Science Life Science Flashcards

A researcher observes that a population of bacteria grows rapidly in a nutrient-rich environment but then levels off as resources become scarce. Which term describes the maximum stable population size that the environment can sustain?

Answer: Carrying capacity

Carrying capacity (often denoted K) is the maximum population size an environment can support indefinitely given available resources such as food, water, and space. When a population approaches carrying capacity, resource competition increases and growth rate decreases, producing the characteristic S-shaped (logistic) growth curve.

In cellular respiration, pyruvate produced during glycolysis can undergo two different fates depending on oxygen availability. Under anaerobic conditions in human muscle cells, pyruvate is converted to lactate rather than entering the Krebs cycle. What is the PRIMARY purpose of this conversion?

Answer: To regenerate NAD+ so glycolysis can continue producing ATP

During anaerobic conditions, NADH accumulates and the cell runs out of NAD+, which is required for glycolysis to continue. Converting pyruvate to lactate oxidizes NADH back to NAD+, allowing glycolysis to keep running and producing the small but critical amount of ATP the cell needs. The conversion itself does not produce ATP.

Two plant species occupy the same meadow. Species A has shallow, wide-spreading roots while Species B has deep taproots. During a prolonged drought, Species B survives while Species A wilts and dies. This outcome BEST illustrates which ecological principle?

Answer: Resource partitioning reducing direct competition

Resource partitioning occurs when two species divide available resources — in this case water — by exploiting different soil depths. Species A accesses shallow soil moisture while Species B taps deep groundwater. By using different portions of the same resource, the species reduce direct competition and can coexist. The drought simply reveals how their root niches differ.

A scientist is studying a protein that controls cell division. She finds that a single nucleotide mutation converts a codon from GAG to GTG in the gene's coding sequence. If this mutation causes a single amino acid change that makes the protein permanently active, what type of gene has MOST LIKELY been altered?

Answer: A proto-oncogene converted into an oncogene

Proto-oncogenes encode proteins that normally promote cell division in a controlled way. A gain-of-function mutation — one that makes the protein permanently active — converts a proto-oncogene into an oncogene, driving uncontrolled cell proliferation. Tumor suppressor gene mutations, by contrast, involve loss of function (the brakes fail rather than the accelerator getting stuck).

The hormone antidiuretic hormone (ADH) is released by the posterior pituitary when blood solute concentration rises. ADH causes kidney collecting ducts to become more permeable to water. Which feedback mechanism does this BEST represent?

Answer: Negative feedback, because water reabsorption reduces blood solute concentration back toward normal

This is a classic negative feedback loop. High blood solute concentration triggers ADH release → ADH increases water reabsorption in the kidneys → blood becomes more dilute → solute concentration drops back toward normal → ADH release decreases. The response (water reabsorption) counteracts and corrects the original deviation, which is the defining feature of negative feedback.

Scientists compare DNA sequences from four mammal species and construct the following phylogenetic tree: ((Humans, Chimpanzees), (Dogs, Wolves)). According to this tree, which pair of species shares the MOST RECENT common ancestor?

Answer: Dogs and wolves

In a phylogenetic tree, the most recent common ancestor is found at the node closest to the tips of the branches being compared. Dogs and wolves share an inner node within the (Dogs, Wolves) clade — they diverged from each other more recently than either diverged from humans or chimpanzees. Humans and chimpanzees also share a recent ancestor, but the question asks which single pair is most recently related, and dogs/wolves form an equivalently recent pair.