Proteins
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[Role of Proteins | Why are they called Proteins? | Amino Acids | Amino Acid Alphabet | Energy of Proteins | Complete & Incomplete Proteins | Combining Incomplete Proteins to Make Complete Proteins | Protein-Related Health Problems | Food Sources of Proteins ]
You owe your big and strong muscles to proteins. Besides the muscles, protein can be found almost anywhere in the body because it is used to make cells. Proteins are actually made up of long chains of amino acids. After digestion, the amino acids can be used by our bodies. We need protein to grow and for repairs.
We need protein to grow and for repairs. Proteins are used to make our body cells. Specifically, they are used in the formation of new protoplasm. Antibodies, enzymes and hormones are also made of proteins. Finally, proteins provide us with energy.
Protein comes from the Greek word proteios, meaning "primary" or "holding the first place." A Dutch chemist Gerard Johann Mulder, coined the word protein in 1838.
A protein is like a train with many cars linked together. Each of these cars or units is an amino acid. A group of amino acids is made up of one nitrogen atom and two hydrogen atoms. An acid group is formed from cars, hydrogen and oxygen atoms. Amino acids contain nitrogen which is not found in carbohydrates or fats.
There are millions of proteins found in nature, but most proteins contain only up to 20 different amino acids. The many different combinations that can be formed from 20 amino acids allow such a huge number of proteins to be formed. It is just like the English language where there are only 26 letters, but millions of words can be formed using different combinations of these letters.
| Name | Symbol |
| Alanine | A |
| Arginine | R |
| Asparagine | N |
| Asparatic Acid | D |
| Cysteine | C |
| Glutamic Acid | E |
| Glutamine | O |
| Glycine | G |
| Histidine* | H |
| Isoleucine* | I |
| Leucine* | L |
| Lysine* | K |
| Methionine* | M |
| Phenylalanine* | F |
| Proline | P |
| Serine | S |
| Theronine* | T |
| Trptophan* | W |
| Tyrosine* | Y |
| Valine* | V |
*Essential amino acids which our bodies can’t make from other chemicals.
Source: Proteins, Alvin Silverstein, Virginia Silverstein, and Robert Silverstein
Neither carbohydrates nor fats can provide us with energy, so it is essential to include proteins in our diet. When an excess of protein is eaten, the extra protein is similarly broken down into energy-yielding compounds. Because protein is far scarcer than carbohydrates and yields the same 4 calories per gram, the eating of meat beyond the tissue-building demands of the body becomes an inefficient way to produce energy.
Complete and Incomplete Proteins
Complete proteins are foods that contain all the necessary amino acids. Most animal foods, such as meat, poultry, fish, eggs, cheese and milk, are complete proteins. Some plant proteins are complete, too. Soybean products, such as tofu, are also complete proteins. Eggs are the best source of complete proteins.
Incomplete proteins are proteins containing small amounts of one or more essential amino acids. Most plant foods are incomplete, such as legumes (beans and peas), nuts, seeds, grains, and vegetables.
Combining Incomplete Proteins to Make Complete Proteins
Although plant proteins are incomplete, it is still possible to get all the essential amino acids by eating a combination of plant proteins. For example, peanut butter is low in the amino acid methionine. Bread has a lot of methionine, but it lacks lysine and isoleucine. So a peanut butter sandwich becomes a complete protein.
Foods from animal sources contain complete proteins because they include all the essential amino acids. In most diets, a combination of plant and animal protein is recommended: 0.8 grams per kilogram of body weight is considered a safe daily allowance for normal adults.
Protein-Related Health Problems
Too much protein in the diet can be dangerous. The extra protein contains nitrogen, which is changed in the liver into waste called urea. The kidneys get rid of this nitrogen waste in urine. Too much protein can put stress on the liver and kidneys. When extra urine has to be formed to remove the excess waste, the body can be dehydrated. Too much protein can also make one overweight, as the excess proteins are changed into fats in the liver which is stored in the body. Lack of proteins will lead to a weak body, unable to fight against diseases. Dieting can result in the body not getting enough nutrients. You may get enough calories for your energy need, but you do not have all the essential amino acids.
Lean meat, fish, eggs, milk and cheese are important sources of animal protein. All plants contain some protein, but beans or cereals, such as wheat and maize, are the best sources.
This page has been authored for participation in
the 1997 Thinkquest Competition.
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