Water and Biological Molecules

Life is built from a surprisingly small set of chemical building blocks. This lesson examines water, the medium of life, and then the four major classes of biological macromolecules that make up living things.

The remarkable properties of water

Water is a polar molecule: oxygen pulls electrons more strongly than hydrogen, giving oxygen a slight negative charge and the hydrogens a slight positive charge. This polarity allows water molecules to form hydrogen bonds with one another and with other polar substances, and these weak bonds explain water's life-supporting properties.

• Cohesion: water molecules stick to one another, allowing columns of water to be pulled up through plant xylem.
• Adhesion: water sticks to other surfaces, aiding capillary movement.
• High specific heat: water resists tem