Science Life Science Flashcards

A researcher studying cellular respiration notes that a cell produces ATP through substrate-level phosphorylation without consuming oxygen. Which metabolic pathway is exclusively responsible for this process?

Answer: Glycolysis

Glycolysis is the only metabolic pathway that produces ATP through substrate-level phosphorylation without requiring oxygen. It occurs in the cytoplasm and produces a net gain of 2 ATP molecules by directly transferring phosphate groups to ADP. Oxidative phosphorylation and the electron transport chain require oxygen, and while the citric acid cycle also uses substrate-level phosphorylation, it requires pyruvate from glycolysis and occurs in the mitochondrial matrix under aerobic conditions.

In a population of wildflowers, the allele for red flowers (R) is incompletely dominant over white flowers (r). A cross between two pink-flowered plants produces 120 offspring. Approximately how many offspring would be expected to have red flowers?

Answer: 30

Pink-flowered plants have the genotype Rr (heterozygous). A cross between two Rr plants yields the ratio 1 RR : 2 Rr : 1 rr, meaning 25% red (RR), 50% pink (Rr), and 25% white (rr). With 120 offspring, 25% of 120 = 30 offspring would be expected to have red flowers.

Which of the following best explains why the meristematic cells in plant root tips are especially vulnerable to damage from ionizing radiation?

Answer: They are actively dividing, making DNA replication errors more likely to become permanent mutations

Meristematic cells divide rapidly and continuously, meaning their DNA is frequently being replicated and separated. Ionizing radiation can cause double-strand breaks and other DNA damage; when a cell is actively replicating, there is less time for repair mechanisms to correct lesions before they are copied and fixed as permanent mutations. Non-dividing cells have more opportunity to repair damage before it is passed on.

A marine biologist observes that a species of deep-sea fish produces its own light through a chemical reaction involving luciferin and luciferase. This phenomenon is best classified as an example of which biological concept?

Answer: Bioluminescence

Bioluminescence is the production and emission of light by living organisms through biochemical reactions, typically involving the substrate luciferin and the enzyme luciferase. Phosphorescence refers to the physical property of materials that absorb and re-emit light—it is not a biological process. Chemosynthesis involves using chemical energy to produce organic compounds (not light), and photosynthesis uses light energy to produce glucose.

During an experiment, cells are treated with a drug that prevents the formation of the mitotic spindle. At which checkpoint in the cell cycle would these cells most likely arrest?

Answer: Spindle assembly checkpoint (M checkpoint)

The spindle assembly checkpoint (also called the M checkpoint) monitors whether all chromosomes are properly attached to spindle fibers before anaphase begins. If the mitotic spindle cannot form, chromosomes cannot attach correctly, and the spindle assembly checkpoint will halt the cell in metaphase to prevent unequal chromosome segregation. The G1 and G2 checkpoints assess DNA integrity and cell size, while the S phase checkpoint monitors DNA replication—none of these would detect a spindle formation failure.

A carbon atom in a molecule of atmospheric CO₂ is absorbed by a plant during photosynthesis. Following the most direct pathway, which molecule would this carbon atom be incorporated into first within the Calvin cycle?

Answer: 3-phosphoglycerate (3-PGA)

In the Calvin cycle, CO₂ is first fixed by the enzyme RuBisCO, which attaches CO₂ to ribulose-1,5-bisphosphate (RuBP), a 5-carbon molecule. This reaction produces an unstable 6-carbon intermediate that immediately splits into two molecules of 3-phosphoglycerate (3-PGA), a 3-carbon compound. Therefore, 3-PGA is the first stable product containing the newly fixed carbon. G3P is produced later when 3-PGA is reduced using ATP and NADPH, and glucose is assembled from G3P molecules downstream.