Fats are one kind of lipid. This lesson meets the rest of the family, one group at a time: phospholipids that build every cell membrane, cholesterol and plant sterols, sphingolipids and waxes, the eicosanoid signals your body makes from fatty acids, and the fat-soluble vitamins, then puts them all in one table. Every molecule is drawn at the same scale with every hydrogen shown. The figure is on the left and the lesson script on the right; the quiz is at the end.
Oily molecules that don’t dissolve in water. Fats are one branch.
Which ones count as “fat” on a food label
Total fat (a macronutrient, 9 kcal per gram)Triglycerides, plus phospholipids and the fatty acids in them. Triglycerides are about 95% of the lipid we eat.
Cholesterol (its own line, in milligrams)A sterol: no calories, so it isn’t counted in total fat or in the calories.
Vitamins A, D, E, K (micronutrients)Needed in micrograms to milligrams, listed with the vitamins and minerals. Module 2.
A fat with one fatty acid swapped for a water-loving head.
One phospholipid
12345Water-loving headWater-avoiding tails
1Choline: the N+ group at the very top. Swap it and you get other phospholipids.
2Phosphate: phosphorus (orange) with four oxygens; it carries a negative charge.
3Glycerol: the same 3-carbon backbone as in a triglyceride.
4Fatty acid 1: palmitic acid, straight.
5Fatty acid 2: oleic acid; its cis double bond kinks the tail.
Thousands of them together: the cell membrane
Water outside the cell
Water inside the cell
A cartoon, not to scale: each ball is one water-loving head and each pair of lines its two tails. The heads face the water on both sides; the tails hide from it in the oily middle. That double layer, two molecules thick, wraps every cell you have.
carbon hydrogen oxygen nitrogen phosphorus · every molecule on this page is drawn at the same scale with every hydrogen shown · tap one to enlarge it
1OH: one hydroxyl group, the only water-loving bit.
2Four fused rings: three six-carbon rings and one five-carbon ring, flat and stiff.
3Short branched tail: 8 carbons.
A triglyceride (from the Fats lesson), C55H100O6
Same scale. The fat has a glycerol backbone, three fatty acids and three ester links (red); cholesterol has none of them, which is why it isn’t a fat. Orange marks C=C double bonds, as on the Fats page.
Plant sterols
Beta-sitosterol, C29H50O
1
1The only difference: an extra two-carbon branch on the tail. Close enough to cholesterol to compete with it in the gut.
carbon hydrogen oxygen nitrogen phosphorus · every molecule on this page is drawn at the same scale with every hydrogen shown · tap one to enlarge it
A different backbone, and a lipid we can’t digest.
A ceramide (sphingolipid) C36H71NO3
123
A wax: cetyl palmitate C32H64O2
ABC
1Sphingosine: an 18-carbon backbone with two OH groups and an N, used instead of glycerol.
2Amide link (N–C=O) holding one fatty acid.
3Fatty acid: here stearic acid.
AEster link, the same kind that holds fatty acids to glycerol.
BFatty acid: palmitic acid.
CLong-chain alcohol: cetyl alcohol, 16 carbons ending in O.
Ceramide + phosphate-choline head = sphingomyelin
Sphingomyelin (d18:1/18:0) C41H83N2O6P
12345
1Choline (N⁺) head.
2Phosphate: P with four O, negative charge, bonded to sphingosine’s C1 oxygen.
3Sphingosine backbone: 18 carbons, the trans C4=C5 double bond, the C3 OH.
4Amide link (N–C=O).
5Fatty acid: stearic acid.
The same ceramide with a phosphate-choline head, the head phosphatidylcholine has in Figure 2. A major part of myelin, the insulating wrap around nerve fibres.
carbon hydrogen oxygen nitrogen phosphorus · every molecule on this page is drawn at the same scale with every hydrogen shown · tap one to enlarge it
Orange and red colour; beta-carotene becomes vitamin A, lycopene does not
Carrots, sweet potato, tomatoes, leafy greens
Vitamin D
Vitamin D3 (cholecalciferol), C27H44O
123
1Opened ring: in your skin, UVB light breaks the second ring of a close relative of cholesterol; the gap is here.
2OH: the same one as on cholesterol.
3Tail: the same eight carbons as cholesterol’s. Vitamin D2 (ergocalciferol), from UV-exposed mushrooms and yeast, has one extra double bond and one extra methyl here.
In your body: your liver and kidneys turn it into the active form, which helps you absorb calcium and phosphorus for bones.
Found in: oily fish (salmon, trout, sardines), egg yolks, fortified milk; UV-exposed mushrooms (as D2); made in your skin in sunlight.
Vitamin E
Vitamin E (alpha-tocopherol), C29H50O2
123
1OH: gives up its hydrogen to a free radical and stops it damaging fatty acids.
2Chromanol ring: holds the OH in place.
3Saturated tail: 16 carbons counting its 3 methyl branches; anchors the vitamin among the fatty acids of a membrane.
In your body: an antioxidant that protects the fatty acids in your cell membranes.
Found in: almonds, sunflower seeds, hazelnuts, peanuts; sunflower and safflower oil, wheat germ; some in spinach and broccoli.
Vitamin K: two forms
Vitamin K1 (phylloquinone), C31H46O2
12
Vitamin K2 (menaquinone-4, MK-4), C31H40O2
13
1Naphthoquinone ring: two fused rings with two C=O groups; the same head on every form of vitamin K.
2K1 tail: mostly saturated, one double bond. Made by plants.
3K2 tail: repeating units, each with a double bond (four in MK-4). Made by bacteria, and in your body from K1.
In your body: switches on the proteins that make blood clot, and others that work in bone.
Found in: K1: kale, spinach, collards, broccoli; soybean and canola oil. K2: meat, egg yolk, cheese; natto (fermented soybeans).
Two carotenoids, same formula
Beta-carotene, C40H56
1223
1The middle: an enzyme in your gut cuts here, and each half becomes vitamin A (provitamin A).
2Rings: the same ring as in retinol, one at each end.
3Alternating double bonds: 11 in a row; they absorb blue light, so it looks orange.
In your body: made into vitamin A as you need it; also an antioxidant pigment.
Found in: carrots, sweet potatoes, pumpkin, cantaloupe, mango; spinach and kale.
Lycopene, C40H56
454
4Open ends: no rings, so your body can’t make vitamin A from it.
5Same kind of chain, 11 alternating double bonds in a row with no rings twisting it, so it absorbs slightly different light and looks red.
In your body: an antioxidant pigment; not a vitamin.
Found in: tomatoes (better absorbed from cooked tomato with a little fat), watermelon, pink grapefruit, guava.
carbon hydrogen oxygen nitrogen phosphorus · every molecule on this page is drawn at the same scale with every hydrogen shown · tap one to enlarge it
Triglycerides and phospholipids give energy because of their fatty acids. Sterols, waxes, eicosanoids and the vitamins don’t: their jobs are building, signalling and protecting.
Most lipids have a water-loving end
Phospholipids, cholesterol and ceramides each have a small water-loving part and a large oily part. That two-sided shape is what lets them build membranes and barriers.
General education, not medical advice. Facts follow mainstream sources (US Dietary Guidelines, NIH Office of Dietary Supplements, standard biochemistry texts).
Quiz
Tap a question to see the answer
1. What makes a molecule a lipid?
It is oily and doesn’t dissolve in water (Figure 1). Fats are one kind of lipid.
2. About what share of the lipids we eat are triglycerides?
About 95% (Figure 1).
3. How is a phospholipid different from a triglyceride?
It has 2 fatty acids instead of 3; the third glycerol slot holds a phosphate head, often with choline on it (Figure 2).
4. Why do phospholipids form a double layer in water?
Their heads are water-loving and their tails water-avoiding, so the tails hide in the middle and the heads face the water on both sides (Figure 2).
5. What is lecithin, and why is it added to food?
Phosphatidylcholine (or a phospholipid mix) from egg yolk, soy or sunflower; it is an emulsifier, E322, that keeps oil and water mixed (Figure 2).
6. Why isn’t cholesterol a fat?
It has no glycerol, no fatty acids and no ester links: it is four fused rings with one OH. It isn’t burned for energy, so it has no calories (Figure 3).
7. Name three things your body makes from cholesterol.
Steroid hormones (estrogen, testosterone, cortisol), vitamin D and bile acids; it also goes into cell membranes (Figure 3).
8. Which foods contain cholesterol, and how do plant sterols differ?
Only animal foods. Plant sterols such as beta-sitosterol, in nuts, seeds and oils, look almost the same and compete with cholesterol for absorption (Figure 3).
9. What do ceramides do, and what is a wax made of?
Ceramides help seal the skin barrier; a wax is a fatty acid joined by an ester link to a long-chain alcohol, and we can’t digest it (Figure 4).
10. Which fatty acids are eicosanoids made from?
The 20-carbon fatty acids arachidonic acid (omega-6) and EPA (omega-3) (Figure 5).
11. Why do vitamins A, D, E and K need some fat in the meal?
They are lipids, so they are absorbed along with fat (Figure 6).