The term symport is used to denote an integral membrane protein that simultaneouly transports two substances across membrane in the same direction.
The example below depicts a model for the transport of Na+ ions (Na) and glucose (G) from the lumen of the small intestine to the interstitial fluid and finally into the blood.
The Na+ in the lumen of the intestine comes from dietary intake and from the secretion of the pancrease.
The Na+ concentration in the lumen of the small intestine is greater than in the cytosol of the epithelial cells.
Na+ ions are transported from the lumen of the intestine into the epithelial cell cytoplasm with the aid of the Na-glucose symport. The Na+ is traveling down its concentration gradient.
However, in order for the Na+ to be transported into the cell by the symport, a molecule of glucose must be transported at the same time.
The intracellular concentration of Na+ is kept low by the Na-K ATPase which is an active transport protein.
As long as the Na-K ATPase is functioning the intracellular concentration of Na+ is kept low and the symport will transport Na and glucose into the cell.
The continued transport of glucoe into the cell results in a relatively high concentration of glucose in the cytoplas m.
The concentration of glucose in the cytoplasm is greater than in the interstital spaces. The glucose is transported down this concentration gradient through the glucose permease.
study the cartoon below