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Uitbreiding college energy delivery
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Uitrbeiding college energy deliveryBlz 842 Guyton
Beta oxidation
Adipose tissue and liver are the major tissues of the body where fat is being stored.
Many of the carbohydrates ingested are being converted into triglycerides.
For using triglycerides for energy, they first need to be hydrolysed into fatty acids and glycerol. Then,
both fatty acids and glycerol are transported in the blood to the active tissues, where they will be
oxidized to give energy. Except brain tissue and red blood cells for example, almost all cells can use
fatty acids for energy.
Glycerol is immediately changed by intracellular enzymes into glycerol-3-phosphate, when it enters
the active tissue. This enters the glycolytic pathway for glucose breakdown and is thus used for
energy.
Degradation and oxidation of fatty acids occur only in the mitochondria first step is transport into
the mitochondria. This is a carrier mediated process that uses carnitine as the carrier substance. Once
inside the mitochondria, fatty acids split away from carnitine and are degraded and oxidized.
The fatty acid molecule is degraded in the mitochondria by progressive release of 2-carbon segments,
as acetyl-CoA this is the beta-oxidation. In addition to the released acetyl-CoA molecules, 4 atoms of
H= are released from the fatty acid molecule at the same time, entirely separate from the acetyl-CoA.
The acetyl-CoA molecules enter immediately into the citric acid cycle, combining first with oxaloaxetic
acid to form citric acid, which then is degraded into carbon dioxide and hydrogen atoms.
The hydrogen is subsequently oxidized by the chemiosmotic oxidative system of the mitochondria.
a lot of energy comes free from this beta oxidation.
Blz 830 Guyton
Glucose metabolism
Immediately on entry into the cells, glucose combines with a phosphate radical, and it forms glucose-
6-phosphate.
This phosphorylation is promoted mainly by glucokinase in the liver, and by hexokinase in most other
cells. This phosphorylation is almost completely irreversible, except in the liver cells, the renal tubular
epithelial cells, and the intestinal epithelial cells. These cells also have the enzyme glucose
phosphatase: when this is activated, it can reverse the reaction. In most tissues of the body,
phosphorylation serves to capture the glucose in the cell: except from the cells which have
phosphatase.
After absorption, glucose can be used immediately, or it can be stored in the liver in the form of
glycogen: a polymer of glucose. Liver cells can store up to 5-8 % of their weight as glycogen, and
muscle cells 1-3%.
Glycogenlosysis: glycogen glucose, is being catalysed by thee enzyme phosphorylase. Under resting
conditions, this phosphorylase is in inactive form: glycogen remains stored. Triggers to activate the
phosphorylase are 2 hormones. The initial effect of each of these hormones is to promote the
formation of cyclic AMP in the cells, which then initiates a cascade of chemical reactions that
activates the phosphorylase:
- Epinephrine: When sympathic nervous system is being stimulated increase availability of glucose
for rapid energy metabolism. Both in liver and muscle.
- Glucagon: secreted by the alpha cells of the pancreas when the blood glucose concentration falls
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