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

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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: A molecule without 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

Monosaccharides exist as D- and L- enantiomers, based on the configuration of the chiral carbon furthest from the carbonyl group.

D-form: Hydroxyl group on the right (most common in nature).
L-form: Hydroxyl group on the left.

Example: D-glucose is the naturally occurring form in biological systems.

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 on the anomeric carbon is trans to the CH2OH group.
β-anomer: The OH on the anomeric carbon is cis to the CH2OH group.
Anomeric carbon: The carbon that was the carbonyl carbon in the open-chain form.
Free anomeric carbon: If the anomeric carbon's OH is not involved in a glycosidic bond, it is 'free' and can act as a reducing sugar.

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: Contain a free anomeric carbon that can act 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 but soluble in organic solvents.

Fatty acids: Long, straight-chain carboxylic acids with an even number of carbons.
Steroids: Lipids with a characteristic four-ring structure.

Fatty Acids

Saturated fatty acids: No double bonds; straight chains; solid at room temperature.
Monounsaturated fatty acids: One double bond.
Polyunsaturated fatty acids: Two or more double bonds.
Geometric isomerism: Naturally occurring unsaturated fatty acids are in the cis configuration.
Essential fatty acids: Polyunsaturated fatty acids that must be obtained from the diet (e.g., linoleic acid).

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 and Cholesterol

Steroids: Lipids with a four-ring core structure.
Cholesterol: Contains hydroxyl, alkyl, and double bond functional groups; precursor to bile salts and steroid hormones.

Comparison Table

Type	Structure	Function
Triacylglycerols	Glycerol + 3 fatty acids	Energy storage
Fatty acids	Long hydrocarbon chain + carboxylic acid	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 a central (α) carbon, an amino group, a carboxyl group, a hydrogen atom, and a variable R group.

α carbon: The central carbon atom.
R group: Side chain that determines the amino acid's properties.

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; must be obtained from the diet. Complete foods contain all essential amino acids; incomplete foods lack one or more.

Protein Structure

Primary structure: Sequence of amino acids linked by peptide (amide) bonds.
Peptide bond: Amide bond 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).
Hydrogen bonding is the main force stabilizing secondary structures.

Tertiary and Quaternary Structure

Tertiary structure: 3D folding due to interactions between R groups (hydrophobic, hydrophilic, ionic, disulfide bonds).
Quaternary structure: Association of two or more polypeptide chains.
Difference: Tertiary is the 3D shape of a single chain; quaternary involves multiple chains.

Denaturation and Enzymes

Denaturation: Loss of secondary, tertiary, or quaternary structure without breaking peptide bonds (primary structure remains intact).
Enzymes: Biological catalysts; have 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 with sugars.

Differences Between DNA and RNA

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

Nucleosides and Nucleotides

Nucleoside: Base + sugar.
Nucleotide: Base + sugar + phosphate group.

Nucleic Acid Sequence and Complementarity

DNA strands are complementary: A pairs with T, G pairs with C.
RNA: A pairs with U, G pairs with 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, Transcription, and Translation

DNA replication: The process by which DNA makes a copy of itself.
Transcription: Synthesis of RNA from a DNA template.
Translation: Synthesis of proteins from mRNA using the genetic code.
Genetic code: Set of three-nucleotide codons that specify amino acids.

Example: If the DNA sequence is 5'-ATG-3', the mRNA sequence is 5'-AUG-3', which codes for methionine.

Mutations

Mutation: A change in the nucleotide sequence of DNA.
Types: Substitution, insertion, deletion, frameshift, silent, missense, nonsense.