HELP! ITS TIMED! When would a cell need to use active transport instead of passive transport? A. To transport ions with a high internal cell concentration out of the cell using channels
B. To move water to the external environment
C. To take in nutrients that have a lower concentration outside the cell
D. To diffuse oxygen across the cell membrane

The gene that causes sickle-cell disease is present in a higher percentage of residents of sub-saharan africa than among those of african descent living in the united states. even though this gene causes sickle-cell disease, it also provides some protection from malaria, a serious disease that is widespread in sub-saharan africa but absent in the united states. discuss an evolutionary process that could account for the different percentages of the sickle-cell gene among residents of the two regions.

In which situations is the principle of cross-cutting relationship useful in determining relative age?

What is a photon? a. part of a ribosome b. a light particle c. a carbon dioxide molecule d. part of a chloroplast b. a light particle