Proteins

Proteins are polymers with at least one long chain of amino acids linked by peptide bonds.
BOX on protein structure
Peptide chains are formed by linking single molecules of amino acids (called monomers) by eliminating one molecule of water. As a result a long peptide or protein chain is formed. In the figure, -R1 and -R2 are alkyl groups which for simplicity can be taken to be -H. The -OH in the -COOH group of the first molecule interacts with the H in the first amine (NH2) in NH2-CH2-COOH to form NH2-CH2-CONH-CH2-COOH. That is a peptide bond OCNH in the middle, and one molecule of water (H2O) has been removed, so the reaction is called a condensation.
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A peptide bond is not very strong. In our bodies, proteins are eventually ``degraded'' or broken down into amino acids, sometimes within minutes, and on an average within a day or two. But proteins are needed for much of our body's metabolism. For example muscle activation is a process that requires a lot of protein. Haemoglobin is a crucial protein transporting oxygen in red blood cells. Long stiff proteins are used in making up cell walls. Many proteins are used as enzymes for other reactions.

Sources of proteins
Unlike plants or bacteria, we cannot make our own proteins. So we have to keep eating food every day and our digestion processes break the food up and get the proteins. Protein is also converted to glucose and stored as fat, which can be reconverted to protein when a person is starving.

Vegetarian foods with proteins are milk, cheese and curds (which also have calcium and vitamin D), beans and soyabean (which also have fibre), lentils (dal), especially bengal gram or chickpea (chana) (which also have fibre and minerals), and nuts such as almonds (which also have vitamin E and minerals)/ Certain vegetables such as peas, spinach, broccoli, cabbage, cauliflower (which also have vitamin C, vitamin K, fibre and minerals) also contain proteins.

Meat also has a lot of proteins but one has to watch out for cholesterol, a lipid (or fat-like substance) which is good for making hormones, vitamin D and digestion. But too much of cholesterol can thicken your arteries.

Lean meat has long been recognized as a source of proteins and minerals. Mutton, pork, beef, and chicken also have protein. The skin has saturated fats, so removing it is better for getting the proteins. Fish and prawns are also good for proteins and are low in fats. Egg whites are rich in protein, egg yellow (yolk) is also rich in cholesterol.

Understanding the structure of proteins has posed many challenges to biology. The famous ``protein folding problem'' is to determine for how long polypeptides can take a compact shape, which commonly happens within our bodies.