Final answer:
To transport molecules that cannot pass freely through the plasma membrane, cells use transport proteins and expend ATP for active transport processes. This selective permeability is critical for cellular homeostasis and function.
Step-by-step explanation:
The plasma membrane is essential in maintaining a cell's homeostasis by being selectively permeable. This means that while the plasma membrane allows certain small molecules like oxygen and carbon dioxide to pass freely, it restricts others, especially large or charged molecules. To facilitate the transport of these vital but non-permeable molecules, the membrane employs transport proteins and sometimes uses energy in the form of ATP to move substances against their concentration gradients.
Polar substances and ions such as sodium, potassium, calcium, and chloride need special mechanisms to cross the membrane due to their inability to pass through the lipid bilayer. Large polar molecules, like simple sugars and amino acids, also require assistance for transport, which is provided by various transmembrane proteins that form channels or act as carriers. This process is crucial since it allows cells to acquire necessary nutrients and maintain crucial ion balances, hence ensuring cell survival and proper function.