STPM Biology – Chapter 1: Molecules of Life

A comprehensive set of question–and–answer flashcards covering the unique properties of water, carbohydrates (mono-, di- and polysaccharides), lipids (triglycerides, phospholipids, steroids), amino acids, and protein structure, functions and denaturation as outlined in STPM Biology Chapter 1.

Why is water indispensable for life on Earth?

It makes up 60–90 % of the fresh mass of living cells and provides the external environment for aquatic organisms.

Which three structural features of water give rise to its unique properties?

Its small molecular size, polarity, and extensive hydrogen bonding between molecules.

What is the bond angle between the two O–H bonds in a water molecule?

104.5 °

Why is a water molecule described as a dipole?

Because the more-electronegative oxygen atom carries a partial negative charge while the two hydrogens carry partial positive charges, creating two poles.

What type of intermolecular bond links adjacent water molecules?

Hydrogen bonds.

How many hydrogen bonds can one water molecule form simultaneously?

Up to four—two through its hydrogens and two through lone pairs on oxygen.

Why is water called the ‘universal solvent’?

Its bipolarity allows it to surround and separate ions or polar molecules, dissolving a wide range of substances.

How does water act as a transport medium in organisms?

Dissolved ions and molecules can move freely in its aqueous solutions, enabling transport in blood, lymph, xylem, phloem, etc.

Define cohesion in the context of water.

The attraction between like water molecules due to hydrogen bonding.

Which property of water results in its high surface tension?

Strong cohesive forces between surface water molecules.

How does cohesion aid water transport in plants?

It helps maintain an unbroken column of water in xylem vessels during transpiration pull.

Why is liquid water difficult to compress?

Close packing of cohesive water molecules resists volume change, providing hydrostatic support.

Give two biological examples where water provides hydrostatic support.

Turgidity in plant cells and the hydrostatic skeleton of earthworms.

State the specific heat capacity of water.

4.2 kJ kg⁻¹ K⁻¹

How does water’s high specific heat benefit living organisms?

It buffers temperature changes, keeping cellular and aquatic environments relatively constant.

What is the latent heat of vaporisation of water?

2 260 kJ kg⁻¹

How does a high latent heat of vaporisation aid temperature regulation?

Large amounts of heat are removed during evaporation (sweating, panting) with minimal water loss, cooling the body.

Define water’s latent heat of fusion and give its value.

340 kJ kg⁻¹—the heat needed to melt ice or released when water freezes.

Why does ice float on water?

Below 4 °C water expands as a crystal lattice forms, making ice less dense than liquid water.

Explain one ecological advantage of ice floating.

Floating ice insulates underlying water, preventing complete freezing and allowing aquatic life to survive winter.

Why does liquid water have low viscosity?

Continuous breaking and reforming of weak hydrogen bonds let molecules slide past each other easily.

Give two biological fluids where water acts as a lubricant.

Synovial fluid in joints and mucus in digestive and respiratory tracts.

What is adhesion in relation to water, and where is it important in plants?

Attraction between water molecules and other polar surfaces; it helps maintain water columns in xylem by sticking to vessel walls.

Why is the transparency of water biologically significant?

It allows light penetration, enabling photosynthesis in aquatic environments and within leaf tissues.

Name two metabolic roles of water in cells.

(1) Reactant in hydrolysis reactions; (2) Substrate for photolysis during photosynthesis.

What is the general chemical formula for carbohydrates?

Cₓ(H₂O)ᵧ

Which functional groups make all carbohydrates reducing in nature?

Aldehyde (-CHO) or ketone (>C=O) groups plus multiple hydroxyl (-OH) groups.

List the three main classes of carbohydrates by complexity.

Monosaccharides, disaccharides, polysaccharides.

Define a monosaccharide.

A single sugar unit that cannot be hydrolysed into simpler carbohydrates.

Give the general formula of a monosaccharide.

(CH₂O)ₙ

How are monosaccharides classified?

By carbon number (triose, pentose, hexose) and by functional group (aldose or ketose).

Name an aldose and a ketose triose.

Glyceraldehyde (aldose) and dihydroxyacetone (ketose).

Give an example of a pentose aldose and pentose ketose.

Ribose (aldose) and ribulose (ketose).

State one hexose aldose and one hexose ketose.

Glucose (aldose) and fructose (ketose).

Why are all monosaccharides reducing sugars?

Their free carbonyl groups can donate electrons to reduce Benedict’s reagent.

How do α-glucose and β-glucose differ structurally?

The orientation of the ‑OH on C-1: below the ring in α, above the ring in β.

List four common physical properties of monosaccharides.

Sweet-tasting, crystalline, polar, highly soluble in water.

Give two key functions of pentoses in cells.

Components of nucleic acids (ribose, deoxyribose) and ATP/coenzymes; CO₂ acceptor (ribulose bisphosphate).

State two functions of hexoses.

Primary energy source (e.g., glucose) and building blocks for di- and polysaccharides.

How is a disaccharide formed?

By condensation of two monosaccharides forming a glycosidic bond with loss of one water molecule.

What is the general formula of disaccharides?

C₁₂H₂₂O₁₁

Define a glycosidic bond.

A covalent bond linking two monosaccharide residues via an O atom (-O-).

Which two monosaccharides form maltose, and what bond links them?

Two α-glucose units joined by an α-1,4-glycosidic bond.

Which sugars combine to form sucrose, and is it reducing?

α-Glucose and fructose via an α-1,2-glycosidic bond; sucrose is non-reducing.

Why is sucrose suited for translocation in plants?

It is very soluble yet chemically unreactive, allowing high-concentration transport without entering metabolic pathways.

How can disaccharides be hydrolysed in the lab?

By boiling with dilute acid or by specific enzymes (maltase, sucrase).

Name the monomeric unit of starch, glycogen, and cellulose.

Glucose.

What are the two components of starch?

Amylose and amylopectin.

Describe the structure of amylose.

Unbranched α-1,4 chains forming a helical coil; turns blue-black with iodine.

What characterises amylopectin?

Branched α-1,4 chains with α-1,6 branch points; gives red-violet colour with iodine.

Give two reasons starch is a good storage compound in plants.

Insoluble and therefore osmotically inactive, yet compact and easily hydrolysed to glucose.

How does glycogen differ from amylopectin?

It has shorter chains and more frequent α-1,6 branches, making it more highly branched and readily mobilised.

Why is glycogen an ideal storage polysaccharide for animals?

Its high branching allows rapid enzymatic breakdown; it is compact and relatively soluble.

Describe the structural features of cellulose.

Linear β-1,4 glucan chains with every second glucose flipped 180 °, forming hydrogen-bonded microfibrils with high tensile strength.

Why can’t humans digest cellulose?

They lack the enzyme cellulase necessary to hydrolyse β-1,4 glycosidic bonds.

Explain how cellulose confers rigidity to plant cell walls.

Many hydrogen bonds between parallel chains assemble into micro- and macrofibrils that resist stretching and provide tensile strength.

What is a triglyceride?

An ester formed by condensation of one glycerol molecule with three fatty acids.

Differentiate saturated from unsaturated fatty acids.

Saturated fatty acids have no C=C bonds; unsaturated have one or more double bonds in the hydrocarbon chain.

Why are most unsaturated triglycerides liquids (oils) at room temperature?

Cis double bonds kink the chains, preventing tight packing and lowering melting point.

Define an essential fatty acid and give one example.

A fatty acid the body cannot synthesise (e.g., linoleic acid) and must obtain from the diet.

State two advantages of triglycerides over carbohydrates as energy stores.

They yield about twice the energy per gram (38 kJ g⁻¹ vs 17 kJ g⁻¹) and produce more metabolic water upon oxidation.

How do triglycerides aid thermal insulation?

Subcutaneous fat layers reduce heat loss, vital for aquatic and cold-climate mammals.

What structural modification distinguishes a phospholipid from a triglyceride?

One fatty acid is replaced by a phosphate group, creating a hydrophilic head and hydrophobic tails.

What arrangement do phospholipids adopt in aqueous environments?

They form bilayers (cell membranes) or micelles with hydrophilic heads outward and hydrophobic tails inward.

Name a biologically important phospholipid in membranes.

Lecithin (phosphatidylcholine).

What is the basic skeleton of a steroid molecule?

Four fused carbon rings with variable side chains.

How does cholesterol influence membrane fluidity?

It intercalates between phospholipids, preventing them from packing too tightly and thus increasing fluidity.

What cardiovascular risk is associated with excess cholesterol?

Deposition in arterial walls (arteriosclerosis) can block coronary arteries, leading to heart attacks.

What type of bond links two amino acids?

A peptide bond formed by condensation between ‑COOH and ‑NH₂ groups.

How many common amino acids are found in proteins?

Twenty.

What is a zwitterion?

An amino acid form that carries both a positive (-NH₃⁺) and a negative (-COO⁻) charge simultaneously.

List the four categories of amino acids based on R group properties.

Non-polar (hydrophobic), polar uncharged, acidic (negatively charged), basic (positively charged).

Define the primary structure of a protein.

The linear sequence of amino acids in its polypeptide chain(s).

Name three common secondary structures in proteins.

α-helix, β-pleated sheet, and collagen triple helix.

What stabilises secondary structures?

Hydrogen bonds between backbone ‑C=O and ‑N-H groups.

Identify four interactions that stabilise tertiary protein structure.

Hydrogen bonds, ionic bonds, disulphide bridges, and hydrophobic interactions.

Give an example of a protein with quaternary structure.

Haemoglobin (2 α- and 2 β-chains plus haem groups).

Contrast fibrous and globular proteins.

Fibrous: long, insoluble, structural; Globular: compact, soluble, functional (enzymes, hormones, antibodies).

What is meant by protein denaturation?

Loss of a protein’s native three-dimensional conformation, destroying its biological activity without breaking peptide bonds.

List four factors that can denature proteins.

High heat (>40 °C), extreme pH, organic solvents/detergents/heavy metals, and strong radiation or mechanical agitation.

Can a denatured protein renature?

Sometimes—if damaging conditions are removed, the protein may refold into its functional shape.

What is a conjugated protein?

A globular protein covalently or tightly bound to a non-protein prosthetic group (e.g., glycoproteins, haemoproteins).

Name the fibrous protein forming connective tissue and state its secondary structure.

Collagen; it has a triple-helix configuration.

Why are hydrophilic amino acids found on the exterior of globular proteins?

Their polar side chains interact with water, rendering the protein soluble in aqueous environments.

Explain how proteins act as buffers in blood plasma.

Their amino and carboxyl groups accept or donate H⁺ ions, resisting changes in pH.