Science Flashcards

A scientist observes that a population of bacteria doubles every 20 minutes under ideal conditions. If she starts with 500 bacteria, approximately how many will there be after 2 hours?

Answer: 32,000

In 2 hours (120 minutes), the bacteria double 120/20 = 6 times. Starting with 500: 500 × 2⁶ = 500 × 64 = 32,000. Each doubling multiplies the population by 2, so after 6 doublings the result is 32,000.

During cellular respiration, glucose is broken down to release energy. Which of the following correctly describes what happens to the carbon atoms in glucose (C₆H₁₂O₆) by the end of aerobic respiration?

Answer: They are released as CO₂ through the breakdown of pyruvate in the mitochondria

In aerobic respiration, glucose is first split into pyruvate during glycolysis, then pyruvate enters the mitochondria where it is fully oxidized. The 6 carbon atoms are released as 6 molecules of CO₂. Carbon does not end up in ATP (which contains phosphate groups), in water (hydrogen and oxygen bond to form water), or in glycogen (respiration, not storage, is occurring).

A neutral atom has 17 protons. It gains one electron to become a stable ion. Which of the following best describes the resulting ion?

Answer: It has 18 electrons and a charge of −1

A neutral atom has equal numbers of protons and electrons, so it starts with 17 electrons. Gaining one electron brings the total to 18 electrons. Since protons (positive, 17) are now outnumbered by electrons (negative, 18), the net charge is −1. This forms the chloride ion (Cl⁻), which has a stable noble-gas electron configuration.

A researcher plants two groups of identical tomato seedlings. Group A is grown under full-spectrum light, and Group B is grown under only green light. After four weeks, Group B plants are stunted and pale. What is the MOST likely explanation?

Answer: Chlorophyll reflects green light rather than absorbing it, so Group B plants cannot efficiently perform photosynthesis

Chlorophyll primarily absorbs red and blue wavelengths of light for photosynthesis. Green light is largely reflected by chlorophyll (which is why plants appear green), meaning plants under only green light receive very little usable energy for photosynthesis. This leads to insufficient glucose production, explaining the stunted growth and pale color. Green light frequency is not damaging, and the other options mischaracterize how pigments function.

Two tectonic plates composed of oceanic crust collide. Based on the properties of oceanic crust, what is the MOST likely geological outcome?

Answer: One plate subducts beneath the other, forming a deep ocean trench and potentially a volcanic island arc

When two oceanic plates collide, the denser (usually older and cooler) plate subducts beneath the other. As it sinks into the mantle, it forms a deep ocean trench. The subducted material melts and the magma rises, creating a chain of volcanic islands called an island arc (e.g., the Aleutian Islands). Oceanic-oceanic collisions do not form continental mountain ranges or mid-ocean ridges, and transform faults involve plates sliding past each other rather than colliding.

A scientist measures the half-life of a radioactive isotope and finds it to be 5,730 years (the half-life of Carbon-14). A fossil sample contains 12.5% of the original Carbon-14 amount. Approximately how old is the fossil?

Answer: 17,190 years

Each half-life reduces the remaining C-14 by half. Starting at 100%: after 1 half-life → 50%; after 2 half-lives → 25%; after 3 half-lives → 12.5%. So the sample has undergone 3 half-lives. 3 × 5,730 = 17,190 years. The fossil is approximately 17,190 years old.