In this video, we will learn about
the light-dependent reactions of photosynthesis. We will learn about the electron
transport chain and the series of reactions that occur to produce ATP in plant
Plants like all living organisms
need food to keep them alive and healthy. But unlike animals, plants cannot
move around to hunt or gather their food; instead they carry out photosynthesis. Photosynthesis is the process by
which plants convert carbon dioxide and water into glucose and oxygen. We can learn a little bit about
photosynthesis just from the word alone. Photo- means light and synthesis
means to make. So, photosynthesis refers to a
process that makes food using light.
Photosynthesis in plants has two
main stages: one stage that is completely dependent on light being available and one
that is not. So even though one stage of
photosynthesis happens independently of light, the overall reaction will not happen
unless there is a light source present. Let’s take a look at the stage of
photosynthesis which directly uses light energy, which we refer to as the
light-dependent reactions. The light-dependent reactions take
place in the chloroplasts of plant cells. Chloroplasts are organelles
primarily found within the cells of plants and some algae.
The light-dependent stage of
photosynthesis relies on photosystems to absorb light, and these photosystems are
found within the membranes of the thylakoids inside the chloroplast. So this is where the
light-dependent reactions take place. Let’s have a look at the structures
involved in the light-dependent reactions in a bit more detail.
Within the thylakoid membrane, we
find two photosystems. These photosystems contain
photosynthetic pigments, primarily the pigment chlorophyll. These pigments absorb the light
needed for the light-dependent reactions. There are also other structures;
these include electron carriers, proton pumps, the enzyme NADP+ reductase, and the
enzyme ATP synthase. We’ll learn more about the role
these structures play throughout the video. So, let’s go through the reactions
that make up the light-dependent stage of photosynthesis.
First, photosystem two takes in
light energy. This may seem confusing: why is
photosystem two appearing before photosystem one? This is actually just because of
the order in which they were discovered, with photosystem one being discovered and
named before they found the second one.
The absorption of light energy by
photosystem two initiates two events. First, electrons within the
photosystem become excited and move to a higher energy level. They are then transferred to the
next electron acceptor in the electron transport chain.
Secondly, a molecule of water is
split into an oxygen atom and two hydrogen ions. The splitting of water using light
energy is called photolysis or photolysis. The photolysis of water also
releases electrons. These electrons replace those which
have moved from photosystem two to the next electron acceptor and just like before
can absorb light energy and become excited.
Now, we can follow the progress of
these electrons. These electrons move down the
structures that make up the electron transport chain, and energy is released during
this process. This energy is used to actively
transport hydrogen ions from the stroma of the chloroplast to the inside of the
thylakoid, otherwise known as the thylakoid space. The electrons are then moved to
photosystem one by an electron carrier. Photosystem one, much like
photosystem two, absorbs light energy. Then, the electrons in photosystem
one become excited. They are passed to electron
carriers and then to the enzyme NADP+ reductase. Here, the coenzyme NADP+ is
reduced, but what does this mean?
In chemistry, reduction is the gain
of electrons. In this reaction, the coenzyme
NADP+ gains two electrons and a hydrogen ion to form reduced NADP, also called
NADPH. This is an important reaction, as
NADPH is a key coenzyme in the next stage of photosynthesis, the light-independent
stage. You might remember that earlier on
in the process, hydrogen ions were being actively transported from the stroma and
into the thylakoid space. This means that the interior of the
thylakoid contains a higher concentration of hydrogen ions than the stroma. The difference in the electrical
charge and the number of hydrogen ions across the thylakoid membrane is called an
We would expect the hydrogen ions
to move down their electrochemical gradient by diffusion. This means they would move from an
area of high concentration to an area of low concentration. However, the hydrogen ions cannot
just diffuse through the membrane; instead the hydrogen ions move through the inner
channel of the enzyme ATP synthase. This movement of ions down their
electrochemical gradient is called chemiosmosis. But why is this important? The movement of hydrogen ions
through ATP synthase is coupled to the phosphorylation of ADP. In this process, ADP, or adenosine
diphosphate, gains a phosphate group to form ATP or adenosine triphosphate.
ATP is the energy-carrying molecule
of all living cells, plants included. The breakdown of ATP and associated
bond formations act as a constantly available and rapid supply of energy. Because this phosphorylation of ADP
is coupled with the movement of hydrogen ions through ATP synthase, we can say that
ATP is synthesized using chemiosmosis. So now, we’ve reached the end of
the light-dependent reactions. Let’s quickly recap the major
reactants and products.
We know that a molecule of water,
H2O, is split using light energy into oxygen, hydrogen ions, and electrons. This oxygen is actually the oxygen
that we as humans breathe. Later, NADP+ accepts two electrons
and a hydrogen ion to form reduced NADP or NADPH. Finally, ADP gains a phosphate
group to form ATP via chemiosmosis. The process that we have been
looking at so far can also be referred to as noncyclic photophosphorylation. Let’s break this down to understand
what that means.
Photo- refers to light, and
phosphorylation refers to the addition of a phosphate group. We know that in a light-dependent
reactions, light energy is required and ADP is phosphorylated to form ATP. Noncyclic means that this process
is linear in that it only proceeds in one direction. However, there can also be cyclic
photophosphorylation. In cyclic photophosphorylation,
only photosystem one is used.
The electrons that are excited by
the absorption of light are essentially recycled. These electrons do not get passed
on to NADP+ reductase to be gained by NADP+. Instead, the electrons that were
excited by light energy in photosystem one move through electron carriers and return
to photosystem one to repeat the cycle again. Hydrogen ions continue to be
actively transported into the thylakoid space and then move through ATP
synthase. So, ATP is still generated.
Now that we’ve learned about the
light-dependent reactions of photosynthesis, let’s try some practice questions.
What is the creation of ATP from
the phosphorylation of ADP using light energy known as? (A) Dephosphorylation, (B)
oxidative phosphorylation, (C) photophosphorylation, (D) chemiosmosis.
To help us answer this question,
let’s break down the reactions being described. Phosphorylation refers to the
addition of a phosphate group. When ADP is phosphorylated, it
forms the energy-carrying molecule ATP. In the light-dependent stage of
photosynthesis, this phosphorylation of ADP is catalyzed by the enzyme ATP
synthase. This process will only happen when
light is available. This is actually where we get the
photo- part of photosynthesis from. Photo- means light and synthesis
means to make. So, we can assume from this that
reactions which require light energy will typically have the word part photo-
Looking back at our answers, we can
see that one option summarizes the process of phosphorylating a molecule using light
energy. Therefore, the creation of ATP from
the phosphorylation of ADP using light energy is known as photophosphorylation, or
answer choice (C).
The diagram provided shows a basic
outline of the light-dependent reactions. As electrons move from photosystem
two to photosystem one, how are they replaced? (A) By the reduction of NADP+, (B)
by the photolysis of water, (C) by the absorption of light energy, (D) by the
movement of H+ ions.
The light-dependent stage of
photosynthesis uses light energy to initiate a series of reactions, which eventually
results in the production of ATP. First, photosystem two absorbs
light energy, which excites the electrons contained within the chlorophyll pigment
in the photosystem. The electrons then move from one
structure of the electron transport chain to another. This means the electrons that were
transported from photosystem two need to be replaced.
When light energy is absorbed by
photosystem two, it’s used to excite electrons, but it’s also used to split water
molecules. Each water molecule is split into
oxygen, two hydrogen ions, and two electrons. The splitting of water using light
energy is called photolysis. The electrons produced by the
photolysis of water replace the electrons that move from photosystem two to
photosystem one. Therefore, our correct answer is
answer choice (B). The electrons are replaced by the
photolysis of water.
Let’s conclude our video with some
key points. Photosynthesis is the process by
which plants convert carbon dioxide and water into oxygen and glucose using light
energy. The light-dependent reactions of
photosynthesis occur primarily in the thylakoid membrane. These reactions are initiated by
the absorption of light by photosystems. ATP is formed by the
phosphorylation of ADP, which is coupled to the movement of hydrogen ions through
ATP synthase. The formation of ATP using light
energy is known as photophosphorylation.