Learning Objectives
- Define lipids and describe their chemical nature.
- Explain the structure and classification of fatty acids and lipids.
- Describe biological functions of lipids in plants and animals.
- Explain structural and storage roles of lipids in plants.
- Relate lipid metabolism to horticultural performance and crop quality.
6.1 General Considerations of Lipids
Definition
Lipids are a heterogeneous group of biomolecules that are insoluble in water but soluble in non-polar organic solvents such as chloroform, benzene, and petroleum ether.
Key Properties
- Hydrophobic or amphipathic molecules
- Chemically diverse compounds
- Often composed of fatty acids and their derivatives
- Found in membranes, storage tissues, and lipoprotein complexes
Major Biological Functions
- Energy storage (fats and oils)
- Structural components of membranes (phospholipids, sterols)
- Thermal insulation and mechanical protection
- Precursors of hormones and signaling molecules
- Electron carriers and pigments
- Solubilization of fat-soluble vitamins (A, D, E, K)
Applied Horticulture Insight:
Lipids influence seed oil content, fruit aroma, stress tolerance, membrane stability, and post-harvest quality in horticultural crops.
6.2 Fatty Acids and Glycerol: Building Blocks of Lipids
Fatty Acids
- Straight-chain hydrocarbons with a terminal carboxyl group (–COOH)
- General formula: R–COOH
- Usually contain even numbers of carbon atoms
- Exist as free fatty acids or esterified forms
Classification of Fatty Acids
A. Based on Chain Length
- Short-chain: 4–6 carbons
- Medium-chain: 8–12 carbons
- Long-chain: >12 carbons
B. Based on Degree of Unsaturation
- Saturated fatty acids: no double bonds
- Monounsaturated fatty acids: one double bond
- Polyunsaturated fatty acids (PUFAs): two or more double bonds
Examples of Important Fatty Acids
| Fatty Acid |
Notation |
Source |
Biological Importance |
| Oleic acid |
18:1 (ω9) |
Olive oil |
Membrane fluidity, nutrition |
| Linoleic acid |
18:2 (ω6) |
Sunflower, soybean oil |
Essential fatty acid |
| Linolenic acid |
18:3 (ω3) |
Leafy plants, flaxseed |
Essential fatty acid |
| Arachidonic acid |
20:4 (ω6) |
Animal tissues |
Hormone precursors |
Essential Fatty Acids
- Linoleic acid (ω6)
- Linolenic acid (ω3)
Physical Properties of Fatty Acids
- Longer chains → higher melting point
- More unsaturation → lower melting point
- Saturated fats → solid at room temperature
- Unsaturated fats → liquid at room temperature
6.3 Classification of Lipids
Based on Composition
- Simple lipids: Triacylglycerols, waxes
- Complex lipids: Phospholipids, glycolipids, lipoproteins
- Derived lipids: Steroids, fatty acids, lipid-soluble vitamins
Plant Oils in Horticulture
Plant oils are abundant in seeds and fruits. Major oil crops include:
- Sesame, sunflower, safflower, soybean, peanut, olive, coconut, castor bean
6.4 Triacylglycerols (TAGs)
Triacylglycerols are the main storage lipids composed of glycerol and three fatty acids.
Types of TAGs
- Simple TAGs: same fatty acids
- Mixed TAGs: different fatty acids
Horticultural Application:
TAG composition determines oil quality, flavor, shelf life, and industrial value of crops such as olive, sesame, and sunflower.
6.5 Waxes in Plants
Definition
Waxes are esters of long-chain fatty acids and long-chain alcohols.
Types of Plant Waxes
- Epicuticular wax
- Cuticular wax
- Cutin
- Suberin
Functions of Waxes
- Water repellency
- Protection against pathogens and UV radiation
- Reduction of transpiration
- Mechanical protection
6.6 Structural Lipids in Biological Membranes
Membrane Lipid Types
- Glycerophospholipids
- Sphingolipids
- Galactolipids and sulfolipids (important in chloroplasts)
- Sterols
- Archaeal ether lipids
Membrane Structure
- Lipid bilayer structure
- Amphipathic nature of lipids
- Dynamic membrane fluidity
Applied Plant Biochemistry:
Membrane lipid composition affects photosynthesis, cold tolerance, drought resistance, and post-harvest stability of horticultural crops.