Video Transcript
The diagram shows a basic outline
of the light-dependent reactions. At the beginning, light is absorbed
by photosystem II. What happens to the electrons in
this photosystem? (A) The electrons use light energy
to divide and form new electrons. (B) The electrons gain energy and
move into the interior of the thylakoid. (C) The electrons become excited
and release energy that is used to split water into hydrogen and oxygen. Or (D) the electrons become excited
and move to a higher energy level, leaving photosystem II.
Let’s remove the answer choices for
now and talk through how to answer this question.
Photosynthesis occurs within the
chloroplasts of plants. The light-dependent reactions,
shown in the diagram, occur within the thylakoid membrane of the chloroplast. In the diagram, we can see the
electron transport chain, which is a series of photosystems, enzymes, proton pumps,
and electron carriers found within the thylakoid membrane. In this series of reactions, light
energy is absorbed by photosystems. But what happens to the electrons
in these photosystems?
Light first hits photosystem II,
and its energy is absorbed by the pigments within this photosystem. This excites the electrons, and
they move to a higher energy level. The electrons then exit the
photosystem and move down the electron transport chain. As electrons move from one
component to the next, they release energy. The electrons that have left
photosystem II need to be replaced, and these electrons are provided by the
photolysis of water. Water is split into oxygen and
hydrogen by light energy, which also releases electrons.
Let’s look back at our answer
choices to see if we can identify the correct choice. We know that electrons do not use
light energy to divide and multiply, so we can eliminate option (A). We also know that electrons move
down the electron transport chain and are not moved into the interior of the
thylakoid, so we can eliminate option (B).
The electrons do become excited as
light energy is absorbed by the photosystem, but they do not release this energy to
split water. The light energy is used to split
water in a process called photolysis, which replaces electrons that have moved from
photosystem II. The correct answer is (D). The electrons become excited and
move to a higher energy level, leaving photosystem II.
