Students in a class measured the mass of various living organisms. They then kept the organisms in the dark for 24 hours before remeasuring them. None of the organisms were provided with nutrients during the 24-hour period. The data are as follows.

Organism Masses Before and After Experiment
Organism Starting mass (g) Final mass (g)
Elodea (submerged aquatic plant) 15.10 14.01
Goldfish 10.10 9.84
Sea anemone 25.60 24.98

Which of the following is the best explanation for the pattern of change in mass of the organisms over time?

Question

Students in a class measured the mass of various living organisms. They then kept the organisms in the dark for 24 hours before remeasuring them. None of the organisms were provided with nutrients during the 24-hour period. The data are as follows.

Organism Masses Before and After Experiment
Organism	Starting mass (g)	Final mass (g)
Elodea (submerged aquatic plant)	15.10	14.01
Goldfish	10.10	9.84
Sea anemone	25.60	24.98

Which of the following is the best explanation for the pattern of change in mass of the organisms over time?

BlackTom AI · Accepted Answer

The question presents a scenario where organisms are kept in the dark without nutrients for 24 hours, and all show a decrease in mass from the starting to the final measurement. This sets up a pattern consistent with energy production processes that do not require light.

To evaluate possible explanations, imagine what happens to living cells in the absence of light and nutrients. Without photosynthesis (which requires light), the organisms cannot generate new biomass from inorganic carbon sources. Instead, they rely on stored organic molecules and cellular respiration to meet energy needs. During cellular respiration, organic matter is broken down to produce ATP, releasing carbon dioxide and water as byproducts. The loss of mass over time reflects the loss of the original organic material used in respiration, albeit some water movement or osmotic adjustments can complicate the exact amount.

Option: Cellular respiration (the process that releases energy by breaking down sugars and other organic molecules) accounts for the observed mass decrease, because:
- It occurs in the dark and does not require light-driven processes like photosynthesis.
- It uses stored cellular resources, leading to consumption of mass from the organisms.
- The byproducts (CO2 and H2O) leave the organism or are redistributed, contributing to a net decrease in mass.

Why other, unstated options would be less plausible in this context:
- If an option claimed that mass loss was due to photosynthesis in the dark, that would be incorrect because photosynthesis requires light and would add biomass rather than subtract it.
- If an option suggested that mass remained constant, that would contradict the observed decreases across all three organisms and ignore ongoing respiration and metabolic use of stored materials.
- If an option argued that mass changes were solely due to osmotic water movement unrelated to metabolism, it would fail to explain consistent mass loss across diverse organisms, and would not account for the consumption of stored organic matter tied to energy production.

Overall, the pattern of declining mass in the absence of light and nutrients aligns most directly with cellular respiration as the best explanation for the observed data.

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