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. They are classified based on the number of sugar units present.

• Monosaccharides: Single sugar units (e.g., glucose, fructose, galactose).

• Disaccharides: Two monosaccharide units joined by a glycosidic bond (e.g., maltose, lactose, sucrose).

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

Example: Glucose is a monosaccharide; sucrose is a disaccharide composed of glucose and fructose.

Stereoisomers and Chirality

Stereoisomers are compounds with the same molecular formula and sequence of bonded atoms but different three-dimensional orientations.

• Optical isomers (enantiomers): Non-superimposable mirror images due to the presence of a chiral carbon (a carbon atom attached to four different groups).

• Chiral vs. Achiral: Chiral compounds have at least one chiral carbon; achiral compounds do not.

Example: D- and L-glucose are enantiomers.

Classification of Monosaccharides

Monosaccharides are classified by:

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

• Type of carbonyl group: Aldose (aldehyde group), ketose (ketone group).

• Combined classification: e.g., aldotriose, ketotetrose.

D and L Enantiomers

Monosaccharides exist as D- and L- enantiomers, based on the configuration around the chiral carbon farthest from the carbonyl group. Only D-forms occur naturally in most biological systems.

Haworth Projections and Anomers

Monosaccharides can cyclize to form ring structures, represented by Haworth projections.

• α (alpha) and β (beta) anomers: Differ in the orientation of the -OH group on the anomeric carbon (carbon 1 in aldoses).

• Anomeric carbon: The carbon derived from the carbonyl group during cyclization; can be free (reducing) or involved in a glycosidic bond (nonreducing).

Important Monosaccharides

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

Important Disaccharides

Name	Monosaccharide Components	Glycosidic Bond	Reducing?
Maltose	Glucose + Glucose	α(1→4)	Yes
Lactose	Galactose + Glucose	β(1→4)	Yes
Sucrose	Glucose + Fructose	α,β(1→2)	No

Important Polysaccharides

Name	Monosaccharide Unit	Glycosidic Bond	Branched?	Digestible?
Amylose	Glucose	α(1→4)	No	Yes
Amylopectin	Glucose	α(1→4), α(1→6)	Yes	Yes
Glycogen	Glucose	α(1→4), α(1→6)	Yes (highly)	Yes
Cellulose	Glucose	β(1→4)	No	No (humans)

Note: Cellulose cannot be digested by humans due to the β(1→4) linkage.

Reducing and Nonreducing Sugars

• Reducing sugars: Have a free anomeric carbon capable of acting as a reducing agent (e.g., glucose, maltose, lactose).

• Nonreducing sugars: Both anomeric carbons are involved in glycosidic bonds (e.g., sucrose).

Chapter 15: Lipids

General Properties and Classification

Lipids are a diverse group of hydrophobic, nonpolar molecules, insoluble in water. They serve as energy storage, structural components, and signaling molecules.

• Fatty acids

• Triacylglycerols (triglycerides)

• Steroids

Fatty Acids

• Long, straight hydrocarbon chains with an even number of carbons and a terminal carboxylic acid group.

• Saturated fatty acids: No double bonds.

• Monounsaturated fatty acids: One double bond.

• Polyunsaturated fatty acids: Two or more double bonds.

• Geometric isomerism: Naturally occurring unsaturated fatty acids are usually cis isomers.

• Essential fatty acids: Polyunsaturated fatty acids that must be obtained from the diet (e.g., linoleic acid).

Comparison: Saturated vs. Unsaturated Fatty Acids

Property	Saturated	Unsaturated
Double Bonds	None	One or more
Physical State (room temp)	Solid (fats)	Liquid (oils)
Source	Animal	Plant

Triacylglycerols (Triglycerides)

• Esters formed from glycerol and three fatty acids.

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

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

Chemical Properties of Triacylglycerols

• Hydrogenation: Addition of hydrogen to unsaturated bonds, converting oils to fats.

• Hydrolysis: Breakdown into glycerol and fatty acids (e.g., during digestion).

• Saponification: Hydrolysis with a base to produce soap and glycerol.

Steroids

• Characterized by a four-ring structure.

• Cholesterol: Contains hydroxyl, alkyl, and double bond functional groups; precursor to bile salts and steroid hormones.

Comparison: Triacylglycerols, Fatty Acids, and Steroids

Property	Triacylglycerols	Fatty Acids	Steroids
Structure	Glycerol + 3 fatty acids	Long hydrocarbon + COOH	Four fused rings
Function	Energy storage	Building blocks	Hormones, membranes

Chapter 16: Proteins

Amino Acids

Amino acids are the building blocks of proteins, each containing a central (α) carbon, an amino group, a carboxyl group, a hydrogen atom, and a variable R group.

• α carbon: Central carbon to which all groups are attached.

• R group: Side chain that determines the amino acid's properties.

Classification of Amino Acids

• Nonpolar: Hydrophobic side chains.

• Polar neutral: Uncharged, hydrophilic side chains.

• Polar acidic: Side chains with carboxylic acid groups.

• Polar basic: Side chains with amino groups.

Essential amino acids: Cannot be synthesized by the body and must be obtained from the diet.

Protein Structure

• Primary structure: Sequence of amino acids linked by peptide (amide) bonds.

• Peptide bond: Amide linkage between the carboxyl group of one amino acid and the amino group of another.

• N-terminus: Free amino group at one end; C-terminus: Free carboxyl group at the other.

• Secondary structure: Local folding into α-helix, β-sheet, or triple helix, stabilized by hydrogen bonds.

• Tertiary structure: Overall 3D shape, stabilized by interactions between R groups (hydrophobic, hydrophilic, ionic, disulfide bonds).

• Quaternary structure: Association of multiple polypeptide chains.

Denaturation: Loss of secondary, tertiary, or quaternary structure without breaking peptide bonds (primary structure remains intact).

Enzymes

• Biological catalysts, usually proteins.

• Have optimal temperature and pH for activity.

Chapter 17: Nucleic Acids

Components of DNA and RNA

• Bases: Purines (adenine, guanine), pyrimidines (cytosine, thymine in DNA; uracil in RNA).

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

• Phosphoric acid: Forms the backbone via phosphodiester bonds.

Nucleosides and Nucleotides

• Nucleoside: Base + sugar.

• Nucleotide: Base + sugar + phosphate group.

Differences Between DNA and RNA

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

Nucleic Acid Sequence and Complementarity

• DNA strands are antiparallel and complementary (A pairs with T, G with C).

• RNA uses uracil (U) instead of thymine (T).

• 3’ hydroxy end: Free -OH group on the 3’ carbon of the sugar.

• 5’ phosphate end: Free phosphate group on the 5’ carbon.

DNA Replication, Transcription, and Translation

• Replication: DNA makes a copy of itself.

• Transcription: DNA is used to synthesize RNA.

• Translation: RNA directs the synthesis of proteins (amino acid sequence).

Types of RNA

• mRNA: Messenger RNA, carries genetic code from DNA to ribosome.

• tRNA: Transfer RNA, brings amino acids to ribosome.

• rRNA: Ribosomal RNA, structural and catalytic component of ribosomes.

Genetic Code and Mutations

• Genetic code: Triplet codons in mRNA specify amino acids.

• Mutation: Change in DNA sequence; can be substitution, insertion, or deletion.