Which of the following tables
correctly summarizes what happens to substances involved in the light-dependent
The light-dependent reactions make
up the first stage of photosynthesis. And they take place in the
chloroplast, specifically in the thylakoid membrane. Let’s take a closer look at these
reactions to help us answer this question.
Here, we have the different
structures that make up what is called the electron transport chain in the thylakoid
membrane. The electron transport chain
includes photosystems, proton pumps, electron carriers, and an enzyme called ATP
synthase. At the start of the light-dependent
reactions, light energy is absorbed by photosystem II. This light energy excites
electrons, which then begin to move down the electron transport chain.
The absorption of this light energy
by photosystem II also initiates the breakdown of water molecules into oxygen and
hydrogen ions. This breakdown of water using light
energy is called photolysis.
Let’s look back at what is
happening to our electrons. As the electrons move down the
electron transport chain, they release energy. This energy is used by proton pumps
in the membrane to transport hydrogen ions from the stroma and into the thylakoid
space. When electrons reach photosystem I,
they are again excited by the light energy that the photosystem absorbs. These electrons are passed to an
enzyme called NADP+ reductase. This enzyme catalyzes the reaction
that forms NADPH, also called reduced NADP.
NADPH is formed when the coenzyme
NADP+ gains a hydrogen ion and two electrons. Because NADP+ has gained electrons,
we say it has been reduced.
So far, we have seen that water is
broken down in a process called photolysis, and NADP+ is reduced to form NADPH. For the final part of the
light-dependent reactions, let’s take a closer look at ATP synthase.
We saw earlier that hydrogen ions
are transported from the stroma and into the thylakoid space by proton pumps. This results in there being a high
concentration of hydrogen ions in the thylakoid space and a low concentration of
hydrogen ions in the stroma. These ions move from a high
concentration to a low concentration. They cannot just simply move
through the membrane, but they are able to move down the channel of ATP
ATP synthase couples this movement
of hydrogen ions to the phosphorylation of ADP. When ADP gains a phosphate group,
it forms ATP. We can also say that ATP is
synthesized, as synthesized means “to make.”
Now we should have enough
information to go back and answer our question. We know that in the light-dependent
reactions, water is broken down by photolysis, ATP is synthesized, and NADP+ is
reduced to form NADPH. Table (C) is the only table which
correctly summarizes all of these reactions. Therefore, our correct answer is