Leave a stick of butter and a bottle of olive oil on a warm kitchen counter. The butter keeps its shape; the oil pours. Both are mostly fats built from the same kinds of parts, so why do they behave so differently? The answer is a detail of molecular shape, and shape is the big idea of this unit: how a molecule is built decides what it can do.
Water: polarity and hydrogen bonds
In a water molecule, oxygen and hydrogen share electrons in polar covalent bonds. Oxygen pulls harder, so its end is partly negative and the hydrogen ends partly positive. Neighboring molecules attract each other through hydrogen bonds, and that gives water its key properties:
- Cohesion, adhesion and surface tension: water sticks to itself and to other polar surfaces, and its surface acts like a thin film.
- High specific heat: water warms and cools slowly, which helps organisms hold a steady body temperature.
- High heat of vaporization: evaporating water takes a lot of energy with it. That's why sweating cools you.
Hydrogen bonds aren't only between water molecules. They also form within big biological molecules, holding DNA's two strands together and helping proteins fold.
The elements of life
Carbon, hydrogen and oxygen are the most common elements in biological molecules. Carbon makes a good backbone because each carbon atom forms four covalent bonds, so it can build chains, branches and rings. Three more elements are worth tracking:
- Sulfur: in proteins.
- Phosphorus: in nucleic acids and phospholipids.
- Nitrogen: in nucleic acids, and in the amine group of every amino acid.
Building and breaking polymers
Cells join monomers by dehydration synthesis: an H comes off one monomer and an OH off the other, the equivalent of a water molecule is lost, and a covalent bond forms. Repeating this is polymerization. Hydrolysis runs the other way: water is added across the bond, its H goes to one monomer and its OH to the other, and the bond breaks.
Carbohydrates and lipids
Monosaccharides such as glucose are the monomers of polysaccharides, which can be linear or branched. Starch stores energy in plants, glycogen stores it in animals, and cellulose builds plant cell walls. You don't need to draw their molecular structures for the AP exam.
Lipids are mostly nonpolar and hydrophobic. A saturated fatty acid has only single bonds between carbons, so its tail is straight and packs tightly. An unsaturated one has at least one double bond, which kinks the tail. The more double bonds, the more liquid the fat is at room temperature. Olive oil is rich in unsaturated fats and butter in saturated ones: there's your kitchen counter. Other lipids include phospholipids (the bilayer of membranes), cholesterol (stability in animal cell membranes) and steroid hormones.
Nucleic acids: direction matters
Each nucleotide has a five-carbon sugar, a phosphate and a nitrogenous base. A strand has a 5′ end (phosphate) and a 3′ end (hydroxyl), and new nucleotides are added only at the 3′ end. DNA's two strands are antiparallel, running in opposite directions, with A pairing with T and C with G. This is the simple version; you'll use the 3′ end again in unit 6, when DNA is copied.
Proteins: four levels of shape
Amino acids join by peptide bonds between the carboxyl group of one and the amine group of the next. Each amino acid has a variable R group that is hydrophobic, hydrophilic or ionic.
- Primary: the sequence of amino acids.
- Secondary: alpha helices and beta-pleated sheets, from hydrogen bonds in the backbone.
- Tertiary: the 3D shape, from hydrogen bonds, hydrophobic and ionic interactions, and disulfide bridges between R groups.
- Quaternary: two or more polypeptide chains working together.
Change one amino acid and you may change the fold, and with it the function.
Common mistakes students make
- Mixing up specific heat and heat of vaporization. One explains slow temperature change; the other explains cooling by evaporation.
- Saying "unsaturated fats are solid". It's the reverse: double bonds kink the tails, so unsaturated fats tend to be liquid.
- Forgetting sulfur. In AP Biology, sulfur is the element that points to proteins.
- Adding nucleotides to the 5′ end. Strands grow at the 3′ end.
Frequently asked questions
What are the four biological macromolecules?
Carbohydrates, lipids, proteins and nucleic acids. Lipids aren't true polymers of a repeating monomer.
What is the difference between hydrolysis and dehydration synthesis?
Dehydration synthesis builds a polymer and releases water. Hydrolysis uses water to break a polymer apart.
Why is water polar?
Oxygen pulls the shared electrons more strongly than hydrogen, so the molecule has a partly negative end and partly positive ends.
What determines a protein's shape?
Its amino acid sequence. The R groups' interactions fold the chain into its final 3D shape.
Try it
Say each answer before you flip the card. Then predict: what would you expect to see if you put a spoonful of olive oil and a spoonful of soft butter in the fridge for a few hours?
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