Questions: Biomolecules

Q1. What is a carbohydrate? Give one example of each class.

Answer (English): A carbohydrate is a polyhydroxy aldehyde or ketone, or a compound that gives one on hydrolysis. Monosaccharides cannot be hydrolysed to a smaller sugar, and examples are glucose and fructose, both C6H12O6. Disaccharides contain two units joined by a glycosidic linkage, such as sucrose, maltose and lactose. Polysaccharides contain many units, such as starch, cellulose and glycogen. The group is also described by its behaviour with Tollens reagent: sugars that reduce it are reducing sugars, and sucrose is the standard non reducing sugar.

Hinglish: Carbohydrate polyhydroxy aldehyde ya ketone hota hai, ya aisa compound jo hydrolysis par woh de. Monosaccharide chhota nahi hota, jaise glucose aur fructose, dono C6H12O6. Disaccharide mein do units glycosidic linkage se judi hoti hain, jaise sucrose, maltose, lactose. Polysaccharide mein bahut units hoti hain, jaise starch, cellulose, glycogen. Tollens se pata chalta hai kaun reducing sugar hai; sucrose non reducing hai.

Q2. How does the structure of glucose explain its reaction with bromine water and with nitric acid?

Answer (English): Glucose is an aldohexose, so it has a terminal aldehyde group that is easily oxidised. Bromine water is a mild oxidant and takes only the aldehyde group to a carboxyl group, so the product is gluconic acid and the chain length stays the same. Nitric acid is a stronger oxidant and can also oxidise the primary alcohol at carbon 6, so both ends of the six carbon chain become carboxyl groups and the product is saccharic acid. The two products together show that the aldehyde is at one end and the primary alcohol at the other.

Hinglish: Glucose aldohexose hai, isliye uske end mein aldehyde group hai jo aasani se oxidise hota hai. Bromine water mild oxidant hai, sirf aldehyde ko carboxyl banata hai, isliye gluconic acid banta hai aur chain ki lambai wahi rehti hai. Nitric acid strong oxidant hai jo C6 ke primary alcohol ko bhi oxidise kar deta hai, isliye dono ends carboxyl ban jate hain aur saccharic acid milta hai.

Q3. Why is sucrose a non reducing sugar while maltose is reducing?

Answer (English): A sugar reduces Tollens or Fehling reagent only if it has a free anomeric carbon that can open back to the aldehyde form in solution. In sucrose the anomeric carbon of glucose and the anomeric carbon of fructose are both used in the glycosidic linkage, so neither ring can open and no aldehyde appears. In maltose only one anomeric carbon is used in the linkage and the other stays free, so the ring can open and the sugar reduces the reagent. Lactose behaves like maltose.

Hinglish: Sugar tabhi Tollens ya Fehling reduce karta hai jab uske paas free anomeric carbon ho jo solution mein aldehyde form mein khul sake. Sucrose mein glucose aur fructose dono ke anomeric carbons linkage mein busy hain, isliye koi ring nahi khulti aur aldehyde nahi banta. Maltose mein ek anomeric carbon free rehta hai, isliye woh reduce karta hai. Lactose bhi maltose jaisa hai.

Q4. What is a peptide bond? How many peptide bonds are there in a tetrapeptide?

Answer (English): A peptide bond is the amide linkage formed when the carboxyl group of one amino acid and the amino group of another lose a molecule of water. The bond is written as CO-NH and it holds the residues together in a protein. A tetrapeptide has four amino acid residues, so there are three joins between consecutive residues and three peptide bonds. In general a chain of n residues has n minus one peptide bonds, and each hydrolysis of one bond splits the chain into two pieces.

Hinglish: Peptide bond woh amide linkage hai jo ek amino acid ke carboxyl group aur doosre ke amino group se paani nikalne par banta hai. Isse likhte hain CO-NH aur yahi protein ke residues ko jodta hai. Tetrapeptide mein chaar residues hote hain, isliye teen joins aur teen peptide bonds hote hain. n residues wali chain mein n-1 peptide bonds hote hain.

Q5. Explain the four levels of protein structure.

Answer (English): The primary structure is the sequence of amino acids in the chain, held by peptide bonds. The secondary structure is the local folding of the backbone into regular shapes, the alpha helix and the beta pleated sheet, held by hydrogen bonds between backbone NH and CO groups. The tertiary structure is the overall three dimensional shape of a single chain, held by disulphide bridges, hydrophobic interactions, salt bridges and hydrogen bonds between side chains. Quaternary structure is the assembly of two or more folded chains into one functional protein, as in haemoglobin with four subunits.

Hinglish: Primary structure amino acids ka sequence hai jo peptide bonds se juda hota hai. Secondary structure backbone ka local folding hai, alpha helix aur beta pleated sheet, jo NH aur CO ke beech hydrogen bonds se banta hai. Tertiary structure ek chain ka poora three dimensional shape hai, jismein disulphide bridges, hydrophobic interactions aur salt bridges kaam karte hain. Quaternary structure do ya zyada folded chains ka assembly hai, jaise haemoglobin ke chaar subunits.

Q6. What are enzymes and why are they called specific and efficient catalysts?

Answer (English): Enzymes are proteins that catalyse the reactions of living systems. They are specific because each enzyme has an active site whose shape and chemical groups fit one substrate or one family of substrates, so a single enzyme does not act on everything. They are efficient because they speed up reactions by a very large factor under mild conditions of temperature, pH and pressure that a cell can provide, where an industrial catalyst would need high temperature or pressure. Like all catalysts they are not consumed in the reaction, and they lower the activation energy without changing the equilibrium.

Hinglish: Enzymes proteins hain jo living systems ke reactions catalyse karte hain. Woh specific hain kyunki har enzyme ke active site ka shape aur groups ek substrate ke liye fit hote hain. Woh efficient hain kyunki mild temperature, pH aur pressure par bhi reaction ko bahut tez kar dete hain, jahan industrial catalyst ko kathin conditions chahiye. Catalyst ki tarah reaction mein khapat nahi hote aur activation energy kam karte hain.

Q7. Why must water soluble vitamins be taken regularly, while fat soluble vitamins need not be?

Answer (English): Water soluble vitamins, the B group and vitamin C, are not stored in the body to any useful extent. They dissolve in the body fluids and any excess is filtered out by the kidneys and leaves in urine, so the body needs a fresh supply every day. Fat soluble vitamins, A, D, E and K, are stored in the liver and in fatty tissue, so the body can draw on a reserve and does not need them every day. The downside of storage is that an excessive dose of a fat soluble vitamin can build up to a harmful level, while extra water soluble vitamin is simply excreted.

Hinglish: Water soluble vitamins (B group aur C) body mein store nahi hote; woh body fluids mein ghulate hain aur excess kidneys se urine mein nikal jata hai, isliye roz chahiye. Fat soluble vitamins (A, D, E, K) liver aur fatty tissue mein store hote hain, isliye reserve se kaam chal jata hai. Storage ka nuksan ye hai ki zyada dose jama ho sakta hai, jabki water soluble vitamin extra nikal jata hai.

Q8. How does a nucleotide differ from a nucleoside?

Answer (English): The smaller unit, a nucleoside, is a nitrogenous base joined to a five carbon sugar. Adding a phosphate group on the sugar converts it into a nucleotide. So a nucleotide is a nucleoside plus phosphate, and the phosphate is what lets nucleotides link to each other through phosphodiester bonds to build the nucleic acid chain. The sugar is 2-deoxyribose in DNA and ribose in RNA, and the bases are adenine, guanine, cytosine and thymine in DNA, with uracil replacing thymine in RNA.

Hinglish: Nucleoside nitrogenous base aur five carbon sugar ka combination hai. Jab sugar par phosphate group bhi jud jata hai to woh nucleotide ban jata hai. Phosphate ki wajah se nucleotide phosphodiester bond se jude kar chain banate hain. DNA mein sugar 2-deoxyribose aur RNA mein ribose hoti hai; DNA mein thymine hai aur RNA mein uski jagah uracil.

Q9. Why does DNA form a double helix with A pairing with T and G pairing with C?

Answer (English): The bases stick out from the sugar phosphate backbone and can hydrogen bond with a base on another strand, but only in particular combinations. Adenine offers two hydrogen bonding sites and thymine offers two that match, so A pairs with T using two hydrogen bonds. Guanine and cytosine match at three sites, so G pairs with C using three hydrogen bonds. A larger purine always pairs with a smaller pyrimidine, which keeps the distance between the two backbones constant, and the matching shapes mean the two strands are complementary. The strands must be antiparallel for the pairing to work, and they wind into a right handed helix with the bases inside.

Hinglish: Bases sugar phosphate backbone se bahar nikle hote hain aur doosri strand ke base se hydrogen bond bana sakte hain, par sirf khaas jodon mein. Adenine aur thymine ke do sites match karte hain isliye A-T ke do hydrogen bonds bante hain. Guanine aur cytosine ke teen sites match karte hain isliye G-C ke teen. Bada purine chhote pyrimidine ke saath baithta hai, isliye backbones ka distance same rehta hai. Strands antiparallel hoti hain aur right handed helix banati hain.

Q10. Why cannot humans digest cellulose even though both starch and cellulose are glucose polymers?

Answer (English): The difference is the linkage. Starch has alpha glycosidic linkages, and human digestive enzymes such as amylase have active sites that fit that geometry and can hydrolyse it. Cellulose has beta linkages, which tilt the next glucose unit the other way, so the chain is straight and rigid instead of coiled. Human enzymes do not have a site that fits the beta linkage, so the bonds cannot be broken and the cellulose passes through the digestive tract. Ruminants and termites can use cellulose because their gut contains microorganisms that make the enzyme cellulase.

Hinglish: Farq linkage ka hai. Starch mein alpha glycosidic linkage hota hai, aur human digestive enzymes jaise amylase ka active site usi geometry mein fit hota hai. Cellulose mein beta linkage hota hai jisse agla glucose unit doosri taraf jhuk jata hai, chain seedhi aur sakht ho jati hai. Human enzymes beta linkage ko tod nahi sakte, isliye cellulose digest nahi hota. Ruminants aur termites cellulose kha sakte hain kyunki unki gut mein cellulase banane wale microorganisms hote hain.

Quiz below: five questions on sugars, proteins, vitamins and nucleic acids. Check the classification list before you start.