Osmosis

Definition

The net passive movement of solvent molecules (typically water) across a selectively permeable membrane from a region of higher water potential to lower water potential, driven by differences in solute concentration and tending toward equalization of solute concentrations on either side. In arthropod physiology, osmosis governs water uptake, loss, and internal homeostasis across membranes of the gut, Malpighian tubules, rectum, and other epithelial tissues.

Etymology

From Greek ōsmos 'push, thrust', from ōthein 'to push'

Example

Desert beetles such as Onymacris rely on rectal osmosis to reclaim water from fecal pellets, extracting moisture against steep concentration gradients to survive arid conditions; similarly, the cryptobiontic midge Polypedilum vanderplanki tolerates extreme desiccation by accumulating trehalose, which upon rehydration drives rapid osmotic water influx across cell membranes.

Related Terms

• osmotic pressure
• Osmoregulation
• water potential
• selectively permeable membrane
• Malpighian tubules
• rectal pads
• Cryptobiosis
• desiccation tolerance

Usage Notes

Distinguished from diffusion, which describes solute movement; osmosis specifically describes solvent movement. Osmotic pressure—the external pressure required to prevent net solvent flow—is a colligative property dependent on total solute concentration, not solute identity. In entomological contexts, osmosis is often discussed alongside active transport mechanisms, as many arthropod epithelia couple osmotic gradients with ion pumps to achieve directional water movement. The term is sometimes misapplied to any water movement across membranes; specialists reserve it for passive, gradient-driven flow.