Introduction

Lipids are one of the four major groups of biological molecules found in living organisms. Although they are often called fats, the lipid group also includes oils, waxes, and phospholipids. Lipids perform many essential functions, including storing energy, insulating the body, protecting organs, forming waterproof coatings, and building the membranes that surround every living cell.

Unlike carbohydrates, which provide energy that can be used quickly, lipids are mainly used for long-term energy storage. Because lipids contain much more energy per gram than carbohydrates, they are an efficient way for organisms to store energy for future use.

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What Are Lipids?

Lipids are a group of organic molecules made mainly of:

• Carbon (C)
• Hydrogen (H)
• Oxygen (O)

Compared with carbohydrates, lipids contain:

• Much more hydrogen.
• Less oxygen.

Lipids are generally:

• Insoluble in water.
• Soluble in many organic solvents.

The major types of lipids include:

• Fats
• Oils
• Waxes
• Phospholipids

Although these molecules have different structures, they all belong to the lipid family.

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Figure 1. Lipids include fats, oils, waxes, and phospholipids, each with specialised biological functions.

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The Structure of Lipids

Most fats and oils are made from:

• One glycerol molecule.
• Three fatty acid molecules.

Together they form a molecule called a triglyceride.

Scientists often represent triglycerides as:

• Glycerol "backbone"
• Three fatty acid "tails"

This structure allows lipids to store large amounts of chemical energy.

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Figure 2. Most fats and oils are triglycerides made from one glycerol molecule and three fatty acids.

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Lipids as Long-Term Energy Storage

One of the main functions of lipids is long-term energy storage.

Animals store excess energy as fat.

Plants store oils, especially in:

• Seeds
• Nuts
• Some fruits

When food is scarce, stored lipids are broken down to release energy.

Lipids store more than twice as much energy per gram as carbohydrates, making them an efficient energy reserve.

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Comparing Lipids and Carbohydrates

Both lipids and carbohydrates provide energy, but they are used differently.

Cells usually use carbohydrates first because they provide energy more rapidly.

Lipids become especially important during prolonged exercise or periods without food.

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Figure 3. Carbohydrates provide quick energy, while lipids are the body's long-term energy store.

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Fats

Fats are lipids that are usually solid at room temperature.

Examples include:

• Butter
• Animal fat
• Lard

Functions:

• Long-term energy storage.
• Thermal insulation.
• Protection of internal organs.
• Energy reserve during fasting.

Many mammals use body fat to help maintain their body temperature.

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Oils

Oils are lipids that are usually liquid at room temperature.

Examples include:

• Olive oil
• Sunflower oil
• Canola oil
• Fish oil

Plants commonly store energy as oils inside their seeds.

When seeds germinate, these oils provide energy for the young plant before it begins photosynthesis.

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Waxes

Waxes are protective lipids.

Functions include:

• Preventing water loss.
• Protecting surfaces.
• Providing waterproof coatings.

Examples:

• Plant leaf cuticle.
• Earwax.
• Beeswax.
• Waterproof coatings on fruits.

The waxy cuticle helps reduce water loss from leaves.

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Figure 4. Different types of lipids have specialised roles in living organisms.

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Phospholipids

Phospholipids are specialised lipids that form the cell membrane.

Unlike triglycerides, phospholipids have:

• One glycerol molecule.
• Two fatty acid tails.
• One phosphate-containing head.

The phosphate head is:

• Hydrophilic ("water-loving")

The fatty acid tails are:

• Hydrophobic ("water-repelling")

Because of these properties, phospholipids naturally arrange themselves into a phospholipid bilayer, forming the basic structure of all cell membranes.

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Figure 5. Phospholipids form the bilayer that surrounds every living cell.

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Lipids and Cell Membranes

Every living cell is surrounded by a cell membrane.

The membrane:

• Controls what enters the cell.
• Controls what leaves the cell.
• Protects the cell.
• Maintains internal conditions.

The phospholipid bilayer creates a flexible barrier that allows cells to function properly.

Proteins are also embedded within the membrane, helping transport substances across it.

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Other Functions of Lipids

In addition to storing energy, lipids:

• Insulate the body.
• Cushion organs.
• Form cell membranes.
• Waterproof leaves, feathers, and skin.
• Help absorb vitamins A, D, E, and K.
• Serve as building blocks for some hormones.

Lipids therefore perform many important functions beyond energy storage.

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Worked Example

Question

Complete the table.

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Real-World Connection

Many animals rely on stored fat to survive periods when food is scarce. Bears, for example, build up large fat reserves before hibernation. During the winter, they break down these stored lipids to provide energy while they remain inactive for several months. Similarly, migrating birds store fat before long flights because lipids provide a lightweight, energy-rich fuel for extended journeys.

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Did You Know?

Lipids provide about 9 kilocalories (kcal) of energy per gram, while carbohydrates provide only about 4 kilocalories per gram. This means lipids store more than twice as much energy in the same mass, making them the most efficient long-term energy storage molecules in the body.

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Key Terms

Cell membrane – A thin barrier surrounding the cell that controls the movement of substances into and out of the cell.

Fat – A lipid that is usually solid at room temperature and stores energy in animals.

Fatty acid – A long hydrocarbon chain that forms part of most lipid molecules.

Glycerol – A three-carbon molecule that forms the backbone of triglycerides and phospholipids.

Hydrophilic – Attracted to water.

Hydrophobic – Repelled by water.

Lipid – An organic molecule used for energy storage, insulation, waterproofing, and cell membrane structure.

Oil – A lipid that is usually liquid at room temperature.

Phospholipid – A specialised lipid that forms the main structure of cell membranes.

Triglyceride – A lipid made of one glycerol molecule joined to three fatty acids.

Wax – A protective lipid that reduces water loss and provides waterproof coatings.

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Key Takeaways

• Lipids are organic molecules made mainly of carbon, hydrogen, and oxygen.
• Most fats and oils are triglycerides, consisting of one glycerol molecule and three fatty acids.
• Lipids are the body's main form of long-term energy storage and contain more than twice as much energy per gram as carbohydrates.
• Fats, oils, waxes, and phospholipids each have specialised functions in living organisms.
• Phospholipids form the cell membrane, which controls the movement of substances into and out of cells.
• In addition to storing energy, lipids provide insulation, protection, waterproofing, and structural support for cells.