This document discusses proteins, their structure and functions in the body. It covers how proteins are made through the joining of amino acids, and how protein synthesis relies on obtaining all essential amino acids. The document also summarizes that proteins are necessary for cell growth and repair, acting as enzymes and hormones, and transporting nutrients. It notes that a varied diet can provide sufficient protein and that vegetarian diets require planning but can meet protein needs if done appropriately.

1. CHAPTER
5
Proteins: Crucial Components of
All Body Tissues
Chapter 5 Outline
I. What Are Proteins?
A. Proteins are large, complex molecules with a variety of critical functions in the body.
B. How do proteins differ from carbohydrates and lipids?
1. Genetic material dictates the structure of each protein molecule.
2. Proteins contain nitrogen.
C. The building blocks of proteins are amino acids.
1. Proteins are made from different combinations of 20 amino acids.
2. Our bodies can produce nonessential amino acids but not essential amino acids.
II. How Are Proteins Made?
A. Protein shape determines function.
1. Amino acids join together by means of a peptide bond.
2. When amino acids join together, they form peptides.
a. Two amino acids joined together form a dipeptide.
b. Three amino acids joined together form a tripeptide.
c. Four to nine amino acids are referred to as an oligopeptide.
d. A polypeptide is 10 or more amino acids bonded together.
3. The three-dimensional protein shape is critical to its function.
4. Denaturation is the loss of shape of a protein.
5. A denatured protein loses its function.
C. Protein synthesis can be limited by missing amino acids.
1. All essential amino acids must be available for protein synthesis to occur.
2. The amino acid that is missing or in the smallest supply is the limiting amino acid.
3. Without the proper amount of essential amino acids, proteins cannot be made.
4. An incomplete protein does not contain all of the essential amino acids in sufficient
amounts to support growth and health.
5. A complete protein contains all nine essential amino acids.
D. Protein synthesis can be enhanced by mutual supplementation.
1. Two or more incomplete complementary proteins can be combined to simulate a
complete protein.
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2. III. Why Do We Need To Eat Proteins?
A. Proteins contribute to cell growth, repair, and maintenance.
1. Cells are constantly being broken down, repaired, and replaced.
B. Proteins act as enzymes and hormones.
1. Enzymes are chemicals that help to speed up chemical process but are not apparently
changed during those processes.
2. Some hormones are made from amino acids.
a. Hormones are substances that act as chemical messengers in the body.
C. Proteins help maintain fluid and electrolyte balance.
1. Proteins attract fluid.
2. When protein intake is deficient, it can cause edema.
D. Proteins transport nutrients and other substances.
1. Transport proteins help to maintain electrolyte balance, transport vitamins and
minerals, and move glucose into cells.
E. Proteins help maintain acid-base balance.
1. Proteins are excellent buffers so they help maintain a constant blood pH.
F. Proteins help maintain a strong immune system.
1. Antibodies are proteins that defend the body against foreign substances.
G. Proteins serve as an energy source.
1. In healthy people, protein contributes little to energy needs.
2. When proteins are needed for energy, they are taken from the blood and body tissues.
3. To protect our body tissues, it is important to eat an adequate amount of fat and
carbohydrate.
4. Our bodies cannot store excess protein.
IV. What Happens To The Proteins We Eat?
A. Dietary proteins must be digested into smaller particles before they can be absorbed.
B. Stomach acids and enzymes begin breaking down protein.
1. Hydrochloric acid in the stomach begins protein digestion by denaturing the protein.
2. Hydrochloric acid also converts inactive pepsinogen to its active form, pepsin.
3. Pepsin begins breaking the protein into single amino acids and polypeptides.
C. Enzymes in the small intestine break polypeptides into single amino acids.
1. Proteases from the pancreas and small intestine continue the breakdown of protein in
the small intestine.
2. Amino acids, dipeptides, and tripeptides are absorbed into the cells of the intestinal
wall.
3. Enzymes in the intestinal cells further break down dipeptides and tripeptides into
single amino acids.
4. Amino acids are transported into the bloodstream.
D. Protein digestibility affects protein quality.
1. Digestibility of a protein refers to how well our bodies can digest it.
V. How Much Protein Should We Eat?
A. Most Americans meet or exceed the RDA for protein without any effort.
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3. 1. The RDA for sedentary people is 0.8 g per kg of body weight per day.
2. Protein needs are higher for some groups, such as children, adolescents, pregnant or
lactating women, vegetarians, and athletes.
3. The recommended percentage of energy that should come from protein is 12 to 20
percent of total energy intake.
B. Protein-energy malnutrition can lead to debility and death.
1. Marasmus results from grossly inadequate energy intake, essentially starvation.
2. Kwashiorkor results from a low-protein diet.
C. Can too much dietary protein can be harmful?
1. High protein intake is associated with high cholesterol and heart disease.
2. High protein intake may contribute to bone loss.
3. High protein intake can be harmful for people with kidney disease.
D. A shopper’s guide: good food sources of protein.
1. Good protein sources include meats, dairy products, soy products, legumes, whole
grains, and nuts.
E. Protein and amino acid supplements: any truth to the hype?
1. Taking amino acid supplements does not enhance muscle strength.
VI. Can A Vegetarian Diet Provide Adequate Protein?
A. There are many types of vegetarian diets.
1. Vegetarian diets are varied depending on what types of foods are excluded.
2. The more restrictive the diet, the more challenging it becomes to achieve adequate
protein intake.
B. Why do people become vegetarians?
1. Some are vegetarians for religious, ethical, or food safety reasons.
2. Some vegetarians are concerned about the effect of the meat industry on the global
ecology.
4. Some become vegetarians due to demonstrated health benefits.
a. Vegetarians have a reduced risk for obesity, type 2 diabetes, heart disease, some
cancers, kidney disease, and gallstones.
b. Vegetarians have lower blood pressure.
c. Vegetarians have fewer intestinal problems.
C. What are the challenges of a vegetarian diet?
1. There is potential for inadequate intakes of some vitamins and minerals.
2. It is important for vegetarians and nonvegetarians to consume a varied and
adequate diet.
3. Appropriate planning can result in a healthful and nutritionally adequate vegetarian
diet.
D. Using MyPlate on a vegetarian diet.
1. Although the USDA has not designed a version of MyPlate specifically for people
following a vegetarian diet, healthy eating tips for vegetarians are available at MyPlate
online
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