Biochemistry Exam Study Guide: Carbohydrates, Lipids, and Proteins

Carbohydrates

Classification and Structure

Carbohydrates are organic molecules composed of carbon, hydrogen, and oxygen, and are essential for energy storage and structural functions in living organisms.

• Monosaccharides: Simple sugars such as glucose, fructose, and galactose. They are the basic building blocks of carbohydrates.

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

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

• Heteropolysaccharides: Polysaccharides composed of more than one type of monosaccharide.

Example: Starch is a polysaccharide made up of glucose units and serves as an energy reserve in plants.

Isomerism in Carbohydrates

Isomerism refers to the existence of molecules with the same molecular formula but different structural arrangements.

• Structural Isomers: Differ in the connectivity of atoms (e.g., glucose vs. fructose).

• Stereoisomers: Same connectivity but differ in spatial arrangement (e.g., D- and L-glucose).

• Anomers: Isomers differing at the anomeric carbon (α and β forms of glucose).

Example: D-glucose and L-glucose are mirror images and differ in their biological activity.

Reducing and Non-Reducing Sugars

Reducing sugars have a free aldehyde or ketone group capable of acting as a reducing agent.

• Reducing sugars: Glucose, maltose, lactose

• Non-reducing sugars: Sucrose

Test: Benedict's test is used to identify reducing sugars.

Functions of Carbohydrates

• Energy source (e.g., glucose in cellular respiration)

• Structural components (e.g., cellulose in plant cell walls)

• Cell recognition and signaling (e.g., glycoproteins)

Lipids

Physical Properties of Fatty Acids

Fatty acids are carboxylic acids with long hydrocarbon chains. Their properties depend on chain length and degree of saturation.

• Saturated fatty acids: No double bonds; solid at room temperature (e.g., palmitic acid).

• Unsaturated fatty acids: One or more double bonds; liquid at room temperature (e.g., oleic acid).

Classification of Lipids

• Simple lipids: Esters of fatty acids with alcohols (e.g., triglycerides).

• Compound lipids: Contain additional groups (e.g., phospholipids, glycolipids).

• Derived lipids: Substances derived from simple and compound lipids (e.g., steroids).

Functions of Lipids

• Energy storage (e.g., triglycerides)

• Structural components of membranes (e.g., phospholipids)

• Signaling molecules (e.g., steroid hormones)

Proteins

Classification and Structure

Proteins are polymers of amino acids and perform a wide range of functions in biological systems.

• Fibrous proteins: Structural roles (e.g., collagen, keratin)

• Globular proteins: Functional roles (e.g., enzymes, hemoglobin)

Levels of Protein Structure

• Primary structure: Sequence of amino acids in a polypeptide chain.

• Secondary structure: Local folding patterns such as α-helix and β-sheet, stabilized by hydrogen bonds.

• Tertiary structure: Overall 3D shape of a single polypeptide, stabilized by various interactions (hydrophobic, ionic, disulfide bonds).

• Quaternary structure: Association of multiple polypeptide chains (e.g., hemoglobin).

Example: Hemoglobin is a quaternary protein composed of four polypeptide subunits.

Properties and Functions of Proteins

• Catalysis (enzymes)

• Transport (hemoglobin, membrane channels)

• Structural support (collagen)

• Regulation (hormones, transcription factors)

Protein Isolation and Analysis

• Isolation techniques: Salting out, chromatography, electrophoresis

• Analysis: Determination of amino acid sequence, structure, and function

Enzymes

Classification and Mechanism

Enzymes are biological catalysts that accelerate chemical reactions in living organisms.

• Classification: Based on the type of reaction catalyzed (e.g., oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases)

• Mechanism: Enzymes lower activation energy and increase reaction rate

Equation:

$E + S \rightleftharpoons ES \rightarrow E + P$

Where E is enzyme, S is substrate, ES is enzyme-substrate complex, and P is product.

Enzyme Kinetics

• Michaelis-Menten equation:

$v = \frac{V_{max}[S]}{K_m + [S]}$

Where v is reaction velocity, Vmax is maximum velocity, Km is Michaelis constant, and [S] is substrate concentration.

Enzyme Inhibition

• Competitive inhibition: Inhibitor competes with substrate for active site.

• Non-competitive inhibition: Inhibitor binds elsewhere, altering enzyme activity.

• Uncompetitive inhibition: Inhibitor binds only to enzyme-substrate complex.

Enzyme Isolation and Classification

• Enzymes can be isolated from biological samples using techniques such as centrifugation, chromatography, and electrophoresis.

• Enzymes are classified based on the reactions they catalyze and their substrate specificity.

Enzyme Reactions: Reversible vs. Non-reversible

• Reversible reactions: Can proceed in both directions under physiological conditions.

• Non-reversible reactions: Proceed in one direction; often regulated and rate-limiting in metabolic pathways.

HTML Table: Comparison of Biomolecule Classes

Biomolecule	Monomer	Main Function	Example
Carbohydrate	Monosaccharide	Energy, structure	Glucose, cellulose
Lipid	Fatty acid	Energy storage, membrane structure	Triglyceride, phospholipid
Protein	Amino acid	Catalysis, structure, transport	Enzyme, hemoglobin

Additional info: Some context and explanations have been expanded for clarity and completeness based on standard biochemistry curriculum.