Introduction: The Fuels and Structures of Life
Welcome to Lesson 4. We now begin our exploration of the four major classes of biological macromolecules. This lesson focuses on carbohydrates and lipids, the primary sources of energy and key structural components of all living organisms. We will examine their molecular structures and relate them directly to their diverse biological functions.
Learning Objectives
- LO 4.1: Differentiate between monosaccharides, disaccharides, and polysaccharides, providing examples of each.
- LO 4.2: Describe the formation of a glycosidic linkage and an ester linkage through dehydration synthesis.
- LO 4.3: Compare and contrast the structures and functions of starch, glycogen, cellulose, and chitin.
- LO 4.4: Explain the difference between saturated, unsaturated (cis/trans), and polyunsaturated fatty acids.
- LO 4.5: Describe the structure of phospholipids and steroids, and explain how their properties lead to the formation of biological membranes and signaling molecules.
Part 1: Carbohydrates - Sugars and Their Polymers
Carbohydrates are molecules made of carbon, hydrogen, and oxygen, typically with the empirical formula $(CH_2O)_n$. They serve as a primary source of metabolic energy and as structural materials.
1.1 Monosaccharides: The Simplest Sugars
Monosaccharides (e.g., glucose, fructose) are the monomers of carbohydrates. Glucose ($C_6H_{12}O_6$) is the most important monosaccharide for life. In aqueous solutions, glucose exists in equilibrium between a linear (chain) form and a more stable ring form.
Diagram: α-Glucose and β-Glucose Ring Structures
The orientation of the hydroxyl (-OH) group on Carbon-1 determines the isomer: α-glucose (OH down) or β-glucose (OH up). This seemingly small difference has enormous structural consequences.
1.2 Disaccharides and the Glycosidic Linkage
A disaccharide is formed when two monosaccharides are joined by a glycosidic linkage, a type of covalent bond formed through a dehydration synthesis (condensation) reaction, where a molecule of water is removed.
Diagram: Formation of Maltose (a Disaccharide)
1.3 Polysaccharides: Storage and Structure
Polysaccharides are polymers of hundreds to thousands of monosaccharides. Their function depends on the type of monomer (α or β glucose) and the branching pattern.
| Polysaccharide | Monomer | Linkage Type | Structure | Function & Organism |
|---|---|---|---|---|
| Starch | α-Glucose | α(1→4) and α(1→6) | Helical. Unbranched (amylose) or moderately branched (amylopectin). | Energy storage in plants. |
| Glycogen | α-Glucose | α(1→4) and α(1→6) | Helical, highly branched. More compact than starch. | Energy storage in animals (liver, muscles). |
| Cellulose | β-Glucose | β(1→4) linkages | Straight, unbranched chains that form hydrogen-bonded microfibrils. | Structural component of plant cell walls. |
| Chitin | Modified Glucose (N-acetylglucosamine) | β(1→4) linkages | Similar to cellulose; straight chains form hydrogen-bonded fibrils. | Structural component of fungi cell walls and arthropod exoskeletons. |
Part 2: Lipids - Diverse Hydrophobic Molecules
Lipids are a diverse group of hydrophobic molecules, meaning they do not mix well with water. They include fats, phospholipids, and steroids.
2.1 Fatty Acids and Triglycerides
A triglyceride (fat or oil) consists of a glycerol molecule bonded to three fatty acids via ester linkages. Fatty acids are long hydrocarbon chains.
- Saturated Fatty Acids: Have no double bonds in their hydrocarbon chains. They are straight and pack tightly, making them solid at room temperature (e.g., butter).
- Unsaturated Fatty Acids: Have one or more double bonds. Cis double bonds cause "kinks" in the chain, preventing tight packing (e.g., olive oil). Trans double bonds do not cause significant kinks and are associated with negative health effects.
Diagram: Saturated vs. Unsaturated Fatty Acids
2.2 Phospholipids and Cell Membranes
A phospholipid is structurally similar to a triglyceride but has a phosphate group and only two fatty acids attached to glycerol. This makes them amphipathic: they have a hydrophilic (water-loving) head and two hydrophobic (water-fearing) tails. When placed in water, phospholipids spontaneously self-assemble into a bilayer, forming the basic structure of all cell membranes.
Diagram: Phospholipid Bilayer Structure
2.3 Steroids: Lipids with a Ring Structure
Steroids are a class of lipids characterized by a core structure of four fused carbon rings. They are distinguished from one another by the different chemical groups attached to this ring system. Unlike other lipids, they are not built from fatty acids.
Cholesterol is the most common steroid and is a crucial component of animal cell membranes, where it helps regulate membrane fluidity. It is also the precursor from which other steroids, such as the sex hormones testosterone and estradiol, are synthesized.
Diagram: The Steroid Nucleus and Cholesterol
Interactive Practice Quiz
Test your understanding of carbohydrates and lipids. Choose the best answer for each question (A-E) and then submit to see your results.