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: Simple sugars containing a single sugar unit (e.g., glucose, fructose, galactose).

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

• Polysaccharides: Large molecules formed by the polymerization of many monosaccharide units (e.g., starch, glycogen, cellulose).

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

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 that is superimposable on its mirror image.

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 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 to the CH2OH group.

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

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

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?	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 for β(1→4) bonds)

Reducing and Nonreducing Sugars

• Reducing sugars: Contain 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 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; essential fatty acids are polyunsaturated and must be obtained from the diet.

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

Comparison: Saturated vs. Unsaturated Fatty Acids

Property	Saturated	Unsaturated
Double Bonds	None	One or more
Shape	Straight	Kinked (cis)
State at Room Temp	Solid	Liquid

Triacylglycerols (Triglycerides)

• Esters formed from glycerol and three fatty acids.

• Major form of energy storage in animals.

Fats and Oils

• Both are triacylglycerols.

• 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-fused 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-chain carboxylic acid	Four fused rings
Function	Energy storage	Energy, membrane component	Hormones, membrane component

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 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; must be obtained from the diet.

• Complete foods: Contain all essential amino acids (e.g., animal proteins).

• Incomplete foods: Lack one or more essential amino acids (e.g., most plant proteins).

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 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 stabilizing force.

Tertiary Structure

• Three-dimensional folding due to interactions between R groups.

• Stabilized by hydrophobic interactions, hydrophilic interactions, hydrogen bonds, ionic bonds, and disulfide bridges.

Quaternary Structure

• Association of two or more polypeptide chains.

• Difference from tertiary: Tertiary is the 3D structure of a single chain; quaternary involves multiple chains.

Denaturation of Proteins

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

Enzymes

• Biological catalysts, usually proteins.

• 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, DNA only), Uracil (U, RNA only).

• 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

Nucleoside and Nucleotide

• 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 hydroxyl group on the 3’ carbon of the sugar.

• 5’ phosphate end: The end with a free phosphate group on the 5’ carbon.

DNA Replication

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

RNA and Types of RNA

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

• tRNA (transfer RNA): Brings amino acids to the ribosome 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 proteins from mRNA sequence.

• Genetic code: Set of three-nucleotide codons in mRNA that specify amino acids.

Mutations

• Changes in the DNA sequence.

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