The Macromolecules of the Cell: Study Notes for Cell Biology

The Macromolecules of the Cell

Introduction

Macromolecules are essential components of all cells, forming the structural and functional basis of life. They are synthesized from small molecules through condensation reactions, resulting in polymers with unique properties. The four major classes of macromolecules are proteins, nucleic acids, polysaccharides, and lipids.

Proteins

Classes and Functions of Proteins

Proteins are versatile macromolecules found throughout the cell, classified into nine major functional groups:

• Enzymes: Catalysts that accelerate chemical reactions.

• Structural proteins: Provide physical support and shape.

• Motility proteins: Enable contraction and movement.

• Regulatory proteins: Control and coordinate cellular functions.

• Transport proteins: Facilitate movement of substances across membranes.

• Signaling proteins: Mediate communication between cells.

• Receptor proteins: Allow cells to respond to environmental stimuli.

• Defensive proteins: Protect against disease.

• Storage proteins: Serve as reservoirs of amino acids.

Amino Acids: The Monomers of Proteins

Proteins are polymers of amino acids, of which 20 types are used in protein synthesis. Each amino acid has a central (α) carbon, an amino group, a carboxyl group, and a unique side chain (R group). The properties of amino acids are determined by their R groups, which can be nonpolar, polar, acidic, or basic.

• Nonpolar (hydrophobic) R groups: 9 amino acids

• Polar or charged (hydrophilic) R groups: 11 amino acids

Polypeptides and Protein Structure

Amino acids are linked by peptide bonds formed via dehydration reactions, resulting in polypeptides with directionality (N-terminus and C-terminus). Proteins may be monomeric (single polypeptide) or multimeric (multiple polypeptides, e.g., hemoglobin).

Bonds and Interactions in Protein Folding

• Covalent bonds: Disulfide bonds between cysteine residues stabilize protein structure.

• Noncovalent interactions: Hydrogen bonds, ionic bonds, van der Waals forces, and hydrophobic interactions collectively influence folding and stability.

• Molecular chaperones: Assist in proper protein folding.

Levels of Protein Structure

• Primary structure: Linear sequence of amino acids.

• Secondary structure: Local folding patterns (α helix, β sheet) stabilized by hydrogen bonds.

• Tertiary structure: Three-dimensional conformation determined by R group interactions.

• Quaternary structure: Assembly of multiple polypeptides into a functional unit.

Example: Hemoglobin is a tetramer with two α and two β subunits.

Protein Domains and Motifs

Domains are discrete units of tertiary structure with specific functions. Motifs are recurring combinations of α helices and β sheets, such as the β–α–β motif and helix-turn-helix motif.

Nucleic Acids

Structure and Function

Nucleic acids (DNA and RNA) are linear polymers of nucleotides that store, transmit, and express genetic information. DNA contains deoxyribose, while RNA contains ribose.

Nucleotides: The Monomers

• Components: Five-carbon sugar, phosphate group, nitrogenous base (purine or pyrimidine).

• Purines: Adenine (A), Guanine (G)

• Pyrimidines: Thymine (T), Cytosine (C), Uracil (U in RNA)

Polymerization and Structure

Nucleotides are linked by 3′,5′ phosphodiester bonds, giving nucleic acids directionality (5′ to 3′). DNA is a double-stranded helix with antiparallel strands, stabilized by complementary base pairing (A-T, G-C).

• Base pairing: A-T (2 hydrogen bonds), G-C (3 hydrogen bonds)

RNA is usually single-stranded but forms local base-paired regions.

Polysaccharides

Structure and Function

Polysaccharides are long-chain polymers of monosaccharides, serving structural and storage roles. The repeating units are monosaccharides, which may be aldoses (aldehyde group) or ketoses (ketone group).

Classification of Sugars

• Trioses: 3 carbons

• Tetroses: 4 carbons

• Pentoses: 5 carbons

• Hexoses: 6 carbons (e.g., d-glucose)

• Heptoses: 7 carbons

Glucose and Its Forms

d-glucose exists in both linear and ring forms, with α and β anomers based on the orientation of the hydroxyl group on carbon 1.

Disaccharides and Glycosidic Bonds

• Maltose: Two glucose units (α glycosidic bond)

• Lactose: Glucose and galactose (β glycosidic bond)

• Sucrose: Glucose and fructose

Storage and Structural Polysaccharides

• Starch: Plant storage polysaccharide (amylose and amylopectin)

• Glycogen: Animal storage polysaccharide, highly branched

• Cellulose: Structural polysaccharide in plant cell walls (β-d-glucose)

• Chitin: Structural polysaccharide in exoskeletons and fungal cell walls (GlcNAc units)

Glycosidic Bond Types and Structure

α and β glycosidic bonds result in different structural properties: α bonds (starch, glycogen) form loose helices; β bonds (cellulose) form rigid rods and microfibrils.

Lipids

General Features

Lipids are hydrophobic macromolecules, not formed by linear polymerization. They are important for energy storage, membrane structure, and signaling.

Main Classes of Lipids

• Fatty acids: Long hydrocarbon chains with a carboxyl group; can be saturated (no double bonds) or unsaturated (one or more double bonds).

• Triacylglycerols: Glycerol with three fatty acids; main energy storage form.

• Phospholipids: Amphipathic molecules forming membranes; include phosphoglycerides and sphingolipids.

• Glycolipids: Lipids with carbohydrate groups; important in membrane structure.

• Steroids: Four-ringed structures; include cholesterol and steroid hormones.

• Terpenes: Isoprene-based compounds; include vitamins and pigments.

Fatty Acids and Their Properties

Saturated fatty acids have straight chains and pack tightly; unsaturated fatty acids have bends due to double bonds and are less tightly packed. Trans fats are a type of unsaturated fatty acid with less pronounced bends, associated with health risks.

Triacylglycerols

Triacylglycerols are formed by esterification of three fatty acids to glycerol. They are solid (fats) if saturated and liquid (oils) if unsaturated.

Phospholipids

Phospholipids are major membrane components. Phosphoglycerides have a glycerol backbone, two fatty acids, and a phosphate group linked to an alcohol. Sphingolipids are based on sphingosine and are found in membrane microdomains (lipid rafts).

Glycolipids

Glycolipids contain carbohydrate groups and are found mainly in the outer leaflet of the plasma membrane.

Steroids

Steroids have a four-ring structure. Cholesterol is the most common steroid in animal cells and is a precursor for steroid hormones (estrogens, androgens, glucocorticoids, mineralocorticoids).

Terpenes

Terpenes are synthesized from isoprene units and include compounds such as vitamin A, carotenoids, and electron carriers (coenzyme Q, plastoquinone).

Tables

Common Small Molecules in Cells

Type	Example	Function
Amino acids	Glycine, Alanine	Protein synthesis
Nucleotides	ATP, GTP	Genetic information, energy
Monosaccharides	Glucose	Energy, structure
Fatty acids	Palmitate	Energy storage, membrane structure
Additional info:	Other small molecules (vitamins, cofactors)	Enzyme function, signaling

Abbreviations for Amino Acids

Amino Acid	3-Letter Code	1-Letter Code
Alanine	Ala	A
Arginine	Arg	R
Asparagine	Asn	N
Aspartic acid	Asp	D
Additional info:	See full table for all 20 amino acids

Levels of Organization of Protein Structure

Level	Description
Primary	Amino acid sequence
Secondary	Local folding (α helix, β sheet)
Tertiary	Three-dimensional conformation
Quaternary	Assembly of multiple polypeptides

The Bases, Nucleosides, and Nucleotides of RNA and DNA

Base	Nucleoside	Nucleotide
Adenine	Adenosine	AMP, ADP, ATP
Guanine	Guanosine	GMP, GDP, GTP
Cytosine	Cytidine	CMP, CDP, CTP
Thymine (DNA)	Thymidine	TMP, TDP, TTP
Uracil (RNA)	Uridine	UMP, UDP, UTP

Some Common Fatty Acids in Cells

Name	Number of Carbons	Saturation
Palmitate	16	Saturated
Stearate	18	Saturated
Oleate	18	Monounsaturated
Linoleate	18	Polyunsaturated

Key Equations

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

• Peptide bond formation: $\text{Amino acid}_1 + \text{Amino acid}_2 \rightarrow \text{Dipeptide} + H_2O$

• Phosphodiester bond in nucleic acids: $\text{Nucleotide}_1 + \text{Nucleotide}_2 \rightarrow \text{Dinucleotide} + H_2O$

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