What is the direct source of energy used to move glucose into the epithelial cell from the intestinal lumen?

Glucose is imported against its concentration gradient from the intestinal lumen across the apical surface of the epithelial cells by a two Na⁺ glucose symporter located in the microvillar membranes. This symporter couples the energetically unfavorable inward movement of one glucose molecule to the energetically favorable inward transport of two Na⁺ ions .

In the steady state, all the Na⁺ ions transported from the intestinal lumen into the cell during Na⁺-glucose symport, or the similar process of Na⁺/amino acid symport, are pumped out across the basolateral membrane, often called the serosal (blood-facing) membrane. Thus the low intracellular Na⁺ concentration is maintained.

The Na⁺/K⁺ ATPase that accomplishes this is found in these cells exclusively on the basolateral surface of the plasma membrane. The coordinated operation of these transporters allows uphill movement of glucose and amino acids from the intestine into the cell, and ultimately is powered by ATP hydrolysis by the Na⁺/K⁺ATPase.

a- all cells come from pre-existing cells

enzyme active site and substrate specificity

enzymes bind with chemical reactants called substrates. there may be one or more substrates for each type of enzyme, depending on the particular chemical reaction. in some reactions, a single-reactant substrate is broken down into multiple products