Biology

Biological Molecules

5,517 words, 10 sections

Biological molecules are the chemistry of a living cell: carbohydrates, proteins, lipids and nucleic acids, the conjugated molecules built out of them, and the water in which all of it happens. Biology carries 81 of the 180 marks in this paper, and this unit alone has been asked 510 distinct times across 94 official papers between 2000 and 2026 by six boards, Balochistan, DUHS, Federal, KMU, NUMS and UHS. Its mean item difficulty is 1.39, so it is heavily sampled and easy, and the practice bank holds 2320 active questions on it. A student who has mastered the chapter can read a partial description of a molecule and name it, name the bond that joins its subunits, and say which property of water the effect being tested depends on.

Everything here is built from one repeating unit: a nucleotide of a pentose sugar, a phosphate group and a nitrogenous base, which is what a report of a molecule containing a sugar, a phosphate and a nitrogenous base keys. The composition is fixed too: a nucleotide may carry one to three phosphate groups, which is how AMP, ADP and ATP differ, while one to five bases, one or two pentoses and hexose sugar are all rejected.

The base sits on the 1st carbon of the pentose and the phosphate esterifies with a sugar hydroxyl, which is why phosphoric acid, $H_3PO_4$ , is the answer when the question asks what carries the phosphate. In a chain the phosphate links one sugar to the next by a phosphodiester bond, and nucleotides differ only in the base. Adenine and guanine are two-ringed purines, cytosine and thymine one-ringed pyrimidines, and uracil cannot occur in DNA at all.

The two strands are held by hydrogen bonds between complementary bases, and pairing is always purine against pyrimidine, which is what holds the width constant. Cytosine pairs with guanine. The backbone sits outside with the bases pointing inward, and it is the backbone DNA and messenger RNA share. Adjacent base pairs are $0.34\ \text{nm}$ apart along the axis. Only Punjab carries the experiment behind the model: Meselson and Stahl in 1958, bacteria grown in $^{15}N$ then moved to $N^{14}$ and separated on a caesium chloride gradient, where the hybrid band ruled out the conservative and the dispersive models.

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Introduction

1. Nucleic acids: the nucleotide and the double helixEverything here is built from one repeating unit: a nucleotide of a pentose sugar, a phosphate group and a nitrogenous base, which is what a report of a molecul