Question
In a stable natural ecosystem, how is the supply of nutrients and energy maintained?
A. Nutrients and energy are efficiently cycled within the ecosystem.
B. Energy from the Sun is cycled through the food chains, while nutrients are constantly lost.
C. Nutrients are efficiently cycled within the ecosystem, while there is a constant flow of energy.
D. Energy is cycled through photosynthesis and respiration, while food chains ensure that nutrients flow through the ecosystem.
▶️Answer/Explanation
Answer: C. Nutrients are efficiently cycled within the ecosystem, while there is a constant flow of energy.
Explanation:
In a stable natural ecosystem:
- Nutrients (like carbon, nitrogen, and phosphorus) are recycled through processes such as decomposition, excretion, and uptake by plants. These elements are used again and again within the ecosystem.
- Energy, however, does not cycle. It flows in one direction—from the Sun, through producers (via photosynthesis), to consumers, and finally to decomposers. At each step, energy is lost as heat, mainly due to metabolic processes. So, new energy must continuously enter the system from sunlight.
Answer Evaluation:
A. Incorrect – While nutrients are cycled, energy is not. It flows through the system and cannot be reused once lost as heat.
B. Incorrect – This reverses the truth. Nutrients are recycled, not constantly lost, while energy flows and is not recycled.
C. Correct – This correctly describes that nutrients are recycled within the ecosystem, and energy flows in from the Sun and is lost, needing constant input.
D. Incorrect – Energy is not cycled through photosynthesis and respiration; instead, it is transformed and eventually lost as heat.
Question
The diagram shows part of a food chain. The left box represents producers and the right box primary consumers. Arrows show energy flows.
Which forms of energy are represented by X and Y?
▶️Answer/Explanation
Answer: D. light (X), chemical (Y)
Explanation:
The diagram shows energy flow into producers (e.g., plants) and from producers to primary consumers. The energy types are as follows:
Interpreting the Arrows:
- X (into producers):
- The main source of energy for producers (like green plants) is light energy from the Sun.
- Therefore, X = light.
- Y (from producers to primary consumers):
- Energy is transferred in the form of chemical energy stored in the biomass of producers (e.g., glucose).
- Therefore, Y = chemical.
Evaluation of Other Options:
| Option | X | Y | Explanation |
|---|---|---|---|
| A | heat | kinetic | Heat is lost, not used as input; kinetic is not the main energy form transferred in food chains. |
| B | heat | chemical | Heat is not the energy entering producers; they use light. |
| C | light | heat | Correct for X, but Y is chemical, not heat. |
| D | light | chemical | Matches the actual energy transformations in a food chain. |
Question
The diagram shows interactions between food chains in an ecosystem in the African savannah.
Which arrows indicate the flow of chemical energy?
A. I and II only
B. I and III only
C. II and III only
D. I, II and III
▶️Answer/Explanation
Answer: C. II and III only
Explanation:
This diagram represents energy transfer in a food web.
Understanding energy types:
Arrow I: Sunlight → Star grass
- This is the flow of light energy, not chemical.
- Plants use light energy for photosynthesis to create chemical energy.
Arrows II & III:
- II: Star grass → Herbivores (Hare, Gazelle, Grasshopper)
- III: Herbivores → Carnivores (Cheetah, Shrike)
- These arrows represent the transfer of chemical energy stored in the biomass from one organism to another (via consumption).
Evaluation of Options:
| Option | Arrows Included | Explanation |
|---|---|---|
| A | I and II | I is light energy, not chemical |
| B | I and III | I is not chemical, so incorrect |
| C | II and III | Both represent chemical energy transfer |
| D | I, II and III | Includes I incorrectly |
Question
Which processes occur in an ecosystem?
I. Biomass increases in each successive trophic level.
II. Inorganic nutrients are recycled.
III. Chemical energy is stored in carbon compounds.
A. I and II only
B. I and III only
C. II and III only
D. I, II and III
▶️Answer/Explanation
Answer: C. II and III only
Explanation:
To understand which processes actually occur in ecosystems, let’s briefly define the key terms in the statements.
I. Biomass increases in each successive trophic level
Incorrect – This is false. Biomass decreases at each successive trophic level because energy is lost as heat through respiration, movement, and waste. Only a small percentage (typically about 10%) of energy is passed on to the next level. So the amount of biomass (living material) generally gets smaller as you move up the food chain.
II. Inorganic nutrients are recycled
Correct – Elements like carbon, nitrogen, and phosphorus are recycled in ecosystems. Decomposers break down dead organisms and waste, releasing nutrients back into the soil and atmosphere for reuse by producers.
III. Chemical energy is stored in carbon compounds
Correct – During photosynthesis, plants capture energy from sunlight and store it as chemical energy in carbon-based molecules like glucose. This energy is then passed along the food chain.