Which of the following identifies what happens by day 21 of cattle gestation? the heart begins to beat
the teeth begin to form
the placenta is formed
the calf is covered with fur

organ system-an organ system is a group of organs that work together to perform one or more functions. each does a particular job in the body, and is made up of certain tissues.

many other transport proteins, often called pumps, move materials against a concentration gradient. active transport drives molecules across a membrane from a region of lower concentration to a region of higher concentration. this process, shown in figure 5.1, uses transport proteins powered by chemi- cal energy.

its probably to later but whats the question