By forming over 100 pinocytic vesicles per minute, a macrophage ingests over 10% of its own volume of fluid every half an hour. This is equivalent to ingesting about 100% of the macrophage's plasma membrane. If the average volume of a macrophage is about 450 µm³ and its average surface area is about 750 µm², what is the average diameter of the early pinocytic vesicles in the cell? Assume spherical vesicles. The formulas for the volume (V) and surface area (S) of a sphere of radius r are V = 4/3 × ? × r³, and S = 4 × ? × r², respectively. Write down your answer in nanometers, with no decimals, e.g. 830 nm.

Question

Quizplus · Accepted Answer

The cell internalizes 3000 vesicles every half an hour (i.e. 100 vesicles/min × 30 min), which is equivalent to 45 µm³ in volume (i.e. 10% × 450 µm³) and 750 µm² in surface area (i.e. 100% × 750 µm²). Thus, the volume and surface area of each vesicle is, on average, about 0.015 µm³ and 0.250 µm², respectively. The ratio of volume to surface area for a sphere is equal to r/3 where r is the radius. Hence, the diameter (2r) is equal to (0.015 µm³/0.250 µm²) × 6 = 0.36 µm = 360 nm.

By Forming Over 100 Pinocytic Vesicles Per Minute, a Macrophage