Comprehensive Study Notes: Carbohydrates, Lipids, Proteins, and Nucleic Acids

Chapter 13: Carbohydrates

Types of Carbohydrates

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, and are classified based on their complexity:

• Monosaccharides: Simple sugars with a single unit (e.g., glucose).

• Disaccharides: Composed of two monosaccharide units joined by a glycosidic bond (e.g., sucrose).

• Polysaccharides: Long chains of monosaccharide units (e.g., starch).

Example: Glucose (monosaccharide), lactose (disaccharide), cellulose (polysaccharide).

Stereoisomers, Optical Isomers, and Enantiomers

Stereoisomers are molecules with the same molecular formula but different spatial arrangements. Optical isomers (enantiomers) are a type of stereoisomer that are non-superimposable mirror images.

• Chiral Carbon: A carbon atom bonded to four different groups, leading to chirality.

• Chiral Compounds: Have at least one chiral carbon; achiral compounds do not.

• Enantiomers: Pair of molecules that are mirror images but not superimposable.

Example: D- and L-glucose are enantiomers.

Classification of Monosaccharides

Monosaccharides are classified by:

• Number of Carbons: Triose (3), tetrose (4), pentose (5), hexose (6), etc.

• Carbonyl Group: Aldose (aldehyde group), ketose (ketone group).

• Combined Classification: Aldotriose, ketotetrose, etc.

D and L Enantiomers of Monosaccharides

Monosaccharides exist as D- and L- forms, based on the configuration of the chiral carbon furthest from the carbonyl group. Only D-forms are commonly found in nature.

Haworth Projections and Anomers

Haworth projections represent cyclic forms of monosaccharides. Anomers are isomers differing at the anomeric carbon.

• α-Anomer: The OH group on the anomeric carbon is trans to the CH2OH group.

• β-Anomer: The OH group is cis to the CH2OH group.

• Anomeric Carbon: The carbon derived from the carbonyl group during ring formation.

Important Monosaccharides

Monosaccharide	Carbons	Chiral Carbons	Aldose/Ketose	D/L
Glucose	6	4	Aldose	D
Fructose	6	3	Ketose	D
Galactose	6	4	Aldose	D

Important Disaccharides

Disaccharide	Components	Glycosidic Bond	Reducing/Nonreducing
Maltose	Glucose + Glucose	α(1→4)	Reducing
Lactose	Glucose + Galactose	β(1→4)	Reducing
Sucrose	Glucose + Fructose	α,β(1→2)	Nonreducing

Important Polysaccharides

Polysaccharide	Components	Glycosidic Bond	Branched?	Digestibility
Amylose	Glucose	α(1→4)	No	Digestible
Amylopectin	Glucose	α(1→4), α(1→6)	Yes	Digestible
Glycogen	Glucose	α(1→4), α(1→6)	Yes (more branched)	Digestible
Cellulose	Glucose	β(1→4)	No	Indigestible (humans lack enzyme)

Reducing and Nonreducing Sugars

Reducing sugars have a free anomeric carbon capable of acting as a reducing agent. Nonreducing sugars (e.g., sucrose) do not have a free anomeric carbon.

Chapter 15: Lipids

General Properties of Lipids

Lipids are a diverse group of hydrophobic, nonpolar molecules, insoluble in water but soluble in organic solvents.

• Nonpolar and insoluble in water.

• Serve as energy storage, structural components, and signaling molecules.

Classification of Lipids

• Fatty acids: Long, straight-chain carboxylic acids.

• Steroids: Four fused ring structures (e.g., cholesterol).

Fatty Acids

• Long, straight chains with an even number of carbons.

• Contain a carboxylic acid functional group.

Saturated, Monounsaturated, and Polyunsaturated Fatty Acids

• Saturated: No double bonds.

• Monounsaturated: One double bond.

• Polyunsaturated: Two or more double bonds.

Geometric Isomerism in Unsaturated Fatty Acids

Unsaturated fatty acids exhibit cis-trans isomerism. Only cis-isomers occur naturally, causing kinks in the chain.

Differences Between Saturated and Unsaturated Fatty Acids

• Saturated: Solid at room temperature, straight chains.

• Unsaturated: Liquid at room temperature, bent chains due to cis double bonds.

Essential Fatty Acids

Polyunsaturated fatty acids that cannot be synthesized by the body and must be obtained from diet (e.g., linoleic acid).

Triacylglycerols

Esters formed from glycerol and three fatty acids (triester).

• Fats: Solid at room temperature, more saturated fatty acids.

• Oils: Liquid at room temperature, more unsaturated fatty acids.

Chemical Properties of Triacylglycerols

• Hydrogenation: Addition of hydrogen to unsaturated bonds.

• Hydrolysis: Splitting into glycerol and fatty acids.

• Saponification: Hydrolysis with base to produce soap.

Steroids

Steroids are lipids with a characteristic four-ring structure.

Cholesterol and Bile Salts

• Cholesterol: Contains hydroxyl, alkyl, and double bond functional groups.

• Bile Salts: Derived from cholesterol, aid in fat digestion.

Differences Between Triacylglycerols, Fatty Acids, and Steroids

Type	Structure	Function
Triacylglycerols	Glycerol + 3 fatty acids	Energy storage
Fatty acids	Long chain carboxylic acids	Building blocks, energy
Steroids	Four fused rings	Hormones, membrane structure

Chapter 16: Proteins

Amino Acids

Amino acids are the building blocks of proteins, each containing an α-carbon, an amino group, a carboxyl group, and an R group.

• α-Carbon: Central carbon atom.

• Functional Groups: Amino (-NH2), carboxyl (-COOH), and R group.

• R Group: Side chain that determines amino acid properties.

Classification of Amino Acids

• Nonpolar: Hydrophobic side chains.

• Polar Neutral: Hydrophilic, uncharged side chains.

• Polar Acidic: Side chains with negative charge.

• Polar Basic: Side chains with positive charge.

Essential Amino Acids

Amino acids that cannot be synthesized by the body and must be obtained from diet. Complete foods contain all essential amino acids; incomplete foods lack one or more.

Primary Structure of Proteins

The sequence of amino acids in a polypeptide chain, linked by peptide bonds (amide bonds).

• N-terminus: Amino end.

• C-terminus: Carboxyl end.

• Dipeptide, tripeptide, tetrapeptide: Chains of 2, 3, 4 amino acids, respectively.

Secondary Structure of Proteins

Regular folding patterns stabilized by hydrogen bonds:

• α-Helix: Spiral structure.

• β-Sheet: Sheet-like structure.

• Triple Helix: Three polypeptide chains wound together.

Tertiary Structure of Proteins

Three-dimensional folding due to interactions between R groups:

• Hydrophobic interactions

• Hydrophilic interactions

• Salt bridges

• Disulfide bonds

Quaternary Structure of Proteins

Association of multiple polypeptide chains. Differs from tertiary structure, which involves a single chain.

Denaturation of Proteins

Loss of structure due to heat, pH, or chemicals. Primary structure (peptide bonds) remains intact.

Enzymes

Proteins that catalyze biochemical reactions. Each enzyme has an optimum temperature and pH for activity.

Chapter 17: Nucleic Acids

Components of DNA and RNA

• Bases: Adenine, guanine, cytosine, thymine (DNA), uracil (RNA).

• Sugars: Deoxyribose (DNA), ribose (RNA).

• Phosphoric acid: Forms the backbone.

Differences Between DNA and RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, G, C, T	A, G, C, U
Strands	Double	Single

Nucleoside and Nucleotide

• Nucleoside: Sugar + base.

• Nucleotide: Sugar + base + phosphate.

Nucleic Acid Sequence

DNA strands are complementary. The sequence of one strand determines the other. RNA is transcribed from DNA.

• 3' Hydroxy End: The end with a free -OH group.

• 5' Phosphate End: The end with a free phosphate group.

DNA Replication

Process by which DNA makes a copy of itself before cell division.

RNA and Types of RNA

• mRNA: Messenger RNA, carries genetic code.

• tRNA: Transfer RNA, brings amino acids.

• rRNA: Ribosomal RNA, forms ribosomes.

Transcription and Translation

• Transcription: DNA is copied into RNA.

• Translation: RNA directs protein synthesis.

Genetic Code and Amino Acid Sequence

The genetic code is a set of three-base codons in mRNA that specify amino acids.

Mutations

• Types: Substitution, insertion, deletion.

• Can alter protein function or cause disease.