Final answer:
The plasma membrane of a cell is selectively permeable and only allows specific substances to pass through, necessitating specialized transport mechanisms for water-soluble substances to maintain cellular processes.
Step-by-step explanation:
In living cells, the plasma membrane does not allow the free passage of water-soluble substances due to its selectively permeable nature. This property is crucial as it allows a cell to maintain its internal environment by regulating the entry and exit of substances. The plasma membrane consists of a phospholipid bilayer, where the hydrophobic tails face inward, creating a barrier to water-soluble substances.
Certain molecules and ions may require specific transport mechanisms such as protein channels and carriers to cross the membrane. This is an essential function since substances, like glucose and electrolytes, must be regulated to sustain cellular processes. Transport across cell membranes can either be passive, not requiring energy, or active, requiring the cell to expend energy in the form of ATP.