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.

• Chiral carbon: A carbon atom attached to four different groups.

• Achiral compound: Does not have a chiral center; its mirror image is superimposable.

Example: D- and L-glucose are enantiomers; they differ in the arrangement around the chiral carbon furthest from the carbonyl group.

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.

Example: Glucose is an aldohexose; fructose is a ketohexose.

D and L Enantiomers of Monosaccharides

Monosaccharides exist as D- and L- enantiomers, 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 the cyclic structure of monosaccharides. Anomers are isomers differing at the anomeric carbon (the carbon derived from the carbonyl group during ring formation).

• α-anomer: The OH group on the anomeric carbon is trans (opposite) to the CH2OH group.

• β-anomer: The OH group on the anomeric carbon is cis (same side) to the CH2OH group.

• Anomeric carbon: The carbon that was the carbonyl carbon in the open-chain form.

Example: In α-D-glucose, the anomeric OH is down; in β-D-glucose, it is up (in Haworth projection).

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/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

Name	Monosaccharide Component	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 (more than amylopectin)	Yes
Cellulose	Glucose	β(1→4)	No	No (humans lack enzyme to hydrolyze β(1→4) bonds)

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 do not have a free anomeric carbon (e.g., sucrose).

Chapter 15 – Lipids

General Properties and Classification

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

• Fatty acids

• Triacylglycerols (triglycerides)

• Steroids

Fatty Acids

Fatty acids are long, straight-chain carboxylic acids with an even number of carbon atoms.

• Saturated fatty acids: No double bonds between carbon atoms.

• Monounsaturated fatty acids: One double bond.

• Polyunsaturated fatty acids: Two or more double bonds.

• Geometric isomerism: Naturally occurring unsaturated fatty acids are usually in the cis configuration.

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

Triacylglycerols (Triglycerides)

Triacylglycerols are 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 fatty acids, converting double bonds to single bonds.

• Hydrolysis: Breakdown of triacylglycerols into glycerol and fatty acids (can be catalyzed by acids, bases, or enzymes).

• Saponification: Hydrolysis of triacylglycerols with a strong base to produce glycerol and soap (salts of fatty acids).

Steroids

Steroids are lipids with a characteristic four-ring structure. Cholesterol is the most common steroid in animals.

• Cholesterol: Contains a hydroxyl group, a double bond, and a hydrocarbon tail.

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

Comparison Table: Triacylglycerols, Fatty Acids, and Steroids

Property	Triacylglycerols	Fatty Acids	Steroids
Structure	Glycerol + 3 fatty acids	Long-chain carboxylic acid	Four fused rings
Function	Energy storage	Energy, membrane component	Hormones, membrane component
Solubility	Insoluble in water	Insoluble in water	Insoluble in water

Chapter 16 – Proteins

Amino Acids

Amino acids are the building blocks of proteins. Each contains a central (α) carbon, an amino group (–NH2), a carboxyl group (–COOH), a hydrogen atom, and a variable R group (side chain).

• α carbon: The central carbon atom to which all groups are attached.

• R group: Determines the identity and properties of the amino acid.

Classification of Amino Acids

• Nonpolar: Hydrophobic side chains.

• Polar neutral: Uncharged polar side chains.

• Polar acidic: Side chains with a carboxylic acid group.

• Polar basic: Side chains with an amino group.

Essential amino acids: Cannot be synthesized by the body and must be obtained from the diet. Foods containing all essential amino acids are called complete proteins.

Protein Structure

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

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

• N-terminus: Free amino group at one end of the peptide chain.

• C-terminus: Free carboxyl group at the other end.

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

Secondary Structure

• α-helix: Right-handed coil stabilized by hydrogen bonds.

• β-sheet: Sheet-like arrangement stabilized by hydrogen bonds.

• Triple helix: Three polypeptide chains woven together (e.g., collagen).

• Attractive force: Hydrogen bonding between backbone atoms.

Tertiary Structure

• Overall 3D shape of a single polypeptide chain.

• Stabilized by: Hydrophobic interactions, hydrophilic interactions, hydrogen bonds, ionic bonds, and disulfide bridges between R groups.

Quaternary Structure

• Association of two or more polypeptide chains (subunits).

• Difference from tertiary: Tertiary is the 3D shape of one chain; quaternary is the arrangement of multiple chains.

Denaturation of Proteins

Denaturation is the loss of secondary, tertiary, or quaternary structure due to heat, pH changes, or chemicals. The primary structure (peptide bonds) remains intact.

Enzymes

Enzymes are biological catalysts, usually proteins, that speed up chemical reactions. Each enzyme has an optimum temperature and pH for activity.

Chapter 17 – Nucleic Acids

Components of DNA and RNA

• Bases: Adenine (A), Guanine (G), Cytosine (C), Thymine (T, in DNA), Uracil (U, in RNA).

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

• Phosphoric acid: Forms the backbone of nucleic acids.

Differences Between DNA and RNA

Feature	DNA	RNA
Sugar	Deoxyribose	Ribose
Bases	A, G, C, T	A, G, C, U
Strands	Double	Single
Function	Genetic information storage	Protein synthesis, gene expression

Nucleoside and Nucleotide

• Nucleoside: Base + sugar.

• Nucleotide: Base + sugar + phosphate group.

Nucleic Acid Sequence and Complementarity

The sequence of nucleotides in DNA or RNA determines genetic information. Complementary base pairing allows prediction of the sequence of the other strand.

• DNA base pairing: A–T, G–C.

• RNA base pairing: A–U, G–C.

3’ hydroxy end: The end of the nucleic acid strand with a free –OH group on the 3’ carbon of the sugar. 5’ phosphate end: The end with a free phosphate group on the 5’ carbon.

DNA Replication

DNA replication is the process by which DNA makes a copy of itself before cell division. It is semi-conservative, meaning each new DNA molecule contains one old and one new strand.

RNA and Types of RNA

• mRNA (messenger RNA): Carries genetic code from DNA to ribosomes.

• tRNA (transfer RNA): Brings amino acids to ribosomes during protein synthesis.

• rRNA (ribosomal RNA): Structural and catalytic component of ribosomes.

Transcription and Translation

• Transcription: Synthesis of RNA from a DNA template.

• Translation: Synthesis of a protein from an mRNA template, using the genetic code.

Genetic code: The set of three-nucleotide codons in mRNA that specify amino acids.

Mutations

• Mutation: A change in the nucleotide sequence of DNA.

• Types of mutations: Substitution, insertion, deletion, frameshift, silent, missense, nonsense.