705
CHAPTER EIGHTEEN
Nutrition and Metabolism
Proteins are classif
ed as complete or incomplete based
on the amino acid types they provide.
Complete proteins
have adequate amounts oF the essential amino acids to main-
tain human body tissues and promote normal growth and
development. Certain proteins in milk, meat and eggs are
complete. Incomplete proteins cannot by themselves main-
tain human tissues or support normal growth and develop-
ment. Zein in corn, For example, has too little oF the essential
amino acids tryptophan and lysine to be complete. A
par-
tially complete protein
does not have enough amino acid
variety to promote growth, but it has enough to maintain
liFe. A protein in wheat called gliadin is partially complete
because it has very little oF the amino acid lysine.
Many proteins in plants do not provide enough of one or more essen-
tial amino acids for the human diet. However, combining appropriate
plant foods can supply an adequate diversity of dietary amino acids.
For example, beans are low in methionine but have enough lysine.
Rice lacks lysine but has enough methionine. A meal of beans and
rice o±
ers enough of both types of amino acids.
Plants can be genetically modi²
ed to make their protein more
“complete.” For example, genetic instructions for producing the
amino acid tryptophan inserted into corn cells can compensate for
the low levels of this nutrient normally found in corn.
PRACTICE
14
How do cells use proteins?
15
Which foods are rich sources of protein?
16
Why are some amino acids called essential?
17
Distinguish between a complete protein and an incomplete protein.
Nitrogen Balance
In a healthy adult, proteins are continuously built up and
broken down. This occurs at diFFerent rates in diFFerent tis-
sues, but the overall gain oF body proteins equals the loss,
producing a state oF
dynamic equilibrium
(di-nam
ik e
kwı˘-
lib
re-um). Proteins have a high percentage oF nitrogen, so
dynamic equilibrium also brings
nitrogen balance
(ni
tro-jen
Protein Sources
±oods rich in proteins include meats, f sh, poultry, cheese,
nuts, milk, eggs, and cereals. Legumes, including beans and
peas, contain less protein.
The human body can synthesize many amino acids
(nonessential amino acids). However, eight amino acids the
adult body needs (ten required For growing children) can-
not be synthesized suFf
ciently or at all, and they are called
essential amino acids.
This term reFers only to dietary
intake, because all amino acids are required For normal pro-
tein synthesis.
Table 18.3
lists the amino acids in Foods and
indicates those that are essential.
All twenty types oF amino acids must be in the body at
the same time For growth and tissue repair to occur. In other
words, iF the diet lacks one essential amino acid, the cells
cannot synthesize protein. Essential amino acids are not
stored, so those not used in protein synthesis are oxidized as
energy sources or are converted into carbohydrates or Fats.
FIGURE 18.7
The body digests proteins from foods into amino
acids, but must deaminate these smaller molecules before they can be
used as energy sources.
Heat
CO
2
++
H
2
O
Energy
Citric
acid
cycle
ATP
Proteins from foods
Amino acids
Acetyl
coenzyme A
+
–NH
2
groups
Deaminated portions
(by various pathways)
Glucose
Fat
Urea
Hydrolysis
Deamination
TABLE
18.3
|
Amino Acids in Foods
Alanine
Glycine
Proline
Arginine (ch)
Histidine (ch)
Serine
Asparagine
Isoleucine (e)
Threonine (e)
Aspartic acid
Leucine (e)
Tryptophan (e)
Cysteine
Lysine (e)
Tyrosine
Glutamic acid
Methionine (e)
Valine (e)
Glutamine
Phenylalanine (e)
Eight essential amino acids (e) cannot be synthesized by human cells and must be provided in the diet.
Two additional amino acids (ch) are essential in growing children.
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