From Real Exams Quiz
Secondary 4 Combined Science Biology Ecology Quiz
Free Sec 4 Comb Sci Bio Ecology quiz, Qwen3.6 Exam version, with questions, answers, and O Level-style practice for Singapore students.
These static practice materials are generated from the site's syllabus and paper-generation workflow, with source and model context shown so students and parents can evaluate the material before use.
Questions
Free quiz and exam paper access
Enter your details to view this paper
Your access is remembered on this device.
Answers
Secondary 4 Combined Science Biology Quiz - Ecology (Answer Key)
Total Marks: 40
Section A: Multiple Choice Questions
1. B
Energy flows from producers to consumers and is eventually lost as heat. It is not recycled. [1]
2. C
Photosynthesis removes CO2 from the atmosphere to produce glucose. [1]
3. B
Ladybirds are predators of aphids. An increase in predators leads to a decrease in prey. [1]
4. D
Bacteria are living organisms (biotic). pH, temperature, and water are abiotic. [1]
5. B
Energy is lost as heat during respiration and in excretory products/uneaten parts. [1]
Section B: Structured Questions
6.
(a) Rabbit and Grasshopper. [1]
(b) Tertiary consumer (or Quaternary trophic level). [1]
(c) Energy is lost at each trophic level (as heat/waste). [1] There is insufficient energy remaining to support a viable population at higher levels. [1]
7.
(a) Nitrogen fixation. [1]
(b) Plants absorb nitrates from the soil through their roots (via active transport). [1] Nitrates are used to synthesize amino acids, which are then joined to form proteins. [1]
8.
(a) The populations oscillate/cycle. The peak of the predator (Y) follows the peak of the prey (X). [1]
(b) With fewer prey (X) available, there is less food for the predators (Y). [1] This leads to starvation/decreased reproduction in Y, causing their population to drop. [1]
9.
(a) Fewer trees are available to absorb carbon dioxide via photosynthesis. [1] (Or: Burning trees releases stored carbon as CO2).
(b) Tree roots hold the soil together. [1] Without roots, rain/wind washes/blows the topsoil away more easily. [1]
10.
(a) Light intensity (or distance from the lamp). [1]
(b) Temperature / CO2 concentration / Type of plant. [1]
(c) Another factor becomes limiting (e.g., CO2 concentration or temperature). [1] The enzymes involved in photosynthesis are working at their maximum rate. [1]
11.
(a) Excess fertilizers (nitrates/phosphates) leaching into water bodies. [1]
(b) Algae grow rapidly (algal bloom) blocking light. [1] Submerged plants die due to lack of light/photosynthesis. [1] Decomposers break down dead matter, using up oxygen via respiration. [1] Fish die due to lack of dissolved oxygen. [1] (Max 3 marks)
12.
(a) Bacteria or Fungi. [1]
(b) They break down dead organic matter/waste. [1] Releasing minerals/nutrients (like nitrates) back into the soil for plants to use. [1]
13.
(a) Efficiency = (Energy in Frog / Energy in Grasshopper) × 100
= (200 / 2000) × 100 = 10%. [2]
(b) 1. Energy lost as heat during respiration. [1]
2. Energy lost in waste (feces/urine) or uneaten parts. [1]
14.
(a) The variety of different species of organisms in an ecosystem. [1]
(b) Source of new medicines / drugs. [1] (Or: Tourism / Genetic resources for crops).
15.
(a) Plants take in CO2 for photosynthesis and release O2. [1] Insects/animals take in O2 for respiration and release CO2. [1] This creates a balanced cycle within the sealed system.
(b) Plants cannot perform photosynthesis without light. [1] They will die, stopping the production of oxygen and food for the insects. [1] (Note: Question asks for 1 mark, so "No photosynthesis occurs" is sufficient).
Section C: Free Response Questions
16.
Marking Points:
- Photosynthesis removes CO2 from the atmosphere/water. [1]
- Converts it into organic compounds (glucose/carbohydrates) in producers. [1]
- Respiration (by producers, consumers, and decomposers) releases CO2 back into the atmosphere. [1]
- This maintains the balance of carbon in the ecosystem. [1]
(Max 4 marks)
17.
Marking Points:
- Pesticides are not specific; they may kill beneficial insects (e.g., bees/pollinators). [1]
- Bioaccumulation: Pesticides build up in the tissues of organisms. [1]
- Biomagnification: Concentration increases up the food chain, harming top predators (e.g., birds of prey). [1]
(Max 3 marks)
18.
Marking Points:
- Energy Flow: Energy enters as sunlight, is converted by producers, passed to consumers, and is eventually lost as heat. It does not cycle back to the sun. [2]
- Nutrient Cycling: Nutrients (C, N, P) are taken up by plants, passed to animals, and returned to the soil/atmosphere by decomposers/excretion. They are reused by producers. [2]
(Max 4 marks)
19.
(a) Sewage contains organic matter. [1] Decomposers (bacteria) multiply to break down the sewage. [1] These bacteria use up dissolved oxygen during aerobic respiration, lowering oxygen levels for fish. [1]
(b) Treat sewage before discharge (e.g., sewage treatment plant). [1] (Or: Reduce fertilizer use / Create buffer zones).
20.
Marking Points:
- Biological Control Advantages: Specific to the pest (less harm to non-targets); No chemical pollution; Long-term solution if established. [2]
- Biological Control Disadvantages: Slow acting; Introduced agent might become a pest itself; May not completely eradicate the pest. [1]
- Comparison: Chemical pesticides act fast but cause pollution/resistance. Biological control is more environmentally friendly but requires careful planning. [1]
(Max 4 marks. Award marks for balanced evaluation.)