The heart is a muscular organ that pumps blood. blood that contains waste materials is brought to the heart by blood vessels. this blood enters the and then moves down. the presence of the prevents the backflow of blood inside the heart

Biology students used raw eggs in an experiment on tonicity and osmosis. the students put their eggs into vinegar to remove the shells. the vinegar was used to remove the shell from the raw egg. what other effect(s) did the vinegar have on the egg?

As energy flows through an ecosystem, each trophic level.

Which of the following is true? species on earth today are but a fraction of all species that ever lived. the number of species existing at one time has decreased throughout history. extinctions of past species has happened gradually and on a small scale. most organisms present early in earth's prehistory were more complex than modern organisms. "bacteria represent a newer form of life, not present during the early prehistory of earth."