AZSCI Systems, Energy, and Matter 2 — Questions and Answers
Question 1: A student sets up an experiment with a sealed terrarium containing soil, plants, and a small amount of water. The terrarium is placed in direct sunlight. After several weeks, the plants have grown, but the total mass of the sealed terrarium has remained virtually unchanged. Which statement best explains this observation?
- The plants produced new matter from sunlight, which has no mass, perfectly balancing the mass lost through respiration.
- Matter was converted into energy by the plants during photosynthesis and then converted back into matter, resulting in no net change.
- The Law of Conservation of Mass applies to this closed system; atoms from the water and carbon dioxide were rearranged into glucose and oxygen, but no atoms were created or destroyed. (Correct answer)
- Small measurement errors are expected; the mass of the terrarium must have decreased as the plants used up nutrients from the soil.
Correct answer: The Law of Conservation of Mass applies to this closed system; atoms from the water and carbon dioxide were rearranged into glucose and oxygen, but no atoms were created or destroyed.
This scenario is a direct application of the Law of Conservation of Mass in a closed system. Photosynthesis takes carbon dioxide from the air and water from the soil and, using light energy, rearranges these atoms to create glucose (for growth) and oxygen. Cellular respiration does the reverse. Because the terrarium is sealed, no matter can enter or leave. Therefore, the total number of atoms inside remains constant, and so does the total mass, even as the atoms are rearranged into different molecules.
Question 2: In a specific aquatic ecosystem, the Net Production Efficiency (NPE) of endothermic mammals (like otters) is significantly lower than that of ectothermic fish (like trout). What is the primary reason for this difference in energy transfer efficiency?
- Otters are larger than trout and therefore require more biomass to sustain themselves.
- Trout are more effective hunters and capture a higher percentage of available prey.
- Otters expend a large amount of ingested energy to maintain a constant internal body temperature, which is then unavailable for biomass production. (Correct answer)
- The process of cellular respiration is fundamentally less efficient in mammals than it is in fish.
Correct answer: Otters expend a large amount of ingested energy to maintain a constant internal body temperature, which is then unavailable for biomass production.
Net Production Efficiency (NPE) measures how efficiently organisms convert the energy they consume into biomass. Endotherms (warm-blooded animals) like otters must use a substantial portion of their metabolic energy to maintain a stable, high internal body temperature (thermoregulation). This energy is largely lost as metabolic heat and is not converted into new tissue (biomass). Ectotherms (cold-blooded animals) like trout have metabolic rates that conform to the ambient temperature, so they lose far less energy to heat maintenance, allowing a larger percentage of their consumed energy to be allocated to growth.
Question 3: A forest ecosystem experiences a prolonged drought, causing many primary producer plants to die. This leads to a sharp decline in the herbivore population. However, the carnivore population that preys on the herbivores declines more slowly. This scenario illustrates which ecological concept?
- The 10% rule of energy transfer, because the carnivores are more energy-efficient.
- A trophic cascade, because the removal of producers impacts all subsequent levels. (Correct answer)
- A biogeochemical feedback loop, where the drought is intensified by the lack of plant transpiration.
- Top-down control, where predator populations dictate the size of prey populations.
Correct answer: A trophic cascade, because the removal of producers impacts all subsequent levels.
A trophic cascade is an ecological phenomenon triggered by the addition or removal of top predators and involving reciprocal changes in the relative populations of predator and prey through a food chain, which often results in dramatic changes in ecosystem structure and nutrient cycling. In this advanced scenario, the cascade is initiated from the bottom-up (loss of producers), but the cascading effect through the trophic levels is the core concept being demonstrated. The decline in producers directly causes a decline in primary consumers (herbivores), which in turn causes a decline in secondary consumers (carnivores).
Question 4: Which of the following describes a critical distinction between the flow of energy and the cycling of matter in an ecosystem?
- Energy is recycled by decomposers and returned to producers, while matter flows in one direction and is eventually lost as heat.
- Matter and energy are both cycled repeatedly through the ecosystem, but matter changes form while energy does not.
- Energy flows in one direction from the sun to producers to consumers and is lost as heat at each transfer, while matter is cycled continuously between living and nonliving components. (Correct answer)
- Matter is created by producers through photosynthesis, while energy is transferred from the sun; both are then destroyed by consumers.
Correct answer: Energy flows in one direction from the sun to producers to consumers and is lost as heat at each transfer, while matter is cycled continuously between living and nonliving components.
This question addresses a fundamental principle of ecosystem dynamics. Energy enters an ecosystem, primarily from the sun, and flows directionally from one trophic level to the next. At each transfer, a significant amount of energy is lost as metabolic heat, in accordance with the second law of thermodynamics, and it cannot be recycled. In contrast, matter (elements like carbon, nitrogen, water) is finite and is continuously recycled. Decomposers play a key role in returning these elements from dead organic material back to the soil and atmosphere, making them available for producers again in what are known as biogeochemical cycles.
Question 5: A scientist is studying a reaction in an open beaker where a solid chemical is dissolved in water, producing a gas that escapes into the room. The scientist observes that the total mass of the beaker and its contents decreases. How does this observation relate to the Law of Conservation of Mass?
- It proves the Law of Conservation of Mass is only applicable to reactions involving solids and liquids.
- The law is violated because mass was destroyed when the gas was produced.
- The law is not violated because the system is open; the mass of the escaped gas, if captured, would account for the observed decrease. (Correct answer)
- The decrease in mass is due to the conversion of matter into heat energy during the reaction.
Correct answer: The law is not violated because the system is open; the mass of the escaped gas, if captured, would account for the observed decrease.
The Law of Conservation of Mass states that mass is neither created nor destroyed in a chemical reaction. A common misconception arises when observing reactions in open systems where a gaseous product can escape. The system's mass (the beaker and its contents) decreases because matter in the form of gas has left the system boundary. If the experiment were conducted in a closed, sealed system, the total mass would remain constant because the gas would be trapped. The law itself is not violated; the measurement simply doesn't account for all the products.
Question 6: During cellular respiration, a glucose molecule (C6H12O6) is broken down. During photosynthesis, a glucose molecule is synthesized. Which statement accurately compares the net energy change in these two processes?
- Both processes are endergonic, requiring a net input of energy to proceed.
- Photosynthesis is exergonic, releasing energy into the ecosystem, while cellular respiration is endergonic, storing energy in ATP.
- Both processes are exergonic, releasing a net amount of energy as heat and ATP.
- Photosynthesis is endergonic, storing light energy in the chemical bonds of glucose, while cellular respiration is exergonic, releasing that stored energy. (Correct answer)
Correct answer: Photosynthesis is endergonic, storing light energy in the chemical bonds of glucose, while cellular respiration is exergonic, releasing that stored energy.
Photosynthesis is an endergonic process because it requires a net input of energy (from sunlight) to build complex, high-energy molecules (glucose) from simpler, low-energy ones (CO2 and H2O). It stores energy. Cellular respiration is the opposite; it is an exergonic process that breaks down a complex, high-energy molecule (glucose) into simpler ones, releasing the stored chemical energy to produce ATP and heat. The processes are complementary parts of the carbon and energy cycles.
AZSCI Systems, Energy, and Matter 2
A student sets up an experiment with a sealed terrarium containing soil, plants, and a small amount of water.
The terrarium is placed in direct sunlight.
After several weeks, the plants have grown, but the total mass of the sealed terrarium has remained virtually unchanged.
Which statement best explains this observation?