Water properties & transport

Transport across the cell membrane, water potential, osmosis and properties of water.

Water structure

V-shaped. The oxygen atom is slightly negative compared with the hydrogen atoms. Therefore the molecule has a dipole – oxygen end is relatively negative, hydrogen end is positive.  

Hydrogen bonds in water molecules

form between slightly negative oxygen in one water molecule and slightly positive hydrogen in another. When water hydrogen bonds to itself = cohesion but when water hydrogen bonds to other stuff = adhesion.  

Polarity of water

allows it to dissolve compounds like sodium chloride because water molecules are dipoles so they attract charged particles  e.g. ions and other polar molecules e.g. glucose.  

High specific heat capacity of water

means a large amount of heat energy is needed to raise its temperature. This is due to the hydrogren bonds between water molecules restricting their movement, resisting an increase in kinetic energy and therefore, resisting a temperature increase. Prevents large fluctuations in water temperature, important in keeping aquatic habitats stable, so that organisms don’t have to adapt to extremes of temperature. Also allows enzymes within cells to work efficiently. 

High latent heat of vaporisation of water

means a lot of heat energy is needed to change it from a liquid to a vapour. This is important, for example, in temperature control, where heat is used to vaporise water from sweat on the skin from a leaf’s surface. As the water evaporates, the body cools.  

High surface tension of water

at ordinary temperatures water has the highest surface tension of any liquid except mercury. In a pond, cohesion between water molecules at the surface produces surface tension so that the body of an insect, such as the pond skater, is supported. 

Osmosis

The passive movement of water from an area of less negative water potential to an area of more negative water potential across a partially permeable membrane.

Water potential

A measure of how freely water molecules can move

When placed in pure water, an animal cell has a…

more negative water potential. Water moves in by osmosis and the cell bursts.  

When placed in salt solution, an animal cell has a…

more negative water potential. Water moves in by osmosis and the cell bursts.  

When placed in pure water, a plant cell has a…

less negative water potential. Water moves out by osmosis and the cell shrivels.  

When placed in salt solution, a plant cell has a…