Triglycerides store twice as much energy per gram as carbohydrate and repel water completely. Swap one fatty acid for a phosphate group and you get a molecule with a water-loving end and a water-hating end, the building block of every cell membrane.
A triglyceride is one glycerol joined to three fatty acids by three ester bonds, each formed by condensation between an OH of glycerol and the COOH of a fatty acid. Triglycerides are non-polar and hydrophobic. Saturated fatty acids have no C=C double bonds and straight tails; unsaturated ones have at least one C=C, which kinks the tail. Their many C-H bonds make triglycerides an energy-dense, insoluble store. In a phospholipid one fatty acid is replaced by a phosphate group, giving a hydrophilic (polar) head and two hydrophobic (non-polar) tails.
Fig. 1: a triglyceride with one unsaturated fatty acid, and a phospholipid with its phosphate head
Section 01
Glycerol plus three fatty acids.
Step through the saturated fatty acid, the unsaturated fatty acid, the triglyceride, the phospholipid and the bilayer. Look for the carboxyl head, the kink and the three ester bonds.
Glycerol: a three-carbon alcohol with an OH group on each carbon.
Fatty acid: a carboxyl group (COOH) attached to a long hydrocarbon chain, often 14 to 22 carbons.
Ester bond: the COOH of a fatty acid and an OH of glycerol condense, losing H2O. Three fatty acids give a triglyceride and three water molecules; hydrolysis reverses it.
Non-polar: the long hydrocarbon tails have no charged or polar groups exposed, so triglycerides are hydrophobic and insoluble in water.
Saturated
Unsaturated
C=C double bonds
none: every carbon carries the maximum hydrogen
one (monounsaturated) or more (polyunsaturated)
tail shape
straight
kinked at each C=C
at room temperature
usually solid (fats, mainly animal)
usually liquid (oils, mainly plant)
Stage 1 · Learn
Check what the sim just showed you
Four quick checks. Each correct answer earns XP and records this skill on your profile.
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In the simulator, what puts the kink in the tail of the unsaturated fatty acid?
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Why are fats made of saturated fatty acids usually solid at room temperature?
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The triglyceride model shows how many ester bonds, and how many water molecules are released when it forms?
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In the bilayer view, where are the fatty acid tails?
Section 02
Structure to function.
Every role of a triglyceride traces back to two facts: it is packed with C-H bonds, and it does not mix with water.
Energy store: a high proportion of C-H bonds and little oxygen, so oxidation releases about 37 kJ g−1, roughly twice the 17 kJ g−1 of carbohydrate. More energy per unit mass suits animals that move and seeds that must be light.
Insoluble: large stores do not affect the water potential of cells, and fat droplets do not dissolve away.
Metabolic water: oxidising triglycerides produces a lot of water, valuable to desert animals and developing embryos in eggs.
Insulation and buoyancy: adipose tissue under the skin and blubber in whales are poor heat conductors, and lipid is less dense than water, helping aquatic animals float. Fat around organs such as the kidneys also cushions them.
Section 03
Phospholipids: heads and tails.
Return to the simulator and choose the phospholipid, then the bilayer.
Structure: glycerol, two fatty acids joined by ester bonds, and a phosphate group in place of the third fatty acid.
Head: the phosphate group is charged, so the head is polar and hydrophilic: it interacts with water.
Tails: the two fatty acid chains are non-polar and hydrophobic.
In water: phospholipids form a bilayer, heads facing the water on both sides and tails hidden in the middle. This is the basis of cell membranes (topic 4).
Examiner trap
A triglyceride is not a polymer: glycerol and fatty acids are different molecules, not a chain of repeating monomers. And do not call the phospholipid head "hydrophobic because it contains phosphorus": it is hydrophilic because the phosphate group is charged.
Stage 2 · Exam
Exam-style questions
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Why does 1 g of triglyceride release more energy when respired than 1 g of carbohydrate?
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Which groups react to form an ester bond in a triglyceride?
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Why is the head of a phospholipid hydrophilic?
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How many water molecules are needed to completely hydrolyse one triglyceride, and what are the products?
Skill unlocked
Lipids, mastered.
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