NEET-Pattern Practice Questions

How This Chapter Is Asked in NEET

Biomolecules is one of the highest-yield chapters in the whole paper, and it is also one of the most predictable. Almost every question is one of four shapes: name the compound or the class from a description, give the number, the range or the example, state the one function or property, or spot the odd one out. There is nothing here to work out. Every mark is decided before you enter the hall, by how exactly you learnt the wording.

The traps repeat, and in this chapter they are always the same ones:

• Acid-soluble pool against acid-insoluble fraction. Grind the tissue in trichloroacetic acid, $\mathrm{Cl_3CCOOH}$, and strain the slurry. The filtrate is the acid-soluble pool. The retentate is the acid-insoluble fraction. The option that calls the filtrate the insoluble fraction is built out of this swap.
• Wet weight against dry weight against ash. Wet weight is the fresh tissue. Dry weight is what is left after all the water evaporates. Ash is what is left after full burning, when all the carbon compounds are oxidised to $\mathrm{CO_2}$ and water vapour and are removed. Ash therefore holds the inorganic elements, calcium and magnesium among them.
• Nucleoside against nucleotide. The phosphate is the whole difference. Base plus sugar is a nucleoside - adenosine, guanosine, thymidine, uridine, cytidine. Base plus sugar plus phosphate is a nucleotide - adenylic acid, thymidylic acid, guanylic acid, uridylic acid, cytidylic acid.
• Purines against pyrimidines. Adenine and guanine are substituted purines - two of them. Cytosine, uracil and thymine are substituted pyrimidines - three of them. Two purines, three pyrimidines, and the option that puts thymine with adenine is the trap.
• Ribose against 2-deoxyribose. Ribose, a monosaccharide pentose, gives RNA. 2-deoxyribose gives DNA. Read the prefix, not the word.
• Homopolymer against heteropolymer. A homopolymer has only one type of monomer repeating "n" number of times - cellulose, a polymer of glucose alone. A protein is a heteropolymer, because 20 types of amino acid go into it.
• Starch against cellulose with iodine. Starch forms helical secondary structures and can hold $\mathrm{I_2}$ molecules in the helical portion, and the starch-$\mathrm{I_2}$ complex is blue. Cellulose does not contain complex helices and hence cannot hold $\mathrm{I_2}$.
• Collagen against RuBisCO. Collagen is the most abundant protein in the animal world. Ribulose bisphosphate Carboxylase-Oxygenase (RuBisCO) is the most abundant protein in the whole of the biosphere. The two phrases are not interchangeable, and the paper swaps them.
• Primary against secondary metabolite. Amino acids, sugars, fatty acids, glycerol, nucleotides, nucleosides and nitrogen bases are primary metabolites, and they have identifiable functions and play known roles in normal physiological processes. Alkaloids, flavonoids, rubber, essential oils, antibiotics, coloured pigments, scents, gums and spices are secondary metabolites, and we do not at the moment understand the role of all of them in the host organism.
• Lipids sit in the acid-insoluble fraction but are NOT strictly macromolecules. Their molecular weights do not exceed 800 Da. They separate with the insoluble pool only because on grinding, the membranes break into vesicles which are not water soluble.
• The four levels of protein structure, and which one is "absolutely necessary". Tertiary structure is absolutely necessary for the many biological activities of proteins. The distractors offer primary or quaternary.
• Only right handed helices. In proteins, only right handed helices are observed. An option saying left handed, or both, is always wrong.
• Activation energy. It is the difference in average energy content of $S$ from that of the transition state - not the difference between $S$ and $P$. This one option costs more marks in this chapter than any other.
• Low temperature against high temperature. Low temperature preserves the enzyme in a temporarily inactive state. High temperature destroys enzymatic activity, because proteins are denatured by heat. Only one of the two is permanent.
• Competitive inhibitor. It closely resembles the substrate in molecular structure and competes with the substrate for the substrate-binding site. Malonate against succinic dehydrogenase, because malonate closely resembles succinate.
• The six enzyme classes. Oxidoreductases, transferases, hydrolases, lyases, isomerases and ligases - 6 classes, each with 4 to 13 subclasses, named by a four-digit number. Ligases join, lyases remove groups leaving double bonds, isomerases interconvert isomers.
• Prosthetic group against co-enzyme. A prosthetic group is an organic compound tightly bound to the apoenzyme - haem in peroxidase and catalase. A co-enzyme is an organic compound whose association with the apoenzyme is only transient, usually during catalysis - NAD and NADP, which contain niacin. Tightly bound against transiently associated is the whole test.

One habit pays more than any other here. In a "which is NOT" question, the wrong option is almost never invented - it is a real fact borrowed from a neighbouring molecule. Fructose offered as the monomer of cellulose, thymine offered as a purine, glycerol offered as a secondary metabolite. Ask "which molecule does this actually belong to" rather than "have I heard of this".

Before You Start

Give yourself 45 minutes for 45 questions and look nothing up. Mark every question where you were choosing between two options rather than certain - those marked questions, not only the wrong ones, are your revision list.

The facts most likely to decide your score, by topic:

Analysis and composition. Grind in trichloroacetic acid, $\mathrm{Cl_3CCOOH}$, strain: filtrate is the acid-soluble pool, retentate is the acid-insoluble fraction. Extract, separate, isolate and purify, then use analytical techniques for the molecular formula and probable structure. All the carbon compounds we get from living tissues can be called biomolecules. Wet weight, dry weight, ash. Ash holds the inorganic elements, calcium and magnesium; sulphate and phosphate also appear in the acid-soluble fraction. Oxygen 65.0 per cent, carbon 18.5, hydrogen 9.5 in the human body; silicon 27.7 per cent of the crust and negligible in the body. Carbon and hydrogen are relatively far more abundant in living organisms than in the crust. Inorganic constituents - $\mathrm{Na^+}$, $\mathrm{K^+}$, $\mathrm{Ca^{2+}}$, $\mathrm{Mg^{2+}}$, $\mathrm{H_2O}$, and compounds such as $\mathrm{NaCl}$, $\mathrm{CaCO_3}$, $\mathrm{PO_4^{3-}}$ and $\mathrm{SO_4^{2-}}$.

Amino acids. An amino group and an acidic group as substituents on the same carbon, the $\alpha$-carbon - hence $\alpha$-amino acids. Substituted methanes, four substituent groups: hydrogen, carboxyl group, amino group, and a variable R group. Only twenty types occur in proteins. R group hydrogen - glycine; methyl - alanine; hydroxy methyl - serine. Acidic - glutamic acid. Basic - lysine. Neutral - valine. Aromatic - tyrosine, phenylalanine, tryptophan. The $\mathrm{-NH_2}$ and $\mathrm{-COOH}$ groups are ionizable, so the structure changes with the pH of the solution; the zwitterionic form carries both a positive and a negative charge.

Lipids. Generally water insoluble. A simple fatty acid is a carboxyl group $\mathrm{-COOH}$ attached to an R group; the R group runs from 1 carbon to 19 carbons. Palmitic acid 16 carbons, arachidonic acid 20 carbons, both including the carboxyl carbon. Saturated - no double bond. Unsaturated - one or more $\mathrm{C=C}$ double bonds. Glycerol is trihydroxy propane. Fatty acids esterified with glycerol give monoglycerides, diglycerides and triglycerides. Fats and oils are separated by melting point - oils have a lower melting point, for example gingelly oil, and remain oil in winters. Phospholipids have phosphorous and a phosphorylated organic compound, are found in the cell membrane, and lecithin is one example; they contain in addition a phosphorylated nitrogenous compound. Neural tissues have lipids with more complex structures.

Nitrogen bases, nucleosides, nucleotides. Heterocyclic rings. Adenine and guanine are substituted purines; cytosine, uracil and thymine are substituted pyrimidines; the skeletal rings themselves are purine and pyrimidine. Base plus sugar equals nucleoside - adenosine, guanosine, thymidine, uridine, cytidine. Base plus sugar plus phosphate esterified to the sugar equals nucleotide - adenylic acid, thymidylic acid, guanylic acid, uridylic acid, cytidylic acid. Nucleic acids consist of nucleotides only; DNA and RNA function as genetic material.

Metabolites. Primary metabolites in animal tissues - amino acids, sugars, fatty acids, glycerol, nucleotides, nucleosides, nitrogen bases; identifiable functions, known roles in normal physiological processes. Secondary metabolites in plant, fungal and microbial cells - alkaloids, flavonoids, rubber, essential oils, antibiotics, coloured pigments, scents, gums, spices. Pigments - carotenoids, anthocyanins. Alkaloids - morphine, codeine. Terpenoides - monoterpenes, diterpenes. Essential oils - lemon grass oil. Toxins - abrin, ricin. Lectins - concanavalin A. Drugs - vinblastin, curcumin. Polymeric substances - rubber, gums, cellulose. Many are useful to human welfare and some have ecological importance.

Biomacromolecules. Acid soluble pool - 18 to around 800 daltons. Acid insoluble fraction - only four types: proteins, nucleic acids, polysaccharides and lipids; with the exception of lipids they are ten thousand daltons and above and are polymeric. Less than one thousand dalton - micromolecules or simply biomolecules. In the acid insoluble fraction - macromolecules or biomacromolecules. Lipids do not exceed 800 Da and separate as membrane vesicles, so they are not strictly macromolecules. The acid soluble pool is roughly the cytoplasmic composition. Water is the most abundant chemical in living organisms. Average composition of cells as per cent of total cellular mass - water 70 to 90, proteins 10 to 15, carbohydrates 3, lipids 2, nucleic acids 5 to 7, ions 1.

Proteins. Polypeptides - linear chains of amino acids linked by peptide bonds. A heteropolymer, not a homopolymer, because a homopolymer has only one type of monomer repeating "n" number of times. Essential amino acids come through the diet; non-essential ones the body can make. Collagen - intercellular ground substance. Trypsin - enzyme. Insulin - hormone. Antibody - fights infectious agents. Receptor - sensory reception of smell, taste and hormone. GLUT-4 - enables glucose transport into cells. Collagen most abundant in the animal world; RuBisCO most abundant in the whole of the biosphere.

Polysaccharides. Long chains of sugars, threads with monosaccharides as building blocks. Cellulose is a homopolymer of glucose. Starch is the store house of energy in plant tissues; glycogen is the animal variant; inulin is a polymer of fructose. In glycogen the right end is the reducing end and the left end the non-reducing end, and it has branches. Starch holds $\mathrm{I_2}$ in its helices and the complex is blue; cellulose has no complex helices and cannot hold $\mathrm{I_2}$. Plant cell walls, paper and cotton fibre are cellulosic. Complex polysaccharides use amino-sugars and modified sugars - glucosamine, N-acetyl galactosamine; chitin is in the exoskeletons of arthropods; these complex polysaccharides are mostly homopolymers.

Nucleic acids. Polynucleotides. A nucleotide has three chemically distinct components - a heterocyclic compound, a monosaccharide, and a phosphoric acid or phosphate. The sugar is either ribose, a monosaccharide pentose, or 2-deoxyribose; deoxyribose gives DNA, ribose gives RNA. They carry hereditary information and are passed on from parental generation to progeny.

Structure of proteins. Four levels. Primary - the sequence of amino acids, the positional information; the first is the N-terminal amino acid, the last is the C-terminal amino acid. Secondary - the thread folded as a helix like a revolving staircase, only in some portions; only right handed helices are observed; the $\alpha$-helix and the $\beta$-pleated sheet. Tertiary - folded upon itself like a hollow woolen ball, a three dimensional view, and absolutely necessary for the many biological activities of proteins. Quaternary - the arrangement of more than one polypeptide or subunit; adult human haemoglobin has 4 subunits, two of $\alpha$ type and two of $\beta$ type.

Enzymes and reactions. Almost all enzymes are proteins; some nucleic acids behave like enzymes and are called ribozymes. The active site is a crevice or pocket made by the criss-crossing of the chain in the tertiary structure. Enzymes get damaged above $40^\circ\mathrm{C}$; thermophile enzymes stay stable up to $80^\circ$ to $90^\circ\mathrm{C}$. Physical change - a change in shape without breaking of bonds, or a change of state. Chemical reaction - bonds broken and new bonds formed. Rate is the amount of product formed per unit time; rate is velocity if the direction is specified; rate doubles or halves for every $10^\circ\mathrm{C}$ change. Carbonic anhydrase - about 200 molecules in an hour without it, about 600,000 molecules every second with it, about 10 million times faster. Glucose to pyruvic acid takes ten different enzyme catalysed reactions; lactic acid in skeletal muscle under anaerobic conditions, pyruvic acid under aerobic conditions, ethanol in yeast during fermentation.

Activation energy and enzyme action. Substrate $S$ becomes product $P$. An obligatory, transient $ES$ complex forms; the transition state structure is formed while the substrate is bound. All other intermediate structural states are unstable. Activation energy is the difference in average energy content of $S$ from that of the transition state, and enzymes bring this energy barrier down. $E + S \rightleftharpoons ES \rightarrow EP \rightarrow E + P$. The catalytic cycle - the substrate binds the active site; the enzyme alters its shape to fit more tightly; the bonds are broken and the enzyme-product complex forms; the products are released and the free enzyme takes another substrate molecule.

Factors, classes and co-factors. Anything that alters the tertiary structure alters activity - temperature, pH, substrate concentration, binding of regulating chemicals. Optimum temperature and optimum pH, with activity declining on both sides. Low temperature - temporarily inactive. High temperature - denatured, activity destroyed. Velocity rises with substrate concentration to $V_{max}$, because enzyme molecules are fewer than substrate molecules and become saturated; $K_m$ is the substrate concentration giving half of $V_{max}$. A competitive inhibitor closely resembles the substrate and competes for the substrate-binding site - malonate against succinic dehydrogenase, used in the control of bacterial pathogens. 6 classes, 4 to 13 subclasses each, a four-digit number: oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases. Co-factors make the apoenzyme catalytically active - prosthetic groups tightly bound, haem in peroxidase and catalase; co-enzymes transiently associated, NAD and NADP containing niacin; metal ions forming coordination bonds, zinc for carboxypeptidase. Catalytic activity is lost when the co-factor is removed.