Cellular Energy Metabolism

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Lecture 3
Cellular Energy Metabolism
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ATP is the single source for all of the energy
required within the cell.
During exercise ATP is required for
? Muscle contraction - cross-bridge cycling
powerstroke
? For the re-uptake of Ca2 into the sarcoplasmic
reticulum
? Active transport across membranes e.g. Na, K-
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ATPase
But there is only 60 g of ATP in the body.
Enough for about 6 s of steady state running if
it were the only fuel source.
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? ATP demand and supply are tightly coupled to
prevent a fall in ATP and rigour.
? Even at maximal intensities only become
slightly uncoupled - the ATP conc. is not
allowed to fall below 50 of the resting
level.
? ATP conc. at rest is 25 mmol.kg-1 dm,
After 30 s of max exercise only falls to 18
mmol.kg-1 dm (20 ) (Bogdanis et al.,
1993). Despite 314 ATP mmol.kg-1 dm being
used.
? Therefore for muscle contraction and exercise
to continue ATP must be resynthesised
(re-made) very quickly.
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Kinetic Energy (Movement) Heat Energy
Chemical Energy
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How is ATP resynthesised ?
? PCr in the cytoplasm.
? CHO - Anaerobic glycolysis in the
cytoplasm. - Oxidative phosphorylation in
the mitochondria.
? Fat Oxidative phosphorylation in the
mitochondria.
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At rest PCr is 80 mmol.kg-1 dm
PCr could supply fuel for only 5-6 s of
maximal work if the only energy source.
CK
After 30 s of maximal exercise falls to 15
mmol.kg-1 dm
For every 1 PCr degraded 1 ATP is produced.
PCr levels are 10 - 20 higher in FT fibres
than ST.
PCr levels can be manipulated to some extent by
training and Creatine loading.
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Glucose (blood)
Rate limiting enzyme Posphofructokinase (PFK)
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Lactic acid once formed immediately dissociates
into H and lactate
They do not easily diffuse out of muscle tissue,
and in high intensity exercise accumulate in
muscle. The ? H conc. causes a fall in muscle
pH, which
1. Inhibits the use of ATP (contractile
mechanism) - prevents Ca2 release from the
SR - cross-bridges are less sensitive to Ca2
2. Inhibits the supply (resynthesis) of ATP -
Reduced rate of glycolysis due to inhibition of
PFK
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? The fall in muscle pH stimulates pain receptors
in the muscle.
? Within muscle H are mainly buffered by
phosphates and proteins, which react with H
and transport them out of the muscle fibre.
? The blood lactate conc. peaks 5mins after
sprint exercise.
- The fall in blood pH causes nausea and
dizziness.
? Within the blood bicarbonate and haemoglobin
are the main buffers, that reduce the fall
in blood pH.
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Reconverison to Pyruvate
When there is insufficient oxygen, some pyruvate
is converted to lactic acid.
Causing an accumulation of H ions.
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Reconversion to Pyruvate
When there is sufficient oxygen, and little
pyruvate produced by glycolysis.
glucose
Then lactate can be converted back to pyruvate
and used as a fuel.
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The fate of lactate 2 The Cori Cycle
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Pyruvate
2 ATP
34 ATP
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Overview of CHO Metabolism
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Metabolism of Carbohydrate
From 1 glucose molecule cleaved from muscle
glycogen.
Anaerobic glycolysis 3 ATP (in the
cytoplasm)
Aerobic phosphorylation (in the
mitochondria) - Krebs cycle 2 ATP -
Electron transport chain 34 ATP
Total 39 ATP
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Fat Metabolism
ATP
Mitochondria
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Energy yield from triglyceride oxidation
ATP
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Glycerol
441
3 Fatty Acid molecules ?-oxidation, Krebs
cycle Electron transport chain.
Total 460
Per O2 molecule CHO yields 6.3 ATP Fat
yields 5.6 ATP. ? When O2 is limited CHO is
the prefered fuel.
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Simultaneous production by energy pathways
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Energy pathways during maximal exercise
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Basic Reading
McArdle, WD, Katch, FI and Katch, VL (1996)
Sports exercise Nutrition. Chapter 4.