Levorotation (rotation to the left) is indicated by a negative sign. D(-)-fructose has a negative specific rotation (≈ −92°) and is therefore levorotatory. The other listed sugars are dextrorotatory.
Erythrose has the two chiral centres with identical configurations (meso-like arrangement in the D/L sense) while threose has opposite configurations. Matching the conventional D/L assignment for the pictured configurations gives option (d). (Note: exact mapping depends on the given Fischer projections; answer chosen as the conventional correct sequence.)
Sucrose is non-reducing because the glycosidic bond involves the anomeric carbons of both glucose and fructose (α-1,β-2 linkage), so no free hemiacetal/hemiketal is available to be oxidized.
Addition of HCN to the aldehydic group gives a cyanohydrin (adds one carbon). Hydrolysis of the nitrile (−CN) gives the corresponding carboxylic acid (one carbon longer than the original sugar aldehyde). Subsequent treatment with HI and heat reduces/decarboxylates to the corresponding alkane. Starting from a 6‑carbon aldose (glucose open chain), the final alkane is a 7‑carbon alkane, heptane.
Sucrose is dextrorotatory (+34°). Hydrolysis yields equimolar glucose (≈ +52.7°) and fructose (≈ −92.4°). The algebraic sum (+52.7 − 92.4 ≈ −39.7°) is levorotatory, so the reason correctly explains the assertion.
The central dogma: DNA is transcribed to RNA, which is translated to protein (DNA → RNA → Protein).
Amino acids in proteins are joined by peptide (amide) bonds between the α-carboxyl of one amino acid and the α-amino of the next.
2‑Methylglycine (commonly sarcosine, N‑methylglycine) has an α‑carbon that is CH2 (two identical H substituents), so it has no stereogenic centre and is achiral. The other listed amino acids have a chiral α‑carbon.
RNA contains D‑ribose (has an OH at 2′) while DNA contains 2′‑deoxy‑D‑ribose (lacking the 2′‑OH).
Amino acids in aqueous neutral solution exist predominantly as zwitterions: H3N+–CH(R)–COO− (protonated amino group and deprotonated carboxylate).
Most vitamins are essential nutrients obtained from the diet (although some can be synthesized by gut flora to a limited extent); in general the human body does not synthesize all required vitamins.
Open‑chain D‑fructose has one sp2 carbon (the ketone carbon C‑2) and the remaining five carbons are sp3, so sp2:sp3 = 1:5.
Vitamin B2 is commonly called riboflavin.
The pyrimidine bases in DNA are cytosine and thymine (often misspelled 'thiamine' in some texts). Uracil is found in RNA, not DNA.
The question refers to structural diagrams that are not present in the provided text. L‑serine is the enantiomer with the amino group on the left in the standard Fischer projection (i.e. S configuration at the α‑carbon for serine). Without the actual structures/diagrams it is not possible to identify which labeled option corresponds to L‑serine.
Secondary structure refers to local ordered conformations of the polypeptide backbone (e.g. α‑helix, β‑sheet) stabilized mainly by backbone hydrogen bonds — i.e. fixed configurations of the backbone. Option (d) is a specific example (α‑helix).
The B‑group vitamins are water‑soluble. Vitamins A, E and K are fat‑soluble.
Cellulose is a polymer of β‑D‑glucose; complete hydrolysis yields D‑glucose.
Denaturation disrupts the native three‑dimensional structure of a protein, usually causing loss (not gain) of biological activity. Thus statement (c) is incorrect.
Glucose, being an aldehyde (aldose), forms oximes and osazones and gives a positive Tollens test (is reducing). However, unprotected glucose has multiple acidic OH groups that destroy Grignard reagents, so under ordinary conditions glucose does not react with Grignard reagents unless the hydroxyls are protected. Therefore (b) is correct.
Base pairing: A–T and G–C. Complementary of 5'-ATGCTTGA-3' is 3'-TACGAACT-5', read 5'→3' as TACGAACT.
Insulin is a peptide hormone composed of amino acids (a protein); it consists of two polypeptide chains linked by disulfide bonds.
α and β forms differ only in configuration at the anomeric carbon (C‑1) of the cyclic sugar; such stereoisomers are called anomers.
Epimers differ at a single stereogenic carbon. Glucose and galactose differ at C‑4; glucose and mannose differ at C‑2. Hence both pairs are epimers.
Glycine has two hydrogen atoms on the α‑carbon (R = H) and therefore has no chiral center; it is achiral.
Nucleotides in a single DNA strand are joined by 3'→5' phosphodiester bonds between the 3'‑OH of one deoxyribose and the 5'‑phosphate of the next. Bases are attached to sugars by N‑glycosidic bonds.
Phosphodiester linkages (and N‑glycosidic bonds between sugar and base).
Primary structure = linear sequence of amino acids joined by peptide (amide) bonds; determines all higher structures. Secondary structure = regular local conformations (α‑helix, β‑pleated sheet, turns) stabilized mainly by hydrogen bonds between backbone C=O and N‑H groups; does not involve side‑chain interactions.
Primary: amino acid sequence linked by peptide bonds. Secondary: local folding (α‑helix, β‑sheet) stabilized by H‑bonds between backbone C=O and N‑H.
Rickets is caused by deficiency of vitamin D (impaired Ca2+ absorption and bone mineralization). Scurvy is caused by deficiency of vitamin C, needed for collagen hydroxylation.
i) Vitamin D ii) Vitamin C (ascorbic acid)
At physiological pH alanine exists as a zwitterion with the amino group protonated and the carboxyl deprotonated: \(\mathrm{H_3N^+\!-CH(CH_3)-COO^-}\).
Zwitterion: H3N+–CH(CH3)–COO−
Additional differences: DNA is more stable, stores genetic information; RNA functions in transfer and expression (mRNA, tRNA, rRNA).
1) Sugar: DNA has deoxyribose, RNA has ribose. 2) Bases: DNA uses thymine (T), RNA uses uracil (U). 3) Structure: DNA usually double‑stranded helix, RNA usually single‑stranded.
Formation: condensation (dehydration) reaction giving –CO–NH– link. Resonance between lone pair on N and carbonyl gives partial double bond, restricting rotation and making peptide bond planar; central to protein backbone stability.
Peptide bond is an amide linkage formed between the carboxyl of one amino acid and the amino group of another with elimination of water; it has partial C–N double‑bond character and is planar.
Also: hormones can be peptides, steroids or amines; vitamins are organic compounds classified as fat‑ or water‑soluble.
1) Origin/function: Hormones are endogenous chemical messengers produced by endocrine glands to regulate physiology; vitamins are dietary micronutrients required for normal metabolism. 2) Quantity and role: Hormones act at low concentrations as signaling molecules; vitamins act mainly as coenzymes/antioxidants and are required in small dietary amounts.
Denaturation disrupts noncovalent interactions (H‑bonds, hydrophobic, ionic) but usually does not break primary peptide bonds. Example: cooking egg white causes albumin denaturation and precipitation.
Denaturation = loss of native 3D structure (secondary/tertiary/quaternary) caused by heat, pH, solvents, detergents or heavy metals, resulting in loss of biological activity; may be reversible or irreversible.
Test: reducing sugars give positive Fehling's/Tollens' tests. In sucrose both anomeric carbons are involved in the glycosidic bond, so no free reducing group.
Reducing sugars have a free aldehyde or free ketose (in open chain) that can reduce mild oxidants (e.g., glucose, fructose, maltose); non‑reducing sugars lack a free anomeric hydroxyl (e.g., sucrose).
The presence of multiple stereogenic centers (except in achiral cases) causes rotation of plane‑polarized light; e.g., glucose has several asymmetric carbons.
Because most carbohydrates contain one or more chiral (asymmetric) carbon atoms, giving stereoisomerism and optical activity.
Disaccharides (sucrose, lactose, maltose) are classified under oligosaccharides (short chains), fructose is a simple monosaccharide, starch is a polymer of glucose (polysaccharide).
i) Starch — polysaccharide; ii) Fructose — monosaccharide; iii) Sucrose — disaccharide (oligosaccharide); iv) Lactose — disaccharide (oligosaccharide); v) Maltose — disaccharide (oligosaccharide).
Fat‑soluble vitamins are stored in body fat and liver; water‑soluble vitamins are not stored long and must be regularly ingested.
Vitamins are classified as fat‑soluble (A, D, E, K) and water‑soluble (vitamin C and B‑complex).
Hormones act at low concentrations, have specific receptors on target cells and regulate growth, metabolism, reproduction and homeostasis.
Hormones are chemical messengers secreted by endocrine glands into the blood to regulate target organs; examples: insulin (peptide), adrenaline (amine), estrogen/testosterone (steroids).
General peptide structure: H2N–CH(R1)–CO–NH–CH(R2)–COOH. Substituting R = H for Gly and R = CH3 for Ala gives: Gly–Gly: H2N–CH(H)–CO–NH–CH(H)–COOH; Gly–Ala: H2N–CH(H)–CO–NH–CH(CH3)–COOH; Ala–Gly: H2N–CH(CH3)–CO–NH–CH(H)–COOH; Ala–Ala: H2N–CH(CH3)–CO–NH–CH(CH3)–COOH.
Four dipeptides: Gly–Gly, Gly–Ala, Ala–Gly, Ala–Ala.
They have active sites that bind substrates, form enzyme–substrate complexes, and increase reaction rates often by many orders of magnitude; some enzymes are RNA (ribozymes).
Enzymes are biological catalysts (mostly proteins) that accelerate biochemical reactions without being consumed, by lowering activation energy and showing substrate specificity.
α-D-Glucopyranose is the cyclic hemiacetal of D-glucose (pyranose form). In the Haworth representation the ring oxygen is between C1 and C5. For the D-series groups that are on the right in the Fischer projection appear down in the Haworth. Thus the substituents around the ring are: C1 — OH down (α-anomer), C2 — OH down, C3 — OH up, C4 — OH down, C5 — CH2OH up. (Right-handed chair form places the anomeric OH axial down.)
α-D-Glucopyranose: six-membered (pyranose) ring with the anomeric OH (C1) axial (down) and CH2OH at C5 up.
Common cellular RNAs and their functions: - mRNA (messenger RNA): carries genetic code from DNA to ribosomes for translation. - tRNA (transfer RNA): delivers specific amino acids to the ribosome; has anticodon for codon recognition. - rRNA (ribosomal RNA): structural and catalytic components of ribosomes. - snRNA (small nuclear RNA): involved in pre-mRNA splicing (spliceosome). - snoRNA (small nucleolar RNA): guides chemical modification of rRNA. - miRNA (microRNA) and siRNA (small interfering RNA): regulate gene expression by RNA interference. - piRNA: involved in transposon silencing in germ cells.
Major types: messenger RNA (mRNA), transfer RNA (tRNA), ribosomal RNA (rRNA); other types: snRNA, snoRNA, miRNA, siRNA, piRNA.
Formation of the α-helix: - The polypeptide backbone coils into a right-handed helix (common form). - Stabilization arises from internal hydrogen bonds between the carbonyl oxygen of residue i and the amide hydrogen of residue i+4. - Geometry: ~3.6 amino acids per turn, pitch ≈ 5.4 Å (axial rise ≈ 1.5 Å per residue). - Side chains (R groups) point outward from the helix, minimizing steric clashes and allowing specific interactions. - α-Helices are common in proteins and contribute to structural stability and function (e.g., membrane-spanning helices are often hydrophobic).
α-Helix is a right-handed protein secondary structure stabilized by hydrogen bonds between C=O of residue i and N–H of residue i+4; it has ~3.6 residues/turn and a pitch of ~5.4 Å.
Functions of lipids: - Energy storage: triglycerides store large amounts of energy per gram and are mobilized when needed. - Structural components: phospholipids and cholesterol are key constituents of biological membranes, controlling fluidity and permeability. - Insulation and protection: subcutaneous fat provides thermal insulation and adipose tissue cushions organs. - Signalling and regulation: steroid hormones (e.g., estrogen, testosterone), prostaglandins and other lipid mediators regulate physiological processes. - Transport and vitamins: lipids are carriers for fat-soluble vitamins (A, D, E, K) and enable their absorption. - Essential fatty acids: components of membranes and precursors for signalling molecules. - Buoyancy in some aquatic organisms and electrical insulation in nerve cells (myelin).
Lipids function as energy stores, membrane components, thermal/electrical insulators, protective padding, signalling molecules (hormones), and sources of fat-soluble vitamins and essential fatty acids.
Interpretation: The Fischer projection reads from top (aldehyde) to bottom (CH2OH) with three internal stereocentres shown as H left, OH right at each line given (i.e. the penultimate carbon has OH on the right). The configuration of the penultimate (next-to-last) chiral carbon determines D or L. Since the OH on the penultimate carbon is on the right, this is a D-sugar.
D-sugar