Bell pepper - useful for osmosis experiments

What is osmosis?

Throughout the rest of this week we will be asking you to evaluate different teaching strategies for helping students get to grips with osmosis, starting with the use of animations in biology, then practical work, and finally modelling. But first, we’ll begin with a quick overview of what osmosis is.

Here’s the definition of osmosis that you will see in most text books:

In biology, osmosis is the movement of water molecules from a solution with a high concentration of water molecules to a solution with a lower concentration of water molecules, through a cell’s partially permeable membrane.

A partially permeable membrane (sometimes called a selectively permeable membrane) only allows certain molecules or ions to cross it

Black rectangle with white centre split vertically in two by a thick dotted black line representing a partially permeable membrane. Blue circles on the left of the line representing a high concentration of water molecules in a dilute solution. On the right of the line area represents a concentrated solution containing a low concentration of water molecules and there are a few water molecules and some medium sizes red circles representing sucrose molecules. Blue arrows some pointing left, some right are shown in the gaps between dots in black central line. A large blue arrow pointing to the right labelled Net movement of water molecules is shown at the bottom of the diagram pointing to the right below the rectangle.

In the diagram above, the higher concentration of water molecules to the left of the partially permeable membrane makes it likely that a large number of water molecules will collide with the membrane and pass through it.

The lower concentration of water molecules on the right hand side of the partially permeable membrane in the diagram makes it likely that fewer water molecules will collide with the membrane and pass through it.

This means that more water molecules move from left to right on this diagram, than move from right to left, and so the overall movement (net movement) is to the right. It is important, though, to stress to students that water molecules are moving in both directions.

You will often see this described as movement ‘down the concentration gradient’, meaning the water is moving from a higher concentration of water (in this case, the dilute sucrose solution), to a lower concentration of water (the concentrated sucrose solution).

If a plant cell is surrounded by a solution that contains a higher concentration of water molecules than the solution inside the cell, water will enter the cell by osmosis and the plant cell will become turgid (firm). The pressure that develops inside a plant cell when it becomes turgid is called turgor pressure. Turgid plant cells help a stem to stay upright.

If a plant cell is surrounded by a solution that contains a lower concentration of water molecules than the solution inside the plant cell, water will leave the cell by osmosis and the plant cell will become flaccid (soft). If the cells in a plant stem become flaccid the turgor pressure inside them will decrease and the stem will wilt.

If a plant cell is surrounded by a solution that contains the same concentration of water molecules as the solution inside the plant cell, there is no overall net flow of water. Movement of water molecules into and out of the cell, through the partially permeable membrane, balances out.

Transpiration keeps the water moving

In plants, water enters the root cells by osmosis, and moves into tubes called xylem vessels to be transported to the leaves. Water molecules inside the xylem cells are strongly attracted to each other because of hydrogen bonding (this is called cohesion). When water evaporates from the leaves (through tiny pores called stomata), more water is drawn up from the root xylem cells to replace that which has been lost. A continuous column of water is therefore pulled up the stem in the xylem vessels by evaporation from the leaves. This is called transpiration.

Find and share

This type of diagram and explanation is fairly common in biology text books, but the movement of molecules is quite abstract and this can be a difficult to concept for students to imagine.

Have a look online for a resource which helps to explain osmosis to students, and share a link below with an explanation of why it’s helpful.

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This article is from the free online course:

Teaching Biology: Inspiring Students with Plant Science

National STEM Learning Centre

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