Describe how natural variation is used in artificial selection

What are the doorways into and out of cells, attached to the membrane, and built in the rough er.

How can antibodies "remember" a particular bacterial invader or "antigen" on that invader?

The carbon cycle is best defined as a process in which a. carbon changes from inorganic forms to organic forms and back b. carbon is changed into other elements such as oxygen or nitrogen c. carbon is continually created from the sun’s energy by plants d. carbon is consumed and regenerated from other elements such as oxygen and nitrogen