
Occurs mainly
in adipose
tissue and liver.
28
Lipid Metabolism
•
Lipolysis:
– Breakdown of fat.
lipase
•
•
Triglycerides
glycerol + free fatty acids
Free fatty acids serve as blood-borne
energy carriers.
– Free fatty acids serve as the major
energy source derived from
triglycerides.
29
Beta-Oxidation
•
•
Enzymes remove
2-carbon acetic acid
molecules from acid
end of fatty acid
chain.
– Forms acetyl
CoA.
Acetyl CoA enters
Krebs Cycle.
– ATP, NADH and
FADH2 produced.
30
10
Brown Fat
•
•
•
Amount of brown fat greatest at time of birth.
Major site for thermogenesis in the newborn.
Brown fat produces an uncoupling protein, causing
H+ to leak out of inner mitochondrial membrane.
– Less ATP produced, causes electron transport
system to be more active.
31
Ketone Bodies
•
•
Triglycerides in adipose tissue broken down and
resynthesized.
– Ensure the blood will contain sufficient levels of
fatty acids for aerobic respiration.
™ May be hydrolyzed to glycerol and fatty acids.
If ATP sufficient, acetyl CoA channeled into
alternate pathway.
– Converted to ketone bodies.
32
Amino Acid Metabolism
•
•
•
•
•
Nitrogen is ingested primarily as protein.
Excess nitrogen is excreted mainly as urea.
Nitrogen balance:
– Amount of nitrogen ingested minus amount
excreted.
+ N balance:
– Amount of nitrogen ingested more than amount
excreted.
- N balance:
– Amount of nitrogen excreted greater than
ingested.
33
In healthy adults the amount of nitrogen
11
Transamination
Adequate amounts of amino acids are required
for growth and repair. A new amino acid can be
obtained by transamination.
– Amine group (NH2 ) transferred from one
amino acid to form another amino acid and a
keto acid.
– Catalyzed by a specific enzyme
(transaminase).
•
34
Transamination
(continued)
35
Oxidative Deamination
•
•
Process by which excess amino acids are eliminated.
Amine group from glutamic acid removed, forming a
keto acid and ammonia.
– Keto acid can be used in the Krebs Cycle.
– Ammonia is converted to urea and excreted.
36
12
Uses of Different Energy Sources
•
•
•
Not all cells can use glucose as the energy
source.
Blood contains a variety of energy sources:
– Glucose and ketone bodies, fatty acids, lactic
acid, and amino acids.
Brain uses glucose as its major source of
energy.
– Blood [glucose] maintained as many organs
spare glucose.
37
Uses of Different Energy Sources
(continued)
38
Formation of ATP
•
•
•
Formation of ATP requires
the input of a large amount
of energy.
– bond produced by
joining Pi to ADP must
contain a part of this
energy.
Energy is released when
ATP converted to ADP and
Pi .
ATP is the universal energy
carrier of the cell.
39
13
Cellular Respiration
•
•
•
Process that produced ATP
Glucose breakdown requires sub-pathways.
– Glycolysis.
– Transition reaction
– Citric Acid Cycle.
– Electron Transport System.
Altogether, the breakdown of one glucose
molecule results in (about) 36 ATP molecules.
40
Oxidation-Reduction
•
•
(continued)
May involve the transfer of H+ rather than
free electrons.
Molecules that serve important roles in the
transfer of hydrogen are NAD and FAD.
– Coenzymes that function as hydrogen
carriers.
41
Oxidation-Reduction
(continued)
42
14
43
Fermentation
•
Fermentation is an anaerobic process that
results in the buildup of lactate.
– Lactate is toxic to cells and causes
muscle cramps and fatigue.
– Only produces two ATP per glucose
molecule.
44
45
15