Science Life Science Flashcards

A researcher observes that a particular enzyme's activity drops sharply when the pH falls below 6.5 or rises above 7.5, but peaks at pH 7.0. Which cellular organelle most likely produces this enzyme, given its optimal pH range?

Answer: Cytoplasm/cytosol

The cytosol (cytoplasm) maintains a near-neutral pH of approximately 7.0–7.4, which matches the enzyme's optimal activity range. Lysosomes are highly acidic (pH ~4.5–5.0), mitochondria have a slightly alkaline matrix (~pH 8), and vacuoles in plant cells can be quite acidic. An enzyme peaking at pH 7.0 is best suited to the cytosolic environment.

During meiosis II in a human cell, a nondisjunction event occurs in one secondary oocyte. How many of the resulting egg cells (and polar bodies) will have an abnormal chromosome number?

Answer: 2 out of 4

Meiosis II nondisjunction in one secondary oocyte causes sister chromatids to fail to separate correctly, producing 2 abnormal cells (one with an extra chromosome, one missing a chromosome) from that one oocyte. The other secondary oocyte (from the first polar body lineage, though typically non-functional) would divide normally. Because meiosis II affects only the products of that one cell division, 2 of the 4 resulting cells (egg + 1 polar body from that oocyte) carry an abnormal number.

A plant species growing in a waterlogged, oxygen-poor soil switches from aerobic respiration to lactic acid fermentation in its root cells. What is the MOST immediate consequence of this metabolic shift for the root cells?

Answer: Accumulation of lactic acid lowering intracellular pH

When root cells shift to lactic acid fermentation, pyruvate is converted to lactate (lactic acid) to regenerate NAD+ and keep glycolysis running. This accumulation of lactic acid lowers the intracellular pH, which can inhibit enzymes and disrupt cell function. ATP yield drops dramatically compared to aerobic respiration (only 2 ATP per glucose vs. ~36–38), glucose catabolism continues (it does not halt), and oxygen consumption decreases rather than increases.

In a population of beetles, shell color is determined by a single gene with two alleles. Brown (B) is incompletely dominant over green (G), making heterozygotes yellow. After a severe drought kills most green beetles, the surviving population is 80% brown and 20% yellow. What evolutionary mechanism BEST explains this change in allele frequency?

Answer: Natural selection favoring the B allele

The drought differentially killed green beetles (GG genotype), meaning organisms with the G allele were less fit in the drought environment. This systematic, non-random removal of one phenotype due to an environmental pressure is the definition of natural selection. Genetic drift involves random changes unrelated to fitness, gene flow requires migration from another population, and mutation introduces new alleles rather than changing existing frequencies directionally.

A biologist studying a temperate forest food web notes that when a top predator (wolves) is removed, deer populations explode, overgrazing shrubs and young trees. This in turn reduces songbird populations that nest in those shrubs. This chain of effects is BEST described as:

Answer: A trophic cascade

A trophic cascade occurs when a change at one trophic level (removal of a top predator) triggers indirect effects that ripple down through multiple lower trophic levels — in this case from wolves → deer → vegetation → songbirds. Competitive exclusion involves two species competing for the same niche. Mutualistic disruption involves breaking a mutually beneficial relationship. Primary succession is the colonization of a lifeless habitat, which is unrelated to this scenario.

A cell biologist treats cultured human cells with a drug that specifically blocks the nuclear pore complex, preventing all transport between the nucleus and cytoplasm. Which cellular process would be MOST immediately and directly disrupted?

Answer: Translation of mRNA into protein at ribosomes

Translation requires mRNA to be exported from the nucleus through nuclear pores to reach cytoplasmic ribosomes. If nuclear pore transport is blocked, newly transcribed mRNA cannot exit the nucleus, so translation at cytoplasmic ribosomes would immediately cease for new proteins. Glycolysis occurs entirely in the cytosol and doesn't require nuclear transport. DNA replication occurs inside the nucleus and doesn't require nuclear pores. Phospholipid synthesis in the ER also doesn't depend on nuclear pore transport directly.