Question 48

A concentration gradient must be present for substances to move across a cell membrane.

For substances to move across a cell membrane, there must be a concentration gradient, which is the difference in concentration of a substance across the membrane. This gradient drives the passive movement of molecules from an area of higher concentration to an area of lower concentration.

A) A concentration gradient

This option is correct because a concentration gradient is essential for the passive transport of substances across the cell membrane. Molecules naturally move from areas of high concentration to low concentration, facilitating the process of diffusion.

B) Negatively charged molecules

This option is incorrect as the presence of negatively charged molecules alone does not ensure movement across the cell membrane. While charge can influence membrane permeability, it is not a requirement for all substances, and movement is primarily driven by concentration gradients.

C) Positively charged molecules

This option is also incorrect. Like negatively charged molecules, the presence of positively charged molecules does not guarantee movement across the membrane. The critical factor for movement remains the concentration gradient rather than the charge of the molecules.

D) A protein channel

While a protein channel can facilitate the transport of certain substances, it is not a prerequisite for all forms of movement across the cell membrane. Some substances can diffuse freely through the lipid bilayer if a concentration gradient exists, making this option insufficient as a standalone requirement.

Conclusion

The requirement for a concentration gradient is fundamental to the movement of substances across cell membranes, making option A the definitive correct choice. Other options, while relevant to specific contexts, do not universally apply to all transport mechanisms, emphasizing the unique role of concentration gradients in cellular transport processes.