BIOMOLECULES
There is vast diversity of living organisms. The chemical compositon and metabolic reactions of the organisms appear to be similar. The composition of living tissues and non-living matter also appear to be similar in qualitative analysis.Closer analysis reveals that the relative abundance of carbon, hydrogen and oxygen is higher in living system. All forms of life are composed of biomolecules only. Biomolecules are organic molecules especially macromolecules like carbohydrates, proteins in living organisms. All living forms bacteria, algae, plant and animals are made of similar macromolecules that are responsible for life. All the carbon compounds we get from living tissues can be called biomolecules.Importaant characteristics of biomolecules are as follows:
- Most of them are organic compounds.
- They have specific shapes and dimension.
- Functional group determines their chemical properties.
- Many of them arc asymmetric.
- Macromolecules are large molecules and are constructed from small building block molecules.
- Building block molecules have simple structure.
- Biomolecules first gorse by chemical evolution.
Some important biomolecules of life
Carbohydrates: Carbohydrates are good source of energy. Carbohydrates (polysaccharides) are long chains of sugars.
Proteins: Proteins are large, complex molecules that play many critical roles in the body. They do most of the work in cells and are required for the structure, function, and regulation of the body’s tissues and organs.
Fats: Fat is one of the three main macronutrients, along with carbohydrate and protein. Fats, also known as triglycerides, are esters of three fatty acid chains and the alcohol glycerol. The terms “oil”, “fat”, and “lipid” are often confused. “Oil” normally refers to a fat with short or unsaturated fatty acid chains that is liquid at room temperature, while “fat” may specifically refer to fats that are solids at room temperature. “Lipid” is the general term, though a lipid is not necessarily a triglyceride. Fats, like other lipids, are generally hydrophobic, and are soluble in organic solvents and insoluble in water.
Nucleic acids: Nucleic acids are biopolymers, or large biomolecules, essential to all known forms of life. They are composed of monomers, which are nucleotides made of three components: a 5-carbon sugar, a phosphate group, and a nitrogenous base.
Structure of carbohydrates
Carbohydrates are molecular compounds made from just three elements: carbon, hydrogen and oxygen. Monosaccharides (e.g. glucose) and disaccharides (e.g. sucrose) are relatively small molecules. They are often called sugars. Other carbohydrate molecules are very large (polysaccharides such as starch and cellulose).
There are mainly three types of carbohydrates :
Monosaccharides: Monosaccharides are the simplest carbohydrates and are often called single sugars. They are the building blocks from which all bigger carbohydrates are made. 
Monosaccharides have the general molecular formula (CH2O)n, where n can be 3, 5 or 6. They can be classified according to the number of carbon atoms in a molecule.
Disaccharides: Disaccharide, also called double sugar , any substance that is composed of two molecules of simple sugars (monosaccharides) linked to each other. Disaccharides are crystalline water-soluble compounds. The monosaccharides within them are linked by a glycosidic bond (or glycosidic linkage), the position of which may be designated ?- or ?- or a combination of the two (?-,?-). Glycosidic bonds are cleaved by enzymes known as glycosidases. The three major disaccharides are sucrose, lactose, and maltose.
Polysaccharides: Monosaccharides can undergo a series of condensation reactions, adding one unit after another to the chain until very large molecules (polysaccharides) are formed. This is called condensation polymerisation, and the building blocks are called monomers.
Functions of carbohydrates
Carbohydrates has some major functions within the body.
Providing energy and regulation of blood glucose: Glucose is the only sugar used by the body to provide energy for its tissues. Therefore, all digestible polysaccharides, disaccharides, and monosaccharides must eventually be converted into glucose or a metabolite of glucose by various liver enzymes. Because of its significant importance to proper cellular function, blood glucose levels must be kept relatively constant.
Flavor and Sweeteners: A less important function of carbohydrates is to provide sweetness to foods. Receptors located at the tip of the tongue bind to tiny bits of carbohydrates and send what humans perceive as a “sweet” signal to the brain. However, different sugars vary in sweetness. For example, fructose is almost twice as sweet as sucrose and sucrose is approximately 30% sweeter than glucose.
Dietary Fiber: Dietary fibers such as cellulose, hemicellulose, pectin, gum and mucilage are important carbohydrates for several reasons. Soluble dietary fibers like pectin, gum and mucilage pass undigested through the small intestine and are degraded into fatty acids and gases by the large intestine. The fatty acids produced in this way can either be used as a fuel for the large intestine or be absorbed into the bloodstream. Therefore, dietary fiber is essential for proper intestinal health.
Biological Recognition Processes: Carbohydrates not only serve nutritional functions, but are also thought to play important roles in cellular recognition processes. For example, many immunoglobulins (antibodies) and peptide hormones contain glycoprotein sequences. These sequences are composed of amino acids linked to carbohydrates. During the course of many hours or days, the carbohydrate polymer linked to the rest of the protein may be cleaved by circulating enzymes or be degraded spontaneously. The liver can recognize differences in length and may internalize the protein in order to begin its own degradation. In this way, carbohydrates may mark the passage of time for proteins.
Structure of proteins
Proteins are biological polymers composed of amino acids. Amino acids, linked together by peptide bonds, form a polypeptide chain. One or more polypeptide chains twisted into a 3-D shape form a protein. Proteins have complex shapes that include various folds, loops, and curves. Folding in proteins happens spontaneously. Chemical bonding between portions of the polypeptide chain aid in holding the protein together and giving it its shape.
There are four distinct levels of protein structure:
Primary structure: It describes the unique order in which amino acids are linked together to form a protein. Proteins are constructed from a set of 20 amino acids.
All amino acids have the alpha carbon bonded to a hydrogen atom, carboxyl group, and amino group. The “R” group varies among amino acids and determines the differences between these protein monomers. The amino acid sequence of a protein is determined by the information found in the cellular genetic code. The order of amino acids in a polypeptide chain is unique and specific to a particular protein. Altering a single amino acid causes a gene mutation, which most often results in a non-functioning protein.
Secondary Structure: It refers to the coiling or folding of a polypeptide chain that gives the protein its 3-D shape. There are two types of secondary structures observed in proteins. One type is the alpha (?) helix structure. This structure resembles a coiled spring and is secured by hydrogen bonding in the polypeptide chain. The second type of secondary structure in proteins is the beta (?) pleated sheet. This structure appears to be folded or pleated and is held together by hydrogen bonding between polypeptide units of the folded chain that lie adjacent to one another.
Tertiary Structure: It refers to the comprehensive 3-D structure of the polypeptide chain of a protein. There are several types of bonds and forces that hold a protein in its tertiary structure. Hydrophobic interactions greatly contribute to the folding and shaping of a protein. The “R” group of the amino acid is either hydrophobic or hydrophilic. The amino acids with hydrophilic “R” groups will seek contact with their aqueous environment, while amino acids with hydrophobic “R” groups will seek to avoid water and position themselves towards the center of the protein. Hydrogen bonding in the polypeptide chain and between amino acid “R” groups helps to stabilize protein structure by holding the protein in the shape established by the hydrophobic interactions. Due to protein folding, ionic bonding can occur between the positively and negatively charged “R” groups that come in close contact with one another. Folding can also result in covalent bonding between the “R” groups of cysteine amino acids. This type of bonding forms what is called a disulfide bridge. Interactions called van der Waals forces also assist in the stabilization of protein structure. These interactions pertain to the attractive and repulsive forces that occur between molecules that become polarized. These forces contribute to the bonding that occurs between molecules.
Quaternary Structure: It refers to the structure of a protein macromolecule formed by interactions between multiple polypeptide chains. Each polypeptide chain is referred to as a subunit. Proteins with quaternary structure may consist of more than one of the same type of protein subunit. They may also be composed of different subunits. Hemoglobin is an example of a protein with quaternary structure. Hemoglobin, found in the blood, is an iron-containing protein that binds oxygen molecules. It contains four subunits: two alpha subunits and two beta subunits.
Functions of Protein
Repair and Maintenance: Protein is termed the building block of the body. It is called this because protein is vital in the maintenance of body tissue, including development and repair. Hair, skin, eyes, muscles and organs are all made from protein. This is why children need more protein per pound of body weight than adults; they are growing and developing new protein tissue.
Energy: Protein is a major source of energy. If you consume more protein than you need for body tissue maintenance and other necessary functions, your body will use it for energy. If it is not needed due to sufficient intake of other energy sources such as carbohydrates, the protein will be used to create fat and becomes part of fat cells.
Hormones: Protein is involved in the creation of some hormones. These substances help control body functions that involve the interaction of several organs. Insulin, a small protein, is an example of a hormone that regulates blood sugar. It involves the interaction of organs such as the pancreas and the liver. Secretin, is another example of a protein hormone. This substance assists in the digestive process by stimulating the pancreas and the intestine to create necessary digestive juices.
Enzymes: The creation of DNA could not happen without the action of protein enzymes. Enzymes are proteins that increase the rate of chemical reactions in the body. In fact, most of the necessary chemical reactions in the body would not efficiently proceed without enzymes. For example, one type of enzyme functions as an aid in digesting large protein, carbohydrate and fat molecules into smaller molecules, while another assists the creation of DNA.
Transportation and Storage of Molecules: Protein is a major element in transportation of certain molecules. For example, hemoglobin is a protein that transports oxygen throughout the body. Protein is also sometimes used to store certain molecules. Ferritin is an example of a protein that combines with iron for storage in the liver.
Antibodies: Antibodies formed by protein help prevent many illnesses and infections. Protein forms antibodies that help prevent infection, illness and disease. These proteins identify and assist in destroying antigens such as bacteria and viruses. They often work in conjunction with the other immune system cells. For example, these antibodies identify and then surround antigens in order to keep them contained until they can be destroyed by white blood cells.
Fats: structure and functions
Fats(Lipids) are important constituent of of the diet because they are a source of high energy value. Lipids are also important because of the fat-soluble vitamins, and essential fatty acids found in the fat of the natural food stuffs. Body fat serves as a very good source of energy, it is stored in adipose tissues. They also act as insulating material in the subcutaneous tissues and are also seen around certain organs. Lipids combined with proteins are important constituents of the cell membranes and mitochondria of the cell. Lipids are not generally macromolecules.
On the basis of chemical structure and constitution, lipids are broadly classified into two categories:
Simple Lipids: Simple lipids contain a trihydric alcohol, glycerol and long chain fatty acids. The carboxyl groups of the fatty acids are ester-linked to the hydroxyl groups of glycerol. The fatty acids present in simple lipids have generally 16 or 18 carbon atoms and they may be saturated or unsaturated. The unsaturated fatty acids, usually have one or two double bonds. Such lipids having three molecules of
fatty acids esterified to glycerol are called triglycerides.
Complex Lipids: In contrast to simple lipids, the complex lipids contain elements like phosphorus, sulfur, nitrogen etc., besides carbon, hydrogen and oxygen which are present in all lipids. Among the complex lipids, phospholipids resemble the simple lipids most closely in their structure.
Phospholipids are a major constituent of the cell membranes of most of the organisms. In a phospholipid molecule, two hydroxyl groups of glycerol are esterified with carboxyl groups of long chain fatty acids as in case simple lipids, while the third hydroxyl group of glycerol is esterified with phosphoric acid. Such a lipid is called a phosphatide. In most of the phospholipids, phosphoric acid is further linked to an organic group.
Functions of lipids
Chemical messengers: All multicellular organisms use chemical messengers to send information between organelles and to other cells. Since lipids are small molecules insoluble in water, they are excellent candidates for signalling. The signalling molecules further attach to the receptors on the cell surface and bring about a change that leads to an action. The signalling lipids, in their esterified form can infiltrate membranes and are transported to carry signals to other cells. These may bind to certain proteins as well and are inactive until they reach the site of action and encounter the appropriate receptor.
Storage and provision of energy: Storage lipids are triacylglycerols. These are inert and made up of three fatty acids and a glycerol. Fatty acids in non esterified form, i.e. as free (unesterified) fatty acids are released from triacylglycerols during fasting to provide a source of energy and to form the structural components for cells. Dietary fatty acids of short and medium chain size are not esterified but are oxidized rapidly in tissues as a source of ‘fuel”. Longer chain fatty acids are esterified first to triacylglycerols or structural lipids.
Maintenance of temperature: Layers of subcutaneous fat under the skin also help in insulation and protection from cold. Maintenance of body temperature is mainly done by brown fat as opposed to white fat. Babies have a higher concentration of brown fat.
Membrane lipid layer formation: Linoleic and linolenic acids are essential fatty acids. These form arachidonic, eicosapentaenoic and docosahexaenoic acids. These for membrane lipids. Membrane lipids are made of polyunsaturated fatty acids. Polyunsaturated fatty acids are important as constituents of the phospholipids, where they appear to confer several important properties to the membranes. One of the most important properties are fluidity and flexibility of the membrane.
Cholesterol formation: Much of the cholesterol is located in cell membranes. It also occurs in blood in free form as plasma lipoproteins. Lipoproteins are complex aggregates of lipids and proteins that make travel of lipids in a watery or aqueous solution possible and enable their transport throughout the body. The main groups are classified as chylomicrons (CM), very low density lipoproteins (VLDL), low density lipoproteins (LDL) and high density lipoproteins (HDL), based on the relative densities Cholesterol maintains the fluidity of membranes by interacting with their complex lipid components, specifically the phospholipids such as phosphatidylcholine and sphingomyelin. Cholesterol also is the precursor of bile acids, vitamin D and steroidal hormones.
Prostaglandin formation and role in inflammation: The essential fatty acids, linoleic and linolenic acids are precursors of many different types of eicosanoids, including the hydroxyeicosatetraenes, prostanoids (prostaglandins, thromboxanes and prostacyclins), leukotrienes (and lipoxins) and resolvins etc. these play an important role in pain, fever, inflammation and blood clotting.
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