AP Biology · Lesson 1

Water and the elements of life

Know Know it

A water molecule is polar. Oxygen pulls the shared electrons closer to itself, so the oxygen end is partly negative and the two hydrogen ends are partly positive.

Because of that uneven charge, the partly positive hydrogen of one water molecule is attracted to the partly negative oxygen of a nearby water molecule. This attraction is a hydrogen bond. Each hydrogen bond is weak, but liquid water has a huge number of them, constantly breaking and re-forming.

Hydrogen bonds explain water's key properties. Cohesion (water sticking to water) gives surface tension and helps pull water up plant xylem. Adhesion (water sticking to other polar surfaces) helps water climb thin tubes. A high specific heat keeps the temperature of oceans and organisms steady. Water is also a strong solvent for polar and charged substances.

Living things are built mainly from carbon, hydrogen, oxygen, nitrogen, phosphorus and sulfur. Carbon makes four covalent bonds, so it forms the chains and rings of every organic molecule. Nitrogen is in amino acids and nucleotide bases. Phosphorus is in nucleic acids, phospholipids and ATP. Organisms take these elements from their surroundings to build new molecules.

Trap Between molecules, not inside one

The bonds inside one water molecule, between oxygen and each hydrogen, are polar covalent bonds. Hydrogen bonds form between neighboring molecules. Boiling water breaks hydrogen bonds; it does not split water into hydrogen and oxygen.

Worked example Easy

Which feature of a water molecule allows it to form hydrogen bonds with other water molecules?

  1. Its total charge is negative
  2. It contains a carbon atom
  3. Its electrons are shared unevenlyAnswer
  4. It is held by ionic bonds

How to solve it

  1. Hydrogen bonds come from attraction between partial charges.
  2. Partial charges exist because oxygen pulls shared electrons toward itself.
  3. So the uneven sharing of electrons is the cause.

Why each choice is right or wrong

  • A. Incorrect. A water molecule is neutral overall, so a negative total charge is not the reason.
  • B. Incorrect. Water is made of oxygen and hydrogen only and has no carbon atom at all.
  • C. Correct. Oxygen pulls electrons closer, leaving partial charges that attract neighboring water molecules.
  • D. Incorrect. The bonds within water are polar covalent bonds, not ionic bonds between ions.

Explain Explain it

Task verbs: explain, predict

To explain a property of water, use a three-link chain: polarity, then hydrogen bonds between molecules, then the property. For example: water is polar, so its molecules form hydrogen bonds with each other, so heat must break many of those bonds before the temperature rises much.

Worked example Medium

A researcher grows algae in a medium that contains every nutrient they need except phosphorus. Which molecule will the algae be least able to build?

  1. Glucose
  2. DNAAnswer
  3. Starch
  4. Cellulose

How to solve it

  1. List the elements each molecule needs.
  2. Glucose, starch and cellulose are all carbohydrates made of C, H and O.
  3. DNA has a phosphate in every nucleotide, so it needs phosphorus.

Why each choice is right or wrong

  • A. Incorrect. Glucose is made only of carbon, hydrogen and oxygen, so it needs no phosphorus.
  • B. Correct. Each DNA nucleotide has a phosphate group, so DNA cannot be made without phosphorus.
  • C. Incorrect. Starch is a chain of glucose units with no phosphate, so it needs no phosphorus.
  • D. Incorrect. Cellulose is also a glucose polymer made of carbon, hydrogen and oxygen only.

Explain Your turn

In winter, a layer of ice forms on top of a pond while liquid water stays below it. Explain, using hydrogen bonds, why ice floats on liquid water.

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A full answer does this

  • Says that in ice, hydrogen bonds hold water molecules in an open, spaced-out arrangement
  • Says this makes ice less dense than liquid water, so it floats

Model answer

In ice, each water molecule forms stable hydrogen bonds with its neighbors, and these bonds hold the molecules in an open, spaced-out lattice. In liquid water the hydrogen bonds keep breaking and re-forming, so the molecules can pack closer together. The same mass takes up more space as ice, so ice is less dense than liquid water, and it floats.

Apply Apply it

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