Question

[Ca2+] within the endoplasmic reticulum (ER) is 10-3M. Cytosolic [Ca2+] is 10-7M. Lysosomes in mammalian cells have an internal pH of 4.0. Cytosolic pH of a typical mammalian cell is 7.0. Which transport reaction has a greater (more positive) Delta Gc, H+ import into lysosomes or Ca2+import into the ER?A. H+ import into the lysosome because it has the steeper concentration gradient.B. Ca2+ import into the ER because it has the steeper concentration gradient.C. H+ import into the lysosome because it is a monovalent cation.D. Ca2+ import into the ER because it is a divalent cation.E. Delta Gc is the same for both ions because their concentration gradients are the same.

Answer 1

B. Ca2+ import into the ER because it has the steeper concentration gradient

Step-by-step explanation:

ΔGt = RT㏑(C₂/C₁)

where ΔGt is the free energy change for transport; R = 8.315 J/mol; T = 298 K; C₂/C₁ is ratio of concentrations inside and outside each organelle.

For Ca²⁺ import

ΔGt = 8.315 J/mol * 298 K * ㏑(10⁻³/10⁻⁷)

ΔGt= 3.42 kJ/mol

For H⁺ import

ΔGt = 8.315 J/mol * 298 K * ㏑ (10⁻⁴/10⁻⁷)

ΔGt = 2.73 kJ/mol

From the above values, ΔGt is greater for Ca²⁺ import because it has a steeper concentration gradient