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Secondary 3 Combined Science Semestral Assessment 2 (End of Year) Paper 5

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TuitionGoWhere Practice Paper – Combined Science Secondary 3

SA2 Examination, Version 5

TuitionGoWhere Secondary School (AI)

Subject: Combined Science (Physics, Chemistry, Biology) Level: Secondary 3 Paper: SA2 (Semester 2 Assessment) Duration: 1 hour 30 minutes Total Marks: 60

Name: _________________________ Class: _________________________ Date: _________________________

Instructions to Candidates

This paper consists of three sections: Section A, Section B, and Section C.
Answer all questions in the spaces provided.
The number of marks is given in brackets [ ] at the end of each question or part question.
You are advised to spend about 30 minutes on Section A, 30 minutes on Section B, and 30 minutes on Section C.
Show all working clearly for calculation questions. Marks will be awarded for correct method even if the final answer is wrong.
Use appropriate scientific terminology and units in your answers.

Section A: Multiple Choice and Short Answer (20 marks)

Answer all questions in this section.

1. Which of the following is the correct statement of the Principle of Conservation of Energy?

A. Energy can be created from nothing in a closed system. B. Energy can be destroyed when work is done. C. Energy cannot be created or destroyed; it can only be converted from one form to another. D. The total energy in an open system always decreases.

[1 mark]

Answer: ________

2. A student investigates the effect of light intensity on the rate of photosynthesis in an aquatic plant. She counts the number of oxygen bubbles produced per minute at different distances from a lamp.

Distance from lamp (cm)	Number of bubbles per minute
10	45
20	28
30	18
40	12
50	8

(a) Using the data in the table, describe the relationship between distance from the lamp and the rate of photosynthesis. [1 mark]

(b) Suggest one variable, other than light intensity, that the student should keep constant during this investigation. [1 mark]

(c) Explain why a glass heat shield is placed between the lamp and the aquatic plant in this experiment. [1 mark]

3. The diagram below shows a simplified cross-section of a human heart. Arrows indicate the direction of blood flow.

Diagram for placeholder 1 (SEC3 Combined Science)

Generated diagram for this question.

(a) Name the blood vessels labelled P and Q in the diagram.

P: _________________________ Q: _________________________

[2 marks]

(b) Describe how oxygenated blood from the lungs is forced into the aorta. [3 marks]

4. A potato cube is placed in a beaker of distilled water. A thin plastic rod is attached to the top of the potato cube. After two hours, the rod is observed to have bent downwards slightly.

(a) Explain why the plastic rod bent downwards. [3 marks]

(b) Describe the state of the potato cells after two hours. [1 mark]

5. The table below shows the relative amounts of amino acids and maltose found in different regions of a person's alimentary canal after a meal containing protein and starch.

Region of alimentary canal	Relative amount of amino acids	Relative amount of maltose
Mouth	0	5
Stomach	2	5
Small intestine (start)	8	12
Small intestine (end)	3	3
Large intestine	1	1

(a) Explain why the amount of amino acids increases, then decreases, in the small intestine. [2 marks]

(b) Name the enzyme responsible for the increase in maltose in the small intestine. [1 mark]

6. A student writes the following word equations for respiration:

Equation 1: Glucose → Lactic acid + Energy Equation 2: Glucose + Oxygen → Carbon dioxide + Water + Energy

(a) Identify which equation represents aerobic respiration. Give a reason for your answer. [2 marks]

(b) State one situation in the human body where Equation 1 is likely to occur. [1 mark]

Section B: Structured Questions (20 marks)

Answer all questions in this section.

7. Zinc is extracted from its ore, zinc blende (ZnS), in a two-step process.

Step 1: Zinc blende is roasted in air to form zinc oxide and sulfur dioxide. Step 2: Zinc oxide is heated with carbon to produce zinc metal and carbon dioxide.

(a) Write a balanced chemical equation, with state symbols, for Step 1. [2 marks]

(b) Write a balanced chemical equation, with state symbols, for Step 2. [2 marks]

(c) In an experiment, 9.70 g of zinc oxide (ZnO) is heated with excess carbon. Calculate the mass of zinc produced.

[Relative atomic masses: Zn = 65, O = 16]

[3 marks]

8. A student performs paper chromatography to test three urine samples (X, Y, and Z) from athletes for the presence of banned substances B1, B2, and B3. The chromatogram obtained is shown below.

Diagram for placeholder 2 (SEC3 Combined Science)

Generated diagram for this question.

(a) Calculate the Rf value of banned substance B2. [2 marks]

(b) State which athlete(s) tested positive for banned substance B3. [1 mark]

(c) Explain why athlete Z's urine sample is of greatest concern to sports authorities. [2 marks]

9. The diagram below shows a plant cell as seen under a light microscope after being placed in a concentrated sucrose solution.

Image pending generation for this question.

(a) Name the state of the cell shown in the diagram. [1 mark]

(b) Explain why the cell has reached this state. [3 marks]

(c) Describe what would happen to the cell if it were transferred to distilled water. [2 marks]

10. A student investigates the reaction between solid zinc carbonate and dilute nitric acid. The reaction produces zinc nitrate solution, carbon dioxide gas, and water.

(a) Write a balanced chemical equation, with state symbols, for this reaction. [2 marks]

(b) Write the ionic equation for this reaction. [2 marks]

Section C: Data Analysis and Extended Response (20 marks)

Answer all questions in this section.

11. A student investigates the pathway of water through a plant. She places a celery stalk in a beaker of water containing red dye. After 30 minutes, she observes that the leaves and the stem have turned red. She then cuts a thin cross-section of the stem and observes it under a microscope.

(a) State the main force responsible for the rise of the red dye in the celery stalk. [1 mark]

(b) Describe the pathway of water through a plant from the soil to the point where it leaves the leaves. [5 marks]

(c) State one function and one adaptation of xylem tissue. [2 marks]

Function: _________________________________________________________________________

Adaptation: _________________________________________________________________________

12. A student investigates the activity of the enzyme glycogen phosphorylase, which catalyses the breakdown of glycogen to glucose-1-phosphate. She sets up four test tubes as shown in the table below and measures the concentration of glucose-1-phosphate produced after 10 minutes.

Test tube	Contents	Temperature (°C)	pH	Glucose-1-phosphate produced (mmol)
A	Glycogen + enzyme	25	7.0	0.8
B	Glycogen + enzyme	35	7.0	1.5
C	Glycogen + enzyme	45	7.0	0.9
D	Glycogen + enzyme	35	3.0	0.2

(a) Using the lock-and-key hypothesis, explain the activity of glycogen phosphorylase. [4 marks]

(b) Explain why the amount of glucose-1-phosphate produced in test tube D is much lower than in test tube B. [2 marks]

(c) Predict the amount of glucose-1-phosphate that would be produced if the experiment were repeated at 55 °C and pH 7.0. Explain your answer. [2 marks]

13. A student investigates the reaction between magnesium ribbon and excess dilute hydrochloric acid. The reaction produces hydrogen gas, which is collected in a gas syringe. The student records the volume of gas collected every 30 seconds.

Time (s)	Volume of hydrogen gas (cm³)
0	0
30	18
60	30
90	38
120	43
150	46
180	48
210	48

(a) Plot a graph of volume of hydrogen gas (y-axis) against time (x-axis) on the grid provided. [3 marks]

[Grid: 10 cm × 10 cm with labelled axes]

(b) Using your graph, determine the volume of hydrogen gas produced after 75 seconds. [1 mark]

(c) Explain why the volume of hydrogen gas stops increasing after 180 seconds. [1 mark]

(d) The student repeats the experiment using the same mass of magnesium powder instead of magnesium ribbon. On the same grid, sketch the curve you would expect to obtain. Label this curve P. [2 marks]

— End of Paper —

Check your work carefully. Ensure all questions are answered.

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Answers

TuitionGoWhere Practice Paper – Combined Science Secondary 3

SA2 Examination, Version 5 – Answer Key and Marking Scheme

TuitionGoWhere Secondary School (AI)

Subject: Combined Science (Physics, Chemistry, Biology) Level: Secondary 3 Paper: SA2 (Semester 2 Assessment) Total Marks: 60

Section A: Multiple Choice and Short Answer (20 marks)

1. C – Energy cannot be created or destroyed; it can only be converted from one form to another. [1 mark]

Marking note: Award 1 mark for correct option only.

2. (a) As the distance from the lamp increases, the number of bubbles per minute decreases / the rate of photosynthesis decreases. [1 mark]

Marking note: Accept any clear statement of inverse relationship. Do not award mark for simply restating data without describing the trend.

(b) Any one of:

Temperature of the water
Concentration of carbon dioxide in the water
Type/species of aquatic plant
Volume of water used

[1 mark]

Marking note: Award 1 mark for any valid controlled variable. Do not accept "light intensity" or "distance from lamp" as these are the independent variable.

(c) To prevent heat from the lamp from affecting the temperature of the water / to ensure that temperature is kept constant / to ensure that only light intensity is changed. [1 mark]

Marking note: Award 1 mark for any answer that clearly identifies the heat shield as controlling temperature as a variable.

3. (a) P: Pulmonary veins [1 mark] Q: Aorta [1 mark]

Marking note: Award 1 mark for each correct label. Accept "pulmonary vein" (singular).

(b) Oxygenated blood returns from the lungs to the left atrium via the pulmonary veins [1 mark]. The left atrium contracts, increasing pressure and forcing blood into the left ventricle [1 mark]. The left ventricle contracts, increasing pressure and forcing blood through the aortic valve into the aorta [1 mark].

Marking note: Award 1 mark for each distinct step. Accept alternative phrasing that clearly describes the pathway and role of pressure/contraction. Do not award marks for descriptions that omit the role of atrial or ventricular contraction.

4. (a) The water potential in the distilled water is higher than the water potential in the potato cells [1 mark]. Water enters the potato cells by osmosis [1 mark]. The potato cells become turgid and expand, exerting pressure on the plastic rod, causing it to bend downwards [1 mark].

Marking note: Award 1 mark for each component. Accept "water concentration" as equivalent to "water potential" at this level. Do not award marks for answers that do not mention osmosis.

(b) Turgid / The cell membrane is pressed against the cell wall. [1 mark]

Marking note: Award 1 mark for "turgid" or an equivalent description. Do not accept "plasmolysed" or "flaccid".

5. (a) The amount of amino acids increases because proteins/polypeptides are digested by protease enzymes into amino acids in the small intestine [1 mark]. The amount then decreases because amino acids are absorbed into the blood through the wall of the small intestine [1 mark].

Marking note: Award 1 mark for digestion explanation and 1 mark for absorption explanation. Accept "protease" or "peptidase" as the enzyme name.

(b) Amylase / Pancreatic amylase [1 mark]

Marking note: Award 1 mark for "amylase". Accept "pancreatic amylase" or "salivary amylase" (though salivary amylase acts mainly in the mouth).

6. (a) Equation 2 represents aerobic respiration [1 mark]. This is because it requires/uses oxygen / it produces carbon dioxide and water (not lactic acid) [1 mark].

Marking note: Award 1 mark for correct identification and 1 mark for a valid reason. Accept any reason that correctly distinguishes aerobic from anaerobic respiration.

(b) During vigorous/strenuous exercise / when oxygen supply to muscles is insufficient / during sprinting or heavy lifting. [1 mark]

Marking note: Award 1 mark for any valid situation where anaerobic respiration occurs in human muscles.

Section B: Structured Questions (20 marks)

7. (a) 2ZnS(s) + 3O₂(g) → 2ZnO(s) + 2SO₂(g) [2 marks]

Marking note: Award 1 mark for correct formulae and balancing, and 1 mark for correct state symbols. Deduct 1 mark if state symbols are missing or incorrect. Accept fractional coefficients if correctly balanced.

(b) 2ZnO(s) + C(s) → 2Zn(s) + CO₂(g) [2 marks]

Marking note: Award 1 mark for correct formulae and balancing, and 1 mark for correct state symbols. Deduct 1 mark if state symbols are missing or incorrect.

(c) Molar mass of ZnO = 65 + 16 = 81 g/mol [1 mark] Moles of ZnO = 9.70 ÷ 81 = 0.1198 mol [1 mark for correct calculation] From equation (b), 2 mol ZnO produces 2 mol Zn, so mole ratio is 1:1. Moles of Zn produced = 0.1198 mol Mass of Zn = 0.1198 × 65 = 7.79 g (to 3 significant figures) [1 mark for correct final answer with units]

Marking note: Award marks for correct method even if final answer is slightly different due to rounding. Accept 7.8 g or 7.79 g. Deduct 1 mark if units are missing from final answer.

8. (a) Rf = distance travelled by substance ÷ distance travelled by solvent front [1 mark] Rf of B2 = 5.0 cm ÷ 10.0 cm = 0.50 [1 mark]

Marking note: Award 1 mark for correct formula or method, and 1 mark for correct answer. Accept 0.5.

(b) Athlete Y and Athlete Z [1 mark]

Marking note: Award 1 mark for both athletes correctly identified. Do not award half marks.

(c) Athlete Z's urine sample contains all three banned substances (B1, B2, and B3) [1 mark], indicating that Athlete Z has taken multiple banned substances / has the most severe doping offence [1 mark].

Marking note: Award 1 mark for identifying that all three substances are present, and 1 mark for explaining the implication. Accept any reasonable explanation of why multiple banned substances is more serious.

9. (a) Plasmolysed [1 mark]

Marking note: Award 1 mark for "plasmolysed". Accept "plasmolysis" or "the cell is plasmolysed".

(b) The concentrated sucrose solution has a lower water potential than the cell sap/cytoplasm of the plant cell [1 mark]. Water moves out of the cell by osmosis, from a region of higher water potential to a region of lower water potential [1 mark]. As water leaves the cell, the vacuole shrinks and the cell membrane/cytoplasm pulls away from the cell wall [1 mark].

Marking note: Award 1 mark for each component. Accept "water concentration" as equivalent to "water potential". Do not award marks if osmosis is not mentioned.

(c) If the cell is transferred to distilled water, the water potential outside the cell would be higher than inside the cell [1 mark]. Water would enter the cell by osmosis, causing the cell to become turgid again / the cell membrane would press against the cell wall [1 mark].

Marking note: Award 1 mark for identifying the direction of water movement and 1 mark for describing the outcome (turgid).

10. (a) ZnCO₃(s) + 2HNO₃(aq) → Zn(NO₃)₂(aq) + CO₂(g) + H₂O(l) [2 marks]

Marking note: Award 1 mark for correct formulae and balancing, and 1 mark for correct state symbols. Deduct 1 mark if state symbols are missing or incorrect.

(b) ZnCO₃(s) + 2H⁺(aq) → Zn²⁺(aq) + CO₂(g) + H₂O(l) [2 marks]

Marking note: Award 1 mark for correct reactants and products (excluding spectator ions), and 1 mark for correct balancing and state symbols. Spectator ions (NO₃⁻) should be omitted. Deduct 1 mark if spectator ions are included.

Section C: Data Analysis and Extended Response (20 marks)

11. (a) Transpiration pull [1 mark]

Marking note: Award 1 mark for "transpiration pull". Accept "transpiration" alone. Do not accept "root pressure" as the main force.

(b) Water is absorbed from the soil by the root hair cells [1 mark]. Water moves through the cortex and endodermis of the root [1 mark]. Water enters the xylem vessels in the root [1 mark]. Water is transported up the stem through the xylem vessels [1 mark]. Water reaches the leaves and exits through the stomata as water vapour by transpiration [1 mark].

Marking note: Award 1 mark for each distinct step. Accept alternative phrasing that clearly describes the pathway. Do not award marks for descriptions that omit the role of root hair cells or stomata.

(c) Function: Transport water and mineral salts from roots to other parts of the plant / Provide mechanical support [1 mark] Adaptation: Lignified walls for strength / Hollow lumen (no cross-walls) for continuous water transport / Narrow diameter for capillarity [1 mark]

Marking note: Award 1 mark for a correct function and 1 mark for a correct adaptation. The adaptation must be linked to the stated function or be a valid structural feature of xylem.

12. (a) The lock-and-key hypothesis states that the active site of an enzyme has a specific shape that is complementary to the shape of its substrate [1 mark]. Glycogen (the substrate) fits into the active site of glycogen phosphorylase, forming an enzyme-substrate complex [1 mark]. The enzyme catalyses the breakdown of glycogen to glucose-1-phosphate [1 mark]. The products are released, and the enzyme remains unchanged and can be reused [1 mark].

Marking note: Award 1 mark for each component. Accept alternative phrasing that clearly explains the lock-and-key model. Do not award marks for descriptions that do not mention the specificity of the active site or the formation of the enzyme-substrate complex.

(b) Test tube D has a pH of 3.0, which is far from the optimum pH of glycogen phosphorylase [1 mark]. The extreme pH denatures the enzyme / changes the shape of the active site, so the substrate (glycogen) can no longer fit into the active site, reducing the rate of reaction [1 mark].

Marking note: Award 1 mark for identifying pH as the factor and 1 mark for explaining denaturation or change in active site shape. Accept "the enzyme is denatured" as sufficient explanation if linked to pH.

(c) Very little or no glucose-1-phosphate would be produced / approximately 0 mmol [1 mark]. At 55 °C, the high temperature would denature the enzyme / the enzyme would lose its specific shape, and the active site would no longer be complementary to the substrate [1 mark].

Marking note: Award 1 mark for correct prediction and 1 mark for explanation involving denaturation at high temperature. Accept "0 mmol" or "negligible amount".

13. (a) Graph plotting [3 marks]:

1 mark for correct axes: x-axis labelled "Time (s)" and y-axis labelled "Volume of hydrogen gas (cm³)" with appropriate scales
1 mark for all points plotted correctly (±1 mm tolerance)
1 mark for smooth curve of best fit drawn through the points

Marking note: Award marks as indicated. If axes are unlabelled or scales are inappropriate, deduct 1 mark. If points are incorrectly plotted, deduct 1 mark per error up to a maximum of 1 mark. If a straight line is drawn instead of a curve, deduct 1 mark.

(b) Approximately 34–35 cm³ (accept 33–36 cm³) [1 mark]

Marking note: Award 1 mark for a value within the acceptable range, read correctly from the candidate's graph. The value must be consistent with the graph drawn.

(c) All the magnesium has been used up / the reaction is complete / the limiting reactant (magnesium) has been completely consumed [1 mark].

Marking note: Award 1 mark for any answer that correctly identifies that the reaction has stopped because the magnesium is used up. Do not accept "the acid has run out" as hydrochloric acid is in excess.

(d) Curve P should:

Start at the origin (0,0) [1 mark]
Rise more steeply than the original curve initially
Reach the same final volume (48 cm³) but in a shorter time
Level off before the original curve

[2 marks]

Marking note: Award 1 mark for a steeper initial gradient, and 1 mark for reaching the same final volume in a shorter time. The curve must be clearly labelled "P". If the curve exceeds the final volume of 48 cm³, do not award the second mark.

End of Answer Key

Total marks: 60