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Essentially, chloroplasts are plastids found in
cells of higher plants (plants with advanced traits with lignified tissue for
transport of water and minerals) and algae as sites of photosynthesis. This
makes them the most important cell organelles given that plants are the primary
producers and the base of all food chains.
Depending on the type of plant or
algae, the number of chloroplasts in a cell may range from 1 to 100. They are located in the cell cytoplasm
and move across the cell cytoplasm along with the cellular fluids.
*Plastids are large organelles commonly found in
plants and algae. These organelles serve as sites of manufacture and storage
(either or both functions) and include chromoplasts (chloroplast is a type of
chromoplast) and leucoplasts such as elaioplast and amyloplast.
* The word chloroplast comes from the Greek words
"chloros" and "plast" meaning green and form respectively.
Compared to other organelles like the
mitochondria, chloroplasts are relatively larger ranging from 4 to 10
micrometers in diameter and about 2 micrometers in thickness.
Their shape also
varies from one plant/algae to another and may appear spherical, ovoid or even
cup-shaped. While they may appear spherical or ovoid in maize
plant, they are seen to appear as spiral coils in spirogya. However, the shape
of a mature chloroplast is always regular.
* The nucleus, for most organisms, is the organelle
that contains DNA. However, DNA is also found in organelles classified as
plastids including mitochondria and chloroplast. The circular DNA of
chloroplast is refered to as cpDNA and helps regulate how the organelle
Compared to other organelles, chloroplasts have
three types of membranes that serve different functions. These include:
The smooth outer membrane
(outer envelope membrane)
The smooth inner membrane
(inner envelope membrane)
Thykaloid membrane system
Outer envelope membrane (OEM) - Being the outer most
membrane, the outer envelope membrane (OEM) plays an important role as the
physical barrier between the organelle and the cytoplasmic environment.
Communication between the inner components of the organelle and the
cytoplasmic environment is mediated by this membrane. Some of the primary
functions of the OEM include the importation of proteins (nuclear-encoded
proteins) movement (diffusion) of other compounds with low molecular weight and
ions, as well as such functions as the site for biosynthesis of lipids.
the most important properties of the outer membrane is that it contains high
amount of lipids than protein (3:1 ratio). This characteristic makes the OEM
the lightest membrane of the three.
* Recent studies have shown that the outer
membrane to contain substrate specific channels, cat-ion-selective channels as
well as a variety of transporters such as the ABC transporters, OEP23 and 16,
21 and 37 OEPs channels. These channels serve to regulate the movement of
molecules and ions in and out of the chloroplast.
Inner envelope membrane - Compared to the outer
membrane that is usually considered to be a more passive barrier, the inner
envelope membrane (IEM) is more selective and only allows some compounds and
metabolites in and out of the organelle.
For the most part, transport across the inner
membrane is regulated by active transport where the proteins located in the
membrane (IEM) actively transport molecules and ions.
Some of the most popular
transporters in the inner membrane include the IEP30, 1EP33 AND IEP45 among
others. These transporters have been shown to perform their function more
effectively when they are hydrophobic (repelling water or not mixing with
Through active transport of metabolites and other ions etc, the inner
membrane ensures that there is equilibrium of raw material (anabolic
precursors) and the final products from the organelle.
Some of the other
functions of the inner envelope membrane include the synthesis of different
types of metabolites and cell division of the organelle. Because the inner
membrane is highly folded with roughly the same protein lipid ratio, it is
heavier compared to the outer envelope membrane.
* The space between the inner and outer membrane
is known as the inter-membrane space and is between 10 and 20 nanometers
Thylakoid membrane system - This system makes up the
internal membrane system. This system appears as flattened disks and is the
site of photosynthesis (on the membrane) the thylakoid membrane enclose thylakoid
that are arranged in stacks (10 to 20 stacks) known as grana. Here, these
stacks are all connected by a single membrane with the stroma thylakoids (stroma
lamellae) connecting the grana. The architecture of thylakoid varies from one
plant to another.
As a result, there are three different models of thylakoid
The form model
Like the other membranes, the thylakoid system
is made up of lipid bi-layers (galactosyl diglyceride is an example of lipids
making up the membrane system) with most of the lipids being those found in
other plastid membranes (galactosyl diglycerides etc). The thylakoid membrane
also encloses the thylakoid lumen, which is a single, large aqueous space.
these different parts of the thylakoid system play an important role in photosynthesis.
The stroma and grana are the two main parts of the thylakoid. As such, they are
also composed of different types and composition of proteins.
grana contain Photosystem II (PSII) as well as LHCII, which is its primary
chlorophyll a/b light harvesting complex. On the other hand, the stroma is
composed of Photosystem I (PSI) and Light Harvest Complex I (LHCI) which are
lacking in the grana.
Photosynthesis (Mechanism in the Thylakoid System)
Basically, photosynthesis is the process through
which plants (and other primary producers) are able to convert energy from
sunlight to chemical energy that is in turn used to convert water,
carbon-dioxide and minerals into organic compounds (glucose).
In the thylakoid
system, this takes place on the thylakoid membrane and stroma. Here, the
photosynthetic pigments are embedded in the thylakoid membrane.
(photosynthesis) involves two major stages including the light phase (light
reactions) and the dark phase (dark reactions). Whereas the light reactions are
involved in the production/synthesis of ATP (Adenosine triphosphate) and NADPH
(Nicotinamide adenine dinucleotide phosphate), dark reaction is involved in the
production of the organic compound by using ATP and DADPH.
* Light reactions occur in the thylakoid membrane while dark reactions take place in the stroma
Electron Flow (Light Reactions)
Photosystems I and II (PSII and PSI) are two of
the most important transmembrane protein complexes involved in electron
transfer in light reactions. These photosystems contain chlorophyll pigments
that absorb light energy.
When sunlight is absorbed by the peripheral chlorophyll
molecules (in photosystem II), it is transported (through Resonance Energy
Transfer (RET)) to the reaction center, which is the central pair of
In the process, the energy causes the electrons to be
exited at a higher state and the subsequent loss of electrons from the
photosystem. These electrons then enter into the electron transfer chain where
they are required for the synthesis of ATP and NADPH.
* Every electron lost from photosystem II is
replaced by electrons obtained from split water molecules. Every time this
photosystem absorbs light photons, it is able to split water molecules to
replace lost electrons (of both PSII and PSI).
Five Protein Complexes involved in Electron Transfer
PSII and PSI - These two photosystems are two of the five
protein complexes. Since they contain chlorophyll (pigment that absorbs
sunlight energy) they release electrons that are then transported through the
electron transfer chain.
Plastoquinone (PQ) - Before the electrons arrive
at the cytochrome bf complex, they have to be carried by carriers to this
destination. This role is carried out by plastoquinone. When the electrons are released
from the photosystems (PSII), they are accepted by plastoquinone (it also
accepts hydrogen ions from the stroma). Electrons from the photosystems are
then transported by plastoquinone to the cytochrome b6f complex while the
hydrogen ions (protons) are transported to the lumen (thylakoid lumen) which is
also important or synthesis and production of ATP.
Cytochrome b6f complex- Electrons carried by plastoquinone
are transported to the cytochrome bf complex, which in turn transfers these
electrons (as well as protons from stroma) to the plastocyanin. During
photosynthesis, this complex enzyme and contributes in the transfer of
electrons to PSI while mediating in the pumping of protons (into thylakoid
lumen space) to contribute in the synthesis of ATP.
Plastocyanin (PC) - From the cytochrome bf
complex, electrons are transferred to the plastocyanin, which acts as a carrier
that in turn transports these electrons to PSI. As with PSII, photons cause the
electrons to become exited and act at higher energy level.
Here, the reaction
center belonging to PSI moves these electrons to a small protein known as ferrodoxin
located in the thylakoid membrane (stromal side) where NADP reductase (an
enzyme) helps synthesize DADPH by moving the electrons in this protein (ferrodoxin
) to NADP ion. Ferredoxin acts as a carrier that accepts the
electrons and consequently reduced to give up the electrons for synthesis of
* This process (transport process) is also
involved in the production of ATP. Here, the protons (Hydrogen ion) transported
in the electron transfer chain provides the energy required to produce ATP from
the phospholylation of ADP (adenosine di-phosphate). ATP synthase enzyme uses
this energy to catalyze ATP from ADP.
The light reaction involves two important steps which include photolysis and photophosphorylation. Whereas photolysis is the process involved in water splitting (releasing oxygen, hydrogen and electrons) photophosphorylation uses these components to produce ATP energy, which is a chemical energy.
Photophosphorylation may occur through the process already described above to produce ATP and NADPH through a process known as Non-cyclic photophosphorylation. However, it can also occur through another process known as cyclic-photophosphorylation (cyclic electron flow) where the end product is only ATP.
Unlike light dependent reactions, light-independent reactions take place in the stroma of the chloroplast which is filled with fluids. As the name suggests, dark reactions do not require light energy and thus take place in the absence of light as such, they are also refered to as light independent reactions. By using the Calvin Cycle, it becomes easier to understand the light independent reaction:
* Calvin cycle is named after Calvin Benson, who discovered it and explains the reactions that produce carbohydrate molecules.
This process takes place in the absence of light (in the dark) it starts with the plant taking in carbon-dioxide through the stomata (pores on the surface of leaves) which moves to the stroma. The processes that follow are divided into three main phases. These include:
Fixation- During fixation, an enzyme known as RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase) in the stroma acts as the catalysts in the reaction between the carbon-dioxide and a molecule known as ribulose bisphosphate (RuBP) which is also present in the stroma of the chloroplast. This reaction results in the production of a compound with six carbon that is then converted into two 3-Phosphoglyceric acid, a compound with three carbons.
Reduction - In this phase, energy from the light dependent phase (in form of ATP and NADPH) is used to convert the 3-Phosphoglyceric acid molecule into Glyceraldehyde 3-phosphate (G3P) which also contains three carbons. In this phase, reduction occurs with NADPH donating electrons to produce the G3P, a three-carbon sugar. Here, ATP is also reduced to ADP.
Regeneration - While some of the G3P molecules go on to form such carbohydrates as glucose, those that remain are recycled to regenerate RuBP to start fixation again.
To view chloroplasts under the microscope,
students can use toluidine blue stain to prepare a wet mount. This simply
involves the following simple steps:
Place a plant sample onto
drop of water on a clean glass slide
Using a dropper, add a drop
of the stain (toluidine blue) on the sample and allow to stand for about a
Add 2 drops of water to
rise the sample and remove any excess liquid using a tissue
Cover the slide with a
cover slip and view under the light microscope
Observation - When viewed under the microscope, students
will be able to distinguish different parts of the cell including the plastids
(chloroplast and mitochondria). On the other hand, a simply wet mount (even
without staining) will show chloroplast to be small green (or dark green)
sports across the cell surface.
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