Thermodynamic analyses have already shown that K+ and Cl− are taken up actively by Enteromorpha intestinalis (L.) Link cells and Na+ is actively excluded in seawater and in low-salinity media (25 mM Cl−). The proton motive force (pmf) has been estimated across the plasmalemma of Enteromorpha intestinalis. The pmf is about zero in plants in seawater but in low-salinity medium there is apparently a proton extrusion pump at the plasmalemma producing a pmf of about − 6 kj mol−1. Secondary active transport cannot be responsible for the active uptake of K+ and Cl− or the active efflux of Na+ by plants in seawater. It is thermodynamically possible (but unlikely) that K+ is taken up by plants in low-salinity medium by an electrogenic K+/H+ coport mechanism but secondary active transport of Na+ or Cl− is not thermodynamically possible. The Hodgkin-Katz equation shows that the membrane potential of Enteromorpha in seawater is consistent with a diffusion potential but in low-salinity medium (ACBSW, 25.5 mM Cl−) the observed membrane potential is more negative than the calculated diffusion potential in both the light and dark and an electrogenic proton extrusion pump appears to be likely in those plants.
Various estimates of the power requirements of active transport of K+in, Cl−in and Na+out have been made and compared to the power available from respiration and photosynthesis. Na+ is very far from electrochemical equilibrium but the power consumption of Na+ transport is less than 0.2 mW m−2 because the Na+ fluxes are very low. Even though both K+ and Cl− are close to electrochemical equilibrium, the active transport of these ions accounts for most of the power consumed in ion transport. The power requirements of active K+ and Cl− transport are approximately equal.
In plants in seawater and low-salinity medium photosynthesis provides about 400 mW m−2 and respiration about 70 mW m−2 of power which is available for the metabolism of the plant. In plants in seawater, the power dissipation necessary to maintain the observed K+, Cl− and Na+ gradients is about 5 to 19 mW m−2 or about 7 to 20% of the total respiratory power available in the dark. The power requirement of ion transport at low-salinity is about 10 to 20 mW m−2 and so would consume about 15 to 30% of the total power available in the dark. In the light the power requirement of ion transport is less than 5% of the total metabolic power available in both seawater and low-salinity. The power requirements of active K+ and Cl− transport are both large and it is not clear why the plant expends so much power on the active transport of Cl−. The metabolic power requirements of the active transport of K+ and Cl− may account for the K+ sensitivity of Enteromorpha in culture and in the field.