There are seven membrane proteins shown in the figure below

There are seven membrane proteins shown in the figure below. a. Based on what you know about the equilibrium potentials of Na^+, K^+ and Cl^-, circle whether each ion on the figure is traveling down its electrochemical gradient (downarrow), or aganist its electrochemical gradient (uparrow) on each side of the intestinal cell. b. Then, next to each protein, indicate whether the transporters are moving the ions based on facilitative diffusion (FD), secondary active transport (SAT), passive diffusion (PD), or primary active transport (PAT).

Solution

27.a. (I) Na+ ions move down the electrochemical gradient on the apical surface, as the concentration of Na+ ions are higher outside the cell as compared to inside the cell. While, on the basal surface, Na+ ions move against the electrochemical gradient, the process which is aided by the Na+/K+ ATPase pump.

(II) The transport of K+ is more like passive diffusion at the apical end, while at the basal end it is transported against the electrochemical gradient

(III) Cl ^- ion flows down its concentration gradient into the lumen via the CL^- channel located on the apical plasma membrane. While it is transported into the epithelial cell by a cotransporter which is located on the basolateral surface, this occurs against the concentration gradient. The CL^- ion concentration is kept high in the cell like K⁺ ions.

27.b The NA+/K+ ATPase pump transports ions by Primary Active transport, while Na⁺/Cl^- cotransporter utilizes secondary active transport mechanism. The influx of sodium occurs via voltage gated ion channels which is an example of facilitative diffusion. The potassium channels and Cl^- ions are also transported by the process of facilitated diffusion. The NA⁺-H⁺ antiporter is an example of secondary active transport.