Notes
Water
Water is polar: oxygen pulls electrons more than hydrogen, so O is slightly negative and H is slightly positive. Neighboring water molecules form hydrogen bonds, which explain:
- Cohesion (water sticks to water, giving surface tension) and adhesion (water sticks to other surfaces, helping capillary action in xylem).
- High specific heat, which keeps temperatures stable, and high heat of vaporization (evaporative cooling).
- Ice floats because hydrogen bonds hold it in a less dense lattice.
- Water is a great solvent for polar and ionic substances.
Elements and macromolecules
Living things are built mostly from C, H, O, N, P and S. Dehydration synthesis joins monomers and releases water; hydrolysis uses water to break polymers apart.
| Macromolecule | Monomer | Key elements | Main jobs |
|---|---|---|---|
| Carbohydrates | Monosaccharides (glucose) | C, H, O | Energy, structure (cellulose) |
| Lipids | Fatty acids + glycerol (not true polymers) | C, H, O (+P in phospholipids) | Energy storage, membranes, hormones |
| Proteins | Amino acids | C, H, O, N, S | Enzymes, structure, transport, signaling |
| Nucleic acids | Nucleotides | C, H, O, N, P | Store and transmit genetic information |
Structure determines function
- Proteins: primary structure is the amino acid sequence; secondary is $\alpha$-helices and $\beta$-sheets from backbone hydrogen bonds; tertiary is the 3D fold from R-group interactions; quaternary is multiple polypeptides. Changing one amino acid can change the shape and function.
- Nucleic acids: a nucleotide is a sugar, a phosphate and a nitrogenous base. DNA uses deoxyribose and A, T, C, G; RNA uses ribose and A, U, C, G. DNA strands are antiparallel ($5'$ to $3'$ paired with $3'$ to $5'$), with A–T and C–G base pairs.
- Lipids: saturated fatty acids have no double bonds (straight, solid at room temperature); unsaturated have C=C double bonds (kinked, liquid). Phospholipids have hydrophilic heads and hydrophobic tails.
Topics in this unit
Each topic has its own short notes and practice.
Practice
Water molecules stick to each other and to xylem walls, pulled up by evaporation from leaves.
A peptide bond forms when a water molecule is removed.
Nucleotides contain phosphate groups; carbohydrates are only C, H and O.
Primary structure determines folding, and folding determines function.
Complementary bases pair through hydrogen bonds; the sugar-phosphate backbone is covalent.
Double bonds create kinks, so the molecules can't pack tightly.
A pairs with T and G with C, and the strands are antiparallel.
Tertiary structure is the overall 3D fold from side-chain interactions.
Free response
Write your answer first, then open the scoring guide.
Free-response question
Phospholipids form the membranes of all cells.
(a) Describe the structure of a phospholipid and identify which part is hydrophilic.
Show the answer and scoring guide
A glycerol attached to two fatty acid tails and a phosphate-containing head group. The phosphate head is polar and hydrophilic; the tails are nonpolar and hydrophobic.
Check off the points you earned:
(b) Explain why phospholipids form a bilayer in water.
Show the answer and scoring guide
The hydrophobic tails avoid water by facing each other inside the bilayer, while the hydrophilic heads face the watery environments on both sides.
Check off the points you earned:
(c) Predict how increasing the proportion of unsaturated fatty acids would affect membrane fluidity, and explain.
Show the answer and scoring guide
Fluidity would increase. Kinks from double bonds keep tails from packing tightly, so the membrane stays more fluid.
Check off the points you earned: