720
UNIT FIVE
The recommended daily allowance of magnesium
is 300 mg for females and 350 mg for males. A typical
diet usually provides only about 120 mg of magnesium
for every 1,000 calories, barely meeting the body’s
needs. Good sources of magnesium include milk and
dairy products (except butter), legumes, nuts, and
leafy green vegetables.
PRACTICE
65
Where are chloride ions most highly concentrated in the body?
66
Where is magnesium stored?
67
What factors inF
uence the absorption of magnesium from the
intestinal tract?
Table 18.10
summarizes the major minerals.
Trace Elements
Trace elements
(microminerals) are essential minerals
found in minute amounts, each making up less than 0.005%
of adult body weight. They include iron, manganese, copper,
iodine, cobalt, zinc, fl
uorine, selenium, and chromium.
Iron
(Fe) is most abundant in the blood, but is stored
in the liver, spleen, and bone marrow and is found to some
extent in all cells. Iron enables
hemoglobin
molecules in red
blood cells to carry oxygen
(f
g. 18.19)
. Iron is also part of
myoglobin,
which stores oxygen in muscle cells. In addition,
With sodium, chlorine helps to regulate pH and maintain
electrolyte balance and the solute concentration of
extracellular fl uids. Chlorine is also essential for the
formation of hydrochloric acid in gastric juice and in the
transport of carbon dioxide by red blood cells.
Chlorine and sodium are usually ingested in table
salt (sodium chloride), and as in the case for sodium,
an ordinary diet usually provides considerably more
chlorine than the body requires. Vomiting, diarrhea,
kidney disorders, sweating, or using diuretics can
deplete chlorine in the body.
7.
Magnesium.
Magnesium (Mg) is responsible for
about 0.05% of body weight and is found in all cells.
It is particularly abundant in bones in the form of
phosphates and carbonates.
Magnesium is important in ATP-forming reactions
in mitochondria, as well as in breaking down ATP to
ADP. Therefore, it is important in providing energy for
cellular processes.
Magnesium absorption in the intestinal tract adapts
to dietary intake of the mineral. When the intake of
magnesium is high, a smaller percentage is absorbed
from the intestinal tract, and when the intake is low, a
larger percentage is absorbed. Absorption increases as
protein intake increases, and decreases as calcium and
vitamin D intake increase. Bone tissue stores a reserve
supply of magnesium, and excess is excreted in the urine.
CH
2
CH
3
CH
2
CH
3
CH
2
CH
3
N
N
N
N
CH
CH
2
CH
3
CH
CH
2
CH
2
COOH
COOH
Fe
(a)
Heme group
(b)
Red
blood
cells
(c)
FIGURE 18.19
Iron in hemoglobin. (
a
) A hemoglobin molecule
contains four heme groups, each of which houses a single iron atom
(±e) that can combine with oxygen. Iron de²
ciency anemia can result
from a diet poor in iron-containing foods. The red blood cells in (
b
) are
normal (1250x), but many of those in (
c
) are small and pale (1250x).
They contain too little hemoglobin, because iron is lacking in the diet.
Vegetarians must be especially careful to consume su³
cient iron.
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