Chemistry of Life Basics

Question 1 (10 marks)

(a) Atomic number = number of protons = 17. This corresponds to chlorine (Cl) on the periodic table. (1 mark atomic number; 1 mark identifying element — since atomic number defines identity.) (2)

(b) Mass number = protons + neutrons.

• Form 1: $17 + 18 = 35$
• Form 2: $17 + 20 = 37$

They are isotopes because they have the same number of protons (same atomic number = same element) but different numbers of neutrons (hence different mass numbers). Chemical identity is set by proton number, so both are still chlorine. (1 mark each mass number; 1 mark correct isotope reasoning.) (3)

(c) Feed the microbe nutrients labelled with the radioactive isotope (radiotracer). The radioactivity can be detected (autoradiography/scintillation counter) as the nutrient is taken up and moves through the cell, allowing tracking without altering the chemical behaviour — because isotopes behave chemically identically, the label follows the same metabolic pathway. (1 mark method; 1 mark detection; 1 mark principle that isotopes are chemically identical.) (3)

(d) Gaining one electron gives a net charge of −1 (an anion). Protons = 17, neutrons = 18, electrons = 18 (17 + 1). (1 mark charge; 1 mark counts.) (2)

Question 2 (12 marks)

(a) (2 marks each: 1 outcome + 1 justification) (6)

• NaCl: dissolves. It is an ionic compound; polar water molecules surround and separate the $\text{Na}^+$ and $\text{Cl}^-$ ions.
• Sugar (sucrose): dissolves. Though covalent, it has many polar –OH groups that hydrogen-bond with water ("like dissolves like").
• Oil: does not dissolve; separates/floats. Oil is nonpolar (hydrocarbon), so it cannot form favourable interactions with polar water (hydrophobic).

(b) Water is polar: oxygen carries a partial negative charge ($\delta^-$), hydrogens partial positive ($\delta^+$). The $\delta^-$ oxygen ends attract $\text{Na}^+$ cations; the $\delta^+$ hydrogen ends attract $\text{Cl}^-$ anions. Water forms hydration shells around each ion, overcoming the ionic attraction and dispersing the ions in solution. (1 mark polarity; 1 mark opposite-charge attraction; 1 mark hydration shell/dissolving.) (3)

(c) "Compound" is incorrect. Definitions: a molecule = two or more atoms chemically bonded; a compound = two or more different elements chemically bonded in fixed ratio; a mixture = two or more substances physically combined, NOT chemically bonded, retaining their own properties and separable. The beaker is a mixture (partly a solution of salt/sugar plus undissolved oil), because the components are not chemically bonded together. (1 mark rejecting "compound"; 1 mark correct definitions; 1 mark identifying as mixture.) (3)

Question 3 (10 marks)

(a) Reactants: $\text{CO}_2$ and $\text{H}_2\text{O}$. Product: $\text{H}_2\text{CO}_3$ (carbonic acid). (1 mark reactants; 1 mark product.) (2)

(b) Rising $\text{H}^+$ lowers pH (pH and $[\text{H}^+]$ are inversely related). Blood becomes more acidic (moves toward the low end, below 7). (1 mark direction of pH; 1 mark acidic.) (2)

(c) pH is a $\log_{10}$ scale, so each 1 unit = ×10 in $[\text{H}^+]$. $\Delta\text{pH} = 7.4 - 6.4 = 1.0 \Rightarrow 10^{1} = 10\text{ times more } \text{H}^+.$ Blood at pH 6.4 is 10 times more acidic. (1 mark ΔpH; 1 mark using power of 10; 1 mark answer = 10.) (3)

(d) The system contains a weak acid ($\text{H}_2\text{CO}_3$) and its conjugate base ($\text{HCO}_3^-$). When $\text{H}^+$ rises, bicarbonate binds the excess $\text{H}^+$ to reform $\text{H}_2\text{CO}_3$, removing free $\text{H}^+$ and preventing a large pH drop; if pH rises, $\text{H}_2\text{CO}_3$ releases $\text{H}^+$. This resistance to pH change keeps the internal environment stable = homeostasis, protecting enzyme function. (1 mark buffer = weak acid/conjugate base pair; 1 mark mechanism of absorbing/releasing H⁺; 1 mark homeostasis link.) (3)

Question 4 (10 marks)

(a) (2 marks each: name + definition) (4)

• Cohesion: attraction between water molecules (via hydrogen bonds); keeps the water column continuous so it can be pulled up.
• Adhesion: attraction between water molecules and the walls of the xylem vessel, helping water cling to and climb the tube (capillary action).

(b) Surface tension. Water molecules at the surface hydrogen-bond to neighbours below and beside them, creating an inward pull that forms a strong "skin"/taut film able to support the insect's weight. (1 mark naming; 1 mark hydrogen-bond cause; 1 mark supporting weight.) (3)

(c) High specific heat capacity. Water absorbs/releases large amounts of heat with only a small temperature change (because energy first goes into breaking hydrogen bonds). This buffers the tree/organism against rapid temperature swings, keeping internal temperature and metabolic processes stable. (1 mark property named; 1 mark H-bond/large heat with small ΔT; 1 mark biological stability.) (3)

Question 5 (8 marks)

(a) CHNOPS = Carbon, Hydrogen, Nitrogen, Oxygen, Phosphorus, Sulphur. (2 marks — all six correct; 1 if minor omission.)

• Phosphorus: in DNA/RNA backbone (phosphate groups) or ATP / phospholipids. (1)
• Sulphur: in certain amino acids (cysteine/methionine) forming disulphide bridges in proteins. (1) (4)

(b) (2 marks each, any two valid reasons) (4)

• Carbon forms four stable covalent bonds, enabling large, complex, branched and ring molecules (versatility).
• Carbon–carbon bonds are strong yet stable at biological temperatures, and carbon readily bonds C, H, O, N to build diverse macromolecules; silicon bonds (e.g. Si–Si, Si–O) are either too weak or, when bonded to oxygen, form rigid solid silica that cannot be used flexibly, and $\text{SiO}_2$ is a solid (not a versatile transportable gas like $\text{CO}_2$).

[
  {"claim": "Mass numbers of the two isotopes are 35 and 37", "code": "m1 = 17 + 18; m2 = 17 + 20; result = (m1 == 35 and m2 == 37)"},
  {"claim": "Anion after gaining one electron has charge -1 with 18 electrons", "code": "protons = 17; electrons = 17 + 1; charge = protons - electrons; result = (charge == -1 and electrons == 18)"},
  {"claim": "pH drop from 7.4 to 6.4 means 10x more H+ concentration", "code": "delta = 7.4 - 6.4; factor = 10**delta; result = abs(factor - 10) < 1e-9"},
  {"claim": "CHNOPS contains exactly 6 elements", "code": "elements = ['C','H','N','O','P','S']; result = (len(elements) == 6)"}
]