Final answer:
Polar molecules, ions, and glucose are osmotically active substances because they affect water movement across membranes and cannot cross the lipid bilayer without assistance. Lipids are typically not osmotically active but are amphipathic, contributing to cell membrane structure.
Step-by-step explanation:
Osmotically active substances are those that can affect the movement of water across a semipermeable membrane, such as the cell membrane. In the context of osmosis, polar molecules, ions, and glucose are osmotically active because they cannot easily cross the lipid bilayer without assistance and therefore exert an osmotic pull-on water molecule.
Lipids are generally not osmotically active because they are non-polar and can pass through the cell membrane with ease. Ions and polar molecules like glucose require special transport mechanisms to move across the membrane due to their hydrophilic nature. This question is related to cell biology, specifically the properties of cell membranes and the transport of substances across these membranes.
Molecules that bear both polar and nonpolar groups, such as phospholipids, are said to be amphipathic. Amphipathic molecules play a crucial role in forming cell membranes where the hydrophilic (water-loving) 'heads' face outward towards the aqueous environment, while the hydrophobic (water-fearing) 'tails' face inward, away from water, forming a bilayer that is critical to the structure and function of the membrane.