Chemical Components of Cells: Amino Acids, Nucleic Acids, Sugars, and Lipids

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Chemical Components of Cells

Common Small Molecules in Cells

Cells are composed of a variety of small molecules that serve as the building blocks for macromolecules. These include amino acids, aromatic bases, sugars, and lipids, each with distinct roles in cellular structure and function.

Kind of Molecules	Number Present	Names of Molecules	Role in Cell
Amino acids	20	See list below	Monomeric units of all proteins
Aromatic bases	5	Adenine, Cytosine, Guanine, Thymine, Uracil	Components of nucleic acids
Sugars	varies	Ribose, Deoxyribose, Glucose	Components of RNA, DNA, energy metabolism
Lipids	varies	Fatty acids, Cholesterol	Components of membranes, energy storage

Table of common small molecules in cells

Amino Acids: Structure and Classification

Amino acids are the monomeric units of proteins. Each amino acid has a central carbon (α carbon) bonded to an amino group, a carboxyl group, a hydrogen atom, and a variable R group. The R group determines the properties and classification of the amino acid.

Basic structure: NH3+–C–COO− with an R group and a hydrogen atom.
Isomerism: Amino acids exist as L and D isomers, but only L isomers are used in protein synthesis.
Classification: Based on the R group, amino acids can be nonpolar (hydrophobic), polar (hydrophilic), or charged (acidic/basic).

L and D isomers of amino acids Handwritten amino acid structure

Amino Acid	Three-Letter Abbreviation	One-Letter Abbreviation
Alanine	Ala	A
Arginine	Arg	R
Asparagine	Asn	N
Aspartate	Asp	D
Cysteine	Cys	C
Glutamate	Glu	E
Glutamine	Gln	Q
Glycine	Gly	G
Histidine	His	H
Isoleucine	Ile	I
Leucine	Leu	L
Lysine	Lys	K
Methionine	Met	M
Phenylalanine	Phe	F
Proline	Pro	P
Serine	Ser	S
Threonine	Thr	T
Tryptophan	Trp	W
Tyrosine	Tyr	Y
Valine	Val	V

Table of amino acid abbreviations

Groups of Amino Acids

Nonpolar (hydrophobic): Glycine, Alanine, Valine, Leucine, Isoleucine, Methionine, Phenylalanine, Proline
Polar (hydrophilic): Serine, Threonine, Cysteine, Tyrosine, Asparagine, Glutamine
Charged: Acidic (Aspartate, Glutamate), Basic (Lysine, Arginine, Histidine)

Amino acid groups: nonpolar, polar, charged Handwritten classification of R groups

Peptide Bond Formation and Polypeptides

Amino acids are linked by peptide bonds formed through dehydration reactions. The resulting polypeptide is a linear, unbranched polymer with an N-terminus and a C-terminus.

Peptide bond: Covalent bond between the carboxyl group of one amino acid and the amino group of another.
Directionality: Polypeptides are synthesized from N-terminus to C-terminus.

Peptide bond formation between glycine and alanine Handwritten polypeptide structure

Levels of Protein Structure

Proteins exhibit four levels of structural organization: primary, secondary, tertiary, and quaternary. Each level is stabilized by specific types of bonds and interactions.

Level of Structure	Basis of Structure	Kinds of Bonds and Interactions Involved
Primary	Amino acid sequence	Covalent peptide bonds
Secondary	Folding into α helix, β sheet, or random coil	Hydrogen bonds between NH and CO groups
Tertiary	Three-dimensional folding of a single polypeptide chain	Disulfide bonds, hydrogen bonds, ionic bonds, van der Waals, hydrophobic interactions
Quaternary	Association of multiple polypeptides	Same as for tertiary structure

Table of protein structure levels Diagram of primary, secondary, tertiary, and quaternary structure

Secondary Structure: α Helix and β Sheet

α Helix: Spiral structure stabilized by hydrogen bonds between NH and CO groups four amino acids apart.
β Sheet: Sheet-like structure stabilized by hydrogen bonds between adjacent polypeptide regions; can be parallel or antiparallel.

α helix structure β sheet structure Common secondary structure motifs

Tertiary Structure

Tertiary structure is the overall three-dimensional shape of a single polypeptide, determined by interactions among R groups.

Hydrophobic residues: Tend to be buried in the interior.
Hydrophilic residues: Tend to be exposed on the surface.
Stabilizing interactions: Disulfide bonds, hydrogen bonds, ionic bonds, van der Waals forces.

Types of bonds in tertiary structure Handwritten diagram of hydrophobic/hydrophilic residue distribution

Quaternary Structure

Quaternary structure refers to the association of two or more polypeptide chains to form a functional protein complex. Hemoglobin is a classic example, consisting of two α and two β subunits.

Hemoglobin tetramer structure

Nucleic Acids: Structure and Components

Nucleic acids (DNA and RNA) are polymers of nucleotides, each composed of a phosphate group, a five-carbon sugar (ribose or deoxyribose), and a nitrogenous base (purine or pyrimidine).

Purines: Adenine (A), Guanine (G)
Pyrimidines: Cytosine (C), Thymine (T, in DNA), Uracil (U, in RNA)
Nucleoside: Sugar + base
Nucleotide: Sugar + base + phosphate group

Handwritten nitrogenous base structures Handwritten sugar structure Handwritten adenine and ribose structure Nucleotide structure Nucleic acid components: sugars, bases, phosphate Handwritten nucleoside and nucleotide structure

Bases	Nucleoside (RNA)	Nucleotide (RNA)	Deoxynucleoside (DNA)	Deoxynucleotide (DNA)
Adenine (A)	Adenosine	Adenosine monophosphate (AMP)	Deoxyadenosine	Deoxyadenosine monophosphate (dAMP)
Guanine (G)	Guanosine	Guanosine monophosphate (GMP)	Deoxyguanosine	Deoxyguanosine monophosphate (dGMP)
Cytosine (C)	Cytidine	Cytidine monophosphate (CMP)	Deoxycytidine	Deoxycytidine monophosphate (dCMP)
Uracil (U)	Uridine	Uridine monophosphate (UMP)	-	-
Thymine (T)	-	-	Deoxythymidine	Deoxythymidine monophosphate (dTMP)

Table of bases, nucleosides, and nucleotides Handwritten purine and pyrimidine structures

Sugars in Cells

Sugars are essential components of nucleic acids and serve as energy sources. Ribose is found in RNA, deoxyribose in DNA, and glucose is a primary energy source.

Handwritten sugar structure

Lipids in Cells

Lipids are hydrophobic molecules that serve as energy storage and structural components of membranes. Fatty acids and cholesterol are key examples.

Fatty acids: Building blocks for other lipids
Cholesterol: Component of membranes

Summary Table: Levels of Organization of Protein Structure

Level of Structure	Basis of Structure	Kinds of Bonds and Interactions Involved
Primary	Amino acid sequence	Covalent peptide bonds
Secondary	Folding into α helix, β sheet, or random coil	Hydrogen bonds between NH and CO groups
Tertiary	Three-dimensional folding of a single polypeptide chain	Disulfide bonds, hydrogen bonds, ionic bonds, van der Waals, hydrophobic interactions
Quaternary	Association of multiple polypeptides	Same as for tertiary structure

Table of protein structure levels

Key Points

Amino acids are the monomers of proteins, classified by their R groups.
Nucleic acids are polymers of nucleotides, which consist of a sugar, a phosphate, and a nitrogenous base.
Sugars are energy sources and structural components of nucleic acids.
Lipids are hydrophobic molecules important for energy storage and membrane structure.