Biological Molecules » Lipid Structure and Functions

What you'll learn this session

Study time: 30 minutes

The chemical structure of lipids
Different types of lipids including triglycerides and phospholipids
The key functions of lipids in living organisms
How to test for the presence of lipids
How lipids relate to health and disease

Introduction to Lipids

Lipids are one of the four main biological molecules essential for life (alongside carbohydrates, proteins and nucleic acids). Unlike other biomolecules, lipids are not polymers but are a diverse group of compounds that share the property of being insoluble in water but soluble in organic solvents like alcohol and ether. This property is crucial for many of their functions in living organisms.

Key Definitions:

Lipids: A diverse group of organic compounds that are insoluble in water but soluble in non-polar solvents.
Hydrophobic: Water-repelling; a property of substances that do not mix with water.
Hydrophilic: Water-loving; a property of substances that readily mix with water.
Fatty acid: A carboxylic acid with a long hydrocarbon chain, a key building block of many lipids.

🔥 Chemical Structure of Lipids

Most lipids contain carbon, hydrogen and oxygen atoms, but in different proportions compared to carbohydrates. Lipids have a much higher proportion of carbon and hydrogen atoms relative to oxygen atoms, which is why they store more energy per gram than carbohydrates.

The basic building blocks of many lipids are fatty acids and glycerol. Fatty acids consist of a carboxyl group (-COOH) attached to a hydrocarbon chain, which can be saturated (no double bonds) or unsaturated (containing one or more double bonds).

🔬 Testing for Lipids

The emulsion test is used to identify lipids in a sample:

Add ethanol to the sample and shake well
Add water to the mixture
If lipids are present, a cloudy white emulsion forms

This happens because lipids dissolve in ethanol but not in water. When water is added, the lipids form tiny droplets suspended in the solution, creating the cloudy appearance.

Types of Lipids

There are several types of lipids, each with specific structures and functions. The main types you need to know for your iGCSE are triglycerides and phospholipids.

🍔 Triglycerides

Triglycerides are formed when three fatty acid molecules combine with one glycerol molecule through a condensation reaction. This creates an ester bond and releases water.

Structure:

One glycerol molecule
Three fatty acid molecules
The fatty acids can be the same or different
They can contain saturated or unsaturated fatty acids

Saturated vs Unsaturated:

Saturated fats have no double bonds between carbon atoms in their fatty acid chains
Unsaturated fats have one or more double bonds in their fatty acid chains
Saturated fats are usually solid at room temperature (like butter)
Unsaturated fats are usually liquid at room temperature (like olive oil)

🟢 Phospholipids

Phospholipids are similar to triglycerides but have a phosphate group replacing one of the fatty acids.

Structure:

One glycerol molecule
Two fatty acid chains (hydrophobic tails)
One phosphate group (hydrophilic head)

This gives phospholipids a unique property: they have a hydrophilic (water-loving) head and hydrophobic (water-repelling) tails. This makes them perfect for forming cell membranes, as they arrange themselves into a bilayer with the hydrophilic heads facing outward toward the watery environments inside and outside the cell and the hydrophobic tails facing inward toward each other.

Functions of Lipids

Lipids perform several vital functions in living organisms. Understanding these functions helps explain why lipids are essential for life.

💪 Energy Storage

Triglycerides are the main energy storage molecules in animals:

They store more than twice as much energy per gram as carbohydrates
They're stored in adipose tissue (fat cells)
They're anhydrous (don't contain water), making them lightweight for energy storage

🧊 Insulation

Lipids provide thermal insulation in animals:

Subcutaneous fat (under the skin) helps maintain body temperature
Animals in cold environments often have thicker fat layers
Blubber in marine mammals can be several inches thick

⚠ Protection

Lipids provide physical protection:

Fat surrounds and cushions vital organs
Helps protect against physical trauma
Waterproofing in plants (waxy cuticle)

👥 Cell Membranes

Phospholipids form the basis of all cell membranes:

Create a selectively permeable barrier
Control what enters and leaves the cell
Form a bilayer with hydrophilic heads facing outward

💊 Hormones

Some lipids function as hormones:

Steroid hormones like testosterone and estrogen
Regulate metabolism, reproduction and development
Can pass through cell membranes due to their lipid nature

🌟 Vitamins

Fat-soluble vitamins are a type of lipid:

Vitamins A, D, E and K
Stored in fatty tissues
Require dietary fat for absorption

Phospholipid Bilayer: The Foundation of Cell Membranes

The phospholipid bilayer is a remarkable structure that forms the basis of all cell membranes. Its unique properties allow cells to maintain their internal environment separate from the external surroundings.

When phospholipids are placed in water, they automatically arrange themselves into a bilayer with:

Hydrophilic phosphate heads facing outward toward the water
Hydrophobic fatty acid tails facing inward away from water

This structure creates a barrier that is selectively permeable, meaning:

Small, non-polar molecules like oxygen and carbon dioxide can pass through easily
Water molecules can pass through in limited amounts
Larger molecules and ions cannot pass through without help from membrane proteins

The fluidity of the membrane is affected by:

Temperature - higher temperatures increase fluidity
Cholesterol content - helps regulate fluidity
Unsaturated fatty acids - create "kinks" in the tails, preventing tight packing and increasing fluidity

Case Study Focus: Lipids and Health

Lipids play a crucial role in human health, but imbalances can lead to health problems:

Cholesterol is a lipid that is essential for cell membrane structure and hormone production. However, high levels of LDL cholesterol ("bad cholesterol") in the blood can lead to atherosclerosis, where fatty deposits build up in arteries, increasing the risk of heart disease and stroke.
Omega-3 fatty acids, found in fish oils and some plant oils, are unsaturated fatty acids that have been shown to have beneficial effects on heart health by reducing inflammation and lowering triglyceride levels.
Trans fats are artificially created unsaturated fatty acids that have been linked to increased risk of heart disease. They're formed when liquid vegetable oils are hydrogenated to make them more solid. Many countries have now banned or restricted the use of trans fats in food.

A balanced diet should include moderate amounts of healthy lipids, particularly unsaturated fats from sources like olive oil, avocados, nuts and fish, while limiting intake of saturated and trans fats.

Summary: Why Lipids Matter

Lipids are essential biomolecules with diverse structures and functions. From energy storage and insulation to forming the very membranes that define our cells, lipids are fundamental to life. Their hydrophobic nature makes them perfect for their roles, whether it's storing energy efficiently or creating barriers between watery environments.

In your iGCSE exam, you'll need to understand the structure of triglycerides and phospholipids, how to test for lipids and the various functions they perform in living organisms. Remember that the unique properties of lipids come from their molecular structure, particularly the balance between hydrophilic and hydrophobic components.

🧠 Test Your Knowledge!