What causes the atrioventricular valves to close during a heartbeat? a Pressure in the atria is higher than in the ventricles. b Pressure in the atria is lower than in the ventricles. c Pressure in the arteries is higher than in the ventricles. d Pressure in the arteries is lower than in the ventricles.

Food provides molecules that serve as fuel and building material for all organisms. plants undergo photosynthesis and make their own food. other organisms, like us and the horse seen above, are consumers and eat food. how do all organisms use food as fuel? be sure to include the name of the process in your answer. me.

Explain why phospholipid molecules form a bilayer. (3 marks)

To answer this question, researchers studied populations of the dusky salamander (desmognathus ochrophaeus) living on different mountain ranges in the southern appalachian mountains. the researchers tested the reproductive isolation of pairs of salamander populations by leaving one male and one female together and later checking the females for the presence of sperm. four mating combinations were tested for each pair of populations (a and b)—two within the same population (female a with male a and female b with male b) and two between populations (female a with male b and female b with male a). the proportion of successful matings for each mating combination was measured. for example, when all the matings of a particular combination were successful, the researchers gave it a value of 1; when none of the matings were successful, they gave it a value of 0. then the researchers calculated an index of reproductive isolation that ranged from 0 (no isolation) to 2 (full isolation). the reproductive isolation value for two populations is the sum of the proportion of successful matings of each type within populations (aa + bb) minus the sum of the proportion of successful matings of each type between populations (ab + ba). the table provides data for the geographic distances and reproductive isolation values for 27 pairs of dusky salamander populations.