Lipids

Lipids are organic compounds mainly composed of carbon (C), hydrogen (H), and oxygen (O), and sometimes phosphorus, nitrogen, or sulphur.

The most important characteristic:
👉 They are hydrophobic (water-insoluble) but soluble in non-polar solvents like alcohol, ether, and chloroform.

Now pause and think: Why this behaviour?

Because lipids are non-polar molecules—they lack charge separation. And in chemistry, “like dissolves like”. So non-polar substances dissolve in non-polar solvents.

Classification of Lipids

Lipids are broadly divided into three categories:

A. Simple Lipids

These are esters of fatty acids and alcohols.

(i) Glycerides

Formed by glycerol + fatty acids.

Based on number of fatty acids:

• Monoglycerides (1 FA) → act as emulsifiers, used in food & skin products
• Diglycerides (2 FA) → also emulsifiers
• Triglycerides (3 FA) → most important biologically

👉 Triglycerides = main fat storage form in the body

They exist as:

• Fats → solid at room temperature (e.g., butter)
• Oils → liquid at room temperature (e.g., olive oil)

🔍 Conceptual clarity:

• Saturated fatty acids → tightly packed → higher melting point
• Unsaturated fatty acids → loosely packed → lower melting point

👉 That is why oils (more unsaturated) are liquid.

Also, remember:
👉 Fats provide more energy than carbohydrates because they are more reduced (less oxygen content), hence yield more energy upon oxidation.

(ii) Waxes

• Esters of long-chain fatty acids + long-chain alcohols
• Function → waterproofing

Examples → Beeswax, Lanolin, Cutin (plant surface)

👉 Think: Nature’s raincoat.

B. Complex Lipids

These contain additional groups like phosphate, carbohydrate, or protein.

(i) Phospholipids

• Structure: glycerol + 2 fatty acids + phosphate group
• Function: major component of cell membrane

👉 They form the lipid bilayer—foundation of life.

Example: Lecithin

(ii) Glycolipids

• Contain carbohydrates + lipids
• Abundant in nervous system

Examples → Cerebrosides, Gangliosides

👉 Important for cell recognition and nerve function

(iii) Lipoproteins

• Lipids + proteins
• Function: transport of fats in blood

👉 Example: HDL, LDL (important in health & disease)

C. Derived Lipids

These are obtained from hydrolysis of simple/complex lipids.

(i) Steroids

Structure: four fused carbon rings

Most important example: Cholesterol

• Maintains membrane fluidity
• Precursor for → Steroid hormones, Vitamin D, Bile salts

👉 Synthesised mainly in the liver

(ii) Steroid Hormones

Sex hormones → Androgens (testosterone), Estrogens (estradiol), Progesterone

Corticosteroids (adrenal cortex):

• Glucocorticoids (cortisol → stress, metabolism)
• Mineralocorticoids (aldosterone → ion balance, BP)

(iii) Vitamin D

• Fat-soluble vitamin
• Synthesised in skin via sunlight
• Essential for calcium absorption

(iv) Fatty Acids

Basic units of lipids → Saturated, Monounsaturated, Polyunsaturated, Trans fats

(v) Glycerol

• Backbone of triglycerides and phospholipids

(vi) Eicosanoids

Derived from 20-carbon fatty acids (arachidonic acid)

Types:

• Prostaglandins → inflammation, pain, fever
• Thromboxanes → clotting, BP regulation
• Leukotrienes → immune & allergic responses

👉 These are local signalling molecules

(vii) Terpenes / Terpenoids

Derived from isoprene units

Functions in plants:

• Growth hormones (gibberellins)
• Pigments (carotenoids)
• Essential oils (limonene)

Functions of Lipids

Let’s connect structure with function:

S. No.	Function	Explanation	Examples/Details
1	Energy Storage	Lipids act as a dense energy source with higher calorific value than carbohydrates and proteins	Provide more energy per unit mass
2	Structural Role	Form essential components of cell membranes	Phospholipids (semi-permeable barrier), Cholesterol (membrane fluidity & stability)
3	Thermal Insulation	Adipose tissue helps maintain body temperature by reducing heat loss	Important in animals in cold climates
4	Protection	Adipose tissue cushions and protects internal organs	Fat surrounding organs
5	Hormone Synthesis	Lipids act as precursors for hormone production	Steroid hormones (testosterone, estrogen, cortisol), Eicosanoids (prostaglandins)
6	Vitamin Absorption	Help in absorption of fat-soluble vitamins	Vitamins A, D, E, K
7	Waterproofing	Prevent water loss and provide protective coatings	Waxes in plant cuticle, feathers, fur
8	Energy Reserve	Stored as long-term energy for later use	Oils in seeds, fats in animals
9	Signaling	Involved in cellular communication processes	Phosphatidylinositol
10	Buoyancy	Help aquatic animals float due to low density	Seen in whales and other marine animals
11	Nerve Function	Form insulating layer around nerve fibres to enhance signal transmission	Sphingolipids in myelin sheath
12	Food Satisfaction	Enhance taste, texture, and satiety of food	Contribute to flavour and fullness