Since carbohydrates are the major part of the diet, they must
be immediately converted into energy, stored as glycogen, or
converted into fats. The introduction has already presented the
facts about the necessity of storing energy as fat. A total of
55% of the carbohydrates are involved in the synthesis of fats.
The total energy content of the diet must be balanced with
the energy requirements of the human body. If excess foods (calories)
are ingested beyond the body's energy needs, the excess foods
(energy) are converted into fat. If insufficient calories are
ingested, the energy deficit is made up by oxidizing fat reserves.
These simple facts provide the key to weight control although
it is probably more easily understood than carried out in practice.
Excessive deposits of lipids lead to an obese condition. Extensive
blood capillary networks in these deposits mean that they are
quite active metabolically. Obesity puts a strain on the heart
by causing it to pump blood through extra capillaries. Generally,
obesity results from overeating, but a few people have malfunctioning
Lipid metabolism is in a constant state of dynamic equilibrium.
This means that some lipids are constantly being oxidized to
meet energy needs, while others are being synthesized and stored.
In rats, the average life-time of a single lipid molecule ranges
from 2 to 10 days. A similar figure probably applies to human
The sequence of reactions involved in the formation of lipids
is known as Lipogenesis. Lipogenesis is not simply the reverse
of the fatty acid spiral, but does start with acetyl CoA and
does build up by the addition of two carbons units. The synthesis
occurs in the cytoplasm in contrast to the degradation (oxidation)
which occurs in the mitochondria. Many of the enzymes for the
fatty acid synthesis are organized into a multienzyme complex
called fatty acid synthetase.
The major points in the overall lipogenesis reactions are:
1) ATP is required
2) The reactions are reductions (addition of H+ and
use of NADPH) which are the reverse of the oxidations in the
fatty acid spiral.
Link to: Fatty
Acid Synthesis (move cursor over arrows)
Jim Hardy, Professor of Chemistry, The University of Akron.