Exhaustive Unit 10 Study Guide: Biomolecules and the Chemistry of Life

Chemical Composition and Logic of Living Systems

A living system is characterized by its ability to grow, sustain, and reproduce itself. Paradoxically, these systems are composed of non-living atoms and molecules. Biochemistry is the domain of science that investigates the chemical reactions and processes occurring within living systems. The fundamental building blocks of these systems are complex biomolecules, including carbohydrates, proteins, nucleic acids, and lipids. Proteins and carbohydrates serve as essential constituents of human nutrition. The interaction among these biomolecules establishes the molecular logic underlying life processes. Beyond complex macromolecules, simple molecules such as vitamins and mineral salts are critical for the physiological functions of organisms. The harmonious and synchronous progress of chemical reactions within the body is the defining characteristic of life.

Classification and Characteristics of Carbohydrates

Carbohydrates are primarily produced by plants and represent a vast group of naturally occurring organic compounds. Common examples include cane sugar, glucose, and starch. Historically, they were defined by the general formula $C_x(H_2O)_y$, leading to the name "hydrates of carbon." While many carbohydrates like glucose ($C_6H_{12}O_6$) fit this formula, others do not. For instance, rhamnose ($C_6H_{12}O_5$) is a carbohydrate that deviates from the formula, while acetic acid ($CH_3COOH$) fits the formula ($C_2(H_2O)_2$) but is not a carbohydrate. Chemically, carbohydrates are defined as optically active polyhydroxy aldehydes or ketones, or compounds that yield such units upon hydrolysis.

Carbohydrates are also known as saccharides, derived from the Greek word "sakcharon," meaning sugar. They are classified into three broad groups based on their behavior during hydrolysis. Monosaccharides are the simplest units that cannot be hydrolyzed further into simpler polyhydroxy aldehydes or ketones; approximately 20 are known, with glucose, fructose, and ribose being common examples. Oligosaccharides yield two to ten monosaccharide units upon hydrolysis and are further classified as disaccharides, trisaccharides, or tetrasaccharides based on the specific count. Sucrose is a common disaccharide yielding one molecule of glucose and one of fructose, while maltose yields two glucose molecules. Polysaccharides yield a large number of monosaccharide units on hydrolysis and include starch, cellulose, glycogen, and gums. Unlike monosaccharides and oligosaccharides, polysaccharides are not sweet and are termed non-sugars.

Another classification method distinguishes between reducing and non-reducing sugars. Reducing sugars are those capable of reducing Fehling’s solution and Tollens’ reagent. All monosaccharides, whether aldoses (containing an aldehyde group) or ketoses (containing a keto group), are reducing sugars. The naming of monosaccharides incorporates the number of carbon atoms: a three-carbon aldose is an aldotriose, a four-carbon ketose is a ketotetrose, and a six-carbon aldose is an aldohexose.

Detailed Structure and Chemical Reactions of Glucose

Glucose, also known as dextrose, is an aldohexose with the molecular formula $C_6H_{12}O_6$. It is the most abundant organic compound on earth and serves as a monomer for starch and cellulose. It is prepared commercially from starch by hydrolysis with dilute $H_2SO_4$ at $393\,K$ under $2-3\,atm$ pressure: $(C_6H_{10}O_5)_n + nH_2O \rightarrow nC_6H_{12}O_6$. In the laboratory, it can be prepared by boiling sucrose with dilute $HCl$ or $H_2SO_4$ in an alcoholic solution, yielding equal amounts of glucose and fructose.

The structure of glucose was elucidated through several chemical evidences. Heating glucose with $HI$ for a prolonged period produces n-hexane, indicating a straight chain of six carbon atoms. Reaction with hydroxylamine yields an oxime, and reaction with hydrogen cyanide yields a cyanohydrin, both confirming the presence of a carbonyl group ($>C=O$). Bromine water, a mild oxidizing agent, oxidizes glucose to gluconic acid, proving that the carbonyl group is an aldehyde. Acetylation with acetic anhydride produces glucose pentaacetate, confirming the presence of five hydroxyl ($-OH$) groups attached to different carbons. Oxidation with nitric acid produces saccharic acid (a dicarboxylic acid) from both glucose and gluconic acid, indicating the presence of a primary alcoholic group.

The configuration of glucose is designated as $D(+)$-glucose. The "D" refers to the relative configuration compared to $(+)$-glyceraldehyde, where the $-OH$ group on the lowest asymmetric carbon (C5 in glucose) is on the right side. The "(+)" indicates its dextrorotatory nature. However, the open-chain structure failed to explain why glucose does not give the Schiff’s test, does not form a hydrogensulphite addition product with $NaHSO_3$, and why glucose pentaacetate does not react with hydroxylamine. These discrepancies led to the discovery of the cyclic hemiacetal structure, where the $-OH$ group at C5 adds to the aldehyde group at C1, forming a six-membered ring called a pyranose structure. This results in two anomers, $\alpha$ and $\beta$, which differ in the configuration at C1 (the anomeric carbon). The $\alpha$-form ($m.p.\,419\,K$) and $\beta$-form ($m.p.\,423\,K$) exist in equilibrium with the open-chain form in solution.

Fructose: Properties and Furanose Structure

Fructose is a ketohexose with the formula $C_6H_{12}O_6$, found in fruits, honey, and vegetables. It is used as a sweetener in its pure form. It belongs to the D-series and is laevorotatory, thus designated as $D(-)$-fructose. Its structure contains a ketonic functional group at C2 and six carbons in a straight chain. Like glucose, it exists in two cyclic forms created by the addition of the $-OH$ at C5 to the carbonyl group at C2. This results in a five-membered ring known as a furanose structure, named after its similarity to the cyclic compound furan.

Disaccharides and the Nature of Glycosidic Linkages

Disaccharides are formed by the joining of two monosaccharide units through an oxide linkage created by the loss of a water molecule. This linkage is specifically called a glycosidic linkage. In sucrose, the linkage occurs between C1 of $\alpha$-D-glucose and C2 of $\beta$-D-fructose. Because the reducing groups of both sugars are involved in the bond, sucrose is a non-reducing sugar. Interestingly, while sucrose is dextrorotatory, its hydrolyzed mixture is laevorotatory because the laevorotation of fructose ($-92.4^{\circ}$) outweighs the dextrorotation of glucose ($+52.5^{\circ}$). Consequently, hydrolyzed sucrose is called invert sugar.

Maltose consists of two $\alpha$-D-glucose units linked by a $C1-C4$ glycosidic bond. Since one glucose unit retains a free aldehyde group at C1 in solution, maltose is a reducing sugar. Lactose, or milk sugar, is composed of $\beta$-D-galactose and $\beta$-D-glucose linked via a $C1-C4$ glycosidic bond. It is also a reducing sugar because the C1 of the glucose unit can produce a free aldehyde group.

Polysaccharides: Starch, Cellulose, and Glycogen

Polysaccharides are high-molecular-weight carbohydrates that serve as storage or structural materials. Starch is the primary storage polysaccharide in plants and the most significant dietary source for humans. It consists of two components: amylose and amylopectin. Amylose is water-soluble, constituting $15-20\%$ of starch, and is a long unbranched chain of $\alpha$-D-(+)-glucose units linked by $C1-C4$ bonds. Amylopectin is water-insoluble, making up $80-85\%$ of starch, and is a branched polymer where branching occurs via $C1-C6$ glycosidic linkages while the main chain uses $C1-C4$ links.

Cellulose is the most abundant organic substance in the plant kingdom, forming the cell walls of plant cells. It is a straight-chain polysaccharide of $\beta$-D-glucose units joined by $C1-C4$ glycosidic linkages. Humans cannot digest cellulose, but it is vital for wood and textile production. Glycogen, known as animal starch, is the storage form of carbohydrates in animals, found in the liver, muscles, and brain. It is highly branched, similar to amylopectin, and is broken down into glucose by enzymes when the body requires energy.

Proteins: Building Blocks and Classification of Amino Acids

Proteins are the most abundant biomolecules in living systems and are essential for growth and maintenance. They are polymers of $\alpha$-amino acids. Amino acids contain both an amino ($-NH_2$) and a carboxyl ($-COOH$) group. Only $\alpha$-amino acids are obtained from protein hydrolysis. They are named based on their properties or sources; for example, glycine is named for its sweet taste, and tyrosine is named after the Greek word for cheese. Amino acids are classified as acidic, basic, or neutral based on the relative count of amino and carboxyl groups.

Essential amino acids are those that the body cannot synthesize and must be obtained from the diet, such as Valine, Leucine, Isoleucine, Arginine, Lysine, Threonine, Methionine, Phenylalanine, Tryptophan, and Histidine. Non-essential amino acids can be synthesized by the body. In aqueous solution, amino acids exist as Zwitter ions—dipolar ions containing both a positive and negative charge, formed by the transfer of a proton from the carboxyl to the amino group. This state makes amino acids amphoteric and gives them salt-like properties (crystalline, high melting, water-soluble). Except for glycine, all natural \alpha$-amino acids are optically active and generally have the L-configuration.\n\n# Structural Organization and Denaturation of Proteins\n\nProteins are formed when amino acids link via peptide bonds (-CO-NH-$), which are amides formed between the carboxyl group of one amino acid and the amino group of another. Structures include dipeptides, tripeptides, and polypeptides (more than ten amino acids). A protein typically has more than 100 amino acid residues and a molecular mass over$10,000\,u, though insulin (51 amino acids) is also classified as a protein.\n\nProteins are classified by shape into fibrous and globular. Fibrous proteins (e.g., keratin, myosin) have parallel polypeptide chains held by hydrogen and disulfide bonds and are water-insoluble. Globular proteins (e.g., insulin, albumin) involve chains coiling into spherical shapes and are water-soluble. Structure is studied at four levels: Primary (sequence of amino acids), Secondary (folding into \alpha$-helix or$\beta-pleated sheets via hydrogen bonding), Tertiary (overall three-dimensional folding stabilized by hydrogen bonds, disulfide linkages, van der Waals, and electrostatic forces), and Quaternary (spatial arrangement of multiple polypeptide subunits).\n\nDenaturation occurs when a protein in its native state is subjected to physical changes (temperature) or chemical changes (pH). This causes the globules to unfold and helices to uncoil, leading to a loss of biological activity. While primary structure remains intact, secondary and tertiary structures are destroyed. Common examples include the coagulation of egg white and the curdling of milk through lactic acid formation.\n\n# Enzymes as Biocatalysts in Living Systems\n\nEnzymes are biocatalysts, mostly globular proteins, that catalyze chemical reactions in the body under mild conditions. They are highly specific to particular reactions and substrates, often named after the substrate with the suffix "-ase" (e.g., maltase hydrolyzes maltose). They can also be named by reaction type, such as oxidoreductases. Enzymes significantly lower the activation energy required for reactions. For example, the acid hydrolysis of sucrose has an activation energy of 6.22\,kJ\,mol^{-1}$, but the enzyme sucrase reduces this to$2.15\,kJ\,mol^{-1}.\n\n# Classification and Biological Role of Vitamins\n\nVitamins are organic compounds required in small amounts for normal growth and health. Most cannot be synthesized by humans and must be obtained through diet. They were originally called "vitamines" (vital amines), but the "e" was dropped when it was discovered that many lacked amino groups. They are classified into fat-soluble vitamins (A, D, E, K), which are stored in the liver and adipose tissue, and water-soluble vitamins (B group and C), which are excreted in urine and must be supplied regularly (except B_{12}).\n\nDeficiency diseases include: Vitamin A (Xerophthalmia, Night blindness); Vitamin B_1$(Beri beri); Vitamin$B_2$(Cheilosis); Vitamin$B_6$(Convulsions); Vitamin$B_{12} (Pernicious anaemia); Vitamin C (Scurvy); Vitamin D (Rickets, Osteomalacia); Vitamin E (Increased RBC fragility, muscular weakness); and Vitamin K (Increased blood clotting time). Sources range from fish liver oil and citrus fruits to green leafy vegetables and exposure to sunlight.\n\n# Nucleic Acids: The Chemical Basis of Heredity\n\nNucleic acids reside in the cell nucleus within chromosomes and are responsible for heredity. They are polynucleotides composed of a pentose sugar, phosphoric acid, and nitrogenous bases. DNA contains \beta$-D-2-deoxyribose, while RNA contains$\beta-D-ribose. Bases include Adenine (A), Guanine (G), and Cytosine (C) in both. DNA uniquely contains Thymine (T), while RNA uniquely contains Uracil (U). A nucleoside is formed by a base attached to C1 of the sugar; a nucleotide is a nucleoside with a phosphate group at C5.\n\nJames Watson and Francis Crick (along with Maurice Wilkins) were awarded the 1962 Nobel Prize for discovering the double helix structure of DNA. In this structure, two strands are wound together and held by hydrogen bonds between complementary base pairs: A pairs with T, and C pairs with G. RNA is generally single-stranded and exists as messenger RNA (m-RNA), ribosomal RNA (r-RNA), and transfer RNA (t-RNA). DNA serves as the reserve of genetic information and the master template for protein synthesis, while RNA molecules physically carry out the synthesis of proteins. DNA fingerprinting is a technique using the unique sequence of bases in an individual's DNA for identification in forensics, paternity testing, and evolutionary studies.\n\n# Hormones: Intercellular Messengers and Physiological Regulation\n\nHormones are produced by endocrine glands and transported via blood to target sites. Chemically, they are steroids (estrogens, androgens), polypeptides (insulin, endorphins), or amino acid derivatives (epinephrine, norepinephrine, thyroxine). Insulin and glucagon regulate blood glucose levels. Epinephrine and norepinephrine facilitate responses to external stimuli. Thyroxine, an iodinated derivative of tyrosine, regulates metabolism; its deficiency causes hypothyroidism (lethargy, obesity) and goiter, which is prevented by using iodized salt (NaI).\n\nThe adrenal cortex produces glucocorticoids (carbohydrate metabolism, stress response) and mineralocorticoids (water/salt excretion). Adrenal cortex dysfunction can lead to Addison’s disease. Gonads produce sex hormones: testosterone for male characteristics, and estradiol and progesterone for female characteristics and pregnancy. Har Gobind Khorana shared the 1968 Nobel Prize for his work on the genetic code and nucleic acids, highlighting the intensive chemical study of these biological regulators.\n\n# Questions & Discussion\n\n**10.1 Glucose or sucrose are soluble in water but cyclohexane or benzene are insoluble. Explain.**\nGlucose and sucrose contain multiple hydroxyl (-OH) groups which can form extensive hydrogen bonds with water molecules, facilitating dissolution. Cyclohexane and benzene are non-polar hydrocarbons that cannot form hydrogen bonds with water.\n\n**10.2 What are the expected products of hydrolysis of lactose?**\nHydrolysis of lactose yields \beta$-D-galactose and$\beta$$-D-glucose.

10.3 How do you explain the absence of aldehyde group in the pentaacetate of D-glucose? The pentaacetate of glucose does not react with hydroxylamine effectively because the aldehyde group is involved in the cyclic hemiacetal formation and is not free when the hydroxyl groups are acetylated.

10.4 The melting points and solubility in water of amino acids are generally higher than that of the corresponding halo acids. Explain. Amino acids exist as Zwitter ions, creating strong electrostatic attractions (salt-like behavior) between molecules, resulting in higher melting points and better solubility in polar solvents like water compared to simple halo acids.

10.5 Where does the water present in the egg go after boiling the egg? During boiling, the protein in the egg undergoes denaturation and coagulation. The water is trapped or absorbed into the denatured protein network, transforming the liquid into a solid mass.

10.6 Why cannot vitamin C be stored in our body? Vitamin C is water-soluble and is readily excreted in urine rather than being stored in adipose tissues.

10.7 What products would be formed when a nucleotide from DNA containing thymine is hydrolysed? Hydrolysis would yield 2-deoxy-D-ribose sugar, phosphoric acid, and the nitrogenous base thymine.

10.8 When RNA is hydrolysed, there is no relationship among the quantities of different bases obtained. What does this fact suggest about the structure of RNA? This suggests that RNA is generally a single-stranded molecule and does not follow the strict base-pairing rules (Chargaff's rules) that apply to the double-stranded helix structure of DNA.