Question

Regarding the cellular membranes which one of the following is incorrect?

• A. Membranes are selectively permeable but a solute's electrical polarity and other physical characteristics may effect its ability to enter or leave.
• B. Water soluble substances pass through it more rapidly than lipids and lipid soluble substances.
• C. Generally smaller molecules pass through cellular membrane more than aggregate molecules.
• D. None of the above.

Answer 1

Answer: (B)

Water soluble substances pass through it more rapidly than lipids and lipid soluble substances.

Every living cell must acquire from its surroundings the raw materials for biosynthesis and for energy production, and must release to its environment the byproducts of metabolism. The plasma membrane contains proteins that specifically recognise and carry into the cell such necessities as sugars, amino acids and inorganic ions. In some cases, these components are brought into the cell against a concentration gradient, i.e., "pumped in". Certain other species are "pumped out". With few exceptions, the traffic of small molecules across the plasma membrane occurs by protein mediated processes via trans membrane channels, carriers and pumps. These channels and pumps handle considerably much more transport of small molecules as compared to large biomolecules by the cell itself. The energy of activation for translocation of a polar solute across the lipid bilayer is so large that pure lipid bilayers are virtually impermeable to polar and charged species over the periods of time span important to cells. This is because basically membranes are made up of lipids which are poor solvents for polar molecules and charged species.