The transportation of molecules or substances in a mammalian cell depends upon the permeability of molecules/ substance across the phospholipid bilayer membrane. The plasma membrane is very permeable; therefore, it lets several materials to arrive and exit the cell while preventing the movement of others that might be harmful altogether. This selectivity is central to the cell’s survival as a cell that is unable to sustain itself may be destroyed. In a mammalian cell, active and passive transportation occurs. Certain cells require specific substances in larger amounts as compared to others. These substances can be obtained through the extra-cellular fluid surrounding the cell passively or a special mechanism might be employed by the cell to ensure transportation. Most mammalian cells utilize energy which is in the form of adenosine triphosphate (ATP), to ensure and create an uneven presence of ions on the two sides of their membranes.
In mammalian cells, direct transports are passive and it is a naturally occurring phenomenon for which the cell does not utilize energy. Guided by diffusion, the molecules of substances more across the concentration gradient that is from a area of higher concentration to a area of lower concentration. On the other hand, active transportation in mammalian cells requires energy in the form of ATP. This energy helps the cell to move the substance against the gradient i.e., from a lower to higher concentration region. Through an active transportation system, ions that have small molecular weight are transported across the membrane, however, some active transport mechanisms may transfer larger molecules as well. Primary active transport is dependent upon ATP in creating a difference in charge by moving the ions across the membrane. Whereas secondary active transport is not dependent upon ATP as the movement of substance is through the electrochemical gradient maintained by primary active transport (Clark, Choi, & Douglas, 2012).
Clark, M. A., Choi, J., & Douglas, M. (2012). Biology 2E. OpenStax.