Biomedical Engineering Reference
In-Depth Information
of adenosine triphosphate is removed by hydrolysis, a substantial amount of free
energy is released.
ATP
+
H O
↔+
ADP
phosphate
+
energy
2
where ADP is adenosine diphosphate. ATP is used throughout the body to store
energy that would otherwise be released as heat. This provides the energy for most
of the energy-consuming activities of the cell. The exact amount of energy released
depends on the conditions, but a value of 7.3 kcal per mole is an average value.
For this reason, this bond is known as a “high-energy” bond. The bonds between
the first and second phosphates are weak bonds with low bond energies. Another
primary source of energy is NADH. Through the mitochondrial electron trans-
port chain, NADH transfers two electrons and a hydrogen ion to oxygen, releasing
energy.
+
+
NADH
++
H
1 / 2O
↔
NAD
+
H O
+
energy
2
2
Notice that water is necessary to supply hydrogen ions in this reaction and
water is also a product. Cells also use the oxidation of flavin adenine dinucleotide
(FAD) to release energy
FADH
+
1 / 2O
↔
FAD
+
H O
+
energy
2
2
2
A muscle fiber contains only enough ATP to power a few twitches. A normal
metabolism cannot produce energy as quickly as a muscle cell can use it, so extra
storage source is needed. The ATP pool is replenished as needed by two other
sources of high energy:
Creatine phosphate
, a phosphorylated form of creatine, is used by muscle
cells to store energy. The phosphate group can be quickly transferred to ADP
to regenerate the ATP necessary for muscle contraction.
Glycogen
, which is a storage form of glucose. Nearly 70% of glycogen is
stored in the muscle, 20% in the liver, and the remaining in blood.
4.4.3 Energy Expenditure Calculations
In many scenarios, energy imbalance occurs either due to a high-energy intake, low-
energy expenditure, or a mixture of the two conditions. This could result in obesity,
which may be treated effectively by increasing energy expenditure. Requirements of
individuals vary in relation to a wide variety of factors, making energy balance an
issue of great importance in weight loss purposes or acute as well as chronic illness
feeding regimens. Indirect calorimetry is a widely used method for assessing:
Resting energy expenditure (REE) or the energy required by the body during
a nonactive period, primarily used to determine the caloric requirements of
patients;
