3. Lemon juice is an acid. What ion is present in lemon juice that
makes it an acid?

Lemon is an acidic fruit with a pH ranging 2 to 3. The acid in lemons are citric acid which makes lemon tart, which is Vitamin C.

The addition of inorganic phosphate helps to sustain the reaction by helping the reaction to occur in a thermodynamically unfavorable manner.

Explanation:

The reaction  of inorganic phosphate with  glucose is an endergonic reaction which is thermodynamically unfavorable as a result reaction need coupling of ATP hydrolysis to take place in biological system because ATP hydrolysis releases huge amount of free energy as a result the overall free energy change of this reaction becomes negative which helps the reaction to proceed in a thermodynamically favorable manner.

       That"s why inorganic phosphate can be used to sustain the rapidly proceeding glycolysis pathway. The addition of inorganic phosphate makes the reaction thermodynamically unvorable.