The big idea: Lipids (fats and oils) are the body's main long-term energy store. The lipid used for storage is the triglyceride — one glycerol joined to three fatty acids.
Where fat is stored: Animals store triglycerides as fat in special storage cells called adipose tissue, often as a layer under the skin and around organs.
A triglyceride as an energy store: a glycerol joined to three long, hydrophobic fatty-acid tails packed with energy-rich C–H bonds.
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Two stores, two jobs: The body keeps two kinds of energy store:
Carbohydrate (glycogen in animals) = the short-term, quick-access store — used first, between meals.
Lipid (triglyceride fat) = the long-term, high-capacity store — used for sustained energy needs and saved for when food is scarce.
- Lipid
- A group of fatty, water-insoluble biological molecules that includes fats, oils and phospholipids.
- Triglyceride
- The lipid used to store energy: one glycerol molecule joined to three fatty acids.
- Adipose tissue
- Animal tissue made of fat-storage cells; it stores triglycerides and forms an insulating layer under the skin.
- Energy store
- A molecule the body builds up when energy is plentiful and breaks down later to release that energy.
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Triglycerides suit energy storage because of three properties, and each one comes straight from their structure.
Learn the property and the reason behind it — the exam rewards the link, not just the fact.
The three storage advantages
- More energy per gram. Triglycerides release about twice as much energy per gram as carbohydrates, because their long fatty-acid tails are packed with energy-rich C–H bonds and very little oxygen.
- Insoluble in water. Fatty-acid tails are hydrophobic (water-repelling), so fat does not dissolve in the cytoplasm and does not draw water into the cell by osmosis — it stores quietly without upsetting the cell's water balance.
- Compact and light. Because they hold so much energy and carry no extra water, fat stores the same energy for less mass — useful for animals that must move or migrate.
A triglyceride as an energy store: a glycerol joined to three long, hydrophobic fatty-acid tails packed with energy-rich C–H bonds.
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Insoluble is an advantage here: Being insoluble sounds like a problem, but for a store it is exactly what you want.
Because the fatty-acid tails are hydrophobic, fat does not dissolve in water and does not add dissolved particles to the cell.
No pull on water: So storing a lot of fat does not lower the cell's water potential or pull water in by osmosis — large amounts can be stored safely.
A soluble store like glucose would affect the cell's water balance.
Fat also keeps you warm and cushioned: The fat layer in adipose tissue does more than store energy:
Thermal insulation — a layer of fat under the skin slows heat loss, helping mammals (and especially animals in cold water, like whales and seals) keep a steady body temperature.
Protection — fat around organs acts as a cushion against knocks.
Fat stored in adipose tissue under the skin: droplets of triglyceride act as a long-term energy store and as a layer of thermal insulation.
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- Hydrophobic
- Water-repelling; a non-polar part of a molecule that does not mix with or dissolve in water.
- Osmosis
- The movement of water across a membrane from a more dilute to a more concentrated solution; a soluble store can affect it, an insoluble one cannot.
- Thermal insulation
- Reducing the loss of heat; a fat layer under the skin slows heat escaping from the body.
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The big idea: Fat under the skin is a heat insulator. Warm-bodied animals in cold habitats need a thicker layer.
The points to remember
- Mammals and birds keep their bodies warm, at about 37 °C or more, whatever the habitat.
- Fat conducts heat poorly, so a layer of adipose tissue under the skin slows heat loss.
- The colder the habitat, the more heat would be lost, so the thicker the fat layer.
- Water takes heat away faster than air, so animals in cold water carry the thickest fat (blubber).
- The same fat is also an energy store, used up in seasons when food is scarce.
Remember it as: Warm body, cold home, thick fat.
Real example: ringed seals: Ringed seals live in Arctic water near 0 °C. A thick layer of blubber under the skin keeps their body warm and stores energy.
How this is tested: On Paper 2 this is usually a 2–4 mark Describe / Outline / Explain question.
What you may be asked
- Describe the structure and properties of triglycerides as energy stores.
- Explain how triglyceride properties suit them for storage.
- Explain why oils are insoluble while glucose dissolves in water. Answer it from polarity: fatty-acid tails are hydrophobic / non-polar; glucose has many polar –OH groups.
- For full marks, always pair the property with its reason (insoluble because hydrophobic; high energy because many C–H bonds).
Exam-style question — triglyceride properties as an energy store
Explain how the properties of triglycerides make them well suited to storing energy in animals. [4]
How to score all four marks
- Pair property 1 with its reason. Triglycerides release more energy per gram than carbohydrates because their long fatty-acid tails contain many energy-rich C–H bonds.
- Pair property 2 with its reason. They are insoluble in water because the fatty-acid tails are hydrophobic — so the store does not draw water in by osmosis and does not affect the cell's water balance.
- Add the consequence. Being energy-dense and water-free, fat stores the same energy for less mass, so it is a compact, long-term store.
- Add the bonus role. Fat in adipose tissue under the skin also gives thermal insulation, reducing heat loss. (Each clearly explained point — property + reason — scores a mark, up to 4.)
Final answer
High energy per gram (many C–H bonds), insoluble because hydrophobic (no osmotic effect), so a compact long-term store; the fat layer also insulates.
Watch the verb: Describe = say WHAT the properties are (high energy density, insoluble, compact).
Explain = also say WHY (because of C–H bonds / hydrophobic tails).
The 'why insoluble' question
A 'why oils are insoluble' question is really asking about polarity.
Fatty-acid tails are non-polar / hydrophobic, so they cannot form bonds with polar water. Glucose has many polar –OH groups that can, so it dissolves.
| Property | Lipid (triglyceride / fat) | Carbohydrate (glycogen / starch) |
|---|---|---|
| Energy released per gram | About twice as much | About half as much |
| Solubility in water | Insoluble (hydrophobic) | Soluble / can affect water balance |
| Effect on the cell's water (osmosis) | None — does not draw water in | Can draw water in / raise osmotic pressure |
| Mass carried for the same energy | Lighter store for the same energy | Heavier store for the same energy |
| How quickly it is released | Slower — a long-term store | Faster — a short-term, ready store |
A two-mark Paper 2 version asks for properties linked to storage:
Explain how the properties of triglycerides make them suitable for energy storage.
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