March Physiology: Module 16
Metabolism is defined as the chemical changes that occur within a cell to
manufacture energy, store energy, or build new cell structures.
The two types of metabolism:
1. Anabolism is the creation of large molecules from small ones with the use of
2. Catabolism is the breakdown of large molecules into small ones resulting in
the release of energy (ATP).
The Building Blocks
Triglycerides (make up most of the fat found in the body) consist of three fatty acid
chains attached to one molecule of glycerol.
Proteins consist of long chains of amino acids linked together
Carbohydrates are long chains of monosaccharides (like glucose) joined together.
Glycogen: the carbohydrate glucose that is stored inside muscle and liver cells
Fats are stored as triglycerides inside fatty tissue called adipose tissue.
The amino acids become structural or functional proteins inside the muscle cells.
-1% of the total energy
requirements of the body
-its reserves (in the form of
glycogen) can last for roughly a
-stored as glycogen (in most
cells of the body)
-found in the liver and skeletal
-common fuel source for all the
cells; one of the only fuel
sources for the brain (stored in
-contribute to 77% of the total
-can last for up to two months depending on the individual
-stored as proteins
-not usually used to create energy
-can account for 22% of the body’s fuel requirements if absolutely necessary
Glucose enters cell and converted to glucose-6-phosphate (G-6-P).
G-6-P can enter glycolysis to produce ATP OR can be converted to glycogen & stored.
When necessary, glycogen can be converted back to G-6-P to enter glycolysis to produce
In glycolysis, G-6-P will undergo a series of reactions that will result in the
production of ATP and the end product of pyruvate.
Pyruvate can either:
1. Enter the citric acid cycle (CAC) and produce lots of ATP (with the help of
2. Enter another shorter reaction to produce a small amount of ATP and the by-
product lactate (lactic acid). Physiology: Module 16
Which reaction will pyruvate take?
If O2 is present: most of the pyruvate will enter the CAC
If not enough O2 present: pyruvate will be converted to lactic acid
During strenuous exercise, your
cardiovascular system may not be able to
supply sufficient oxygen to your working
muscle cells to produce ATP via the citric
acid cycle (CAC).
Glycolysis (anaerobic: can continue
without oxygen to produce ATP; end
With insufficient oxygen: CAC will not be
able to work at full capacity & pyruvate
accumulates. If too much pyruvate
accumulated, glycolysis is slowed.
To keep glycolysis working (so it can
produce ATP), pyruvate will be converted
to lactic acid.
Accumulation of lactic acid causes dilation
The accumulation of of blood vessels and the decreasing pH will
lactic acid causes the unload more oxygen from hemoglobin and
"burning" sensation in thus, increasing blood flow & increasing
the muscle and is oxygen delivery to this working tissue to
believed to interfere reduce the build-up of lactic acid.
with the contractileNote: if oxygen is restored,
fatigue. lactic acid can be
converted back to
pyruvate to enter the
citric acid cycle and
pyruvate can be turned
back into glycogen
through G-6-P and stored
for later use.
Sufficient supply of oxygen:
Citric acid cycle (CAC) can function [at full capacity] and the
pyruvate from glycolysis is converted to acetyl coenzyme A
(acetyl CoA), which then enters the CAC.
The CAC in conjunction with oxidative phosphorylation
-a total of 36 molecules of ATP from one molecule of glucose
-2 ATP from CAC
-34 ATP from oxidative phosphorylation
They also produce the by-products CO2 and H2O. The CO2 will diffuse into the blood
and leaves the body at the lungs.
-stored in cells as proteins
-can be metabolized to create ATP by cells (each one enters the metabolic pathways at
different points) Physiology: Module 16
-can be converted to either pyruvate or acetyl CoA to enter glycolysis or the citric
acid cycle (CAC) (respectively)
These molecules can either produce ATP or can be
converted to glycogen and stored in the cell in
much the same way as glucose.
The FED and Fasted State
Fed state (absorptive
state): the condition the
body is in after a meal
(nutrient levels in the
blood are high). During
this period, the body's
goal is to store this new
fuel (amino acids, fats,
and glucose) for later
Fasted state (or post
absorptive state): the
condition the body is in