IMAT 2013 Q43 [Molecules]

Which one of the following molecules is made in both photosynthesis and respiration?

A. Glucose
B. Reduced NADP
C. Carbon dioxide
D. ATP
E. Oxygen

The correct answer here is choice D, ATP.

Let’s then analyze each of the choices, and point out the differences between photosynthesis and respiration:

  • Glucose and Oxygen: In cellular respiration, glucose and oxygen are the reactants of the processes involved. For example, glucose is used as the initial substrate in which glycolysis will ensue, while oxygen is used in the inner mitochondrial membrane in the electron transport chain for the purpose of oxidative phosphorylation. On the other hand, glucose and oxygen are the by-products of photosynthesis. Thus, this makes choices A and E as incorrect answers.

  • Reduced NADP: This is related more to the process of photosynthesis. Reduced NADP is formed during the first stage of photosynthesis, or the light-dependent stage. When light is absorbed in the photosystems found in the thylakoid membrane, it is then transferred to photocenters. The electrons then move on to the cytochrome bf complex, where ultimately, the electrons are transferred to NADP+ via the NADPH reductase enzyme, which forms NADP or reduced NADP. (Do not be confused, NADP+ is the oxidized form, whereas NADP is the reduced form.)

  • Carbon dioxide: If you can recall, carbon dioxide is produced by the mitochondria during the TCA cycle, which is part of cellular respiration, thus making this choice incorrect.

  • ATP: This is the correct answer as it can be produced in both photosynthesis and cellular respiration. In photosynthesis during the light-dependent stage, the thylakoid membrane has the ability to produce ATP via an ATP synthase, wherein the stored ATP will be used to create sugars during the light-independent cycle of photosynthesis, which involves the Calvin cycle. ATP can also be found in cellular respiration. For example, the process of glycolysis yields 2 ATP, on the other hand, the TCA cycle is also able to produce ATP through its NADH, FADH2, and GTP yields. Thus, due to the presence of the production of ATP in both photosynthesis and respiration, this makes it the correct answer.