Class XI Unit 6 BIOMOLECULES by Prof.Sarjerao Yedekar M.Sc BEd Zoology CTET

BIOMOLECULES

INTRODUCTION

• An analysis of plant tissues, animal tissues and microbial mass indicates that they are made up of almost similar types of elements and compounds.
• Four elements: carbon, hydrogen, oxygen and nitrogen constitutes 97–99% of the body of living organisms.
• All compounds or molecules present in living cells are called biomolecules. It includes large macromolecules such as proteins, polysaccharides, lipids and nucleic acids, as well as small molecules such as primary metabolites, secondary metabolites and natural products.
• Only three types of macromolecules, i.e., proteins, nucleic acids and polysaccharides are found in living systems. Lipids, because of their association with membranes, get separated in the macromolecular fraction.
• Biomacromolecules are polymers. These are large sized, high molecular weight complex molecules that are formed by the condensation of biomacromolecules.
• Proteins are heteropolymers made of amino acids. These serve a variety of cellular functions. Many of them are enzymes, antibodies, receptors, hormones and some others are structural proteins.
• Nucleic acids (RNA and DNA) are composed of nucleotides.
• Polysaccharides are components of cell wall in plants, fungi and also of the exoskeleton of arthropods. They also are storage forms of energy (e.g., starch and glycogen).

HOW TO ANALYSE CHEMICAL COMPOSITION

To know what type of organic compounds are found in living organisms, one has to perform a chemical analysis. We can take any living tissue (a vegetable or a piece of liver, etc.) and grind it in trichloroacetic acid (Cl3CCOOH) using a mortar and pestle. We obtain a thick slurry. If we were to strain through a cheesecloth or cotton, we would obtain two fractions. One is called the filtrate or more technically, the acid-soluble pool, and the second, the retentate or the acid-insoluble fraction. Scientists have found thousands of organic compounds in the acid-soluble pool.

A comparison of elements present in non-living and living matter.

PRIMARY AND SECONDARY METABOLITES

Primary metabolites are involved in growth, development, and reproduction of the organisms. The primary metabolite is typically a key component in maintaining normal physiological processes. These are typically formed during the growth phase as a result of energy metabolism, and are very essential for proper growth. E.g., are ethanol, lactic acid, and certain amino acids.

Secondary metabolites are typically organic compounds produced through the modification of primary metabolites. These do not play a role in growth, development, and reproduction like primary metabolites and are typically formed during the end or near the stationary phase of growth. Many of the identified secondary metabolites have a role in ecological function, including defence mechanism(s), by serving as antibiotics and by producing pigments.

MICROMOLECULES

These are molecules of low molecular weight and have higher solubility. These include minerals, water, amino acids, sugars and nucleotides.

ELEMENTS

On the basis of presence and requirement in plants and animals, these are grouped into major (Ca, P, Na, Mg, S, K, N) and minor (Fe, Cu, Co, Mn, Mo, Zn, I) bioelements.

On the basis of functions, these may be of following types :–

• Framework elements : Carbon, oxygen and hydrogen.
• Protoplasmic elements : Proteins, nucleic acids, lipids, chlorophyll, enzymes, etc.
• Balancing elements : Ca, Mg and K.

BIOLOGICAL COMPOUNDS

• Inorganic compounds : Water 80%, inorganic salts 1-3%.
• Organic compounds : Carbohydrates (1.0%), lipids (3.5%), proteins (12.0%), nucleotides (2.0%), other compounds (0.5%).

CELLULAR POOL

Various kinds of biomolecules aggregated and interlinked in a living system is called cellular pool. It includes over 5000 chemicals.

WATER

Major constituent of cell (about 60-90%) and exists in intracellular, intercellular as well as in vacuoles. In cells, it occurs in free state or bound state (KOH, CaOH etc.).

CARBOHYDRATES

E.g., sugars, glycogen (animal starch), plant starch and cellulose.

SOURCE OF CARBOHYDRATE

It is formed during photosynthesis. It exists only in 1% but constitutes 80% of the dry weight of plants.

COMPOSITION

It consists of carbon, hydrogen and oxygen in the ratio CnH2nOn. It is also called saccharide with sugars being the basic components of saccharides.

CLASSIFICATION OF CARBOHYDRATES

(A)  Monosaccharides

Monosaccharides are colourless, sweet tasting solids that show oxidation, esterification and fermentation. Due to asymmetric carbon, these exist in different isomeric forms. These can rotate polarized light hence are dextrorotatory and laevorotatory.

These are single sugar units which can not be hydrolysed further into smaller carbohydrates. General formula is CnH2nOn, E.g., trioses-3C, (Glyceraldehyde, dihydroxyace-tone etc.), tetroses-4C, pentoses-5C, hexoses-6C etc.

Important Hexoses Glucose : (C6H12O6) are fructose, galactose (important constituent of glycolipids and glycoproteins).

NOTES

Fructose is called fruit sugar (sweetest among natural sugars) and glucose is called “sugar of body” (blood sugar). Normal level of blood glucose is 80-120mg/100ml. If it exceeds then the condition is called “glucosuria”.

(B)  Oligosaccharides

It is formed due to condensation of 2–10 monosaccharide units, the oxygen bridge is known as “glycoside linkage” and water molecule is eliminated. The bond may be α and β.

• Disaccharides C12H22O11 : It is composed of two molecules of the same or different monosaccharide units. Also called “double sugars”.
• Maltose : Also called “malt sugar” stored in germinating seeds of barley, oats, etc. It is formed by enzymatic (enzyme amylase) action on starch. It is a reducing sugar.
• Sucrose : “Cane sugar” or “table-sugar”. It is obtained from sugarcane and beetroot and on hydrolysis splits into glucose and fructose.
• Lactose : “Milk sugar’’, present in mammalian milk. On hydrolysis if yields glucose and galactose. Streptococcus lacti converts lactose into lactic acid and causes souring of milk.

• Trisaccharides C18H32O16 : It is composed of three molecules of sugars.
• Raffinose : Found in sugar beet, cotton and in some fungi. It is made up of glucose, fructose and galactose.
• Tetrasaccharides : It is composed of four molecules of the same or different sugars. Stachyose is found in Stachys tubifera. It is made up of two units of galactose, one units of glucose and one unit of fructose.
• Polysaccharides : General formula is (C6H10O5)n formed by condensation of several molecules (300–1000) of monosaccharides.

LIPIDS

• Fat and its derivatives are together known as lipid. The term 'Lipid' was given by German biochemist, Wilhem Bloor for fat and fat like substances.
• Essential constituents are C, H, O but the ratio of hydrogen and oxygen is not 2 : 1. The amount of oxygen is considerably very less.
• Lipids are insoluble in water and soluble in organic solvents like acetones, chloroform, benzene, hot alcohol, ether etc.
• Lipids occur in protoplasm as minute globules. These require less space for storage as compared to carbohydrate because lipid molecule is hydrophobic and condensed.
• Lipids do not form polymer.
• Lipids provide more than double energy as compared to carbohydrates.
• In animals, fat present in the subcutaneous layer and working as food reservoir and shock-absorber.
• Animals store maximum amount of food in the form of lipid.
• Lipids are not strictly macromolecules.
• There are 3 classes of lipids :
• Simple lipids or neutral fats
• Compound or conjugated lipids
• Derived lipids

SIMPLE LIPIDS OR NEUTRAL FATS

• These are esters of long chain fatty acids with various alcohols. In the majority of simple lipids, the alcohol is a trihydroxy sugar alcohol i.e. glycerol.
• Three molecules of fatty acid linked with one molecule of glycerol. The linkage is called “ester bond”, such type of lipids are called triglycerides. Three molecules of water are released during formation of triglycerides (dehydration synthesis).

• Similar or different fatty acids participate in the composition of a fat molecule. Simple lipids contain two types of fatty acids :

SATURATED FATTY ACIDS are those in which all the carbon atoms of hydrocarbon chain are saturated with hydrogen atoms. Absent of Double Betn C And H

e.g.  Palmitic acid CH3(CH2)14 - COOH

Stearic acid CH3(CH2)16 - COOH

• Simple lipids with saturated fatty acids remain solid at normal room temperature e.g., fats.
• Saturated fatty acids are less reactive so they tend to store in the body and cause obesity.

UNSATURATED FATTY ACIDS are those in which some carbon atoms are not fully occupied by hydrogen atoms.present of Double Bond Betn C and H

e.g. Oleic acid CH3(CH2)7CH = CH(CH2)7 – COOH

Linoleic acid CH3(CH2)4 – (CH = CH – CH2)2 –  (CH2)6 – COOH

Linolenic acid CH3(CH2) – (CH = CH – (CH2)3 –  (CH2)6 – COOH

• Unsaturated fatty acids also called as essential fatty acids because animals are not able to synthesize them.
• Simple lipids with unsaturated fatty acids remain liquid at room temperature e.g., oils.
• Unsaturated fatty acids are more reactive so they tend to metabolise in the body and provide energy.
• Oils with polyunsaturates are recommended by physicians for persons who suffer from high blood cholesterol or cardio-vascular diseases. This is because increasing the proportion of polyunsaturated fatty acids to saturated fatty acids, without raising the fats in the diet tend to lower the cholesterol level in blood.
• Waxes : Waxes are monoglycerides with one molecule of long chain fatty acids and a long chain monohydroxy alcohol. Waxes are more resistant to hydrolysis as compared to triglycerides. Waxes have an important role in protection. They form water insoluble coatings on hair and skin in animals and stem, leaves and fruits of plants.

CONJUGATED OR COMPOUND LIPIDS

PHOSPHOLIPIDS OR PHOSPHATIDE

• It is made of 2 molecules of fatty acid + glycerol + H3PO4 + nitrogenous compounds. Phospholipids are the most abundant type of lipids in protoplasm.
• Phospholipids have both hydrophilic polar end (H3PO4 and nitrogenous compound) and hydrophobic non-polar end (fatty acids). Such molecules are called amphipathic. Due to this property, phospholipids form bimolecular layer in cell membrane.
• Some biologically important phospholipids are as follows :
• Lecithin or phosphatidylcholine : Nitrogenous compound in lecithin is choline. Lecithin occurs in egg yolk, oil seeds and blood. In blood, lecithin functions as carrier molecule. It helps in transportation of other lipids.
• Cephalin or Ethanolamine : Similar to lecithin but the nitrogenous compound is ethanolamine. Cephalin occurs in nervous tissue, egg yolk and blood platelets.
• Sphingolipids or sphingomylins are similar to lecithin but in place of glycerol they contain an amino alcohol sphingosine.

These occur in myelin - sheath of nerves. Other examples of phospholipids are phosphatidyl serine, phosphatidyl inositol, plasmologens.

GLYCOLIPID

It is made of 2 molecules of fatty acid + sphingosine + galactose, e.g. cerebroside which occurs in white matter of the brain.

GANGLIOSIDES

These occur in nerve ganglia and spleen. These also contain N-acetyl neuraminic acid and glucose beside other compounds.

DERIVED LIPIDS

• These are derived from simple or conjugated lipids and are complex in structure. These are insoluble in water and soluble in organic solvents.
• Steroids : Steroids exhibit tetracyclic structure called  “Cyclo pentano perhydrophenanthrene nucleus”.

On the basis of functional group, steroids are of two types,

• Sterols : Lipids without straight chains are called sterols. These are composed of fused hydrocarbon rings and long hydrocarbon side chain, E.g., cholesterol.
• Sterones : They are ketonic steroids, E.g., sex hormones, adreno-corticoids, ecdysone hormone of insects, diosgenin obtained from yam plant (Dioscorea) is used in the manufacture of anti-fertility pills.

FUNCTIONS OF LIPIDS

• Oxidation of lipids yield comparatively more energy in the cell than proteins and carbohydrates.
• The oil seeds such as groundnut, mustard, coconut store fats to provide nourishment to the embryo during germination.
• These function as structural constituent i.e., all the membrane system of the cell are made up of lipoproteins.
• Amphipathic lipids are emulsifiers.
• It works as a heat insulator and is used in the synthesis of hormones.
• Fats provide solubility to vitamins A, D, E and K.

MACROMOLECULES

Macromolecules are polymerisation products of micromolecules, having high molecular weight and low solubility. These include mainly polysaccharide, protein and nucleic acids.

POLYSACCHARIDES

These are branched or unbranched polymers of monosaccharides joined by glycosidic bond.  Their general formula is (C6H10O5)n. Polysaccharides are amorphous, tasteless and insoluble or only slightly soluble in water and can be easily hydrolysed to monosaccharide units.

TYPES OF POLYSACCHARIDES

• On the basis of structure
• Homopolysaccharides : These are made by polymerisation of single kind of monosaccharides. e.g., starch, cellulose, glycogen, etc.
• Heteropolysaccharides : These are made by condensation of two or more kinds of monosaccharides, e.g., chitin, pectin, etc.
• On the basis of functions
• Food storage polysaccharides : These serve as reserve food, e.g., starch and glycogen.
• Structural polysaccharides : These take part in structural framework of cell wall, e.g., chitin and cellulose.

Glycogen : It is a branched polymer of glucose and contains 30,000 glucose units. It is also called animal starch is found as storage product in blue green algae, slime moulds, fungi and bacteria. It is a non-reducing sugar and gives red colour with iodine. In glycogen, glucose molecules are linked by 1→ 4 glycosidic linkage in straight part and 1 → 6 linkage in the branching part. Glycogen has branch points at about every 8-10 glucose units.

Starch (C6H10O5) : Starch is formed in photosynthesis and function as energy storing substance. It is found abundantly in rice, wheat, legumes, potato, banana, etc. Starch is of two types- Straight chain polysaccharides are known as amylose and branched chain as amylopectin. Both are composed of D–glucose units joined by α-1→ 4 linkage and α-1→ 6 linkage. It is insoluble in water and gives blue colour when treated with iodine.

Inulin : It is also called “dahlia starch”(found in roots). It has unbranched chain of 30 – 35 fructose units linked by β-2→1 glycosidic linkage between 1 and 2 of carbon atom of D– fructose unit.

Cellulose : An important constituent of cell wall (20 – 40%), made up of unbranched chain of 6000 β–D glucose units linked by 1 → 4 glycosidic linkage. It is fibrous, rigid and insoluble in water. It doesn’t give any colour when treated with iodine. It is the most abundant polysaccharide.

Chitin : It is a polyglycol consisting of N-acetyl–D–glucosamine units connected with β-1,4 glycosidic linkage. Mostly it is found in hard exoskeleton of insects and crustaceans and sometimes in fungal cell wall. It is the second most abundant carbohydrate and the most abundant heteropolysaccharide.

Agar-Agar : It is a galactan consisting of both D and L galactose and is used to prepare bacterial cultures. It is also used as luxative and obtained from cell wall of red algae e.g., Gracilaria, Gelidium etc.

Pectin : It is a cell wall material in collenchyma tissue that may also be found in fruit pulps, rind of citrus fruits etc. It is water soluble and can undergo sol→gel transformation. It contains arabinose, galactose and galacturonic acid.

Gum : It secreted by higher plants after injury or pathogenic attacks. It is viscous and seals the wound. It involves sugars like L-arabinose, D-galactose, D-glucusonic acid. E.g., gum arabic.

FUNCTIONS

• Cellulose, pectin and chitin are constituents in cell wall of higher plants but peptidoglycan in the cell wall of prokaryotes. These are the reserve food material and form a protective covering.
• Fibres obtained are used in making cloth and rope.
• Nitrocellulose and trinitrate cellulose (gun-cotton) are used as explosives.