Final answer:
Water is the smallest molecule compared to glucose, starch, and proteins, allowing it to easily diffuse through a semipermeable membrane.
Step-by-step explanation:
The molecule that is smaller and therefore more likely to diffuse through a semipermeable membrane is water. Semipermeable membranes are designed to allow only small molecules and water to pass through while blocking larger molecules like starch and proteins. Glucose is small enough to pass through some semipermeable membranes, but when compared to water, glucose molecules are significantly larger and may require transport proteins to facilitate their movement across the cell membrane. In contrast, water molecules are very small and can pass more readily through the pores of semipermeable membranes. Starch and proteins are much larger than glucose and water, making them unable to pass through the membrane without specific mechanisms.