1.4 Properties of Biological Macromolecules

Carbohydrates: fuel and structure

Monosaccharides like glucose provide quick energy. Cells store energy as polysaccharides — starch in plants and glycogen in animals — and build structure with them too: cellulose stiffens plant cell walls and chitin forms insect exoskeletons and fungal walls.

Lipids are hydrophobic

Lipids are nonpolar, so they don't mix with water (hydrophobic). Triglycerides (a glycerol + three fatty acids) store energy. Saturated fatty acids have no C=C double bonds, pack tightly, and are solid at room temperature (animal fats); unsaturated fatty acids have double bonds that kink the chain, so they stay liquid (plant oils).

Phospholipids and steroids

Phospholipids have a hydrophilic (water-loving) phosphate head and two hydrophobic fatty-acid tails — this split personality lets them form the cell membrane's bilayer. Steroids, like cholesterol, are lipids built from four fused carbon rings.

Key terms

Saturated fat

Fatty acids with no C=C double bonds; straight chains pack tightly, solid at room temperature.

Unsaturated fat

Fatty acids with C=C double bonds that kink the chain; liquid oils.

Cellulose

Structural polysaccharide that stiffens plant cell walls.

Glycogen

The polysaccharide animals use to store glucose (in liver and muscle).

Phospholipid

A lipid with a hydrophilic head and two hydrophobic tails; forms the membrane bilayer.

Try an AP-style question

A polypeptide is synthesized from left to right. Which end is made first?

1. The carboxyl (C) terminus
2. Both ends at the same time
3. The amino (N) terminus
4. The R-group end