Question
Biological pigments include a variety of chemical structures with diverse functions.
The graph shows the conversion of hemoglobin to oxyhemoglobin.
$$
\mathrm{Hb}(\mathrm{aq})+4 \mathrm{O}_2(\mathrm{~g}) \rightleftharpoons \mathrm{Hb}\left(\mathrm{O}_2\right)_4(\mathrm{aq})
$$
The partial pressure of oxygen gas, $\mathrm{p}\left(\mathrm{O}_2\right)$ is proportional to its concentration.
a. Explain the shape of the curve at low oxygen partial pressure up to about $5 \mathrm{kPa}$.
b.i. Sketch a graph on the axes above to show the effect of decreasing pH on the binding of oxygen to hemoglobin (the Bohr Effect).
b.ii.Outline the effect of decreasing $\mathrm{pH}$ on the oxygen saturation of hemoglobin.
▶️Answer/Explanation
Markscheme
a. binding of $\mathrm{O}_2$ «to one active site» affects shape of $\mathrm{Hb} /$ other active sites
OR
binding of one $\mathrm{O}_2$ «molecule» affects binding of other $\mathrm{O}_2$ «molecules»
increasing affinity of $\mathrm{Hb}$ to $\mathrm{O}_2$
OR
enhanced binding of «further» $\mathrm{O}_2$ «molecules»
OR
cooperative binding
[2 marks]
b.i.
sketching right shift of curve on graph
[1 mark]
b.ii.decreases «oxygen saturation»
Accept “hemoglobin binds to $\mathrm{O}_2$ with less affinity”.
Question
Changes in physiology can impact living creatures.
a. The graph shows the change in oxygen partial pressure in blood, measured at different $\mathrm{pH}$ values.
Explain the effect of changing $\mathrm{pH}$ on the percentage saturation of hemoglobin at a given partial pressure of oxygen.
b. Explain the biomagnification of the pesticide DDT.
c. Vitamins are organic compounds essential in small amounts.
State the name of one functional group common to all three vitamins shown in section 35 of the data booklet.
▶️Answer/Explanation
Markscheme
a. as $\mathrm{pH}$ decreases, protons $/ \mathrm{CO}_2$ bind to allosteric sites
OR
as $\mathrm{pH}$ decreases, protons $/ \mathrm{CO}_2$ act as non-competitive inhibitor
OR
active/binding site changes shape
saturation decreases
OR
more oxygen released
OR
affinity to oxygen decreases
b. accumulates in fat/tissues/living organisms
OR
cannot be metabolized/does not break down «in living organisms»
OR
not excreted / excreted «very» slowly
passes «unchanged» up the food chain
OR
increased concentration as one species feeds on another «up the food chain»
NOTE: Accept “lipids” for “fat”.
c. hydroxyl
NOTE: Accept “hydroxy” but not “hydroxide”.
Accept “alkenyl”.
Do not accept formula.