Diffusion Limitations in Complex Organisms

Simple diffusion is a passive process where molecules move from an area of high concentration to low concentration. While effective for single-celled organisms or very small multicellular ones, it becomes insufficient as organisms increase in size and complexity.

• Increased Distance: In larger, multicellular organisms, the distance between the external environment (where gases/nutrients enter) and the internal cells becomes substantial. Simple diffusion is too slow to cover these distances efficiently.
• Slow Rate: Diffusion rate decreases significantly with distance. It cannot adequately supply all cells with necessary substances (like oxygen and nutrients) or remove waste products (like carbon dioxide) in a timely manner for a large organism.
• Body Complexity: Increased cellular organization and specialized tissues create internal environments far removed from the surface, making diffusion impractical as the sole transport mechanism.
• Inefficiency of Other Options: Options A, B, and C are incorrect. Cells within a multicellular organism are generally not directly exposed to air; diffusion handles various substances, not just oxygen; and diffusion does not require light.

Therefore, multicellular organisms require specialized transport systems, such as circulatory systems, to overcome the limitations of simple diffusion.