CELLULAR MECHANISMS OF K+ ABSORPTION AND SECRETION (GI Tract)

The pattern of intestinal K⁺ movement parallels that of the kidney: (1) the intestines have the capacity for both K⁺ absorption and secretion, and (2) the intestines absorb K⁺ in the proximal segments but secrete it in the distal segments

Dietary K⁺ furnishes 80 to 120 mmol/day, whereas stool K⁺ output is only ~10 mmol/day. The kidney is responsible for disposal of the remainder of the daily K⁺ intake

Substantial quantities of K⁺ are secreted in gastric, pancreatic, and biliary fluid. Therefore, the total K⁺ load presented to the small intestine is considerably greater than that represented by the diet

CELLULAR MECHANISMS OF Na+ ABSORPTION (GI Tract)

 1. Electrogenic Sodium Absorption

This is an active sodium absorption mechanism present not only in the intestine but also in several other epithelia. Active absorption of sodium creates a negative electrical potential in the compartment which sodium leaves as compared to the one which it enters. In case of the intestinal epithelium, the pump for active absorption is present in the basolateral membrane.

The process may be conceptually broken into two stages:

The process may be broken into two steps: 1 and 2. To understand the process, 2 may be considered before 1. Sodium is actively pumped across the basolateral membrane of intestinal epithelium (2). Since sodium ions are positively charged, it creates a potential difference across the membrane, the outside being positive as compared to inside the cell. As a result of the electrical gradient, sodium ions move passively from the lumen into the enterocyte (1). Active transport (2) is against the electrical and concentration gradient. The electrical gradient generated by the active transport (and hence the name electrogenic) leads to passive transport (1) along the electrical and concentration gradient

Vitamin B12(Cobalamin) Absorption

Cobalamin, or vitamin B₁₂, is synthesized only by microorganisms, not by mammalian cells. Only bacteria and archaea have the enzymes required for its synthesis; neither fungi, plants, nor animals (including humans) are capable of producing vitamin B₁₂. Although many foods (Animal products: meat, fish, shellfish, eggs, and (to a limited extent) milk) are a natural source of B₁₂ because of bacterial symbiosis. The vitamin is the largest and most structurally complicated vitamin and can be produced industrially only through bacterial fermentation-synthesis.