Biolo gically imp ortant substances such as calcium, magnesium and ur ea may
be actively transported in certai n circums tances bu t it is difficult to obtain satisfactory evidence that these are primary instances of such a p ro cess.
Wa ter usually moves readily by osmosis across plasma memb ranes but is no t,
accord ing to conte mporary evidence, subject to active tr anspo rt in vertebrates,
tho ugh it may ind irectly be influenced by such processes. The perm eability of
membranes to wa ter may nevertheless be subject to alteration ; in th e tetrapod kidney and the skin and urinary bladde rs of some Amphibia, neurohypophysial hormon es incre ase th e osmotic perm eability of the membranes.
Th e proce ss of active tr ansport requires ener gy which, in the cases of sodi um
and pota ssium tr ansport, is now generally considered to be deri ved from a labilehigh energy ph osphate group pr esent in adenosine triphosphate (ATP) and whi ch
becomes available wh en it is br oken down to th e diphosph ate (ADP). A very elegant demonstration of th e relationship of ATP to active sodium transfer was made
by CALDWELL, HODGKIN, KEYNES, and SHAW (1960) usin g th e squid axon . Th e
supply of high energy phosph ate in the axon was allowed to disappe ar under th e
influence of cyanide, sodium transport declining simultaneously. Wh en ATP, or
compounds that could be con verted to ATP , we re injected into the axon , active
sodium extr usion was resum ed.
ATP is form ed in cells by the metabol ism of carbohy drates, fats and proteins.
Som e of the interrelationships of the metabolism of such substrates and the formation of ATP is given in F ig. 1.1. One g-mole of glucose, du rin g aerobic metabolism , gives with the aid of 6 g-mo les of oxygen , 36 g-moles of ATP. Thirty of
these result from the aero bic metaboli sm of th e int erme diary pyruvic acid in the
'-------------'--Proteins
Glucose
+
Glycogen
I
Glycolysis
IAnaerob,,)
UDP Glucos e - - - - - - ,
t
Glucose-I-PO, .... t - - - - - - - '
;
Glucose-6 -FO ,..
'iii;
~
-IATP
Fructose-6-PO,
- I ATP ---~;
Gl yceraldehyde-3 - PO, 12 moles)
I
.'ATP ..
t
Alanine---+---_ Pyruvate
Triglycerides
AspLate
• oxa,oL:;::-;:: ' ATP
t
r" ' \ Acetyl CoA Fats
; -. 3ATP
/ -lATPI \"'~~~y /
Malate
Citrate Acetone \ Acids
t Citric Add Cycle l Acetoacetate
\
IAerob,c)
~ ....... 3ATP
Fumarate
£t - Ketoglutarate
~P . ';7\
Succinate
Glut amate
Fig. 1.1. Summary of the metabolic
transformations involved in the
production of ATP from
carbohydrates, fats and proteins.
13
be actively transported in certai n circums tances bu t it is difficult to obtain satisfactory evidence that these are primary instances of such a p ro cess.
Wa ter usually moves readily by osmosis across plasma memb ranes but is no t,
accord ing to conte mporary evidence, subject to active tr anspo rt in vertebrates,
tho ugh it may ind irectly be influenced by such processes. The perm eability of
membranes to wa ter may nevertheless be subject to alteration ; in th e tetrapod kidney and the skin and urinary bladde rs of some Amphibia, neurohypophysial hormon es incre ase th e osmotic perm eability of the membranes.
Th e proce ss of active tr ansport requires ener gy which, in the cases of sodi um
and pota ssium tr ansport, is now generally considered to be deri ved from a labilehigh energy ph osphate group pr esent in adenosine triphosphate (ATP) and whi ch
becomes available wh en it is br oken down to th e diphosph ate (ADP). A very elegant demonstration of th e relationship of ATP to active sodium transfer was made
by CALDWELL, HODGKIN, KEYNES, and SHAW (1960) usin g th e squid axon . Th e
supply of high energy phosph ate in the axon was allowed to disappe ar under th e
influence of cyanide, sodium transport declining simultaneously. Wh en ATP, or
compounds that could be con verted to ATP , we re injected into the axon , active
sodium extr usion was resum ed.
ATP is form ed in cells by the metabol ism of carbohy drates, fats and proteins.
Som e of the interrelationships of the metabolism of such substrates and the formation of ATP is given in F ig. 1.1. One g-mole of glucose, du rin g aerobic metabolism , gives with the aid of 6 g-mo les of oxygen , 36 g-moles of ATP. Thirty of
these result from the aero bic metaboli sm of th e int erme diary pyruvic acid in the
'-------------'--Proteins
Glucose
+
Glycogen
I
Glycolysis
IAnaerob,,)
UDP Glucos e - - - - - - ,
t
Glucose-I-PO, .... t - - - - - - - '
;
Glucose-6 -FO ,..
'iii;
~
-IATP
Fructose-6-PO,
- I ATP ---~;
Gl yceraldehyde-3 - PO, 12 moles)
I
.'ATP ..
t
Alanine---+---_ Pyruvate
Triglycerides
AspLate
• oxa,oL:;::-;:: ' ATP
t
r" ' \ Acetyl CoA Fats
; -. 3ATP
/ -lATPI \"'~~~y /
Malate
Citrate Acetone \ Acids
t Citric Add Cycle l Acetoacetate
\
IAerob,c)
~ ....... 3ATP
Fumarate
£t - Ketoglutarate
~P . ';7\
Succinate
Glut amate
Fig. 1.1. Summary of the metabolic
transformations involved in the
production of ATP from
carbohydrates, fats and proteins.
13