Video Transcript
The diagram provided shows a simple
outline of light-dependent reactions, with key reactants and products removed. What molecules would correctly
replace labels 1 and 2? (A) ADP plus inorganic phosphate
and ATP. (B) Oxygen and water. (C) NADP plus inorganic phosphate
and NADPH. Or (D) FADH2 and FAD.
Plants get their nutrition through
photosynthesis, which uses light energy to convert carbon dioxide and water to
glucose and oxygen. The glucose produced provides the
plant with energy for life processes. Photosynthesis has two main stages,
known as the light-dependent and light-independent stages.
Our question tells us that the
provided diagram outlines the light-dependent reactions, which take place in the
thylakoid membrane of chloroplasts. The light-dependent stage of
photosynthesis involves a series of reactions that rely on the movement of electrons
down an electron transport chain. Let’s walk through the major steps
of these reactions using our diagram as a reference.
The initial step occurs when
chlorophyll pigments in photosystem II absorb light energy. This excites the electrons in the
pigments, causing them to move to a higher energy level. The electrons are then passed down
various components of the electron transport chain. The light energy entering
photosystem II is also used to split water into oxygen and hydrogen ions, releasing
electrons. This process, known as photolysis,
generates the electrons needed to replace the excited electrons that have been
transferred to the primary electron acceptor, which is shown in dark pink in the
diagram.
As the excited electrons move down
the electron transport chain, they release energy. This energy is used by proton
pumps, shown in light pink in the diagram, to actively transport hydrogen ions
across the thylakoid membrane. This creates an electrochemical
gradient, with the concentration of hydrogen ions higher inside the thylakoid than
outside of it.
When the electrons reach
photosystem I, they are once again excited through the absorption of light energy by
chlorophyll. In photosystem I, the electrons are
transferred between the electron acceptors and protein carriers. They eventually make their way to
the enzyme NADP+ reductase, which catalyzes the reduction of NADP+ to NADPH. Once there is a high-enough
concentration of hydrogen ions within the thylakoid space, they diffuse down their
concentration gradient through a molecule called ATP synthase.
You probably recognize ATP as the
energy storage molecule of cells. The word “synthase” is derived from
the term “synthesis,” and the suffix -ase indicates an enzyme. So, we can deduce that ATP synthase
is an enzyme that synthesizes, or produces, ATP. It does this by coupling the
movement of hydrogen ions down their gradient with the addition of a phosphate group
to ADP, a process called phosphorylation. The newly created phosphate bond in
ATP contains the majority of its energy and will be used to synthesize glucose in
light-independent reactions.
Now that we have reviewed some of
the steps of the light-dependent reactions, let’s return to our question. We are asked to replace the labels
1 and 2 in the diagram with the correct molecules. You may have realized that label 1
represents the reactants and label 2 represents the product of the reaction
catalyzed by ATP synthase. So, what molecules should replace
these labels? Well, we just saw that
phosphorylation is catalyzed by ATP synthase, and the reactants of phosphorylation
are ADP and an inorganic phosphate, while the product is ATP. So the correct answer is (A). Label 1 should be replaced by ADP
and inorganic phosphate, and label 2 should be replaced by ATP.
