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Chapter 6
Proteins and Amino Acids
Key Terms
1.
amino acid pool: The amino acids in body tissues and fluids that are available for new
protein synthesis.
2.
incomplete (low-quality) proteins : Proteins that lack one or more essential amino acids.
Also called low-quality proteins.
3.
tripeptide : Three amino acids joined by peptide bonds.
4.
wasting : The breakdown of body tissue such as muscle and organ for use as a protein
source when the diet lacks protein.
5.
deamination: The removal of the amino group (–NH2) from an amino acid.
6.
cystic fibrosis: An inherited disorder that causes widespread dysfunction of the exocrine
gland and results in chronic lung disease, abnormally high levels of electrolytes in sweat,
and deficiency of pancreatic enzymes needed for digestion.
7.
marasmus : A type of malnutrition resulting from chronic inadequate consumption of
protein and energy that is characterized by wasting of muscle, fat, and other body tissue.
8.
acidosis : An abnormally low blood pH (below about 7.35) due to increased acidity.
9.
alkalosis: An abnormally high blood pH (above about 7.45) due to increased alkalinity.
10.
edema : Swelling caused by the buildup of fluid between cells.
11.
buffers: Compounds that can take up and release hydrogen ions to keep the pH of a
solution constant.
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12.
complete (high-quality) proteins : Proteins that supply all of the essential amino acids in
the proportions the body needs. Also known as high-quality proteins.
13.
proenzymes : Inactive precursors of enzymes.
14.
dipeptide : Two amino acids joined by a peptide bond.
15.
chymotrypsinogen/chymotrypsin: A protease produced by the pancreas that is converted
from the inactive proenzyme form to the active form in the small intestine.
16.
complementary proteins : Two or more incomplete food proteins whose assortment of
amino acids make up for, or complement, each other so that the combination provides
sufficient amounts of all the essential amino acids.
17.
kwashiorkor : A type of malnutrition that occurs primarily in young children who have an
infectious disease and whose diets supply marginal amounts of energy and very little
protein. Common symptoms include poor growth, edema, apathy, weakness, and
susceptibility to infections.
18.
protein digestibility-corrected amino acid score (PDCAAS) : A measure of protein
quality that takes into account the amino acid composition of the food and the
digestibility of the protein.
19.
nitrogen equilibrium : Nitrogen intake equals the sum of all sources of nitrogen excretion;
nitrogen balance equals zero.
20.
antibodies : Infection-fighting protein molecules in blood or secretory fluids that tag,
neutralize, and help destroy pathogenic microorganisms (e.g., bacteria, viruses) or toxins.
21.
proteases : Enzymes that break down protein into peptides and amino acids.
22.
polypeptide : More than 10 amino acids joined by peptide bonds.
23.
nitrogen balance : Nitrogen intake minus the sum of all sources of nitrogen excretion.
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24.
immune response : A coordinated set of steps, including production of antibodies, that
the immune system takes in response to an antigen.
25.
oligopeptide : Four to 10 amino acids joined by peptide bonds.
26.
intravascular fluid : The fluid portion (plasma) of the blood contained in arteries, veins,
and capillaries. It accounts for about 15 percent of the extracellular fluid.
27.
extracellular fluid: The fluid located outside of cells.
Fill-in-the-Blank
1.
Proteins that use energy and convert it into some form of mechanical work are motor
proteins. They are active in processes such as cell division, muscle contraction, and
sperm movement.
2.
Celiac disease is a disease that involves an inability to digest gluten, a protein found in
wheat, rye, oats, and barley. If untreated, it causes flattening of the villi in the intestine,
leading to severe malabsorption of nutrients. Symptoms include diarrhea, fatty stools,
swollen belly, and extreme fatigue.
3.
The bond between two amino acids formed when a carboxyl (–COOH) group of one
amino acid joins an amino (–NH2) group of another amino acid, releasing water in the
process, is a peptide bond.
4.
Conditionally indispensable amino acids are amino acids that are normally made in the
body (dispensable) but become essential under certain circumstances, such as during
critical illness.
5.
The most abundant fibrous protein in the body is collagen.
6.
The fluid located outside of cells is extracellular fluid.
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7.
Keratin is a water-insoluble fibrous protein that is the primary constituent of hair, nails,
and the outer layer of the skin.
8.
A condition resulting from long-term inadequate intakes of protein and energy that can
lead to wasting of body tissues and increased susceptibility to infection is protein-energy
malnutrition (PEM).
9.
The main nitrogen-containing waste product in mammals is urea. Formed in liver cells
from ammonia and carbon dioxide, it is carried via the bloodstream to the kidneys, where
it is excreted in the urine.
10.
Deamination is the removal of the amino group (–NH2) from an amino acid.
11.
Denaturation involves a change in the three-dimensional structure of a protein resulting
in an unfolded polypeptide chain that cannot fulfill the protein’s function.
12.
The constant breakdown and synthesis of proteins in the body is called protein turnover.
13.
Indispensable amino acids are amino acids the body cannot make at all or cannot make
in sufficient quantities to meet the body’s needs. These must be supplied in the diet.
14.
Interstitial fluid is the fluid between cells in tissues. It is also called intracellular fluid.
15.
Substances released at the end of a stimulated nerve cell that diffuse across a small gap
and bind to another nerve cell or muscle cell, stimulating or inhibiting it, are
neurotransmitters.
16.
Negative nitrogen balance occurs when nitrogen intake is less than the sum of all
sources of nitrogen excretion.
17.
Positive nitrogen balance occurs when nitrogen intake exceeds the sum of all sources of
nitrogen excretion.
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18.
Amino acids the body can make if supplied with adequate nitrogen are dispensable
amino acids. These do not need to be supplied in the diet.
19.
Hemoglobin is the oxygen-carrying protein that gives blood its red color.
Fill-in-the-Blank Summaries
Amino Acids Are the Building Blocks of Protein
Proteins are sequences of amino acids. There are 20 different amino acids; 9 are essential and 11
are nonessential. Tyrosine and cysteine are both considered conditional amino acids. If
your intake of methionine is too low, your body needs cysteine from your diet to free
methionine for protein formation. People with PKU lack sufficient amounts of an
enzyme that converts phenylalanine to tyrosine. These people must carefully monitor the
amount of phenylalanine in their diets to avoid problems such as irreversible brain
damage.
One amino acid is linked to the next by a peptide bond. When this bond is created, the carboxyl
group of one amino acid binds to the amino group of another amino acid. This reaction
releases water in the process. An oligopeptide is a chain of 4 to 10 amino acids; a
polypeptide contains at least 11 amino acids. Acidity, heat, and oxidation can disrupt the
chemical forces that stabilize a protein’s shape, causing it to denature.
Functions of Body Proteins
Proteins have structural and mechanical functions. Collagen, the most abundant protein in
mammals, gives skin and bones their elastic strength. The proteins that turn energy into
mechanical work are known as motor proteins. These proteins are also involved in cell
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division and sperm swimming. Proteins that catalyze chemical reactions without being
used up are enzymes. Hormones are chemical messengers that are made in one part of
the body but act on cells in other parts of the body. Antibodies are blood proteins that
attack and inactivate bacteria and viruses. If the body does not have enough protein to
maintain normal levels of blood proteins, fluids will leak into surrounding tissues and
cause edema. Proteins help maintain pH levels in body fluids by serving as buffers; they
donate hydrogen ions when conditions are alkaline and pick up extra hydrogen ions when
conditions are acidic.
Evaluating Protein Quality
A simple way to determine a food’s protein quality is to compare its amino acid composition to
that of a reference pattern of amino acids. This method is referred to as chemical
scoring, or amino acid scoring. The amino acid with the lowest score is the limiting
amino acid. The chemical score of the food protein is the score of its limiting amino
acid. The protein efficiency ratio (PER) measures amino acid composition and
accounts for digestibility. Net protein utilization (NPU) measures how much dietary
protein the body actually uses.
The net protein utilization (NPU) method determines how much of the nitrogen absorbed from
a particular food is retained by the body for growth or maintenance.
Short Answer
1.
From the body’s perspective, protein is critically important. Protein is part of every cell,
it is needed in thousands of chemical reactions, and it keeps us “together” structurally. If
you eat more protein than your body needs, what happens to the excess protein?
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When we eat more protein than we need, the excess is either used to make energy or is
stored as fat.
2.
If you do not eat enough protein to meet your body’s needs, how does your body adjust?
When the diet lacks protein, the body breaks down tissue such as muscle and uses it as a
protein source.
3.
Amino acids (with the exception of proline) uniformly consist of a central carbon atom
chemically bonded to one hydrogen atom (H), one carboxylic acid group (–COOH), one
amino (nitrogen-containing) group (–NH2), and one side group unique to each amino acid
(R). In what way does the amino acid side group give each amino acid its identity?
The side group of an amino acid determines that amino acid's shape, size, composition,
electrical charge, and pH.
4.
Insulin is a protein hormone that plays a key role in regulating the amount of glucose in
the blood. People with type 1 diabetes mellitus must take insulin injections to control
blood glucose. Why can’t insulin be taken as a pill?
If insulin were taken as a pill it would be denatured and digested just like any other
protein.
5.
In the case of starvation, when the diet does not provide enough energy to sustain vital
functions, how does the body adapt to make energy and glucose for use by the brain,
lungs, and heart?
The body will break down its own protein from enzymes, muscle, and other tissues to make
energy and glucose for use by the brain, lungs, and heart.
6.
Why is it considered risky for a person to take large amounts of one particular amino
acid?
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If a person consumes a large amount of one particular amino acid, absorption of another
amino acid that shares the same transport system will be deficient. Thus, taking a high dose
amino acid supplement may interfere with the absorption of another amino acid from your
diet.
7.
Compute recommended protein intake for: A) An adult women who weighs 130 pounds;
and b) An adult man who weighs 190 pounds.
A) 130 pounds  2.2 pounds/kg = 59 kg
59 kg  0.8 grams protein/kg = 47 grams of protein
B) 190 pounds  2.2 pounds/kg = 86 kg
86 kg  0.8 grams protein/kg = 69 grams of protein
8.
Under which circumstances do individual protein needs for persons under the age of 50
increase above the general recommendation of 0.8 grams/kg body weight?
Severe physical stress can increase the body’s need for protein. Infections, burns, fevers,
and surgery all increase protein losses, and the diet must replace that lost protein.
9.
Identify the difference between vegetarian diets, lacto-ovo-vegetarian diets, and vegan
diets.
Vegetarian diets eliminate animal products to various degrees. Lacto-ovo-vegetarians
include milk and eggs in their diets, whereas vegans eat no animal foods.
Labeling
Genetic material in the nucleus of every cell is the blueprint for the thousands of proteins needed
to perform life functions. Label the different components necessary for protein synthesis to be
complete
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(clockwise from top): Structural and Mechanical; Enzymes; Hormones; Antibodies; Fluids
Balance; Acid–Base Balance; Channels and Pumps; Transport.
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