VJC 2021 Prelim P4 Suggested Answer

1 Victoria Junior College Biology Department 2021 Prelim Paper 4 Suggested Answer Marking abbreviations: A: Accept R: Reject BOD: benefit of doubt AW: alternative wording AVP: Any valid point NAQ: not answering question Question 1 (a) (i) Suggest why betalain molecules cannot pass from vacuoles of plant cells into the cytoplasm. [2] 1. betalain molecules are polar / charged / hydrophilic; 2. (so) will not be able to diffuse through hydrophobic core of phospholipid bilayer or hydrophobic ‘tails’ of phospholipids; 3. membrane lack membrane proteins / carriers / channels / pumps to transport betalain across membrane; Or 4. betalain molecules are too large; 5. (so) will not be able to diffuse, through hydrophobic core of phospholipid bilayer / AW ; A ref. to hydrophobic ‘tails’ of phospholipids Note: mark will only be awarded once for either points 1 or 4. The second mark is awarded for the explanation which must be related to the first point (i.e. point 1 or 4) that student gave. (ii) Why were beetroot discs washed several times before they were used? [1] • The surface of the beetroot discs is covered by pigments and hence affecting the accuracy of the data collected; Or • Wash away the leaked pigments from cut/ damaged cells so that the leakage of pigment later can be attributed to the effects of alcohol; (iii) State the concentrations of alcohol that you will use in your investigation. [1] • at least five different concentrations of alcohol at regular intervals, including 0% e.g. 0, 20, 40, 60, 80, 100% / 0, 25, 50, 75, 100% R: 10x, 5x, 2x dilution factors, i.e., serial dilution (iv) Use the space provided below to show, in a suitable format, how you prepared the solutions of alcohol. [4] Concentration of alcohol/ % Volume of 100% alcohol / cm3 Volume of distilled water / cm3 Total volume / cm3 0 0.0 0.0 10.0 5.0 10.0 5.0 20 2.0 1.0 8.0 4.0 10.0 5.0 40 4.0 2.0 6.0 3.0 10.0 5.0 60 6.0 3.0 4.0 2.0 10.0 5.0 80 8.0 4.0 2.0 1.0 10.0 5.0 100 10.0 5.0 0.0 0.0 10.0 5.0

2 Marking points: 1. A complete lined table, with independent variable on left-most column; 2. Correct column headings with appropriate units + No units in the body of the table; 3. [Precision] Recording of volume; 4. [Calculation] Correct calculation; Allow error from (iii) carried forward (ECF) here. Calculation is based on the alcohol concentration indicated. (v) State and explain a step that you can take in the preparation of the discs to ensure that you obtained valid results in this investigation. [2] • step taken on preparing discs, e.g. use a cutter with a fixed dimension of 4 mm / measure and discard poorly cut discs; ignore simple use of ruler and scalpel • to ensure constant thickness/ surface area of discs; OR • discs washed, until no trace of pigment / same number of times (minimum of four) or discs handled carefully so not, punctured / damaged; • ensuring no further leakage of pigment / explanation involving cell damage; OR • cut the discs from the same core of beetroot tissue/ select discs with the same colour throughout; • to ensure that beetroot discs have the same concentration of betalain at the start of the experiment; (vi) Use this space to record, in a suitable format, your results from step 6. [4] Concentration of alcohol/ % Concentration of betalain/ % 0 0.0 20 Between 0.0 and 0.1 40 0.5 60 Between 1.0 and 5.0 80 5.0 100 Between 5.0 and 10.0 Another possibility: Concentration of alcohol/ % Concentration of betalain/ % 0 0.0 25 Between 0.5 and 1.0 50 Between 1.0 and 5.0 75 Between 1.0 and 5.0 100 Between 1.0 and 5.0 Marking points: 1. A complete lined table, with independent variable on left-most column; 2. Correct column headings with appropriate units + No units in the body of the table; 3. [Precision] Recording of percentage concentration of betalain according to prepared solutions, no intermediate values (accept middle value, i.e., 0.05, 0.3, 0.75, 3.0, 7.5 only);

3 4. [Accuracy] 0% alcohol is not more than 0.1% betalain, 100% alcohol is between but not including 1 and 10% betalain and correct trend of having increasing concentration of betalain / accept plateau seen above 50% alcohol; (vii) Describe and explain your results. [3] 1. As concentration of alcohol increases from 0 to 100%, concentration of betalain increases from 0 to between 5.0 and 10.0% ; 2. alcohol dissolves / disrupts hydrophobic region / fatty acids / (phospho)lipid ‘tails’ of cell membranes / tonoplasts and cell (surface) membranes; 3. permeability of cell membranes to betalains increases with alcohol concentration OR betalains diffuse, out of vacuoles and cells / through tonoplasts and cell (su rface) membranes; 4. any plausible suggestion as to type of disruption e.g. pore formation / disruption of organisation /denaturation, of proteins in membranes; 5. membrane remains intact up to critical concentration (for those students with plateau); (viii) Based on the apparatus and materials provided, explain how you would increase the confidence in your estimation for 100% alcohol. [2] 1. prepare further concentrations of betalain of regular intervals between 5.0% and 10.0% e.g. 6.0, 7.0, 8.0%, 9.0 to better determine the actual concentration of betalains for 100% alcohol; A: based on student’s results 2. place the tube for 100% alcohol against these standard solutions of betalain for colour matching against the white card; (ix) Describe and explain a suitable control for this investigation. [2] 1. 5 cm3 0% alcohol / distilled water with the 2 discs immersed for 20 minutes; A: 2 glass beads / no beetroot discs in alcohol solutions for 20 minutes 2. so that any leakage in water needs to be taken into account when interpreting results from the alcohol solutions / to show that the permeability of membranes is due to the presence of alcohol; A: to show that the colour of the solutions is due to the effect of alcohol on the permeability of the membranes

4 (b) This procedure can be modified to investigate other factors which influence the permeability of membranes. (i) Use the grid opposite to present your results in a suitable format. [4] Marking points: 1. (A) Axis – labelled with correct units; 2. (S) Scale – graph should occupy at least two-third of the area of the total grid with no odd scale; 3. (P) Points – plotted accurately; 4. (L) Line – point-to-point OR best fit. No extrapolation of graph. (ii) With reference to the graph, explain the effect of temperature on the absorbance of light in the colorimeter. [3] 1. (QV) As temperature increases from 10oC to 80oC, absorbance of green light increases from 16a.u. to 98a.u; 2. Ref increasing kinetic energy of molecules (phospholipids & proteins) which move faster, creating gaps / pores in the membrane; 3. Ref protein molecules denaturing at high temperatures because their hydrogen/ ionic bonds break, OR idea of high temperature disrupting hydrophobic interacti ons between phospholipids creating pores in membranes; 4. Idea of k inetic energy of molecules increases which increases movement of pigment molecules diffusing across the membrane;

5 Question 2 (a) (i) The stem shows regular patterns in the overall shape as well as distribution of vascular bundles. Use Fig. 2.1 as a guide, draw a large, representative plan diagram of one third of the stem. Use one ruled line and label to identify the xylem. Fig. 2.1 Marking points: 1. Neat, continuous lines with no shading. Drawing shows only 1/3 of the specimen (as instructed). 2. Correct number of layers (3) + correct proportion / relative thickness of layers 3. Correct shape and sizes of vascular bundles (must show difference in sizes between the vascular bundles) 4. Ruled line to identify xylem (ii) You are provided wi