Video Transcript
In this video, we will learn how to
describe the structure of the chloroplast. By discussing the different
structures of the chloroplast, we will explain how the chloroplast is adapted for
its function of storing light energy in the form of sugar molecules.
Plants are multicellular organisms
that like humans require nutrition to keep them alive and functioning. Unlike humans, however, plants do
not possess a specialized digestive system to take in and absorb nutrients from food
sources. Amazingly, plants have evolved to
solve this problem by making their own food. They carry out this process, which
is known as photosynthesis, in specialized cell organelles called chloroplasts. If you were to look at some typical
plant cells under a microscope, you’ll probably be able to see some
chloroplasts. Let’s recap the process of
photosynthesis before we talk about chloroplasts in more detail.
Photosynthesis is the process by
which organisms like green plants convert carbon dioxide and water into sugars like
glucose and oxygen. It must take place in the presence
of light energy. This light energy can be from
natural sunlight or from artificial light like from a lamp. The process of photosynthesis
primarily takes place in the leaves, as this is where most of the chloroplasts will
be found. But the parenchyma tissues of plant
stems may also contain chloroplasts, so some photosynthesis can occur here too. Most plants cannot carry out
photosynthesis in their roots. This is because most plant roots
tend to grow below ground and they don’t receive the light required for
photosynthesis.
Let’s take a closer look at a
typical photosynthetic cell. This diagram is a basic outline of
all the major organelles contained within a photosynthetic plant cell. Like all plant cells, it has a cell
wall, supporting the cell and helping it to keep its structure. It also has a vacuole, which helps
the cell maintain water balance. This cell is called a
photosynthetic cell because it contains the light-sensitive pigment chlorophyll
stored in a double–membrane bound organelle called a chloroplast. A plant cell will usually contain
multiple chloroplasts and potentially up to hundreds of them.
Photosynthesis provides major
benefits for the plant. The glucose that it produces is a
sugar and it acts as a source of nutrition for the plant, allowing it to grow,
reproduce, and carry out other essential life processes. One of these essential life
processes is cellular respiration. Cellular respiration occurs in
organelles called mitochondria, and it breaks down the glucose that’s produced in
photosynthesis to release energy. The most common form of cellular
respiration uses oxygen. So some of the oxygen that’s
produced by photosynthesis can be reused in this process. Now, let’s take a closer look at
the organelle in which photosynthesis occurs, the chloroplast.
As we mentioned briefly earlier,
chloroplasts are examples of organelles that are surrounded by a double
membrane. Between these two membranes is an
intermembrane space. This double membrane is only around
10 nanometers thick, which is 0.000001 centimeters. Inside the chloroplast is a fluid
matrix called the stroma. The stroma contains many enzymes,
along with the DNA of the chloroplast and ribosomes. It also contains starch grains,
which act as insoluble stores of glucose. When needed by the plant, these
grains are converted back into the soluble sugar glucose and translocated around the
plant to be used in cellular respiration.
Suspended in the stroma are
disklike structures called thylakoids. These thylakoids form stacks of
around 15 or more disks. A stack of thylakoids is called a
granum or plural grana. These grana are interconnected by
thin membranous folds called lamellae, or a singular lamella. Within each thylakoid is an aqueous
space called the lumen. We already mentioned that the
primary role of the chloroplast is to act as the site of photosynthesis within a
plant cell. To do this, it must be able to
absorb light energy. Let’s look in more detail at the
functions of each of the specialized structures within the chloroplast to explain
how this happens.
Pigments are molecules that absorb
some specific wavelengths of light and are believed to reflect others. The thylakoid membranes within the
chloroplast contain specialized pigments called chlorophyll. Chlorophyll is able to absorb light
energy and utilize it in photosynthesis. It is commonly found in plants in
two forms: chlorophyll A and chlorophyll B.
This image shows an approximate
representation of the visible light spectrum. Chlorophyll A and B can absorb
light energy with different wavelengths. The pigment chlorophyll A will
absorb the blue, most yellow, orange, and red wavelengths of light, but it is much
worse at absorbing green wavelengths. When light energy is used, the
light vanishes. As the blue, yellow, and red light
energy is used, only the green light remains. It has been proposed that this is
why many leaves that contain chlorophyll look green to the human eye.
Chlorophyll has a very complex
structure. Chlorophyll A, for example, has the
chemical formula C55H72O5N4Mg, and a part of it looks like this. The central magnesium ion is
thought to be crucial for the ability of this pigment to absorb light. Interestingly, chlorophyll has a
very similar structure to the heme group that’s found in hemoglobin in animals’ red
blood cells, with the exception that the magnesium ion is replaced with an ion of
iron and hemoglobin.
Chlorophyll is not the only pigment
that may be contained within a chloroplast. Plants may also contain other
photosynthetic pigments, such as xanthophyll and beta carotene, which are
responsible for giving plant structures a yellow, orange, or reddish color. It is advantageous for a plant to
have different photosynthetic pigments because they can use the light energy of
different light wavelengths. These other pigments may also have
functions of protecting chlorophyll from photooxidation. However, these other pigments are
not as common or abundant as chlorophyll pigments. So this explains why the majority
of plant leaves are green.
In the thylakoid membranes,
pigments like chlorophyll are contained within photosystems. Photosystems are functional and
structural units of protein complexes involved in photosynthesis. The primary function of these
structures is to absorb and utilize sunlight for the first stage of photosynthesis,
which is called the light-dependent stage. This stage is made up of a series
of reactions that absorb light energy and use it to split water to generate chemical
energy in the form of ATP and oxygen as a byproduct. The second stage of photosynthesis,
the light-independent stage or Calvin cycle, which uses the chemical energy from the
light-dependent stage to transfer carbon dioxide into glucose, is carried out in the
stroma of chloroplasts. The stroma is well adapted for this
as it contains the necessary enzymes for the chemical reactions involved in the
light-independent stage.
Now that we’ve discussed the
different structures and functions of those structures of a chloroplast, let’s test
our newly acquired knowledge on a couple of practice questions.
Which of the following statements
correctly links the grana to the thylakoids? (A) Circular grana come together in
stacks to form a single thylakoid. (B) Disklike structures called
thylakoids form stacks called grana. Or (C) a single thylakoid cell is
formed of multiple grana components.
Let’s review the structures within
a chloroplast to answer this question. Chloroplasts are cell organelles
that are surrounded by two membranes. The inside of a chloroplast is
filled with a fluid matrix called the stroma. The stroma contains many enzymes
along with the DNA of the chloroplast and ribosomes. It also contains disklike
structures called thylakoids. These thylakoids, which are one of
the key terms used in our question, form stacks of around 15 or more disks. A stack of thylakoids is called a
granum or plural grana. This is the other key term that’s
been used in the question.
The grana are interconnected by
thin membranous folds called lamellae or a singular lamella. Within each thylakoid is an aqueous
space called the lumen. So now we know the relationship
between thylakoids and the grana, which is (B). Disklike structures called
thylakoids form stacks called grana.
Let’s try and solve a second
practice question together.
Absorption spectra for the visible
light spectrum are provided. Which line shows the correct
absorption for the photosynthetic pigment chlorophyll a?
We know that plants can use light
to make their own food through a process called photosynthesis. You might remember that
photosynthesis is the process of turning carbon dioxide and water into glucose and
oxygen using light energy. The glucose sugar provides energy
for growth, reproduction, and metabolism, so light is absorbed for the process of
photosynthesis. This absorption is performed by
photosynthetic pigments, such as chlorophyll a, which reside in the chloroplasts of
a photosynthetic plant cell.
Let’s take a closer look at one of
these chloroplasts. The chloroplast interestingly has
two membranes, and it contains its own DNA within a fluid-filled space called the
stroma. Within the stroma, there are also
grana or a singular granum. Each granum is made up of disklike
structures called thylakoids, and they are linked by lamellae, which in the singular
form is a lamella. It’s the thylakoids within the
chloroplast that contain the pigments that absorb the light energy to initiate
photosynthesis. They contain chlorophyll a and
chlorophyll b. Chlorophyll a is one of the most
abundant photosynthetic pigments in a plant’s leaf. It absorbs red, orange, yellow, and
blue light but not green. In fact, since green light is not
absorbed, most plant leaves look green to our eyes.
Now, let’s take a look at the
absorption spectra provided by the question. Each of the three black traces on
the graph shows the level of light of different wavelengths that are absorbed by
different molecules. So which one of the three black
traces shows an absorption of red, orange, yellow, and blue but not green
wavelengths of light? The solid line, line one, shows
absorption peaks in all colors except green. Therefore, the line that
illustrates the absorption spectrum for chlorophyll a is line one.
Let’s solve one final practice
question together.
How are the thylakoids adapted for
their function? (A) Thylakoids contain
photosynthetic pigments within their membrane to absorb light. (B) Thylakoids have a small
surface-area-to-volume ratio, so reactions can occur more quickly. (C) Thylakoids contain specialized
enzymes that carry out the process of respiration. Or (D) thylakoids have a double
membrane that allows them to control what enters and leaves the chloroplast.
Thylakoids are disklike structures
contained within the chloroplasts of some plant cells. The thylakoids form stacks called
grana or a singular granum, and their function is to assist the chloroplast with
carrying out photosynthesis. The primary way that thylakoids are
adapted to help the chloroplast with this function is by containing photosynthetic
pigments within their membranes. These photosynthetic pigments,
which are most commonly chlorophyll A in most vascular land plants, absorb available
sunlight and transfer it to reaction centers, where the energy is used to carry out
photosynthetic reactions.
Looking at our options, it seems
that option (A) is the correct answer. But let’s double-check this by
discussing the other choices. Option (B) is not correct as the
disklike structures of the thylakoids actually provides them with a large
surface-area-to-volume ratio. It also allows them to contain more
pigments and so increase the rate of photosynthetic reactions. Option (C) is also incorrect, as
although thylakoids may contain specialized enzymes, it is to aid the process of
photosynthesis and not to aid respiration. Respiration is carried out by
different organelles, the mitochondria. Option (D) is also incorrect as the
thylakoids do not have a double membrane, and they also do not control what enters
or leaves the chloroplast. Instead, this is the role of the
double membrane that surrounds the chloroplast itself and not the membrane of the
thylakoids.
So the correct answer is option
(A). Thylakoids contain photosynthetic
pigments within their membrane to absorb light.
Let’s summarize the key points that
we’ve learned in this video about the structure of the chloroplast. Chloroplasts are organelles within
a plant cell that acts as a site of photosynthesis. They have a double membrane and
contain thylakoids arranged into stacks called grana or a singular granum suspended
in a fluid called the stroma. Chloroplasts contain photosynthetic
pigments, which are primarily chlorophyll A and B, but they may also contain
pigments like beta carotene and xanthophyll. The thylakoids are the site of the
light-dependent reactions of photosynthesis and contain photosynthetic pigments for
this function. The stroma is the site of the
light-independent reactions of photosynthesis and contains the required enzymes,
coenzymes, and carbon dioxide.
