Lipids
Introduced| English | Meaning |
|---|---|
| triglyceride/ˈtrɪɡlɪsəraɪd/ | 1 glycerol + 3 fatty acids joined by ester bonds (condensation). |
| glycerol/ˈɡlɪsərɒl/ | A three-carbon alcohol with three hydroxyl groups, forming part of the structure of triglycerides. |
| fatty acid/ˈfæti ˈæsɪd/ | A carboxylic acid with a hydrocarbon chain, commonly a component of lipids such as triglycerides. |
| ester bond/ˈestə bɒnd/ | A covalent linkage formed between a carboxyl-derived group and an oxygen attached to a carbon group, as in fats. |
| hydrophobic/ˌhaɪdrəˈfəʊbɪk/ | Unable to mix readily with water, or tending to avoid contact with it. |
| saturated/ˈsætʃəreɪtɪd/ | Describing a solution unable to dissolve more solute under stated conditions, or an organic molecule with no carbon-carbon double or triple bonds. |
| unsaturated/ʌnˈsætʃəreɪtɪd/ | Containing one or more carbon-carbon double or triple bonds rather than only single bonds. |
| phospholipid/ˈfɒsfəlɪpɪd/ | A lipid with a phosphate-containing hydrophilic head and hydrophobic fatty-acid tails, commonly forming cell-membrane bilayers. |
| bilayer/ˈbɪleɪə/ | A structure made of two layers; a cell membrane contains two layers of phospholipid molecules. |
Energy stored, membranes built
- Lipids are the fats and oils — the body's long-term energy store.
- They do not mix with water, and that single fact explains what they do.
- One special lipid builds every cell membrane.
Triglycerides 甘油三酯
- A triglyceride is one glycerol 甘油 joined to three fatty acids 脂肪酸 by ester bonds 酯键 (each bond forms by condensation, so three waters are removed).
- It is non-polar and hydrophobic 疏水 — it will not mix with water.
- Fatty acids are saturated 饱和 (no C=C double bonds, straight, usually solid fats) or unsaturated 不饱和 (one or more double bonds, kinked, usually liquid oils).
A triglyceride: glycerol joined to three fatty acids by ester bonds
- Step through it below to watch the three ester bonds form.
Building a triglyceride
Watch one glycerol join three fatty acids. Each ester bond forms by condensation, removing one water — three bonds, three waters.
A triglyceride is made from:
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A triglyceride = 1 glycerol + 3 fatty acids, joined by ester bonds (each forms by condensation).
Glycerol joins each fatty acid by an ______ bond, formed by condensation.
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Three ester bonds form, removing three water molecules in total.
Match each fatty acid type to its feature.
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Saturated chains have no C=C and pack tightly (solid); unsaturated chains kink at each C=C, so they stay liquid.
Put the formation of a triglyceride in order.
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Each ester bond forms by condensation; three bonds release three water molecules.
Why fat is a great energy store
- It releases about twice the energy per gram as carbohydrate.
- It is insoluble, so it stays put and has no osmotic effect.
- It stores no extra water, so it adds little mass.
- Under the skin it also gives insulation and protects organs.
One reason triglycerides are good long-term energy stores is that they:
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Lipids release ~2× the energy per gram of carbohydrate, are insoluble, and store no water.
Phospholipids 磷脂 and the bilayer 双层
- A phospholipid is a triglyceride with one fatty acid replaced by a phosphate group.
- This gives it two ends: a hydrophilic ("water-loving") polar phosphate head and two hydrophobic fatty-acid tails.
- In water the heads face out and the tails hide inside — forming the bilayer that makes every cell membrane.
Phospholipids line up with their water-loving heads out and water-hating tails in, forming a bilayer
A phospholipid forms membranes because it has:
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The polar head is attracted to water and the non-polar tails avoid it, so phospholipids arrange into a bilayer.
In a phospholipid bilayer, the hydrophobic tails point inward and the hydrophilic heads face the water.
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Correct — heads face the watery surroundings on both sides; tails meet in the middle, away from water.
You've got it
- triglyceride = 1 glycerol + 3 fatty acids joined by ester bonds (condensation)
- saturated = no C=C (solid fats); unsaturated = has C=C double bonds (liquid oils)
- great energy store: ~2× energy/gram, insoluble, no water, insulation
- phospholipid = hydrophilic head + hydrophobic tails → forms the membrane bilayer