Quiz
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1. How many bonds does carbon make, and how many does hydrogen make?
Carbon 4, hydrogen 1, oxygen 2 (Figure 3).
2. What are the parts of a triglyceride, and which part of a fatty acid joins the glycerol?
A glycerol backbone plus 3 fatty acids. Each fatty acid's acid head (–COOH) joins glycerol through an ester link; the carbon tail sticks out (Figure 3).
3. Are fats and oils different kinds of molecule?
No. Both are triglycerides made of C, H and O. Oils carry more cis-unsaturated, kinked chains, so they melt below room temperature; "fat" or "oil" just names the state at about 20–25 °C (Figure 2).
4. What does "saturated" mean?
Every tail carbon holds as many hydrogens as it can, because there are no double bonds (Figure 4).
5. Why is butter solid at room temperature while olive oil is liquid?
Butter is rich in saturated fatty acids with straight chains that pack tightly. Olive oil is mostly oleic acid, whose cis double bond kinks the chain so it can't pack (Figures 2 and 4).
6. What does the "3" in omega-3 mean, and where do you start counting?
Count from the tail (ω) end, ω1 = last carbon; the first double bond starts at carbon 3 (Figure 6).
7. Which omega-3s come straight from fish or algae?
EPA and DHA. Plant ALA converts to them only poorly (Figure 6, pink box).
8. Why is omega-9 not essential, while omega-3 and omega-6 are?
Human desaturase enzymes can only add double bonds up to carbon 9 counted from the acid end. That is enough to turn stearic acid into oleic acid, an omega-9, so the body makes it. On an 18-carbon chain the omega-6 and omega-3 double bonds sit at carbons 12 and 15, out of reach, so linoleic acid and ALA must come from food (Figure 6, lower diagram).
9. How is industrial trans fat made, and why does it behave like saturated fat?
By partial hydrogenation of vegetable oil, which flips some cis double bonds to trans (Figure 7). A trans double bond keeps the chain nearly straight, so it packs and solidifies like saturated fat (Figures 2 and 4).
10. Name a natural source of trans fat.
Milk, butter, cheese, beef or lamb: vaccenic acid made by bacteria in cows, sheep and goats (Figure 4, note).
11. Do steroid hormones come from saturated fat?
No. They are made from cholesterol, which the liver can build from fat breakdown products (Figure 4, pink box).
12. What happens to a triglyceride when your body needs energy?
Lipases (ATGL, then HSL) cut its 3 ester links, adding water at each, into 1 glycerol and 3 free fatty acids. Glycerol goes to the liver, which can make glucose from it; the fatty acids ride on albumin to muscle and the heart to be burned for fuel, and in a deeper fast the liver turns some into ketones (Figure 9).