Science Test #7 Flashcards

A scientist studying a lake ecosystem notices that after a large algae bloom dies off, dissolved oxygen levels in the water drop sharply. Fish begin dying even though sunlight is still abundant. Which sequence of events BEST explains this oxygen depletion?

Answer: Decomposing bacteria consume oxygen as they break down the dead algae, creating a hypoxic zone

When massive algae blooms die, aerobic decomposing bacteria rapidly multiply and consume dissolved oxygen as they break down the organic matter. This bacterial respiration depletes oxygen far faster than it can be replenished, creating a hypoxic (low-oxygen) or anoxic dead zone that suffocates fish. This process, called eutrophication-driven hypoxia, is distinct from the original bloom itself.

A patient has a mutation that causes their ribosomes to misread the stop codon UAA as an amino acid codon. Which outcome is MOST likely?

Answer: Proteins will be longer than normal because translation continues past the intended endpoint

Stop codons (UAA, UAG, UGA) signal the ribosome to terminate translation and release the polypeptide chain. If a ribosome misreads UAA as a coding codon instead of a stop signal, translation continues into what would normally be the untranslated region (UTR), producing a protein that is longer than normal with extra, often nonfunctional amino acids appended to its C-terminus. Amino acids added before the stop codon are unaffected.

Two identical metal spheres carry charges of +8 μC and −2 μC respectively. They are briefly touched together and then separated. What is the charge on each sphere after separation?

Answer: +3 μC and +3 μC

When two identical conductors touch, charge redistributes equally between them. The total charge is (+8 μC) + (−2 μC) = +6 μC. Since the spheres are identical, this total divides equally: +6 μC ÷ 2 = +3 μC on each sphere. The net charge is conserved (law of conservation of charge), and identical conductors always share charge equally.

A geologist discovers a rock layer containing iridium at unusually high concentrations sandwiched between layers with normal iridium levels. Iridium is rare on Earth's surface but common in asteroids. Which conclusion is BEST supported by this evidence?

Answer: The iridium was deposited by a large asteroid or comet impact, consistent with a global extinction boundary

The iridium anomaly described matches the famous Cretaceous-Paleogene (K-Pg) boundary layer found worldwide. The most widely accepted scientific explanation is that a large asteroid impact vaporized and dispersed iridium-rich material globally, which then settled as a thin layer. While some volcanic activity can bring mantle iridium to the surface (option A is a minority scientific view), the globally uniform thin layer and the correlation with mass extinction events most strongly support an impact event.

During cellular respiration, a cell produces 2 ATP during glycolysis and uses the Krebs cycle and electron transport chain to produce more. If the electron transport chain is blocked by a toxin, which molecules will accumulate in the cytoplasm?

Answer: NADH and pyruvate

The electron transport chain (ETC) regenerates NAD⁺ by accepting electrons from NADH. If the ETC is blocked, NAD⁺ cannot be regenerated, and NADH accumulates. Without sufficient NAD⁺, the Krebs cycle and glycolysis slow or halt, causing pyruvate to accumulate in the cytoplasm. The cell may shift to fermentation to regenerate some NAD⁺, but overall NADH and pyruvate build up. ATP production beyond the glycolytic 2 ATP is also halted.

A star with 20 times the mass of our Sun reaches the end of its life. Which sequence correctly describes its likely stellar evolution endpoint, and WHY does it differ from the Sun's fate?

Answer: It becomes a neutron star or black hole after a supernova, because its core mass exceeds the Chandrasekhar limit

Stellar fate is determined by mass. The Sun (low-mass star) will eventually become a red giant, shed its outer layers as a planetary nebula, and leave behind a white dwarf. A star 20 times the Sun's mass has sufficient core mass after fuel exhaustion to exceed the Chandrasekhar limit (~1.4 solar masses). Its core collapses catastrophically, triggering a supernova explosion. The remnant becomes a neutron star (if core mass is 1.4–3 solar masses) or a black hole (if greater). The critical factor is whether electron or neutron degeneracy pressure can halt the collapse.