Secondary 3 Combined Science Semestral Assessment 2 (End of Year) Paper 1

TuitionGoWhere Practice Paper - Combined Science Secondary 3 (Marking Scheme)

Section A: Multiple Choice Questions [8 marks]

1. B - Energy cannot be created or destroyed, only converted [1]

2. B - Small intestine [1]

3. C - Transpiration pull [1]

4. B - Glucose + Oxygen → Carbon dioxide + Water + Energy [1]

Section B: Structured Response Questions [57 marks]

5.(a) After 2 hours, the plastic rod bends downward. Explain why this happens. [3]

• Higher water potential in water than in potato cube [1]
• Water enters potato cube by osmosis [1]
• Potato becomes turgid, exerting upward pressure on rod, causing it to bend down [1]

5.(b) Predict what would happen to the rod if the potato cube was placed in concentrated sugar solution instead of water. Give a reason for your answer. [2] Prediction: Rod would bend upward / Rod would not bend down [1] Reason: Water would leave potato cube, making it flaccid/plasmolysed, reducing pressure on rod [1]

5.(c) State the name of the process responsible for water movement in this experiment. [1]

• Osmosis [1]

6.(a) Calculate the Rf value for substance B2. [2] Working: Rf = distance travelled by substance ÷ distance travelled by solvent front = 4.5 ÷ 6.0 [1] Answer: 0.75 [1]

6.(b) Which banned substances are present in athlete X's urine sample? [1]

• B1 and B2 [1]

6.(c) State one advantage of using chromatography to test for banned substances. [1]

• Can separate and identify multiple substances in one test / Quick and reliable / Can detect small amounts [1]

7.(a) Write balanced chemical equations for the two main steps in zinc extraction. Include state symbols. [4] Step 1 (Roasting): 2ZnS(s) + 3O₂(g) → 2ZnO(s) + 2SO₂(g) [2] Step 2 (Reduction): ZnO(s) + C(s) → Zn(s) + CO(g) [2] [1 mark each for correct equation, 1 mark each for correct state symbols and balancing]

7.(b) Calculate the volume of carbon dioxide gas produced when 32.6 g of zinc carbonate reacts completely with excess hydrochloric acid. [3] Working: ZnCO₃ + 2HCl → ZnCl₂ + CO₂ + H₂O Moles of ZnCO₃ = 32.6 ÷ 125 = 0.261 mol [1] Moles of CO₂ = 0.261 mol (1:1 ratio) [1] Volume of CO₂ = 0.261 × 24 = 6.26 dm³ [1]

8.(a) Draw a graph of these results on the grid below. [3]

• Correct plotting of all points [1]
• Appropriate smooth curve showing initial increase then leveling off [1]
• Correctly labeled axes with units [1]

8.(b) Describe the relationship between light intensity and oxygen production shown by your graph. [2]

• Oxygen production increases with light intensity initially [1]
• Then levels off/remains constant at higher light intensities [1]

8.(c) Suggest two variables that must be controlled in this experiment. [2]

1. Temperature [1]
2. Carbon dioxide concentration / pH / Type of plant / Volume of water [1]

8.(d) Explain why oxygen production levels off at high light intensities. [2]

• Light intensity is no longer the limiting factor [1]
• Another factor becomes limiting (temperature/CO₂ concentration/chlorophyll concentration) [1]

9.(a) Describe how oxygenated blood from the lungs is pumped to the rest of the body. [4]

• Oxygenated blood returns from lungs via pulmonary veins to left atrium [1]
• Left atrium contracts, pushing blood into left ventricle [1]
• Left ventricle contracts with great force [1]
• Blood is forced through aortic valve into aorta and to the rest of the body [1]

9.(b) Explain why the left ventricle has thicker muscular walls than the right ventricle. [2]

• Left ventricle needs to pump blood to the whole body [1]
• Requires greater pressure/force than right ventricle which only pumps to lungs [1]

10. With reference to aerobic and anaerobic respiration in humans, explain the differences between these two processes using word equations. [6] Aerobic respiration:

• Glucose + Oxygen → Carbon dioxide + Water + Energy [2]
• Requires oxygen / Occurs in mitochondria [1]

Anaerobic respiration:

• Glucose → Lactic acid + Energy [2]
• Does not require oxygen / Occurs in cytoplasm [1]
• Releases less energy than aerobic respiration [1]

11.(a) Structure P is found in large quantities in this cell. Name structure P and explain why it is present in large quantities. [2] Name: Mitochondria [1] Explanation: To provide energy for enzyme production/secretion of digestive enzymes [1]

11.(b) State two other organelles that would be found in large quantities in pancreatic cells and explain why. [4] Organelle 1: Ribosomes/Rough endoplasmic reticulum [1] Explanation: To synthesize digestive enzymes/proteins [1] Organelle 2: Golgi apparatus [1] Explanation: To package and modify enzymes for secretion [1]

12. Describe the pathway of water through a flowering plant from absorption by the roots to loss from the leaves. Include the tissues involved and the forces responsible for water movement. [6]

• Water absorbed by root hair cells from soil [1]
• Moves through cortex and endodermis in root [1]
• Enters xylem vessels in root [1]
• Transported up stem through xylem tissue [1]
• Reaches leaves and moves to mesophyll cells [1]
• Lost through stomata by transpiration / Transpiration pull creates the force for upward movement [1]

Total: 65 marks

Grade Boundaries:

• A: 52-65 marks (80%+)
• B: 46-51 marks (70-79%)
• C: 39-45 marks (60-69%)
• D: 33-38 marks (50-59%)
• E: 26-32 marks (40-49%)
• F: Below 26 marks