Why don't na+ ions move across the plasma membrane of neurons and down their concentration gradient all the time? a. the plasma membrane is not permeable to na+ ions. b. the na+/k+ pump is required and it only operates when atp is plentiful c. a na+ channel is required and it is usually closed. a & c. a, b, & c.

Which of the following occurs most often in a population when genetic variation and overproduction are present? some organisms have a lower reproductive success than others. some organisms have a higher reproductive success than others. some organisms have no reproductive success. some organisms do not have genetic variation.

What is the answer for the complicated molecules that make up living things usually contain carbon. why is carbon so important in these molecules?

Why is it important that fibrin is insoluble in water?