Explain: On what basis are substances transported by the plasma membrane/cell membrane?

(A) $\Rightarrow$ One of the most important functions of the plasma membrane is the transport of molecules across it.
$\Rightarrow$ The membrane is selectively permeable to some molecules present on either side of it.
$\Rightarrow$ Many molecules can move briefly across the membrane without any requirement of energy,and this is called passive transport.
$\Rightarrow$ Neutral solutes may move across the membrane by the process of simple diffusion along the concentration gradient,i.e.,from higher concentration to lower. Water may also move across this membrane from higher to lower concentration. The movement of water by diffusion is called osmosis.
$\Rightarrow$ As polar molecules cannot pass through the nonpolar lipid bilayer,they require a carrier protein of the membrane to facilitate their transport across the membrane.
$\Rightarrow$ $A$ few ions or molecules are transported across the membrane against their concentration gradient,i.e.,from lower to higher concentration. Such transport is an energy-dependent process in which $ATP$ is utilized,and it is called active transport,e.g.,the $Na^{+}/K^{+}$ pump.