Assessment: Photosynthesis and Limiting Factors

Subject: Science | Year: 10

Name: _________________________ Class/Set: ____________ Date: ____________

Section A: Knowledge Check (Multiple Choice)

Mark the correct answer.

Q1: In which specific organelle within a plant cell does the process of photosynthesis occur?

a) ☐ Mitochondrion b) ☐ Chloroplast c) ☐ Ribosome d) ☐ Vacuole

⇨ The following data table shows the results of an experiment measuring the volume of oxygen produced by a piece of Elodea (pondweed) at different distances from a light source.

Distance from lamp (cm)	Volume of oxygen (cm³ / min)
10	45.0
20	11.2
30	5.0
40	2.8

Q2: Based on the data in the table, what is the relationship between the distance from the light source and the rate of photosynthesis?

a) ☐ As distance increases, the rate increases linearly. b) ☐ As distance increases, the rate remains constant. c) ☐ As distance increases, the rate decreases. d) ☐ As distance decreases, the rate decreases.

Q3: Which of the following statements best describes a 'limiting factor' in photosynthesis?

a) ☐ The factor that is in the shortest supply, preventing the rate from increasing further. b) ☐ The pigment that absorbs the most light energy from the sun. c) ☐ The product of the reaction that is released into the atmosphere. d) ☐ The enzyme that catalyses the conversion of glucose into starch.

Q4: According to the inverse square law, if the distance of a light source from a plant is doubled, what happens to the light intensity?

a) ☐ It doubles. b) ☐ It stays the same. c) ☐ It is halved (× 1/2). d) ☐ It is quartered (× 1/4).

Q5: Which row in the table below correctly identifies the reactants of photosynthesis?

Row	Reactant 1	Reactant 2
a) ☐	Oxygen	Glucose
b) ☐	Carbon dioxide	Water
c) ☐	Water	Oxygen
d) ☐	Glucose	Carbon dioxide

Section B: Application (Written Answers)

Answer in the spaces provided.

Q6: A student investigates the effect of light intensity on photosynthesis. At a distance of 10 cm, the light intensity is calculated as 0.01 arbitrary units (a.u.). Calculate the light intensity when the lamp is moved to a distance of 50 cm. Use the formula: Light Intensity ∝ 1/d². [3 marks]

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⇨ The data provided in the table below represents the rate of photosynthesis in a tomato greenhouse at two different temperatures and varying concentrations of carbon dioxide (CO₂).

CO₂ Concentration (%)	Rate of Photosynthesis at 15°C (a.u.)	Rate of Photosynthesis at 25°C (a.u.)
0.00	0	0
0.02	15	20
0.04	25	45
0.06	25	60
0.08	25	65
0.10	25	65

Q7: Analyse the data provided in the table. Explain why the rate of photosynthesis at 15°C remains constant between 0.04% and 0.10% CO₂ concentration, while the rate at 25°C continues to increase. [5 marks]

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Total Marks: _______ / 16

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⚠ TEACHER KEY / MARK SCHEME (DO NOT PRINT FOR STUDENTS)

Section A Answers

Q1: b

Explanation: Chloroplasts contain chlorophyll which absorbs light for photosynthesis. Mitochondria are for respiration.

Q2: c

Explanation: The data shows a clear inverse relationship; as the lamp moves further away (distance increases), the volume of oxygen (rate) drops significantly.

Q3: a

Explanation: A limiting factor is defined by its restriction on the maximum possible rate of a process.

Q4: d

Explanation: Inverse square law: 1/2² = 1/4. Light intensity decreases with the square of the distance.

Q5: b

Explanation: Carbon dioxide and water are the inorganic reactants required to produce glucose and oxygen.

Section B Guidance

Q6 Guidance

• 1 Mark: Correct substitution into formula: 1 / 50² or 1 / 2500.
• 1 Mark: Recognition of the ratio change: (10 / 50)² = (1/5)² = 1/25.
• 1 Mark: Final answer: 0.0004 a.u. (or 4 × 10⁻⁴ a.u.).

Model Answer: "Using the inverse square law, Light Intensity = 1 / d². At 10 cm, the intensity is proportional to 1/100. At 50 cm, the intensity is proportional to 1/50². 1 / 2500 = 0.0004. Therefore, the light intensity at 50 cm is 0.0004 arbitrary units."

Q7 Guidance

• 1 Mark (Analysis): Identify that at 15°C, CO₂ is no longer the limiting factor after 0.04%.
• 1 Mark (Analysis): Identify that temperature is the limiting factor at the lower temperature (15°C).
• 1 Mark (Explanation): Explain that at 25°C, enzymes have more kinetic energy / more frequent collisions.
• 1 Mark (Explanation): Explain that at 25°C, the rate can increase further because the 'temperature bottleneck' has been eased.
• 1 Mark (Evaluation): Note that even at 25°C, the rate eventually levels off at 0.08%, suggesting light or another factor becomes limiting.

Model Answer: "At 15°C, the rate levels off at 25 a.u. because temperature has become the limiting factor; the enzymes responsible for photosynthesis are working at their maximum rate for that temperature. In contrast, at 25°C, the rate reaches 65 a.u. because the higher temperature provides more kinetic energy for enzyme-controlled reactions. Between 0.04% and 0.08% CO₂ at 25°C, CO₂ was still the limiting factor as increasing it increased the rate, whereas at 15°C, the temperature limitation prevented any further gains from additional CO₂."

Pedagogical Pulse:

• Misconception Alert: Many Year 10 students struggle with the Inverse Square Law. Remind them that if distance increases by a factor of 3, light intensity decreases by a factor of 9 (3²).
• AO3 Skills: When students see data 'levelling off' on a graph or table, encourage them to ask: "What else is missing?" This leads them naturally to identify limiting factors other than the one plotted on the x-axis.