Plastids are
specialized organelles enclosed by a membrane, and they contain various
pigments. They are unique to plant cells.
Proplastids:
Precursors of Specialized Plastids
There are three types
of plastids: leucoplasts, chromoplasts, and chloroplasts. All plastids
originate from proplastids, which are precursor organelles found in less
specialized plant cells, especially in growing and undeveloped tissues.
Depending on their specific functions, proplastids can develop into various
specialized mature plastids. Interestingly, even mature plastids can convert
from one form to another under certain conditions, making them highly versatile
organelles.
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Leucoplasts:
Colorless Organelles for Food Storage
Leucoplasts are
colorless plastids that have triangular or tubular shapes. They are usually
found in underground parts of plants and have two main functions: storing food
and synthesizing starch from glucose. Leucoplasts that store starch are called
amyloplasts.
Chromoplasts:
Plastids Responsible for Non-Green Pigments
Chromoplasts are
plastids that give rise to colors other than green, such as red or orange. They
are typically found in petals and ripened fruits, and their main function is
the synthesis and localization of various pigments.
Chloroplasts: Green
Plastids for Photosynthesis
Chloroplasts are green
plastids that are primarily located in the green parts of plants, such as
leaves and stems. Chloroplasts are produced from proplastids when they are
exposed to light.
Structure of
Chloroplasts
Chloroplasts are
characterized by their unique structure. Each chloroplast is enclosed by a
smooth double membrane, with an inner membrane folded into flattened vesicles
called thylakoids. These thylakoids are arranged in stacks called grana, with
each granum consisting of 50 or more thylakoids. There can be hundreds of
granum within a single chloroplast. Chlorophyll, the pigment responsible for
the green color of chloroplasts, is located in the thylakoid membranes.
Additionally, chloroplasts contain DNA.
Chloroplasts: Energy
Conversion Centers
Chloroplasts are
essential for photosynthesis, the process by which plants convert sunlight into
energy. Photosynthesis consists of two sets of reactions: light-dependent
reactions and light-independent reactions.
Light-Dependent
Reactions in Thylakoids
Light-dependent
reactions occur in the thylakoids and require the presence of light. During
these reactions, low-energy electrons are removed from water molecules and
energized by the solar energy absorbed by the pigments in the thylakoid
membranes. These energized electrons move through an electron transport system,
producing ATP (adenosine triphosphate) from ADP (adenosine diphosphate) and phosphate.
The energized electrons are also taken up by NADP (nicotinamide adenine
dinucleotide phosphate), converting it into NADPH.
Light-Independent
Reactions in Stroma
Light-independent
reactions, also known as the Calvin cycle, take place in the stroma of
chloroplasts and do not require light. These reactions use ATP and NADPH, which
were formed during the light-dependent reactions, to reduce carbon dioxide (CO2)
and synthesize organic molecules, including glucose, which are used as energy
sources and building blocks for plant growth.
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