Plastids and Chloroplasts: Study Notes

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Plastids

Introduction:

• Plastids are specialized plant organelles derived from precursor structures called proplastids. They are involved in various synthetic and storage functions.

Definition:

• Membrane-bound, oval-shaped organelles functioning as chemical synthesizers and storage bodies.

Occurrence:

• Found abundantly in plant cells and certain primitive organisms like Euglena.

Types of Plastids

• Chloroplast
  • Definition: Green plastids, scattered in the cytoplasm.
  • Occurrence: Found in parts exposed to light, especially in green leaves.
  • Function: Manufacture food material in the presence of sunlight using CO₂ and H₂O.
• Chromoplast
  • Definition: Colored plastids, often found in petals of flowers and fruits.
  • Occurrence: Present in petals of flowers and fruits.
  • Function: Make petals showy to attract pollinators.
• Leucoplast
  • Definition: Colorless plastids, develop in the absence of sunlight.
  • Occurrence: Found in storage cells of roots and underground stems.
  • Function: Convert sugar into starch and store carbohydrates, proteins, and lipids.

Chloroplasts (Plant Cells)

Membrane:

• Double

Structure:

• Have outer and inner membranes that enclose the stroma
• Stroma is a homogenous matrix containing photosynthetic enzymes, DNA, and ribosomes
• Grana are stacks of membrane-bound, flattened sacs with chlorophyll molecules
• One thylakoid stack is known as a granum, each with chlorophyll on its surface
• Grana are connected by stromal lamellae, acting as the chloroplast's skeleton
• Chloroplasts can make their own proteins due to their own DNA; semi-autonomous
• Proplastids are immature plastids in meristematic cells, maturing into leucoplasts, chloroplasts, or chromoplasts

Functions:

• Thylakoids are the site of light-dependent reactions of photosynthesis
• Stroma is the site of light-independent reactions of photosynthesis

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