BIOLOGICAL TRANSPORT 



relaxing macromolecular structure triggered by an entering solute, 

 we could suppose that ATP in some way introduces the initial 

 strained conformation. 



Mitochondria are known to take up water and swell under 

 certain conditions if respiration can occur simultaneously and to 

 contract when ATP and magnesium ion are added (cf. Lehninger, 

 1960a; 1960b). Raaflaub (1953) suggested that enzymatically active, 

 dilatable proteins might account for these morphologic changes. 

 Lehninger (1959) used the term me c h an o enzymes in extensions of 

 this concept. He found that ATP cleavage occurs during the ATP- 

 induced contraction and ceases when the contraction is maximal. 

 A protein complex with suitable mechanical properties may serve 

 to transduce the bond energy of ATP via mechanical energy into 

 osmotic energy, at the same time acting as an ATPase; such a system 

 may well function more widely by also acting in oxidative phos- 

 phorylation to receive energy from electron transport, converting 

 it into the energy of a pyrophosphate bond in ATP via the potential 

 energy inherent in strained protein structure. 



Lehninger and Neubert (1961) have recently shown that a 

 number of disulfide hormones that modify water and solute trans- 

 port into and across cells also act on the mitochondrion, presum- 

 ably in both cases by a disulfide-sulfhydryl exchange, to stimulate 

 a respiration-dependent uptake of water (see Chapter 8). This 

 behavior tends to show that water transport by the mitochondrion is 

 closely related to transport by the plasma membrane. These authors 

 do not suppose that the hormones produce their physiological effects 

 by acting on the mitochondrion, but that inherent similarities in the 

 various membranes of the cell permit this membrane to be taken as 

 a model. Alkali metal transport into isolated nuclei and the de- 

 pendence of their amino acid transport on sodium ion (Allfrey et 

 al., 1961) may also indicate similarities of this membrane to the 

 plasma membrane. 



Mitchell (1960b; 1961a; 1961b; 1961c) has proposed a model in 

 which electron transport is taken to be linked to oxidative phos- 

 phorylation, the latter occurring as a translocation of OH~ from the 

 phosphate ion, so that the latter may add spontaneously to ADP. He 

 has presented (1961c) his views in part as follows: 



According to the chemiosmotic hypothesis [Mitchell, 1960b; 

 1961a; 1961b], there are three main components of the organelles 

 that catalyse oxidative phosphorylation: an oxido-reduction sys- 



84 



