Paper - 6 General Science and Technology

Life Science

Bio-Molecules

Biomolecules
Although 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.

Biomolecules are molecules that occur naturally in living organisms. Biomolecules include macromolecules like proteins, carbohydrates, lipids and nucleic acids. It also includes small molecules like primary and secondary metabolites and natural products. Biomolecules consists mainly of carbon and hydrogen with nitrogen, oxygen, sulphur, and phosphorus. Biomolecules are very large molecules of many atoms, that are covalently bound together.

Classes of Biomolecules
There are four major classes of biomolecules: 

• Carbohydrates
• Lipids
• Proteins
• Nucleic acids

1. Carbohydrates

Carbohydrates are good source of energy. Carbohydrates (polysaccharides) are long chains of sugars. 

Monosaccharides are simple sugars that are composed of 3-7 carbon atoms. They have a free aldehyde or ketone group, which acts as reducing agents and are known as reducing sugars. 

Example of monosaccharides - glucose, fructose, pentose and ribose.

Disaccharides are made of  two monosaccharides. The bonds shared between two monosaccharides is the glycosidic bonds.

milk sugar (lactose) is made from glucose and galactose whereas the sugar from sugar cane and sugar beets (sucrose) is made from glucose and fructose.

Example of disaccharides - sucrose, maltose and lactose.

Monosaccharides and disaccharides are sweet, crystalline and water soluble substances.

Polysaccharides are polymers of monosaccharides. They are unsweet, and complex carbohydrates.They are insoluble in water and are not in crystalline form. 

Polysaccharides are polymeric carbohydrate molecules composed of long chains of monosaccharide units bound together by glycosidic linkages and on hydrolysis give the constituent monosaccharides or oligosaccharides

Example:- Amylose (starch), Amylopectin, Glycogen (animal storage polymer), Cellulose, etc. Amylose, or starch, is a helical chain of Glucose monomers, which are bonded by glycosidic linkages (Alfa linkages 1 - 4).

2. Lipids

• Lipids are composed of long hydrocarbon chains. Lipid molecules hold a large amount of energy and are energy storage molecules. Lipids are generally esters of fatty acids and are building blocks of biological membranes. Most of the lipids have a polar head and non-polar tail. Fatty acids can be unsaturated and saturated fatty acids. 
• Lipids are a group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E, and K), monoglycerides, diglycerides, triglycerides, phospholipids, and others
• The main biological functions of lipids include storing energy, signaling, and acting as structural components of cell membranes.Lipids have applications in the cosmetic and food industries as well as in nanotechnology.

Lipids present in biological membranes are of three classes based on the type of hydrophilic head present:

• Glycolipids are lipids whose head contains oligosaccharides with 1-15 saccharide residues.  
• Phospholipids contain a positively charged head which are linked to the negatively charged phosphate groups. Note:- All living cells and many of the tiny organelles internal to cells are bounded by thin membranes. These membranes are composed primarily of phospholipids and proteins and are typically described as phospholipid bi-layers.
• Sterols, whose head contain a steroid ring. Example steroid.

Example of lipids: oils, fats, phospholipids, glycolipids, etc.

Saturated and unsaturated compounds

• In organic chemistry, a saturated compound is a chemical compound that has a chain of carbon atoms linked together by single bonds and has hydrogen atoms filling all of the other bonding orbitals of the carbon atoms. Alkanes are an example of saturated compounds. 
• An unsaturated compound is a chemical compound that contains carbon-carbon double bonds or triple bonds, such as those found in alkenes or alkynes, respectively. 
• Saturated and unsaturated compounds need not consist only of a carbon atom chain.They can form straight chain,branched chain or ring. They can have functional groups, as well. It is in this sense that fatty acids are classified as saturated or unsaturated. The amount of unsaturation of a fatty acid can be determined by finding its iodine number.
• In a chain of carbons, such as a fatty acid, a double or triple bond will cause a kink in the chain. These kinks have macro-structural implications. Unsaturated fats tend to be liquid at room temperature, rather than solid, due to the kinks in the chain. The kinks prevent the molecules from packing closely together to form a solid. These fats are called oils and are present in fish and plants.

Hydrogenation and hardening

• Hydrogenation of unsaturated fatty acids is widely practiced to give saturated fatty acids, which are less prone toward rancidification(rancid - (of foods containing fat or oil) smelling or tasting unpleasant as a result of being old and stale). Since the saturated fatty acids are higher melting than the unsaturated relatives, the process is called hardening. This technology is used to convert vegetable oils into margarine. During partial hydrogenation, unsaturated fatty acids can be isomerized from cis to trans configuration.

Trans Fat

• Trans fats, or trans-unsaturated fatty acids, trans fatty acids, are a type of unsaturated fats which are uncommon in nature but can be created artificially.
• Fats contain long hydrocarbon chains, which can either be unsaturated, i.e. have double bonds, or saturated, i.e. have no double bonds. In nature, unsaturated fatty acids generally have cis (as opposed to trans) configurations.In food production, liquid cis-unsaturated fats such as vegetable oils are hydrogenated to produce saturated fats, which have more desirable physical properties, e.g. they melt at a desirable temperature (30–40 °C). The process of hydrogenation upon the unsaturated fat converts some of the "cis" double bonds into "trans" double bonds, which yields a trans fat. Trans fats are a contaminant(contaminants - something that makes a place or a substance (such as water, air, or food) no longer suitable for use) introduced by a side reaction with the catalyst in partial hydrogenation.
• Although trans fats are edible, consumption of trans fats has shown to increase the risk of coronary heart disease in part by raising levels of the lipoprotein LDL (so-called "bad cholesterol"), lowering levels of the lipoprotein HDL ("good cholesterol"), increasing triglycerides in the bloodstream and promoting systemic inflammation.
• In most countries, there are legal limits to trans fat content. Trans fats levels can be reduced or eliminated using saturated fats such as lard, palm oil or fully hydrogenated fats, or by using interesterified fat.
• Hydrogenated oil is not a synonym for trans fat: complete hydrogenation removes all unsaturated fats, both cis and trans forms.

3. Proteins

• Proteins are large biological molecules, or macromolecules, consisting of one or more long chains of amino acid residues. 
• Proteins perform a vast array of functions within living organisms, including catalyzing metabolic reactions, replicating DNA, responding to stimuli, and transporting molecules from one location to another. 
• Proteins differ from one another primarily in their sequence of amino acids, which is dictated by the nucleotide sequence of their genes, and which usually results in folding of the protein into a specific three-dimensional structure that determines its activity.
• A linear chain of amino acid residues is called a polypeptide. A protein contains at least one long polypeptide. Short polypeptides, containing less than about 20-30 residues, are rarely considered to be proteins and are commonly called peptides, or sometimes oligopeptides. The individual amino acid residues are bonded together by peptide bonds and adjacent amino acid residues. The sequence of amino acid residues in a protein is defined by the sequence of a gene, which is encoded in the genetic code. 
• Proteins are macromolecules, ranging from simply large to enormous. Proteins typically make up about half the total weight of biomolecules in a cell (excluding water).

4. Nucleic Acids

• Nucleic acids are polymolecules, or large biomolecules, essential for all known forms of life. Nucleic acids, which include DNA (deoxyribonucleic acid) and RNA (ribonucleic acid), are made from monomers known as nucleotides. 
• Each nucleotide has three components: a 5-carbon sugar(ribose or deoxyribose), a phosphate group, and a nitrogenous base. If the sugar is deoxyribose, the polymer is DNA. If the sugar is ribose, the polymer is RNA.
• Together with proteins, nucleic acids are the most important biological macromolecules; each are found in abundance in all living things, where they function in encoding, transmitting and expressing genetic information—in other words, information is conveyed through the nucleic acid sequence, or the order of nucleotides within a DNA or RNA molecule. Strings of nucleotides strung together in a specific sequence are the mechanism for storing and transmitting hereditary, or genetic information via protein synthesis.

DNA

Deoxyribonucleic acid is a molecule that encodes the genetic instructions used in the development and functioning of all known living organisms and many viruses. DNA is a nucleic acid; alongside proteins and carbohydrates, nucleic acids compose the three major macromolecules essential for all known forms of life. Most DNA molecules consist of two biopolymer strands coiled around each other to form a double helix. The two DNA strands are known as polynucleotides since they are composed of simpler units called nucleotides. Each nucleotide is composed of a nitrogen-containing nucleobase—either guanine (G), adenine (A), thymine (T), or cytosine (C)—as well as a monosaccharide sugar called deoxyribose and a phosphate group. The nucleotides are joined to one another in a chain by covalent bonds between the sugar of one nucleotide and the phosphate of the next, resulting in an alternating sugar-phosphate backbone. According to base pairing rules (A with T and C with G), hydrogen bonds bind the nitrogenous bases of the two separate polynucleotide strands to make double-stranded DNA.

DNA is well-suited for biological information storage. Biological information is replicated as the two strands are separated. A significant portion of DNA (more than 98% for humans) is non-coding, meaning that these sections do not serve as patterns for protein sequences.

The two strands of DNA run in opposite directions to each other and are therefore anti-parallel. Attached to each sugar is one of four types of nucleobases (informally, bases). It is the sequence of these four nucleobases along the backbone that encodes biological information. Under the genetic code, RNA strands are translated to specify the sequence of amino acids within proteins. These RNA strands are initially created using DNA strands as a template in a process called transcription.Transcription is the process by which the information in DNA is copied into messenger RNA (mRNA) for protein production.

Translation is the final step on the way from DNA to protein. In translation, messenger RNA (mRNA)—produced by transcription from DNA—is decoded by a ribosome to produce a specific amino acid chain, or polypeptide. The polypeptide later folds into an active protein and performs its functions in the cell. 

Within cells, DNA is organized into long structures called chromosomes. During cell division these chromosomes are duplicated in the process of DNA replication, providing each cell its own complete set of chromosomes. Eukaryotic organisms (animals, plants, fungi, and protists) store most of their DNA inside the cell nucleus and some of their DNA in organelles, such as mitochondria or chloroplasts.In contrast, prokaryotes (bacteria and archaea) store their DNA only in the cytoplasm.

RNA

Ribonucleic acid (RNA) is a polymeric molecule. It is implicated in various biological roles in coding, decoding, regulation, and expression of genes. DNA and RNA are nucleic acids, and, along with proteins and carbohydrates, constitute the three major macromolecules essential for all known forms of life. Like DNA, RNA is assembled as a chain of nucleotides, but unlike DNA it is more often found in nature as a single-strand folded unto itself, rather than a paired double-strand. Cellular organisms use messenger RNA (mRNA) to convey genetic information (using the letters G, A, U, and C to denote the nitrogenous bases guanine, adenine, uracil and cytosine) that directs synthesis of specific proteins. Many viruses encode their genetic information using an RNA genome.

Some RNA molecules play an active role within cells by catalyzing biological reactions, controlling gene expression, or sensing and communicating responses to cellular signals. One of these active processes is protein synthesis, a universal function whereby mRNA molecules direct the assembly of proteins on ribosomes. This process uses transfer RNA (tRNA) molecules to deliver amino acids to the ribosome, where ribosomal RNA (rRNA) links amino acids together to form proteins.

Function of Biomolecules

• Carbohydrates provide the body with source of fuel and energy, it aids in proper functioning of our brain, heart and nervous, digestive and immune system. Deficiency of carbohydrates in the diet causes fatigue, poor mental function. 
• Each protein in the body has specific functions, some proteins provide structural support, help in body movement, and also defense against germs and infections. Proteins can be antibodies, hormonal, enzymes and contractile proteins.
• Lipids, the primary purpose of lipids in body is energy storage. Structural membranes are composed of lipids which forms a barrier and controls flow of material in and out of the cell. Lipid hormones, like sterols, help in mediating communication between cells. 
• Nucleic Acids are the DNA and RNA, they carry genetic information in the cell. They also help in synthesis of proteins, through the process of translation and transcription.