BCHET 149 Solved Guess Paper 2026 for IGNOU Molecules of Life, Bachelor of Science (General) BSCG

Fluid Mosaic Model of the Plasma Membrane

Exam question: What is the Fluid Mosaic Model? How does it explain membrane fluidity and permeability? Elaborate with a diagram.

Meaning of the fluid mosaic model

The fluid mosaic model, put forward by S.J. Singer and G.L. Nicolson in 1972, is the most widely accepted model of the plasma membrane. It describes the membrane as a lipid bilayer, made mainly of amphipathic phospholipids, in which protein molecules are embedded.

Salient features

1. Bilayer arrangement: phospholipids form a bilayer with polar head groups on the outside and nonpolar hydrophobic tails tucked inside.
2. Mobile proteins: proteins are dispersed in the bilayer and can move sideways in the plane of the membrane; peripheral proteins sit on the surface while integral proteins span the bilayer.
3. Protein functions: selective transport, cell recognition, cell-to-cell communication, cell responses and motility.
4. Carbohydrates: about 2 to 5 percent carbohydrate is present, linked to proteins or lipids.

Membrane fluidity

The membrane is a quasi-fluid structure held together only by noncovalent interactions. Its fluidity depends on the cholesterol content and the unsaturated fatty acids in the phospholipids. The cis double bonds of unsaturated fatty acids put a bend in the chain, which disturbs the ordered packing of fatty-acid chains and so raises fluidity.

Membrane permeability

The membrane acts as a selective barrier: nonpolar molecules cross more readily than polar or charged ones. Glucose moves by facilitated diffusion, while most biomolecules cross by an active process that uses energy.

Diagram in the file: bilayer with polar heads, fatty tails, integral and peripheral proteins and cholesterol, all labelled.

Conclusion: The fluid mosaic model pictures the membrane as a dynamic lipid bilayer with mobile embedded proteins; its fluidity, set by cholesterol and unsaturated fatty acids, and its selective permeability govern transport across the cell.

The Peptide Bond

Exam question: Explain the formation, geometry and planar nature of the peptide bond. Illustrate your answer with a diagram.

Formation of the peptide bond

Amino acids join through an amide linkage, -CO-NH-, called the peptide bond. It forms by a condensation reaction between the alpha-carboxyl group of one amino acid and the alpha-amino group of another, with one molecule of water removed. Glycine and alanine joined this way give the dipeptide glycylalanine.

Geometry and planar nature

1. Amide bond: chemically, the peptide bond is a planar amide bond (-CONH-).
2. Cis and trans forms: it shows geometrical isomerism, and the trans form is favoured because the bulky alpha-carbons are further apart than in the cis form.
3. Bond length: the C-N bond is 133 pm, shorter than a normal C-N single bond (146 pm) but a little longer than a C=N double bond (127 pm), which shows partial double-bond character.
4. Resonance: delocalised electrons give both the C=O and C-N bonds partial double-bond character.
5. Coplanarity: the O, C, N and H atoms of the linkage lie in one plane, with O and H trans to each other, and this rigidity limits how the chain can fold.

Diagram in the file: the rigid planar amide group with the two alpha-carbons, the 133 pm bond and its trans arrangement marked.

Conclusion: The peptide bond is a planar, partly double-bonded amide formed by condensation with loss of water, and its rigid trans geometry limits how proteins fold.

Enzyme Inhibition, pH and Temperature

Exam question: (a) Explain the difference between competitive and non-competitive inhibition of enzymes. (b) How do pH and temperature affect enzyme activity? Illustrate.

(a) Competitive vs non-competitive inhibition

1. Competitive inhibition: the inhibitor looks like the substrate and competes with it for the active site, which stops the enzyme-substrate complex from forming. Adding more substrate reverses it; Vmax stays the same while Km rises. Example: malonic acid inhibiting succinic dehydrogenase, and sulphanilamide.
2. Non-competitive inhibition: the inhibitor binds away from the active site and extra substrate cannot reverse it; Km is unchanged while Vmax falls.

(b) Effect of pH and temperature

1. pH: a plot of activity against pH is bell-shaped, with activity highest only in a narrow optimum range, around 7 for most enzymes and about 1.5 for pepsin. Extreme pH denatures the enzyme and changes the ionisation of active-site side chains.
2. Temperature: activity rises with temperature, because of more collisions, up to an optimum near 37 degrees Celsius, then drops sharply as heat denatures the enzyme protein.

Diagrams in the file: inhibitor binding at the active site vs a separate site, plus plotted pH and temperature curves.

Conclusion: Competitive inhibitors mimic the substrate and are overcome by excess substrate (Km rises), while non-competitive inhibitors bind elsewhere (Vmax falls); enzyme activity peaks at an optimum pH and temperature and is lost on denaturation.

The DNA Double Helix

Exam question: Describe the DNA double helical structure. What are the forces that account for the stability of the DNA double helix? Draw a labelled diagram.

The DNA double helical structure

DNA is made of two complementary polydeoxyribonucleotide strands coiled round a common axis and held together by hydrogen bonds. Watson and Crick proposed this right-handed double helix in 1953.

Structural features

1. Two antiparallel strands: coiled around a common axis into a right-handed helix.
2. Backbone and bases: the sugar-phosphate backbone runs on the outside, and the bases lie inside, perpendicular to the long axis, as hydrogen-bonded pairs.
3. Complementary pairing: adenine pairs with thymine through two hydrogen bonds, and guanine pairs with cytosine through three.
4. Dimensions: the helix is 2 nm wide; neighbouring bases are 0.34 nm apart and turned by 36 degrees, so the structure repeats every ten base pairs, or 3.4 nm.

Forces stabilising the double helix

1. Hydrogen bonding: the A:T and G:C hydrogen bonds decide which bases pair, but add little to the overall stability.
2. Base stacking: most of the stability comes from hydrophobic, van der Waals interactions between the stacked bases.

Diagram in the file: right-handed helix with the backbone, A=T and G≡C pairs and dimensions labelled.

Conclusion: The DNA double helix is a right-handed, antiparallel duplex with A-T and G-C pairs; base stacking keeps it stable, while hydrogen bonding gives pairing its specificity.

The Urea Cycle

Exam question: What is the urea cycle? Describe the steps involved in the urea cycle using a diagram.

Meaning of the urea cycle

Ammonia released during amino acid breakdown is toxic, so the body converts it into urea, which is less toxic and easy to excrete. This conversion of ammonia into urea is called the urea cycle. It runs partly in the mitochondria and partly in the cytoplasm, and it starts in the mitochondria.

Steps of the urea cycle

1. Carbamoyl phosphate: ammonium and bicarbonate ions condense to form carbamoyl phosphate, catalysed by carbamoyl phosphate synthase-I in liver mitochondria, using two ATP.
2. Citrulline: carbamoyl phosphate joins ornithine to form citrulline (ornithine transcarbamoylase), and citrulline moves into the cytosol.
3. Argininosuccinate: citrulline combines with aspartate to give argininosuccinate (argininosuccinate synthase), bringing in the second amino group.
4. Arginine: argininosuccinate splits into arginine and fumarate (argininosuccinase); fumarate enters the TCA cycle, which links the two cycles.
5. Urea: arginase cleaves arginine into urea and ornithine, and ornithine returns to the mitochondrion for the next round.

Diagram in the file: the cycle from ornithine through citrulline, argininosuccinate and arginine, with urea released and fumarate leaving.

Conclusion: The urea cycle turns toxic ammonia into excretable urea through carbamoyl phosphate, citrulline, argininosuccinate and arginine, regenerating ornithine and linking to the TCA cycle through fumarate.