Macromolecules: Carbohydrates, Nucleic Acids, and Lipids – Study Notes for General Biology

The Chemical Content of Life

Introduction to Biological Macromolecules

Cells are composed of a diverse array of molecules, many of which are organized into four major classes of macromolecules: carbohydrates, lipids, proteins, and nucleic acids. Each class has a distinct structure and function, contributing to the complexity and specialization of cellular processes.

• Macromolecules are large, complex molecules formed from smaller organic building blocks.

• Major building blocks include: sugars, fatty acids, amino acids, and nucleotides.

• These monomers are polymerized to form polysaccharides, fats and membrane lipids, proteins, and nucleic acids.

Biological Macromolecules

Polymerization: Formation and Breakdown of Polymers

Macromolecules are assembled and disassembled through specific chemical reactions:

• Dehydration Reaction: Synthesizes polymers by removing a water molecule, forming a new covalent bond between monomers.

• Hydrolysis: Breaks down polymers by adding a water molecule, cleaving the covalent bond between monomers.

Carbohydrates

Definition and Biological Functions

Carbohydrates are organic molecules consisting of carbon, hydrogen, and oxygen, typically with the general formula $C_nH_{2n}O_n$. They serve as both fuel and structural components in cells.

• Include sugars and polymers of sugars.

• Major types: Monosaccharides, Disaccharides, Polysaccharides.

• Functions: Energy source, structural support, cell recognition.

Classification of Monosaccharides

Monosaccharides are the simplest carbohydrates and are classified by:

• Number of carbon atoms in the backbone (e.g., triose, pentose, hexose).

• Location of the carbonyl group: Aldose (aldehyde group) or Ketose (ketone group).

Examples of Monosaccharides

• Glucose ($C_6H_{12}O_6$): A common hexose aldose.

• Galactose ($C_6H_{12}O_6$): An isomer of glucose.

• Ribose ($C_5H_{10}O_5$): A pentose aldose found in RNA.

• Fructose ($C_6H_{12}O_6$): A hexose ketose.

Monosaccharide Structure Table

Type	Example	Formula	Carbonyl Group
Aldose (3C)	Glyceraldehyde	$C_3H_6O_3$	Aldehyde
Ketose (3C)	Dihydroxyacetone	$C_3H_6O_3$	Ketone
Aldose (5C)	Ribose	$C_5H_{10}O_5$	Aldehyde
Ketose (5C)	Ribulose	$C_5H_{10}O_5$	Ketone
Aldose (6C)	Glucose, Galactose	$C_6H_{12}O_6$	Aldehyde
Ketose (6C)	Fructose	$C_6H_{12}O_6$	Ketone

Ring Formation in Monosaccharides

In aqueous solutions, monosaccharides often form ring structures when the carbonyl group reacts with a hydroxyl group on the same molecule.

• Alpha ($oldsymbol{ ext{α}}$) and Beta ($oldsymbol{ ext{β}}$) Glucose: The position of the hydroxyl group on carbon 1 determines whether the glucose is in the alpha or beta form.

• These forms are important for the properties of polysaccharides.

Disaccharides

Disaccharides are formed by the dehydration reaction between two monosaccharides, resulting in a covalent bond called a glycosidic linkage.

• Examples: Sucrose (glucose + fructose), Lactose (glucose + galactose), Maltose (glucose + glucose).

Polysaccharides

Polysaccharides are long chains of monosaccharide units linked by glycosidic bonds. They serve structural and energy storage roles.

• Starch: Storage polysaccharide in plants, composed of α-glucose with 1-4 linkages.

• Glycogen: Storage polysaccharide in animals, highly branched α-glucose polymer.

• Cellulose: Structural polysaccharide in plant cell walls, composed of β-glucose with 1-4 linkages.

Comparison Table: Major Polysaccharides

Polysaccharide	Monomer	Linkage	Function	Organism
Starch	α-glucose	α-1,4	Energy storage	Plants
Glycogen	α-glucose	α-1,4 and α-1,6 (branching)	Energy storage	Animals
Cellulose	β-glucose	β-1,4	Structural support	Plants

Structural Features

• Cellulose molecules are straight and unbranched, allowing hydrogen bonding between parallel chains for strength.

• Starch and glycogen are helical and branched, making them more accessible for enzymatic breakdown.

Lipids

Definition and Classification

Lipids are hydrophobic organic molecules, including fats, oils, phospholipids, and steroids. They are not polymers but are assembled from smaller components.

• Fats (Triglycerides): Composed of glycerol and three fatty acids.

• Phospholipids: Glycerol, two fatty acids, and a phosphate group.

• Steroids: Four fused carbon rings (e.g., cholesterol).

Fatty Acids: Saturated vs. Unsaturated

• Saturated fatty acids: No double bonds, straight chains, solid at room temperature.

• Unsaturated fatty acids: One or more double bonds, bent chains, liquid at room temperature.

• Trans fats: Produced by hydrogenation, associated with increased cardiovascular risk.

Phospholipids and Biological Membranes

Phospholipids are amphipathic molecules, with hydrophobic tails and hydrophilic heads. In water, they form bilayers that are the basis of cell membranes.

• Phospholipid bilayer: Major component of biological membranes, forms a selective barrier.

• Micelles: Spherical arrangements aiding in absorption of fat-soluble compounds.

Steroids

• Cholesterol: Precursor for steroid hormones, amphipathic structure.

• Steroid hormones: Regulate development, metabolism, and behavior.

Nucleic Acids

Definition and Function

Nucleic acids are polymers of nucleotide monomers and include DNA and RNA. They store and transmit genetic information.

• DNA (Deoxyribonucleic acid): Double-stranded helix, stores genetic information.

• RNA (Ribonucleic acid): Single-stranded, involved in protein synthesis and other cellular functions.

Nucleotide Structure

• Each nucleotide consists of a pentose sugar (ribose or deoxyribose), a phosphate group, and a nitrogenous base (purine or pyrimidine).

• Phosphodiester linkage: Connects the 5' carbon of one sugar to the 3' carbon of the next, giving directionality to the nucleic acid strand.

DNA vs. RNA Comparison Table

Feature	DNA	RNA
Strands	Double-stranded helix	Single-stranded
Sugar	Deoxyribose	Ribose
Bases	A, T, G, C	A, U, G, C
Function	Genetic information storage	Protein synthesis, regulation

Central Dogma of Molecular Biology

• Genetic information flows from DNA to RNA to protein.

• This process is called gene expression.

DNA Structure and Base Pairing

• DNA strands are antiparallel: one runs 5' to 3', the other 3' to 5'.

• Base pairing: Adenine (A) pairs with Thymine (T), Guanine (G) pairs with Cytosine (C).

• Chargaff's rules: $[ ext{A}] = [ ext{T}]$, $[ ext{G}] = [ ext{C}]$ in double-stranded DNA.

Key Equations

• General formula for carbohydrates: $C_nH_{2n}O_n$

• Dehydration reaction: $ext{Monomer}_1 + ext{Monomer}_2 ightarrow ext{Polymer} + H_2O$

• Hydrolysis reaction: $ext{Polymer} + H_2O ightarrow ext{Monomer}_1 + ext{Monomer}_2$

Example

• Starch in potatoes serves as an energy reserve for the plant.

• Cellulose in wood provides structural support for trees.

• Phospholipid bilayer forms the boundary of all living cells.

• DNA encodes the instructions for building proteins in all organisms.

Additional info: Some details, such as the full structure of nucleotides and the role of essential fatty acids, were inferred and expanded for completeness.