Carbohydrates are organic molecules composed of carbon (C), hydrogen (H), and oxygen (O), typically in a 1:2:1 ratio (general formula C(cid:0)H₂(cid:0)O(cid:0)). They serve as immediate energy sources, structural scaffolds, and recognition elements on cell surfaces.

1. Monosaccharides (Simple Sugars)
2. General Structure: Single sugar units (e.g., glucose, fructose, galactose).
3. Glucose (C₆H₁₂O₆):
   • Ring Form: In aqueous solution, glucose largely exists as a pyranose ring (six-membered), formed by intramolecular reaction between the C-1 carbonyl and the C-5 hydroxyl.
   • Functional Roles:
   • Primary fuel for cellular respiration.
   • Precursor for synthesis of disaccharides and polysaccharides.
   • Isomers: α-D-glucose (“down” orientation of the anomeric C-1 hydroxyl); β-D-glucose (“up” orientation). These anomers interconvert via mutarotation in solution.

1. Disaccharides
2. Formed by condensation (dehydration) reactions, where two monosaccharides combine by removal of one water molecule and creation of a glycosidic bond.
3. Sucrose (Table Sugar):
   • Composed of α-D-glucose and β-D-fructose linked via an α(1→2) glycosidic bond.
   • Function: Transport form of carbohydrate in many plants; common dietary sugar for humans.
4. Lactose (Milk Sugar):
   • Composed of β-D-galactose and β-D-glucose with a β(1→4) glycosidic bond.
   • Digestion: Requires the enzyme β-galactosidase (lactase) to hydrolyze into monosaccharides.

1. Polysaccharides (Complex Carbohydrates)
2. Long chains of monosaccharide units linked by glycosidic bonds. Two major categories: storage polysaccharides and structural polysaccharides.
3. Starch (Plant Storage):
   • Amylose: Linear polymer of α(1→4) linked glucose residues; helical structure.
   • Amylopectin: Branched polymer; α(1→4) chains with α(1→6) branch points every ~24–30 residues.
   • Function:
   • Plants store excess glucose as starch in plastids (chloroplasts, amyloplasts).
   • During germination, amylases break starch into maltose and glucose for energy.
4. Glycogen (Animal Storage):
   • Similar to amylopectin but more highly branched (branch points every ~8–12 residues).
   • Stored primarily in the liver and skeletal muscle cells.
   • Branching Function:
   • Increases solubility.
   • Provides many 'non-reducing ends' for rapid addition or removal of glucose.
5. Cellulose (Structural):
   • Linear polymer of β(1→4) linked glucose units.
   • Hydrogen Bonding:
   • Every glucose monomer is rotated 180° relative to its neighbor, forming straight, unbranched chains.
   • Adjacent chains hydrogen bond extensively, creating rigid microfibrils.
   • Function:
   • Primary component of plant cell walls.
   • Cannot be digested by most animals (vertebrates lack cellulase), but certain microbes and ruminants can hydrolyze β(1→4) linkages.

1. Structure–Function Relationships in Carbohydrates
2. Branching vs. Linearity:
   • Branched polymers (glycogen, amylopectin) are more soluble and readily mobilized.
   • Linear polymers (amylose, cellulose) have compact helical or rigid structures suitable for storage or structural support.
3. Stereo-isomerism:
   • Orientation of hydroxyl groups (α vs. β) influences enzyme specificity (e.g., α-amylase vs. β-cellulase).
4. Glycoproteins and Glycolipids:
   • Carbohydrate moieties attached to proteins or lipids on cell surfaces serve in cell–cell recognition, signaling, and immune responses.

2.1.2 Introduction to Lipids
- Lipids are a diverse group of hydrophobic (or amphipathic) molecules, primarily composed of long hydrocarbon chains or rings. They are insoluble in water but soluble in nonpolar solvents. Principal classes include fatty acids, triglycerides, phospholipids, and steroids.

1. Fatty Acids
2. General Structure: A hydrocarbon chain (typically 12–20 carbons) with a terminal carboxyl (–COOH) group.
3. Saturated vs. Unsaturated:
   • Saturated Fatty Acids: No double bonds between carbons; straight chains. Example: Palmitic acid (16:0), stearic acid (18:0).
   • Unsaturated Fatty Acids: One or more cis double bonds; introduce 'kinks.'
   • Monounsaturated: One double bond. Example: Oleic acid (18:1 Δ9).
   • Polyunsaturated: Multiple double bonds. Examples: Linoleic acid (18:2 Δ9,12); α-linolenic acid (18:3 Δ9,12,15).
4. Functional Consequences:
   • Saturated chains pack tightly and are solid at room temperature (animal fats).
   • Unsaturated chains, due to kinks, have looser packing and are liquid at room temperature (plant oils).

1. Triglycerides (Triacylglycerols)
2. Structure: Glycerol backbone (three-carbon alcohol) esterified to three fatty acid chains.
3. Function:
   • Major form of long-term energy storage in animals (adipose tissue) and plants (seed oil).
   • Upon hydrolysis (lipolysis), release free fatty acids and glycerol, which can be oxidized to produce ATP.
4. Storage Adaptations:
   • Adipocytes (Fat Cells): Expansive lipid droplets displace cytosol and nucleus.
   • Energy Density: 1 gram of triglyceride yields ~9 kcal of energy, more than double that of carbohydrates or proteins.

1. Phospholipids
2. Amphipathic Nature:
   • Hydrophilic (“Head”): Phosphate group (often linked to choline, ethanolamine, serine).
   • Hydrophobic (“Tails”): Two fatty acid chains (one unsaturated, one saturated in many membranes).
3. Bilayer Formation:
   • In aqueous environments, phospholipids spontaneously assemble into bilayers: hydrophobic tails face inward, hydrophilic heads face aqueous exterior/interior.
   • This arrangement forms the foundational architecture of biomembranes, providing selective permeability and a fluid matrix for embedded proteins.

1. Steroids
2. Core Structure: Four fused hydrocarbon rings (three six-membered rings + one five-membered ring).
3. Cholesterol:
   • Rigid, planar steroid ring with a short hydrocarbon tail and a single hydroxyl at C-3.
   • Intercalates among phospholipid tails in animal cell membranes, modulating membrane fluidity:
   • At moderate temperatures, cholesterol restrains phospholipid movement, decreasing fluidity.
   • At low temperatures, it prevents tight packing of fatty acids, maintaining membrane fluidity.
4. Other Steroids:
   • Steroid Hormones: Testosterone, estradiol, cortisol, etc., derived from cholesterol via enzymatic modifications.
   • Function as signaling molecules, often binding intracellular receptors that regulate gene expression.

1. Lipid Rafts and Microdomains
2. Definition: Small, dynamic assemblies of cholesterol, sphingolipids (e.g., sphingomyelin), and specific proteins in the plasma membrane.
3. Function:
   • Organize signal transduction.
   • Facilitate endocytosis, protein sorting, and cell adhesion.
   • Provide a platform for clustering of receptors (e.g., in immune cell activation).

1. Comparison of Carbohydrates vs. Lipids
2. Property Carbohydrates Lipids
3. Solubility: Water-soluble (due to hydroxyl groups) Insoluble in water; soluble in organic solvents
4. Main Functions: Immediate energy, short-term storage, structural roles Long-term energy storage, membrane structure, signaling
5. Energy Yield: ~4 kcal/g ~9 kcal/g
6. Structural Complexity: Monomers → Oligomers → Polysaccharides Fatty acids + glycerol/phosphate/steroid ring
7. Examples of Dysregulation: Diabetes (impaired glucose regulation) Atherosclerosis (excess cholesterol), obesity (excess adiposity)