Question
State an equation for aerobic respiration.
Answer/Explanation
Answer:
\(C_6H_{12}O_6(aq) + 6O_2(g) \rightarrow 6CO_2(g) + 6H_2O(l)\)
Question
a. Draw the structure of the repeating unit of starch and state the type of linkage formed between these units.
b. Formulate the equation for the complete hydrolysis of a starch molecule, $\left(\mathrm{C}_6 \mathrm{H}_{10} \mathrm{O}_5\right)_n$.
c. Calculate the energy released, in $\mathrm{kJ} \mathrm{g}^{-1}$, when $3.49 \mathrm{~g}$ of starch are completely combusted in a calorimeter, increasing the temperature of $975 \mathrm{~g}$ of water from $21.0^{\circ} \mathrm{C}$ to $36.0^{\circ} \mathrm{C}$. Use section 1 of the data booklet.
d. Explain how the inclusion of starch in plastics makes them biodegradable.
▶️Answer/Explanation
a.
continuation bonds $A N D$-O- attached to just one end $A N D$ both $\mathrm{H}$-atoms on end carbons must be on the same side [ $\boldsymbol{C}$ ]
Note: Square brackets not required.
Ignore ” $n$ ” if given.
Mark may be awarded if a polymer is shown but with the repeating unit clearly identified.
Type of linkage:
glycosidic [ $\sim]$
Note: Accept “ether”.
b. $\left(\mathrm{C}_6 \mathrm{H}_{10} \mathrm{O}_5\right)_n(\mathrm{~s})+n \mathrm{H}_2 \mathrm{O}(\mathrm{l}) \rightarrow n \mathrm{C}_6 \mathrm{H}_{12} \mathrm{O}_6$ (aq) $[\checkmark]$
Note: Accept ” $(n-1) \mathrm{H}_2 \mathrm{O}$ “.
Do not award mark if ” $n$ ” not included.
c. $q=« m c \Delta T=975 \mathrm{~g} \times 4.18 \mathrm{~J} \mathrm{~g}^{-1} \mathrm{~K}^{-1} \times 15.0 \mathrm{~K}=» 61100$ «J $/ 61.1$ «J»] [ «heat per gram $=\frac{61.1 \mathrm{~kJ}}{3.49 \mathrm{~g}}=» 17.5$ «kJ g${ }^{-1} »[\boldsymbol{U}]$
Note: Award [2] for correct final answer.
d. Any two of:
carbohydrate grains swell/break plastic into smaller pieces $[\boldsymbol{W}$
inclusion of carbohydrate makes the plastic more hydrophilic/water soluble $[\boldsymbol{V}]$
carbohydrates are broken down/hydrolysed/digested by bacteria/micro-organisms $[\boldsymbol{\sim}]$
plastic becomes more accessible to bacteria as holes/channels are created in it $[\boldsymbol{\sim}]$
«presence of» carbohydrate weakens intermolecular/London/dispersion forces between polymer chains in the plastic [ $\boldsymbol{\sim}$ ]
Note: Accept “starch” for “carbohydrate” throughout. Do not accept carbohydrates are broken down/hydrolyzed.
Question
Phosphatidylcholine is an example of a phospholipid found in lecithin.
Phosphatidylcholine may be formed from propane-1,2,3-triol, two lauric acid molecules, phosphoric acid and the choline cation.
\text { a(i)Deduce the structural formula of phosphatidylcholine. }
a(ii)dentify the type of reaction in (a).
b. Lecithin is a major component of cell membranes. Describe the structure of a cell membrane.
c. Predict, giving a reason, the relative energy density of a carbohydrate and a lipid of similar molar mass.
d. Lecithin aids the body’s absorption of vitamin E.
Suggest why vitamin $\mathrm{E}$ is fat-soluble.
e. Phospholipids are also found in lipoprotein structures.
Describe two effects of increased levels of low-density lipoprotein (LDL) on health.
▶️Answer/Explanation
Markscheme
a(i).
phosphodiester correctly drawn [ $\boldsymbol{\sim}]$
both ester groups correctly drawn $[\sim]$
Note: Accept protonated phosphate.
Accept phosphodiester in centre position.
a(ii)ondensation [ $\boldsymbol{V}]$
Note: Accept “esterification”.
Accept “nucleophilic substitution/ $S_N$ “.
b. phospholipid bilayer/double layer
OR
two layers of phospholipids [ $\boldsymbol{V}]$
polar/hydrophilic heads facing aqueous environment $A N D$ non-polar/hydrophobic tails facing away from aqueous environment [ $\boldsymbol{D}$ ]
Note: Award [2] for a suitably labelled diagram.
Award [1 max] for a correct but unlabelled diagram.
Accept “polar/hydrophilic heads on outside AND non-polar/hydrophobic tails on inside for M2.
c. carbohydrates less energy dense AND carbohydrates higher ratio of oxygen to carbon/more oxidized/less reduced [ $\boldsymbol{]}$ ]
d. long non-polar/hydrocarbon chain «and only one hydroxyl group»
OR
forms London/dispersion/van der Waals/vdW interactions with fat [ $\boldsymbol{J}]$
Note: Accept “alcohol/hydroxy/OH” for “hydroxyl” but not “hydroxide”.
e. atherosclerosis/cholesterol deposition «in artery walls» [ $\boldsymbol{V}]$
increases risk of heart attack/stroke/cardiovascular disease/CHD [ $\mathcal{V}]$
Note: Accept “arteries become blocked/walls become thicker”, “increases blood pressure”, or “blood clots”.
Do not accept “high cholesterol”.