Digestion and Absorption

Part 1: Four Concentric Layers of the Intestinal Wall (3 Marks):
The transverse wall of the human alimentary canal from the esophagus to the rectum is organized into 4 concentric tunics:

1. Serosa: Outermost protective coat consisting of a thin simple squamous mesothelium (visceral peritoneum) supported by loose areolar connective tissue. It minimizes frictional abrasion during peristaltic contractions.
   
2. Muscularis: Composed of smooth muscle fibers arranged into an outer longitudinal layer and an inner circular layer. Sandwiched between these layers is the Auerbach's (myenteric) plexus, which controls rhythmic peristalsis and gut motility.
   
3. Submucosa: Highly vascularized loose connective tissue containing blood vessels, lymphatic channels, and the Meissner's (submucosal) plexus controlling mucosal glandular secretions.
   
4. Mucosa: Innermost layer bordering the lumen. In the small intestine, it forms millions of finger-like projections called villi, covered with simple columnar enterocytes possessing apical microvilli (expanding the absorptive surface area ~600-fold). It contains Goblet cells that secrete protective mucin, and invaginations termed Crypts of Lieberkühn.
   
   Part 2: Functional Significance of Brunner's Glands and Paneth Cells (2 Marks):
   • Brunner's Glands: Located specifically within the submucosa of the duodenum. They secrete an alkaline, mucus-rich fluid containing bicarbonate ($\text{HCO}_3^-$) that neutralizes acidic gastric chyme entering from the pylorus, protects the duodenal mucosa against peptic ulceration, and creates an alkaline pH optimum ($7.8-8.2$) for pancreatic enzymes.
   • Paneth Cells: Situated at the bases of the Crypts of Lieberkühn in the small intestine. They synthesize and secrete antimicrobial enzymes (primarily lysozyme) and antibacterial peptides (defensins), thereby destroying enteric pathogens and preserving intestinal microbiome homeostasis.

Part 1: Characteristics of Human Dentition (3 Marks):

1. Thecodont: Each individual tooth is embedded within a deep bony socket (alveolus) of the maxillary or mandibular jawbone, anchored firmly by collagenous periodontal fibers. This ensures mechanical stability during high-force mastication.
   
2. Diphyodont: Humans develop two successive sets of teeth across their lifespan:
   • Deciduous / Milk dentition: Temporary set of 20 teeth that erupts during infancy and early childhood.
   • Permanent / Adult dentition: Set of 32 teeth that replaces the deciduous teeth from ~6 years of age onwards.
   
3. Heterodont: Teeth are morphologically and functionally specialized into four distinct anatomical classes:
   • Incisors (I): Chisel-shaped front teeth for cutting and biting food.
   • Canines (C): Dagger-shaped pointed teeth for tearing tough fibrous food.
   • Premolars (PM): Bicuspid teeth with two cusps for crushing and grinding.
   • Molars (M): Broad-crowned multi-cusped teeth for fine grinding.
   
   Part 2: Human Dental Formulas (2 Marks):
   The dental formula represents the number of teeth of each type in one half of the upper jaw over one half of the lower jaw ($\text{I}.\text{C}.\text{PM}.\text{M}$):
   • Adult Human: $\frac{2.1.2.3}{2.1.2.3} \times 2 = 32\text{ teeth}$
   • 5-Year-Old Child (Milk Dentition): $\frac{2.1.0.2}{2.1.0.2} \times 2 = 20\text{ teeth}$
   Critical Observation: Premolars are entirely absent ($\text{PM} = 0$) in primary milk dentition, and the third molar (wisdom tooth) is also absent. In humans, 12 teeth (8 premolars + 4 third molars) erupt only once and are classified as monophyodont.

Part 1: The Enterokinase Trigger & Cascade Activation (2 Marks):
Pancreatic juice contains proteolytic enzymes exclusively in the form of inactive zymogens (proenzymes) to prevent autodigestion of pancreatic acinar tissue. The activation cascade unfolds in the duodenum:

1. Primary Trigger: The duodenal mucosal brush-border enzyme Enterokinase (enteropeptidase) cleaves an inhibitory hexapeptide from inactive Trypsinogen, converting it into active Trypsin.
   
2. Autocatalytic Amplification: Once formed, active trypsin functions as a master activator. It autocatalytically activates remaining trypsinogen molecules and catalyzes the activation of all other pancreatic zymogens:
   • $\text{Trypsinogen} \xrightarrow{\text{Enterokinase}} \text{Trypsin}$
   • $\text{Chymotrypsinogen} \xrightarrow{\text{Trypsin}} \text{Chymotrypsin}$
   • $\text{Procarboxypeptidase} \xrightarrow{\text{Trypsin}} \text{Carboxypeptidase}$
   • $\text{Proelastase} \xrightarrow{\text{Trypsin}} \text{Elastase}$
   
   Part 2: Stepwise Protein Digestion Equations in the Small Intestine (3 Marks):
   Proteins, proteoses, and peptones entering from the stomach are sequentially cleaved into amino acids:
   
3. Action of Pancreatic Endopeptidases (cleave internal peptide bonds):
   

$$\text{Proteins, Peptones, Proteoses} \xrightarrow[\text{pH } 7.8-8.2]{\text{Trypsin / Chymotrypsin}} \text{Polypeptides} + \text{Oligopeptides}$$

2. Action of Pancreatic Exopeptidase (cleaves terminal C-terminal peptide bonds):
   

$$\text{Polypeptides} \xrightarrow{\text{Carboxypeptidase}} \text{Dipeptides} + \text{Free Amino Acids}$$

3. Action of Intestinal Succus Entericus Peptidases (Brush Border Terminal Hydrolysis):
   

$$\text{Dipeptides} \xrightarrow{\text{Dipeptidases}} 2\text{ Amino Acids}$$

$$\text{Aminopeptidases} \text{ cleave N-terminal amino acids to yield absorbable free amino acids.}$$

Part 1: Step-by-Step Mechanism of Lipid Digestion & Absorption (3 Marks):
Because lipids are insoluble in water, their digestion and absorption require specialized physical and chemical modifications:

1. Emulsification by Bile Salts: In the duodenum, bile salts (sodium glycocholate, sodium taurocholate) and lecithin reduce the surface tension of large fat globules, breaking them into submicroscopic droplets (emulsion). This expands surface area exponentially for enzymatic attack.
   
2. Pancreatic Lipase (Steapsin) Action: Pancreatic lipase hydrolyzes emulsified triglycerides in a stepwise sequence:
   

$$\text{Triglycerides} \xrightarrow{\text{Lipase}} \text{Diglycerides} \to \text{Monoglycerides} + 2\text{ Free Fatty Acids}$$

3. Micelle Formation: Insoluble free fatty acids, 2-monoglycerides, and cholesterol unite with bile salts to form tiny water-soluble spherical aggregates called micelles (4–7 nm in diameter). Micelles transport lipids across the unstirred water layer to the enterocyte apical membrane.
   
4. Cellular Resynthesis & Chylomicron Assembly: Fatty acids and monoglycerides dissociate from micelles and passively diffuse into enterocytes. In the smooth endoplasmic reticulum (SER), they are re-esterified into triglycerides. In the Golgi apparatus, triglycerides are enveloped by phospholipids, cholesterol, and a protein coat (ApoB-48) to create water-soluble lipoprotein droplets called chylomicrons (100–500 nm).
   
   Part 2: Why Digested Fats Enter Lacteals instead of Blood Capillaries (2 Marks):
   • Size Barrier: Chylomicrons are large lipoprotein complexes (100–500 nm in diameter). Blood capillaries in the intestinal villi possess a continuous basement membrane with narrow endothelial fenestrations (pores ~50–100 nm), which physically prevent chylomicrons from entering the bloodstream directly.
   • Lacteal Permeability: In contrast, the central lymphatic capillary of each villus—the lacteal—possesses large, highly permeable endothelial overlapping clefts and lacks a continuous tight basement membrane. Chylomicrons readily pass through these open intercellular gaps into the lacteal. The chyle flows via the cisterna chyli and thoracic duct to enter the left subclavian vein into systemic venous circulation.

Part 1: Overview of Gastrointestinal Hormonal Control (1 Mark):
Gastrointestinal motility and secretions are regulated by peptide hormones produced by specialized enteroendocrine cells dispersed within the mucosal epithelium. These hormones act locally via endocrine and paracrine pathways to synchronize glandular secretions with food transit.

Part 2: Specific Hormones, Sources & Functions (4 Marks, 1 Mark each):

1. Gastrin:
   • Source: G-cells located in the pyloric antral mucosa of the stomach and duodenum.
   • Stimulus: Vagal nerve stimulation, stomach distension, and presence of partially digested proteins.
   • Actions: Stimulates parietal (oxyntic) cells to secrete hydrochloric acid ($\text{HCl}$) and chief cells to secrete pepsinogen; stimulates gastric mucosal growth and gastric motility.
   
2. Secretin (First hormone discovered by Bayliss & Starling, 1902):
   • Source: S-cells of the duodenal mucosa.
   • Stimulus: Acidic chyme ($\text{pH} < 4.5$) entering the duodenum from the stomach.
   • Actions: Acts on pancreatic ductal cells to stimulate secretion of watery, bicarbonate-rich ($\text{HCO}_3^-$) alkaline pancreatic juice to neutralize chyme; inhibits gastric acid secretion and gastric motility.
   
3. Cholecystokinin-Pancreozymin (CCK-PZ):
   • Source: I-cells located in the duodenal and jejunal mucosa.
   • Stimulus: Fatty acids, monoglycerides, and amino acids in duodenal chyme.
   • Actions: Stimulates rhythmic contraction of the gallbladder to eject concentrated bile into the common bile duct; stimulates pancreatic acinar cells to secrete enzyme-rich pancreatic juice; relaxes the Sphincter of Oddi.
   
4. Gastric Inhibitory Peptide (GIP) / Glucose-dependent Insulinotropic Peptide:
   • Source: K-cells of the duodenal and jejunal mucosa.
   • Stimulus: Ingested glucose, amino acids, and fatty acids.
   • Actions: Functions as an "enterogastrone" by inhibiting gastric acid secretion and gastric emptying; potently stimulates endocrine $\beta$-cells of the pancreas to release insulin in anticipation of absorbed glucose.