Water: a polar solvent for transport

Why water is polar

In a water molecule the oxygen atom attracts the shared electrons more strongly than the hydrogen atoms do. So the oxygen end carries a slight negative charge (δ-) and the hydrogen ends carry slight positive charges (δ+). A molecule with uneven charge like this is polar.

The δ- oxygen of one molecule is attracted to a δ+ hydrogen of another, forming a hydrogen bond. Each bond is weak, but there are huge numbers of them.

Water as a solvent

Ions and other polar molecules dissolve in water. Water molecules cluster round each particle: the δ- oxygen ends face positive ions (such as Na+) and the δ+ hydrogen ends face negative ions (such as Cl-). The particle is surrounded and separated, so it dissolves and can be carried along.

That is why glucose, amino acids, ions, urea and hormones such as insulin travel dissolved in plasma.

What does not dissolve

Non-polar substances such as triglycerides and cholesterol do not dissolve in water. In blood they are carried inside lipoproteins, which have a water-soluble outer layer of protein and phospholipid.

Hydrogen bonding also makes water molecules stick together (cohesion), which helps water move up xylem as a continuous column.

Worked example

Explain why sodium chloride dissolves in blood plasma but a triglyceride does not.

1. Water is polar: δ- oxygen and δ+ hydrogens.
2. Na+ is attracted to the δ- oxygen ends and Cl- to the δ+ hydrogen ends, so the ions are surrounded and separated.
3. A triglyceride is non-polar, so water molecules are not attracted to it.

Answer: Water's polar molecules surround and separate the charged ions, but cannot attract the non-polar triglyceride.

Check yourself

1. Why is a water molecule polar?
   • It contains ionic bonds between O and H
   • Oxygen pulls the shared electrons more strongly, so O is δ- and H is δ+
   • The hydrogen atoms carry full positive charges
   • It is a straight, symmetrical molecule
   Show answer

   Oxygen pulls the shared electrons more strongly, so O is δ- and H is δ+. Unequal sharing of electrons gives a slight negative charge on O and slight positive charges on H.

2. What type of bond forms between neighbouring water molecules?
   • Covalent bonds
   • Ionic bonds
   • Glycosidic bonds
   • Hydrogen bonds
   Show answer

   Hydrogen bonds. The δ- oxygen of one molecule is attracted to a δ+ hydrogen of another.

3. When a Na+ ion dissolves, which part of the water molecules faces it?
   • The δ+ hydrogen atoms
   • The δ- oxygen atoms
   • The covalent bonds
   • The lone hydrogen ions
   Show answer

   The δ- oxygen atoms. Opposite charges attract, so oxygen (δ-) faces the positive ion.

4. Which of these is carried dissolved in blood plasma?
   • Starch
   • Free triglyceride droplets
   • Glucose
   • Cellulose
   Show answer

   Glucose. Glucose is polar and soluble. Triglycerides are carried in lipoproteins; starch and cellulose are not in blood.

5. Why is cholesterol carried in the blood inside lipoproteins?
   • It is non-polar, so it does not dissolve in water
   • It is too large to fit in a red blood cell
   • It would be broken down by plasma enzymes
   • It is a charged ion
   Show answer

   It is non-polar, so it does not dissolve in water. Non-polar molecules are insoluble in water, so they travel inside lipoproteins.

6. Explain why water is a good transport medium in organisms.
   Show answer

   Water is polar, so ions and polar molecules such as glucose dissolve in it and can be carried in solution.. Polarity lets water dissolve ions and polar molecules, so they can be transported.

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Sources

• Pearson Edexcel International Advanced Level in Biology specification (XBI11/YBI11, first teaching 2018)

Last updated 7 November 2026. Spotted a mistake? Tell us.