Roman Demura
Plant Biology 1030-1035
Report: Plastids
This report will discuss plastids; what they are, what kinds are there, what their
functions are, and their characteristics. There are three different general types of plastids in
plant cells; chromoplasts, leucoplasts, and chloroplasts. Chloroplasts being the most important
plastids are responsible for photosynthesis. But there are also other six types of plastids besides
chloroplasts, chromoplasts, and leucoplasts that will be described. Plastids can morph from one
type to another. At one point in time, plastids were their own separate organisms (some still
are) until they established a symbiotic relationship with eukaryotes. Plastids are cell organelles
that perform cell functions for plant cells, without them a plant cell would not be able to live.
Plastids really are prokaryotes (the most primitive life forms.) Billions of years ago
plastids were their own separate organisms and some still are. Those organisms are called
cyanobacteria; which is a prokaryote that has the ability to perform photosynthesis. During
some time a eukaryotic cell swallowed some cyanobacteria but the eukaryote did not digest it,
instead it developed a symbiotic relationship between itself and the cyanobacteria, and the
cyanobacteria became a plastid for the eukaryote cell. That is how plant cells have chloroplasts
and are able to perform photosynthesis. The idea that a prokaryote was engulfed by a
eukaryote and then both organisms developed a symbiotic relationship is called the
“endosymbiotic theory.”
The general types of plastids which are chloroplasts, leucoplasts, and chromoplasts have
their own individual functions. All of these plastids are necessary to sustain a plant life cycle and
reproduction. These plastids are located in leaves, roots, fruits and flowers of plants.
Chloroplasts sustain life cycles while chromoplasts sustain plant reproduction, and leucoplasts
are storage organelles.
Chloroplasts are made out of green pigmented molecules and gives plats their green
color. Chloroplasts are located in leaf cells; chloroplasts use light, water molecules and carbon
dioxide molecules to make sugars and oxygen. Within the chloroplast membrane there are
structures called thylakoids, and stroma. Thylakoids are structures that look like stacked
pancakes and are made up of membrane. The stacks of pancake-like structures are called
grana. Within the thylakoid is empty space called “thylakoid space.” Stroma is a fluid filled
space within the chloroplast that contains enzymes that help perform photosynthesis.
Thylakoid and stroma are both responsible for performing photosynthesis in the chloroplast.
Photosynthesis is divided into two different types of reactions, light dependent
reactions and light independent reactions. Light dependent reactions take place in the
thylakoids. Light independent reactions take place in the stroma. Despite the name “light
independent reactions” the reaction still happens during the day and never happens during
night. Light independent reactions are performed by the enzymes in the stroma.
Chromoplasts are pigmented plastids that are found in fruit and flower tissues. They
contain carotenoid pigments that give colors to the tissues they reside in. Chromoplasts give
red, orange, and yellow colors to plant tissues. Chromoplast function is to colorize plant parts
such as flowers, so that pollinators get attracted to them to pollinate the flowers. They are also
in fruits to add color to the fruit so that animals would be attracted to eat the fruits and spread
seeds.
Leucoplasts are colorless plastids that function as starch and oil producers and storage.
If leucoplasts are exposed to light they can turn into chloroplasts and vice versa. Leucoplasts
are found in non-photosynthetic tissues such as roots, endosperm, tubers, and cotyledons.
There are other six types of plastids besides chloroplasts, chromoplasts, and leucoplasts.
These are proplastids, amyloplasts, elaioplasts, proteinoplasts, gerontoplasts and etioplasts.
These plastids also can be found in a plant cell. Many of these organelles can differentiate into
one another.
Proplastids are organelles which are found in meristem cells. Proplastids don’t have any
important function, though they can differentiate into any other type of plastids. Proplastids
contain thylakoids, but are smaller than the thylakoids of chloroplasts and the thylakoids in
proplastids are the key for a proplastid to differentiate into chloroplasts. The thylakoids in
proplastids become bigger when proplastids receive sunlight. Proplastids are also found in plant
seeds where they provide form of nutrition to the young plant.
Amyloplasts are organelles that reside in root cells and storage tissues. Amyloplasts are
what guides the root cells down. In root cells the starch grains formed by amyloplasts are
heavier then all of the other cell contents. With this the starch grains sink down to the bottom
of the cell which gives the root cell a sense of where gravity is and which direction it should
grow. Amyloplasts also store starch, starch grains of amyloplasts can be found in potatoes.
Elaioplasts are plastids that store lipids. Elaioplasts can be found in whichever plant
tissues require lipid storage. Elaioplasts are crucial for forming pollen grains in flower anthers.
Elaioplasts accumulate lipids in structures called plastoglobules.
Proteinoplasts are plastids that store protein. Proteinoplasts are found in nuts and
seeds. Other names for proteinoplasts are proteoplasts, aleuroplasts and aleuronaplasts, but
these names are no longer being used.
If chloroplasts break down they become gerontoplasts. The components that
breakdown in chloroplasts which cause them to become gerontoplasts are thylakoids and
stroma. Gerontoplasts accumulate a plastoglobuli which contain indigestible molecules which
are derived from chlorophyll. Gerontoplasts are basically dead chloroplasts.
Etioplasts are plastids which form in plant tissues that don’t receive sunlight. Etioplasts
form from proplastids. Etioplasts contain starch grains and very little thylakoid. However
etioplasts can develop into chloroplasts if they receive light, and chloroplasts can become
etioplasts if those chloroplasts don’t receive enough light.
Plastids can differentiate between one another. Plastid differentiation occurs during
developmental processes in plants. An example of plastid differentiation is how a proplastid can
differentiate into chloroplast as mentioned before. Another example is how chloroplasts can
differentiate into chromoplasts in ripening fruits. This is why immature fruits are green, and
ripe fruits are red, orange, or yellow. Another example of differentiation is when potato cells
become green when they are exposed to light. The amyloplasts in potato cells become
chloroplasts. In flowers plastid differentiation also occurs when flower petals become colorful.
Immature petals contain chloroplasts, but when the flower matures those chloroplasts turn into
chromoplasts giving color to flowers.
All plastids are enclosed by two membranes (outer membrane and inner membrane.)
The outer membrane acts as a barrier to the movement of proteins. The inner membrane of
plastids is a permeability barrier between the cell cytoplasm and the plastid stroma.
It was proven that plastids inside plant cells have their own DNA. But much of the
plastid DNA was moved into the host cell’s nuclear genome through evolution. The fact that
plastids have their own DNA was discovered by Lynne Margulis in 1967. The DNA of a plastid
resides in a protein-DNA structure called a plastid nucleoid.
Plastids divide and reproduce much like prokaryotic organisms like bacteria. Plastids
divide within the cell inside the cell cytoplasm. The process of plastid division is called binary
fission. Keeping a plastid population is necessary for a plant. Plastid division was discovered by
observation of meristem cells in developing leaf mesophyll. Plastids in the leaf cells appeared to
be in a shape of a dumbbell. The dumbbell shapes in the mesophyll cells eventually split apart;
this was evidence of binary fission. When plastids are splitting, there is an electron-dense ring
known as the plastid dividing ring which forms around the middle of a plastid.
Plastids are really the ones that do the photosynthesis and storage for plants, without
plastids a plant would not be able to function and perform metabolism. Plastids really are
prokaryotic cells which lived on their own billions of years ago. The endosymbiosis theory is the
assumption of how a prokaryotic organism established a symbiotic relationship with a
eukaryote and became its organelle. There are nine types of plastids three of which are the
three general plastids. Plastids can differentiate between the nine types of plastids there are, a
good example of plastid differentiation is when plant fruits and flowers develop. Plastids have
their own DNA as well as their own membrane, and plastids also divide within the plant cell by
the process known as binary fission.
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