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Enzymes enhance the rate of reaction by                            [2000] 

(a) Forming a reactant-product complex

(b) Changing the equilibrium point of the reaction

(c) Combining with the product as soon as it is formed

(d) Lowering the activation energy of the reaction

Audio Explanation:


(d) Enzymes enhance the rate of a reaction by just lowering the activation energy (the energy required for substances to react and get converted into product) of a reaction.

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Macromolecule chitin is                                                   [2013]

(a) Nitrogen containing polysaccharide

(b) Phosphorous containing polysaccharide

(c) Sulphur containing polysaccharide

(d) Simple polysaccharide

Audio Explanation:


(a) Macromolecule chitin is a complex polysaccharide containing amino sugars and chemically modified sugars, (e.g. glucosamine, N-acetyl galactosamine, etc). Polysaccharides are long carbohydrate molecules of monosaccharide units joined together by glycosidic bonds. They have a general formula Cx(H2O)y. Chitin is the main component of the cell wall of fungi, the exoskeletons of arthropods, insects and radula of molluscs, etc. 

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One turn of the helix in a B-form DNA is approximately                                [2006]

(a) 0.34 nm

(b) 3.4 nm

(c) 2 nm

(d) 20 nm

Audio Explanation:


(b) B-DNA is helical structure with 20Å and the distance between the two base pairs is 3.4Å and there are 10 base pairs in each turn or pitch (one round). Hence, one turn of the helix is approximately 34Å or 3.4 nm (10Å = 1.0 nm). Z-DNA (in comparison to B-DNA) is left handed double helical structure in which double helix winds to left in zig-zag pattern (instead of right, like B-DNA). 

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Which of the following statements regarding enzyme inhibition is correct?                       [2005]

(a) Competitive inhibition is seen when a substrate competes with an enzyme for binding to an inhibitor protein

(b) Competitive inhibition is seen when the substrate and the inhibitor compete for the active site on the enzyme

(c) Non-competitive inhibition of an enzyme can be overcome by adding large amount of substrate

(d) Non-competitive inhibitors often bind to the enzyme irreversibly 

 

Audio Explanation:


(b) A competitive inhibitor competes with substrate molecule for occupying the active site of an enzyme. These inhibitors have structural resemblance with substrate molecules due to which they easily get active site of an enzyme and form an enzyme-inhibitor complex.

              E        +          I             EI complex

       (enzyme)        (inhibitor)

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If DNA percentage of thymine is 20. What is the percentage of guanine?                         [2002]

(a) 20%

(b) 40%

(c) 30%

(d) 60%

Audio Explanation:


(c) Total DNA (100)= A + T + C + G

                         A = 20% (given)

                         A = T (base pairing rule)

                     100 = 20+ 20 + C + G

                  C + G = 100-40 = 60

                         C = G = 30

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Which is wrong about nucleic acids?                                 [1993]

(a) DNA is single stranded in some viruses

(b) RNA is double stranded occasionally

(c) Length of one helix is 45 Å in B-DNA

(d) One turn of Z-DNA has 12 bases

(c) Length of one helix or the pitch per turn of helix is 34 Å in B-DNA, 25 Å in A-DNA and 46 Å in Z-model of DNA. In Z-DNA sugar molecules are seen in opposite direction. So the 3-5 diester bond forms zig-zag structure in Z-DNA.

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Cellulose, the most important constituent of plant cell wall is made of                     [1998]

(a) Unbranched chain of glucose molecules linked by α 1, 4-glycosidic bond

(b) Branched chain of glucose molecules linked by β 1, 4-glycosidic bond in straight chain and α, 1, 6-glycosidic bond at the site of branching

(c) Unbranched chain of glucose molecules linked by β 1, 4-glycosidic bond

(d) Branched chain of glucose molecules linked by α 1, 6-glycosidic bond at the site of branching

(c) Cellulose (C6H10O5)n is the most abundant organic polymer. It is a polysaccharide and consists of long unbrached chains of glucose residues linked β, 1-4 glycosidic bonds. In plants, cellulose is formed from sugar. It serves as building material in the formation of cell wall.

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Given below is the diagrammatic representation of one of the categories of the small molecular weight organic compounds in the living tissues. Identify the category shown and the one blank component X in it.

                                                          [2012]

      Category                             Component

(a) Cholesterol                            Guanine

(b) Amino acid                            NH2

(c) Nucleotide                             Adenine

(d) Nucleoside                            Uracil

(d) Nucleoside is made up of ribose sugar and nitrogenous base only. Uracil forms nucleoside with only ribose sugar. So, the option with category nucleoside component uracil is correct.

                                

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Cytochrome is                          [2001]

(a) Metallo flavoproteins

(b) Fe containing porphyrin pigment

(c) Glycoprotein

(d) Lipid 

(b) Cytochrome is the respiratory pigment. It is composed of protein, iron and porphyrin ring. It functions as an enzyme in the respiratory chain. Unlike in haemoglobin the metal atom in the porphyrin ring must change it's valency for the molecule to function. Cytochromes are basically located in inner mitochondrial membranes and thylakoids of chloroplasts.

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The most abundant element present in plants is                               [2004]

(a) Carbon

(b) Nitrogen

(c) Manganese

(d) Iron

(a) Carbon is the most abundant element present in the plant. Other framework elements are hydrogen and oxygen.

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