# Lecture 3: Biological Macromolecules I — Carbohydrates and Lipids

## General Biology I — Molecular & Cellular

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## Learning Objectives

By the end of this lecture, students will be able to:

1. Explain the concept of dehydration synthesis and hydrolysis in polymer formation
2. Classify carbohydrates as monosaccharides, disaccharides, or polysaccharides and describe their functions
3. Describe the structure and biological roles of the major classes of lipids
4. Compare and contrast saturated and unsaturated fatty acids
5. Explain the amphipathic nature of phospholipids and its relevance to membrane formation

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## Lecture Content

### I. Macromolecules — Overview

Living organisms are composed primarily of four classes of macromolecules: carbohydrates, lipids, proteins, and nucleic acids. Most of these are **polymers**--large molecules constructed from repeating subunits called **monomers**--though lipids are a notable exception. While lipids are not true polymers built from identical repeating units, they are grouped with the other macromolecules because of their large size and biological importance.

The construction and demolition of polymers follow two complementary reaction types. In **dehydration synthesis** (also called a condensation reaction), monomers are joined together by removing a water molecule--one monomer contributes a hydroxyl group (-OH) and the other contributes a hydrogen atom (-H), forming a covalent bond while releasing H2O. This process requires energy input and is catalyzed by specific enzymes. The reverse process, **hydrolysis**, breaks polymers apart by adding a water molecule across the bond, restoring the -OH and -H groups on the separated monomers. Hydrolysis is generally exergonic in biological contexts, and digestion is fundamentally a hydrolytic process--your body breaks down the polymers in food into monomers that can be absorbed and used.

<image>A side-by-side comparison diagram. Left panel: Dehydration synthesis — two monomers (represented as geometric shapes with -OH and -H groups) come together, a water molecule (H2O) is released, and a covalent bond forms between the monomers. Right panel: Hydrolysis — a polymer bond is broken by the addition of H2O, restoring -OH and -H groups on the separated monomers. Arrows show the direction of each reaction, and enzyme labels indicate catalysis.</image>

### II. Carbohydrates

Carbohydrates follow the general formula (CH2O)n--hence the name, which literally means "carbon" (carbo) plus "water" (hydrate). They serve as quick energy sources, long-term energy reserves, structural materials, and markers for cell recognition.

#### A. Monosaccharides (Simple Sugars)

Monosaccharides are the monomers of carbohydrates--single sugar units that cannot be hydrolyzed into simpler sugars. They are classified by the number of carbons they contain. **Trioses** (three carbons), such as glyceraldehyde and dihydroxyacetone, serve as metabolic intermediates in pathways like glycolysis. **Pentoses** (five carbons) include ribose, the sugar found in RNA, and deoxyribose, its counterpart in DNA. The most biologically important group is the **hexoses** (six carbons): glucose, fructose, and galactose all share the molecular formula C6H12O6 but differ in the arrangement of their atoms, making them structural isomers.

Glucose is the primary fuel for cellular respiration and the most commonly encountered sugar in biology. In aqueous solution, monosaccharides exist predominantly in **ring forms** rather than their linear configurations. The ring-closing reaction at carbon 1 produces two possible arrangements--the **alpha** and **beta** anomers--which differ only in the position of the hydroxyl group on that carbon. This seemingly minor distinction has enormous biological consequences: alpha-glucose monomers form starch, while beta-glucose monomers form cellulose, two polysaccharides with radically different properties. Monosaccharides with a free anomeric carbon are classified as **reducing sugars** because they can donate electrons (reduce) other molecules, a property exploited in clinical diagnostic tests.

#### B. Disaccharides

Disaccharides consist of two monosaccharides joined by a **glycosidic linkage**, a covalent bond formed through dehydration synthesis. The three most common disaccharides each pair glucose with a different partner. **Sucrose** (table sugar), composed of glucose and fructose, is the primary form in which sugars are transported through the phloem of plants. **Lactose** (milk sugar), made of glucose and galactose, requires the enzyme lactase for digestion--a detail of considerable clinical importance, since lactase deficiency leads to lactose intolerance. **Maltose**, consisting of two glucose units, is produced as an intermediate during starch digestion.

#### C. Polysaccharides

Polysaccharides are long chains of monosaccharides--often hundreds to thousands of units--and they fall into two functional categories: storage and structural.

Among **storage polysaccharides**, **starch** is the principal energy reserve in plants. It comes in two forms: amylose, an unbranched chain of glucose units connected by alpha-1,4 glycosidic linkages that coils into a helix, and amylopectin, a branched version with alpha-1,6 linkages at the branch points. **Glycogen** is the animal equivalent, stored primarily in the liver and skeletal muscle. Structurally similar to amylopectin, glycogen is more extensively branched, and this greater branching is functionally significant--more branch points mean more free ends from which glucose can be rapidly mobilized when energy demands spike.

The **structural polysaccharides** exploit a different chemistry. **Cellulose**, the most abundant organic molecule on Earth, is a major component of plant cell walls. It consists of glucose monomers linked by beta-1,4 glycosidic bonds, which produce straight, rigid chains that align side by side and form hydrogen-bonded microfibrils of exceptional tensile strength. Most animals cannot digest cellulose because they lack the enzyme cellulase; ruminants such as cows manage only because symbiotic bacteria in their digestive tracts produce it for them. **Chitin**, the structural material of arthropod exoskeletons and fungal cell walls, is similar to cellulose but features a nitrogen-containing group (N-acetylglucosamine) on each monomer. **Peptidoglycan** is the structural polymer of bacterial cell walls and will be discussed further in the context of prokaryotic cells.

<image>A three-panel figure. Panel A: Structural formulas of glucose in linear form and ring form (both alpha and beta anomers), with the position of the -OH group on carbon 1 highlighted. Panel B: Formation of maltose from two glucose monomers via a glycosidic linkage, showing the dehydration synthesis reaction and the resulting alpha-1,4 bond. Panel C: Comparison of starch (amylose helix and amylopectin branching), glycogen (highly branched), and cellulose (straight chains with hydrogen bonds between parallel chains forming microfibrils). Each structure is labeled with its bond type (alpha-1,4 vs. beta-1,4).</image>

### III. Lipids

Lipids are a diverse group of hydrophobic or amphipathic molecules united not by a shared chemical structure but by their insolubility in water. Unlike the other macromolecules, lipids are not true polymers--they are not assembled from identical repeating monomers. Their biological roles are wide-ranging: energy storage (at 9 kilocalories per gram, lipids store more than twice as much energy as carbohydrates), membrane structure, cell signaling, thermal insulation, and physical protection of organs.

#### A. Fatty Acids

Fatty acids are long hydrocarbon chains with a carboxyl group (-COOH) at one end. **Saturated fatty acids** contain no carbon-carbon double bonds; their straight hydrocarbon tails pack tightly together, which is why saturated fats--such as those found in butter and animal fat--are solid at room temperature. Palmitic acid (16 carbons) and stearic acid (18 carbons) are common examples. **Unsaturated fatty acids** have one or more C=C double bonds, each of which introduces a kink in the chain that prevents tight packing. As a result, unsaturated fats--typically plant oils--are liquid at room temperature. A fatty acid with a single double bond is **monounsaturated** (like oleic acid), while one with multiple double bonds is **polyunsaturated** (like linoleic acid). The geometry around the double bond matters as well: in the naturally occurring **cis** configuration, the hydrogen atoms are on the same side, producing a pronounced kink; in the **trans** configuration, they are on opposite sides, yielding a straighter chain. Trans fats, produced industrially through partial hydrogenation, behave more like saturated fats and have been strongly linked to increased cardiovascular disease risk.

#### B. Triglycerides (Triacylglycerols)

A triglyceride consists of one glycerol molecule bonded to three fatty acid chains through **ester linkages**. Triglycerides are the primary form of long-term energy storage in animals, deposited in adipose tissue where they also provide insulation and cushion vital organs. Because they are hydrophobic, triglycerides can be stored in a compact, anhydrous form--free of the water that accompanies glycogen storage--making them a far more efficient energy reserve on a per-gram basis.

#### C. Phospholipids

Phospholipids resemble triglycerides in basic architecture but with a crucial difference: one of the three fatty acid chains is replaced by a phosphate group, often bearing an additional polar head group such as choline, ethanolamine, serine, or inositol. This gives phospholipids an **amphipathic** character--a hydrophilic head (the polar phosphate region) and two hydrophobic tails (the nonpolar fatty acid chains). This dual nature is the key to membrane biology. When placed in water, phospholipids spontaneously arrange themselves into a **bilayer**, with the hydrophobic tails facing inward (away from water) and the hydrophilic heads facing outward toward the aqueous environment on both sides. This self-assembling bilayer is the structural foundation of all biological membranes.

#### D. Steroids

Steroids share a distinctive structure of four fused carbon rings--the steroid nucleus--decorated with various functional groups. **Cholesterol** is the most important steroid in animals, serving as a component of cell membranes where it modulates fluidity, and as the precursor for the synthesis of steroid hormones, vitamin D, and bile salts. **Steroid hormones**--including estrogen, testosterone, cortisol, and aldosterone--are all derived from cholesterol. Because they are lipid-soluble, steroid hormones can cross cell membranes directly and bind to intracellular receptors, often acting as transcription factors to regulate gene expression.

#### E. Waxes

Waxes consist of a long-chain fatty acid esterified to a long-chain alcohol. They are highly hydrophobic and serve a waterproofing function in a variety of biological contexts: the waxy cuticle on plant leaves reduces water loss, the preen oil on bird feathers provides water repellency, and cerumen (ear wax) protects the mammalian ear canal.

<image>A four-panel figure. Panel A: Comparison of a saturated fatty acid (straight chain, solid at room temperature) and an unsaturated fatty acid (kinked chain at the cis double bond, liquid at room temperature), with space-filling models showing packing differences. Panel B: A triglyceride molecule showing glycerol backbone with three fatty acid chains attached via ester linkages. Panel C: A phospholipid with labeled hydrophilic head (phosphate + head group) and hydrophobic tails (two fatty acid chains), shown next to a phospholipid bilayer cross-section with water on both sides. Panel D: The four-ring steroid structure of cholesterol with hydroxyl group and hydrocarbon tail labeled.</image>

### IV. Clinical and Applied Connections

Several important clinical conditions connect directly to carbohydrate and lipid biology. **Lactose intolerance** arises from a deficiency of the enzyme lactase; undigested lactose passes into the large intestine where gut bacteria ferment it, producing gas and causing discomfort. **Diabetes mellitus** involves impaired regulation of blood glucose--in Type 1, the immune system destroys the insulin-producing beta cells of the pancreas, while in Type 2, target cells become resistant to insulin's signal. **Atherosclerosis** develops when cholesterol-rich plaques accumulate in arterial walls, narrowing the vessels and increasing the risk of heart attack and stroke. **Trans fats** exacerbate cardiovascular risk by raising LDL ("bad") cholesterol and lowering HDL ("good") cholesterol. On a more positive note, **dietary fiber**--largely composed of indigestible polysaccharides like cellulose--promotes gut health, slows glucose absorption, and reduces the risk of colorectal cancer.