Organic Compounds and It’s Importance

Organic compounds all contain covalently bonded carbon and hydrogen atoms and perhaps other elements as well. In the human body there are four major groups of organics compounds: carbohydrates, lipids, proteins, and nucleic acids.

CARBOHYDRATES

A primary function of carbohydrates is to serve as sources of energy in cell respiration. All carbohydrates contain carbon, hydrogen, and oxygen and are classified as monosaccharides, disaccharides, oligosaccharides, and polysaccharides, oligosaccharides, and polysaccharides. Saccharide means sugar, and the prefix indicates how many are present.

Monosaccharides, or single- sugar compounds, are the simplest sugars. Glucose is a hexose, or six- carbon, sugar with the formula C6H12O6. Fructose and galactose also have the same formula, but the physical arrangement of the carbon, hydrogen, and oxygen atoms in each differs from that of glucose. This gives each hexose sugar a different three- dimensional shape. The liver is able to change fructose and galactose to glucose, which is then used by cells in the process of cell respiration to produce ATP.

Another type of monosaccharide is the pentose, or five-carbon, sugar. These are not involved in energy production but rather are structural components of the nucleic acids. Deoxyribose (G5H10O4) i part of DNA, which is the genetic material of chromosomes. Ribose (C5H10O5) is part of RNA, which is essential for protein synthesis.

Disaccharides are double sugars, made of two monosaccharides linked by a covalent bond. Sucrose, or cane sugar, for example, is made of one glucose and one fructose, for example, is made of one glucose and fructose. Others are lactose (glucose and galactose) and maltose (two glucose), which are also present in food. Disaccharides are digested into monosaccharides and then used for energy production.

The prefix oligosaccharides are found on the outer surface of cells, oligosaccharides are found on the outer surface of cell membranes. Here they serve as antigens, which are chemical markers (or”signposts”) that identify cells. The A, B, and AB blood types, for example, are the result of oligosaccharide antigens on the outer surface of red blood cell membranes. All of our cells have “self” antigens, which identify the cells that belong in an individual. The presence of “self” antigens on our own cells enables the immune system to recognize antigens that are “non-self.”

Polysaccharides are made of thousands of glucose molecules, bonded in different ways, resulting in different shapes. Starches are branched chains of glucose and are produced by plant cells to store energy. We have digestive enzymes that split the bonds of starch molecules, releasing glucose. The glucose is then absorbed and used by cells to produce ATP.

Glycogen, a highly branched chain of glucose molecules, is our own storage form for glucose. After a
meal high in carbohydrates, the blood glucose level rises. Excess glucose is then changed to glycogen and
stored in the liver and skeletal muscles. When the blood glucose level decreases between meals, the
glycogen is converted back to glucose, which is released into the blood (these reactions are regulated
by insulin and other hormones). The blood glucose level is kept within normal limits, and cells can take in this glucose to produce energy.

Cellulose is a nearly straight chain of glucose molecules produced by plant cells as part of their cell
walls. We have no enzyme to digest the cellulose we consume as part of vegetables and grains, and it passes through the digestive tract unchanged. Another name for dietary cellulose is “fiber,” and although we cannot use its glucose for energy, it does have a function.

Fiber provides bulk within the cavity of the large intestine. This promotes efficient peristalsis, the waves of contraction that propel undigested material through the colon. A diet low in fiber does not give the colon much exercise, and the muscle tissue of the colon will contract weakly, just as our skeletal muscles will become flabby without exercise. A diet high in fiber provides exercise for the colon muscle and may help prevent chronic constipation.

Lipids

Lipids contain the elements carbon, hydrogen, and oxygen; some also contain phosphorus. In this group
of organic compounds are different types of substances with very different functions. We will consider three types: true fats, phospholipids, and steroids.

True fats (also called neutral fats) are made of one molecule of glycerol and one, two, or three fatty acid
molecules. If three fatty acid molecules are bonded to a single glycerol, a triglyceride is formed. Two fatty
acids and a glycerol form a diglyceride, and one fatty acid and a glycerol form a monoglyceride.

The fatty acids in a true fat may be saturated or unsaturated. Each of these carbons is then bonded to
the maximum number of hydrogens; this is a saturated fatty acid, meaning saturated with hydrogen. The
other fatty acids shown have one or more (poly) double covalent bonds between their carbons and less
than the maximum number of hydrogens; these are unsaturated fatty acids. Many triglycerides contain
both saturated and unsaturated fatty acids, and though it is not as precise, it is often easier to speak of saturated and unsaturated fats, indicating the predominance of one or the other type of fatty acid.

At room temperature, saturated fats are often in solid form, while unsaturated fats are often (not
always) in liquid form. Saturated fats tend to be found in animal foods such as beef, pork, eggs, and cheese, but palm oil and coconut oil are also saturated. Unsaturated fats are found in other plant oils such as corn oil, sunflower oil, and safflower oil, but certain fish oils are also unsaturated, and even pork contains unsaturated fatty acids.

Unsaturated fats may be changed to saturated fats in order to give packaged foods a more pleasing texture or taste, or to allow them to be stored longer without refrigeration (a longer shelf life). These are
hydrogenated fats (meaning that hydrogens have been added), also called trans fats. Trans fats contribute significantly to atherosclerosis of arteries, that is, abnormal cholesterol deposits in the lining that may
clog arteries, especially the coronary arteries of the heart. The triglyceride forms of true fats are a storage
form for excess food, that is, they are stored energy (potential energy). Any type of food consumed in
excess of the body’s caloric needs will be converted to fat and stored in adipose tissue. Most adipose tissue is subcutaneous, between the skin and muscles. Some organs, however, such as the eyes and kidneys, are enclosed in a layer of fat that acts as a cushion to absorb shock.

Phospholipids are diglycerides with a phosphate group (PO4) in the third bonding site of glycerol.
Although similar in structure to the true fats, phospholipids are not stored energy but rather structural
components of cells. Lecithin is a phospholipid that is part of our cell membranes (each phospholipid molecule looks like a sphere with two tails; the sphere is the glycerol and phosphate, the tails are
the two fatty acids). Another phospholipid is myelin,

Phospholipids are diglycerides with a phosphate group (PO4) in the third bonding site of glycerol.
Although similar in structure to the true fats, phospholipids are not stored energy but rather structural
components of cells. Lecithin is a phospholipid that is part of our cell membranes (each phospholipid molecule looks like a sphere with two tails; the sphere is the glycerol and phosphate, the tails are
the two fatty acids). Another phospholipid is myelin, which forms the myelin sheath around nerve cells and provides electrical insulation for nerve impulse transmission.
The structure of steroids is very different from that of the other lipids. Cholesterol is an important
steroid; it is made of four rings of carbon and hydrogen (not fatty acids and glycerol). The liver synthesizes cholesterol, in addition to the cholesterol we eat in food as part of our diet. Cholesterol is another component of cell membranes and is the precursor (raw material) for the synthesis of
other steroids. In the ovaries or testes, cholesterol is used to synthesize the steroid hormones estrogen or
testosterone, respectively. A form of cholesterol in the skin is changed to vitamin D on exposure to sunlight. Liver cells use cholesterol for the synthesis of bile salts, which emulsify fats in digestion. Despite its link to coronary artery disease and heart attacks, cholesterol is an essential substance for human beings.

Proteins

Proteins are made of smaller subunits or building blocks called amino acids, which all contain the elements carbon, hydrogen, oxygen, and nitrogen. Some amino acids contain sulfur, which permits the formation of disulfide bonds. There are about 20 amino acids that make up human proteins. Each amino acid has a central carbon atom covalently bonded to an atom of hydrogen, an amino group (NH2), and a carboxyl group (COOH). At the fourth bond of the central carbon is the variable portion of the amino acid, represented by R. The R group may be a single hydrogen atom, or a CH3 group, or a more complex configuration of carbon and hydrogen. This gives each of the 20 amino acids a slightly different physical shape. A bond between two amino acids is called a peptide bond, and a short chain of amino acids linked by peptide bonds is a polypeptide.
A protein may consist of from 50 to thousands of amino acids. The sequence of the amino acids is
specific and unique for each protein, and is called its primary structure. This unique sequence, and the
hydrogen bonds and disulfide bonds formed within the amino acid chain, determines how the protein will
be folded to complete its synthesis. The folding may be simple, a helix (coil) or pleated sheet, called the secondary structure, or a more complex folding may occur to form a globular protein, called the tertiary structure. Myoglobin, found in muscles, is a globular protein. When complete, each protein has a characteristic three-dimensional shape, which in turn determines its function. Some proteins consist of
more than one amino acid chain (quaternary structure). Hemoglobin, for example, has four amino acid
chains. Notice that myoglobin contains an atom of iron (a hemoglobin molecule has four iron atoms). Some proteins require a trace element such as iron or zinc to complete their structure and permit them to function properly.

Enzymes

Enzymes are catalysts, which means that they speed up chemical reactions without the need for an external source of energy such as heat. The many reactions that take place within the body are catalyzed by specific enzymes; all of these reactions must take place at body temperature.
The way in which enzymes function as catalysts is called the active site theory, and is based on the shape of the enzyme and the shapes of the reacting molecules, called substrates. Notice that the enzyme
has a specific shape, as do the substrate molecules.

The active site of the enzyme is the part that matches the shapes of the substrates. The substrates must “fit” into the active site of the enzyme, and temporary bonds may form between the enzyme and the substrate. This is called the enzyme–substrate complex. In this case, two substrate molecules are thus brought close together so that chemical bonds are formed between them, creating a new compound. The product of the reaction, the new compound, is then released, leaving the enzyme itself unchanged and able to catalyze another reaction of the same type.

As the substrate molecule bonds to the active site of the enzyme, strain is put on its internal bonds,
which break, forming two product molecules and again leaving the enzyme unchanged. Each enzyme is
specific in that it will catalyze only one type of reaction. An enzyme that digests the protein in food, for
example, has the proper shape for that reaction but cannot digest starches. For starch digestion, another
enzyme with a differently shaped active site is needed. Thousands of chemical reactions take place within the body, and therefore we have thousands of enzymes, each with its own shape and active site.

The ability of enzymes to function may be limited or destroyed by changes in the intracellular or extracellular fluids in which they are found. Changes in pH and temperature are especially crucial. Recall that the pH of intracellular fluid is approximately 6.8, and that a decrease in pH means that more H ions are present. If pH decreases significantly, the excess H ions will react with the active sites of cellular enzymes, change their shapes, and prevent them from catalyzing reactions. This is why a state of acidosis may cause the death of cells—the cells’ enzymes are unable to function properly.

Nucleic Acid

DNA and RNA

The nucleic acids, DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), are large molecules made of
smaller subunits called nucleotides. A nucleotide consists of a pentose sugar, a phosphate group, and one of several nitrogenous bases. In DNA nucleotides, the sugar is deoxyribose, and the bases are adenine, guanine, cytosine, or thymine. In RNA nucleotides, the sugar is ribose, and the bases are adenine, guanine, cytosine, or uracil. Notice that DNA looks somewhat like a twisted ladder; this ladder is two strands of nucleotides called a double helix (two coils). Alternating phosphate and sugar molecules form the uprights of the ladder, and pairs of nitrogenous bases form the rungs. The size of the bases and the number of hydrogen bonds each can form the complementary base pairing of the nucleic acids. In DNA, adenine is always paired with thymine (with two hydrogen bonds), and guanine is always paired with cytosine (with three hydrogen bonds).

DNA makes up the chromosomes of cells and is, therefore, the genetic code for hereditary characteristics. The sequence of bases in the DNA strands is actually a code for the many kinds of proteins living things
produce; the code is the same in plants, other animals, and microbes. The sequence of bases for one protein is called a gene. Human genes are the codes for the proteins produced by human cells (though many of these genes are also found in all other forms of life—we are all very much related). RNA is often a single strand of nucleotides, with uracil nucleotides in place of thymine nucleotides. RNA is synthesized from DNA in the nucleus of a cell but carries out a major function in the cytoplasm. This function is protein synthesis.

ATP

ATP (adenosine triphosphate) is a specialized nucleotide that consists of the base adenine, the sugar
ribose, and three phosphate groups. Mention has already been made of ATP as a product of cell respiration that contains biologically useful energy. ATP is one of several “energy transfer” molecules within cells, transferring the potential energy in food molecules to cell processes. When a molecule of glucose is broken down into carbon dioxide and water with the release of energy, the cell uses some of this energy to synthesize ATP. Present in cells are molecules of ADP (adenosine diphosphate) and phosphate. The energy released from glucose is used to loosely bond a third phosphate to ADP, forming ATP. When the bond of this third phosphate is again broken and energy is released, ATP then becomes the energy source for cell processes such as mitosis. All cells have enzymes that can remove the third phosphate group from ATP to release its energy, forming ADP and phosphate. As cell respiration continues, ATP is resynthesized from ADP and phosphate. ATP formation to trap energy from food and breakdown to release energy for cell processes is a continuing cycle in cells.


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