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What Causes Osmosis?

A Fundamental Descri ption of the Cause of Water Flow in the Process of Osmosis.

Date : 20/01/2019


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Uploaded by : Craig
Uploaded on : 20/01/2019
Subject : Biology

Osmosis is described in textbooks as a special case of diffusion, where water molecules move from a high to a lower water potential, across a semi-permeable membrane i.e. they move from where there are lots to where there are fewer molecules. This can also be described as a movement of water from a low concentration solution to a higher concentration solution.

Water molecules can cross the semi-permeable membrane, but sugar (sucrose) and salt molecules cannot (they`re too large to pass through the pores of the membrane).

Water flows across a semi-permeable membrane. There must be a pressure difference between the two compartments that causes water to flow.

What causes this pressure difference? This is a fundamental explanation of why osmosis occurs.

For any fluid to flow, there must be a pressure difference between two regions. A fluid flows from the high to the lower pressure compartment.

Sucrose molecules are larger than water molecules and there are more sucrose molecules in the higher concentration solution. We know that water flows from the lower to the higher concentration solution.

One very useful way of thinking about osmosis is to always appreciate that the whole system is dynamic, i.e. it`s in constant motion, with molecules moving rapidly and in all directions. Water molecules will collide with each other, with the semi-permeable membrane and with sucrose molecules. (Textbooks provide a static picture of osmosis!)

When water molecules collide with sucrose molecules in the higher concentration compartment, their velocities will be reduced (because of a change in momentum during the collision). This will reduce the pressure in the higher concentration compartment (because pressure is force per unit area). Molecules with lower velocities will exert less force per unit area during collisions with each other and with the cell membrane.

Overall, pressure is decreased in the higher concentration compartment, which leads to a pressure difference across the two compartments. This causes water to flow from a higher to a lower water potential (water concentration) during the process of osmosis.

This way of thinking provides a fundamental molecular and kinetic explanation of osmosis, in terms of physics. It`s a very useful & (and different) way of understanding this core biological process.

This resource was uploaded by: Craig

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