[orgo814]

Homework question..

Which of the following is true for both primary and secondary active transporters?
a) They are not very specific with respect to the substrates they transport
b) They catalyze ATP hydrolysis to pump a substance against its concentration gradient
c) They couple the transport of one substance against its gradient with co transport of a second substance down its concentration gradient
d) They require free energy input to move a substance against its concentration gradient

Clearly it is not A and B. I was between C and D but for C the second substance should be going against its concentration gradient in secondary active transport right? But, overall they both require energy so I feel D is correct. Is this reasoning good?

[Yggdrasil]

Yes, D is correct.  B is true only for primary active transport and C is true only for secondary active transport.  C is worded correctly for secondary active transport because the transport of the second substance down its concentration gradient (a thermodynamically favorable process) provides the free energy to move the first substance against its concentration gradient (a thermodynamically unfavorable process).  If a secondary active transporter were to move both substances against their concentration gradient, it would need some additional source of free energy to power the transport.