CSCredit Sprint
Sign upStart free

Credit Sprint / Study guides / Biology Unit 1

Credit Sprint's Guide to Biology Unit 1: Chemistry of Life

AP® weight 8–11%CLEP® weight 8–12%

Life is built from a few elements arranged into four kinds of large molecules. Water's polarity and hydrogen bonding make it the solvent of life, and the structure of carbohydrates, lipids, proteins and nucleic acids explains what each one can do.

Dehydration synthesis and hydrolysisMonomer Monomer –OH H– dehydration → Dimer (covalent bond) H₂Owater RELEASED Polymers are built by removing water and broken by adding it

What you need to know

  • Water is polar. Hydrogen bonds between water molecules give it cohesion, adhesion, high specific heat and surface tension, and make ice less dense than liquid water.
  • Carbon forms four covalent bonds, so it can build long chains, rings and branches: the backbone of every macromolecule.
  • Monomers join by dehydration synthesis (a water molecule is removed) and split by hydrolysis (a water molecule is added).
  • Carbohydrates (monosaccharides → polysaccharides) store energy (starch, glycogen) and build structure (cellulose, chitin).
  • Lipids are hydrophobic. Phospholipids have a hydrophilic phosphate head and two hydrophobic tails, so they form bilayers in water.
  • Proteins are chains of amino acids. Each amino acid has an R group; R-group chemistry (polar, nonpolar, charged) drives folding.
  • Protein structure: primary (sequence) → secondary (α-helix, β-sheet, H-bonds in the backbone) → tertiary (R-group interactions) → quaternary (more than one chain).
  • Nucleic acids are chains of nucleotides (sugar, phosphate, nitrogenous base). DNA uses deoxyribose and thymine; RNA uses ribose and uracil.
  • DNA strands are antiparallel (5′→3′ and 3′→5′); A pairs with T (2 H-bonds) and G pairs with C (3 H-bonds).
  • Nitrogen is found in proteins and nucleic acids; phosphorus is found in nucleic acids and phospholipids.

Equations and rules

Dehydration synthesis: monomer–OH + H–monomer → monomer–monomer + H₂O
Hydrolysis: polymer + H₂O → smaller pieces
Chargaff's rule: %A = %T and %G = %C in double-stranded DNA

Exam traps

Watch forHydrogen bonds hold water molecules to each other; covalent bonds hold the atoms within one water molecule.
Watch forDenaturing a protein changes its shape (secondary to quaternary), not its primary sequence.
Watch forSaturated fats have no C=C double bonds and are solid at room temperature; unsaturated fats have kinks from double bonds.

Key terms

  • Polarity: Uneven sharing of electrons. In water, oxygen is partly negative and hydrogens are partly positive.
  • Hydrogen bond: A weak attraction between a partly positive hydrogen and a partly negative atom (O or N) on another molecule.
  • Cohesion: Water molecules sticking to each other. Gives water surface tension and helps pull water up through xylem.
  • Adhesion: Water sticking to other surfaces, such as the walls of xylem vessels.
  • Specific heat: The energy needed to change temperature. Water's is high, so it buffers temperature changes.
  • Dehydration synthesis: Joining two monomers by removing a water molecule.
  • Hydrolysis: Breaking a polymer by adding a water molecule.
  • Monosaccharide: A simple sugar such as glucose. The monomer of carbohydrates.
  • Phospholipid: A lipid with a hydrophilic head and two hydrophobic tails. Forms cell membranes.
  • Amino acid: The monomer of proteins: an amino group, a carboxyl group and a variable R group.
  • Primary structure: The sequence of amino acids in a protein.
  • Tertiary structure: A protein's 3-D shape from interactions between R groups.
  • Nucleotide: The monomer of nucleic acids: a five-carbon sugar, a phosphate and a nitrogenous base.
  • Antiparallel: DNA's two strands run in opposite directions, 5′→3′ and 3′→5′.

Practice questions

Five of the 24 Unit 1 questions in Credit Sprint. Every choice is explained in the app.

1. Which property of water best explains why a lake freezes from the top down instead of the bottom up?
  1. Water has a high specific heat
  2. Ice is less dense than liquid water because hydrogen bonds hold it in an open lattice
  3. Water is a universal solvent
  4. Covalent bonds in ice are weaker than in liquid water
Show answer

Ice is less dense than liquid water because hydrogen bonds hold it in an open lattice. The open lattice makes ice float, insulating the water below.

2. Digestive enzymes break starch into glucose by
  1. dehydration synthesis
  2. denaturation
  3. hydrolysis
  4. oxidation of glucose
Show answer

hydrolysis. Water is added to break each glycosidic bond.

3. Radioactive phosphorus fed to bacteria would end up mainly in their
  1. starch reserves
  2. proteins
  3. DNA and phospholipids
  4. cellulose walls
Show answer

DNA and phospholipids. Both contain phosphate groups.

4. Which feature distinguishes RNA from DNA?
  1. RNA is made of amino acids
  2. RNA contains ribose and uracil
  3. RNA contains phosphate groups
  4. RNA is always double-stranded
Show answer

RNA contains ribose and uracil. DNA has deoxyribose and thymine.

5. Sweat cools the body because
  1. water has low specific heat
  2. breaking hydrogen bonds as water evaporates absorbs heat from the skin
  3. sweat contains salt
  4. water molecules are nonpolar
Show answer

breaking hydrogen bonds as water evaporates absorbs heat from the skin. Evaporative cooling removes the most energetic molecules.