Which is a function of the cell membrane?
a. performs photosynthesis
b. controls what enters and leaves the cell
c. protects the external structures of the cell
d. provides energy to the cell

B. Controls what enters and leaves the cell.

The cell membrane ( plasma membrane)  surrounds the cell and primarily made up of lipids and proteins. It controls what enters and leaves the cell that is why it is also called as a selectively permeable membrane. It does this by not allowing the useful substances to go out and the harmful substances to enter the cell. In this way, it provides protection to the cell.

Some of the essential cellular processes related to cell membrane are- Cell signaling, endocytosis, exocytosis, cell adhesion etc.

the answer that best describes the relative refractory period in the neuron is the resting membrane potential is restored, but concentrations of sodium and potassium are not restored to their original state.

the refractory period is the time during which another stimulus given to the neuron will not lead to a second potential. thus, because na+ channels are inactivated during this time, additional depolarizing stimuli do not lead to new action potentials.  

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